Mary Jones Richard Fosbery Jennifer Gregory and Dennis TaylorCambridge International AS and A Level

BiologyCoursebookThird editionCAMBRIDGE UNIVERSITY PRESS Cambridge New York Melbourne Madrid Cape Town Singapore Satildeo Paulo Delhi Mexico CityCambridge University Press The Edinburgh Building Cambridge CB2 8RU UKwwwcambridgeorg Information on this title wwwcambridgeorg9781107609211copy Cambridge University Press 2003 2013This publication is in copyright Subject to statutory exception and to the provisions of relevant collective licensing agreements no reproduction of any part may take place without the written permission of Cambridge University PressFirst published 2003 Second edition 2007 Third edition 2013 Reprinted 2013Printed in the United Kingdom by Latimer TrendA catalogue record for this publication is available from the British LibraryISBN978-1-107-60921-1 Paperback with CD-ROM for Windowsreg and MacregCambridge University Press has no responsibility for the persistence or accuracy of URLs for external or third-party internet websites referred to in this publication and does not guarantee that any content on such websites is or will remain accurate or appropriate

ContentsIntroduction vi 1 Cell structure 1Why cells 2 Cell biology and microscopy 2 Animal and plant cells have features in common 4 Differences between animal and plant cells 5 Units of measurement in cell studies 6 Electron microscopes8 Ultrastructure of an animal cell 13 Structures and functions of organelles 13 Ultrastructure of a plant cell 18 Two fundamentally different types of cell 18 Tissues and organs 20 End-of-chapter questions24

2 Biological molecules 29The building blocks of life 30 Monomers polymers and macromolecules 30 Carbohydrates 30 Lipids 37 Proteins 39 Water 47 End-of-chapter questions 493 Enzymes 54Enzymes reduce activation energy 57 The course of a reaction 57 Enzyme inhibitors

62 End-of-chapter questions 634 Cell membranes and transport 69Phospholipids 69 Structure of membranes 70 Transport across the cell surface membrane 73 End-of-chapter questions 815 Cell and nuclear division 86The nucleus contains chromosomes 86 The structure of chromosomes 88 Two types of nuclear division 89 Mitosis in an animal cell 90 Cancer 93 End-of-chapter questions 996 Genetic control 103The structure of DNA and RNA 103 DNA replication 105 Genes and mutations

109 DNA RNA and protein synthesis 109 End-of-chapter questions 1157 Transport in multicellular plants 118The need for transport systems in multicellular organisms 118 The transport of water

120 Transport in multicellular plants 120 Translocation 133 Differences between sieve tubes and xylem vessels 137 End-of-chapter questions 1388 The mammalian transport system 144The mammalian cardiovascular system 144 Blood plasma and tissue fluid 150 Lymph

150 Blood 151 Haemoglobin 154 Problems with oxygen transport 157 End-of-chapter questions 1609 The mammalian heart 164The cardiac cycle166 Control of the heart beat168 End-of-chapter questions 17010 Gas exchange 174Lungs 174 Trachea bronchi and bronchioles 174 Alveoli 177 End-of-chapter questions 17811 Smoking 182Tobacco smoke 182 Lung diseases 182 Cardiovascular diseases 185 Proving the links between smoking and lung disease 188 Prevention and cure of coronary heart disease 192End-of-chapter questions194Contentsiii12 Infectious diseases 197Worldwide importance of infectious diseases 197 Cholera 198 Malaria 200 Acquired immune deficiency syndrome (AIDS) 203 Tuberculosis (TB) 208 Antibiotics

211 End-of-chapter questions 21313 Immunity217Defence against disease 217 Cells of the immune system 218 Active and passive immunity 225 Vaccination 226 Problems with vaccines 227 The eradication of smallpox228 Measles 230 End-of-chapter questions 23114 Ecology 235Energy flow through organisms and ecosystems 236 Matter recycling in ecosystems 241 The nitrogen cycle 241 End-of-chapter questions 24515 Advanced practical skills 248Experiments 248 Variables and making measurements 249 Estimating uncertainty in measurement 257 Recording quantitative results 258 Constructing a line graph 259 Constructing bar charts and histograms 260 Drawing conclusions

261 Describing data 261 Making calculations from data 261 Explaining your results 263 Identifying sources of error and suggestingimprovements 263 Drawings 264 End-of-chapter questions 26516 Energy and respiration 268

The need for energy in living organisms 268 Work 268 ATP 270 Respiration 273 Anaerobic respiration 279 Respiratory substrates 280 End-of-chapter questions 28317 Photosynthesis 287An energy transfer process 287 The light-dependent reactions of photosynthesis 288 The light-independent reactions of photosynthesis 290 Leaf structure and function

290 Chloroplast structure and function 293 Factors necessary for photosynthesis 294 Trapping light energy 295 End-of-chapter questions 29718 Regulation and control 301Homeostasis 302 Excretion 302 The structure of the kidney 303 Control of water content 311 Nervous communication 314 Hormonal communication 329 Plant growth regulators336 Electrical communication in plants 339 End-of-chapter questions 34019 Inherited change 347Meiosis 347 Genetics 348 Genotype affects phenotype 351 Inheriting genes 352 Multiple alleles 354 Sex inheritance 355 Sex linkage 355 Dihybrid crosses 357 The 11130882 (chi-squared) test 359 Mutations 361 Environment and phenotype 363 End-of-chapter questions 36320 Selection and evolution 367Natural selection 368 Evolution 370 The DarwinndashWallace theory of evolution by natural selection 374 Species and speciation 375 Artificial selection 377 End-of-chapter questions 37821 Biodiversity and conservation 382The five-kingdom classification 382 Maintaining biodiversity 385 Endangered species 385 End-of-chapter questions 392ivContents22 Gene technology 395Gene technology 395 Benefits of gene technology 399 Potential hazards of gene technology 400 Social and ethical implications of genetic engineering 402 Electrophoresis 403 Cystic fibrosis 404 The genetic counsellor 407 Genetic screening 409 End-of-chapter questions 41123 Biotechnology 414Mining with microorganisms 414 Large-scale production techniques 416 Advantages of batch and continuous culture 419 How penicillin works 420 Immobilising enzymes 422 Monoclonal antibodies 424 End-of-chapter questions 42724 Crop plants 431Cereal crops 431 Maize 433 C4 plants 434 Adaptations for difficult environments 437 Crop improvement 440 End-of-chapter questions 44725 Aspects of human reproduction 451Gametogenesis 453 Human menstrual cycle 456 Birth control 457 Infertility 459 End-of-chapter questions 46426 Planning analysis and evaluation 468 Planning an investigation

468Analysis conclusionsand evaluation End-of-chapter questions4 7 3 480Appendix 1 Amino acid R groups 484 Appendix2 DNAtripletcodes 485 Glossary 486 Index 501 Acknowledgements 510CD-ROMAdvice on how to revise for and approach examinationsChapter summariesMultiple choice tests for Chapters 1ndash14Answers to self-assessment questions Answers to end-of-chapter questions

Contentsv

IntroductionThis new edition is fully updated for the 2014 syllabus to help you do well in your Cambridge International Examinations AS and A level Biology (9700) courses The book and its accompanying CD-ROM provide a self-contained resource for studying these courses with improved focus on exam preparationbull Chapters 1ndash15 provide complete coverage of the AS level syllabus This is also the first year of study for A level The AS syllabus is designed for studentsbull with O level or IGCSE Biology Chapters 16ndash26 cover all the material for the second year of study for A level This includes the relevant Core material and the Applications of Biology sectionExtension material is clearly marked with the following symbol E

and a dotted line runs down alongside the text to mark this additional contentImportant features of this new edition include the followingThe sequence of chapters mirrors the sequence of topics in the syllabus which makes it easy to navigate (Your teacher may however tackle subjects in a different order) Syllabus sections G and H are split into three and two chapters respectively for additional convenience (see table)LevelApplications of BiologySyllabus section Chapter PSelection and evolution 20 Q Biodiversity and 21conservation R Gene technology 22 S Biotechnology 23 T Crop plants 24 U Aspects of human reproduction 25Planning analysis and evaluation26LevelAS level Core syllabusA levelSyllabus section A Cell structureChapter1There are two new chapters covering practical skills Chapter 15 (AS level) and Chapter 26 (A level) Interesting information that is not required by the syllabus but will aid understanding is marked as lsquoextension materialrsquo by orange dotted barsEach chapter contains self-assessment questions (SAQs) These are to help you think about understand and remember what you have just read Each chapter ends with a set of chapter-related questions ranging from formative questions (requiring simple recall or reference to the text) to more challenging structured or essay questions requiring understanding as well as the other skills tested in examinations Some of the questions are past Cambridge examination questions so you can familiarise yourself with the style of the examination questionsBiology involves many technical terms Each time a new term is introduced it is shown in bold orange and its meaning explained The glossary contains definitions of the key terms used in the book

At the end of your course you will be tested on three sets of Assessment Objectivesbull Knowledge with understanding You are expected to know and understand all the facts and concepts listedbull in the syllabus These are all covered in this book Handling information and solving problems Questions testing these skills expect you to use your knowledge and understanding in an unfamiliar context A good knowledge and understanding of this book will enablebull you to approach new situations with confidence Experimental skills and investigations This involves practical work An examination will test your practical skills so try to do plenty of practical work Key information is provided on some practical aspects of the course in Chapters 15 and 26Additional help and guidance are available on the accompanying CD-ROMF Genetic control G Transport7 8vi IntroductionB Biological molecules 2 C Enzymes 3 D Cell membranes and transport

4 E Cell and nuclear division 56Transport in multicellular plants The mammalian transport system The mammalian heart 9H Gas exchange and smoking Gas exchange 10 Smoking 11I Infectious disease 12 J Immunity 13 K Ecology 14Advanced practical skills 15 L Energy and respiration 16 M Photosynthesis

17 N Regulation and control 18 O Inherited change 19

1 Cell structure By the end of this chapter you

should be able todescribe and interpret drawings and photographs of typical animal and plant cells as seen using the light microscope and make microscopical measurements using an eyepiece graticule and stage micrometerbe familiar with the units used in cell studies explain the meanings of and distinguish betweenthe terms resolution and magnificationdescribe and interpret drawings and photographs of typical animal and plant cells as seen using the electron microscope recognising rough and smooth endoplasmic reticulum (ER) Golgi apparatus mitochondria ribosomes lysosomes cell surface membrane centrioles nucleus (including the nuclear envelope and nucleolus) and microvilli as well asIn the early days of microscopy an English scientist Robert Hooke decided to examine thin slices of plant material He chose cork as one of his examples Looking down the microscope he was struck by the regular appearance ofthe structure and in 1665 he wrote a book containing the diagram shown in Figure 11If you examine the diagram you will see the lsquopore-likersquo regular structures that Hooke called lsquocellsrsquo Each cell appeared to be an empty box surrounded by a wall Hooke had discovered and described without realising it the fundamental unit of all living things

Although we now know that the cells of cork are dead further observations of cells in living materials were made by Hooke and other scientists However it was not until almost 200 years later that a general cell theory emerged from the work of two German scientists In 1838 Schleiden a botanist suggested that all plants are made of cells and a year later Schwann a zoologist suggested the same for animals The cell theory states that the basic unit of structure and function of all living organisms is the cell Now over 170 years later this idea is one of the most familiar and important theories in biology To it has beenthe chloroplasts cell wall large permanent vacuole tonoplast and plasmodesmata of plant cellsoutline the functions of the structures listed abovecompare the structure of typical animal and plant cellscalculate the linear magnification of and the actual sizes of specimens from drawings and photographsdescribe the structure of a prokaryotic cell and compare and contrast the structure of prokaryotic cells with that of eukaryotic cellsexplain how eukaryotic cells may be organised into tissues and organs with reference to transverse sections of stems roots and leavesdraw and label low-power plan diagrams of tissues and organsadded Virchowrsquos theory of 1855 that all cells arise from pre-existing cells by cell divisionFigure 11 Drawing of cork cells published by Robert Hooke in 16651 Cell structure 1

Why cellsA cell can be thought of as a bag in which the chemistry of life is allowed to occur partially separated from the environment outside the cell The thin membrane which surrounds all cells is essential in controlling exchange between the cell and its environment It is a very effective barrier but also allows a controlled traffic of materials across it in both directions The membrane is therefore described as partially permeable If it were freely permeable life could not exist because the chemicals of the cell would simply mix with the surrounding chemicals by diffusion (page 73)Cell biology and microscopyThe study of cells has given rise to an important branch of biology known as cell biology Cells can now be studied by many different methods but scientists began simply by looking at them using various types of microscopeThere are two fundamentally different types of microscope now in use the light microscope and the electron microscope Both use a form of radiation in order to create an image of the specimen being examined The light microscope uses light as a source of radiation while the electron microscope uses electrons for reasons which are discussed latereyepiecelight beamobjective cover slipglass slidecondenseriris diaphragmlight source pathway of lightEyepiece lens magnifies and focuses the image from the objective onto the eyeObjective lens collects light passing through the specimen and produces a magnified image

Condenser lens focuses the light onto the specimen held between the cover slip and slideCondenser iris diaphragm is closed slightly to produce a narrow beam of lightFigure 12 How the light microscope worksLight microscopyThe lsquogolden agersquo of light microscopy could be said to be the 19th century Microscopes had been available since the beginning of the 17th century but when dramatic improvements were made in the quality of glass lenses in the early 19th century interest among scientists became widespread The fascination ofthe microscopic world that opened up in biology inspired rapid progress both in microscope design and equally importantly in preparing material for examination with microscopes This branch of biology is known as cytology Figure 12 shows how the light microscope worksBy 1900 all the structures shown in Figures 13 14 and 15 except lysosomes had been discovered Figure 13 shows the structure of a generalised animal cell and Figure 15 the structure of a generalised plant cell as seen with a light microscope (A generalised cell shows all the structures that are typically found in a cell)Golgi apparatus cytoplasmcentriole ndash always found near nucleus has a role in nuclear divisionsmall structures that are difficult to identifymitochondria cell surface membranenuclear envelopechromatin ndash deeply staining and thread-likenucleolus ndash deeply stainingnucleusFigure 13 Structure of a generalised animal cell (diameter about 20 1113088m) as seen with a very high quality light microscope22 1 Cell structuretonoplast ndash membrane surrounding vacuolecell surface membrane (pressed against cell wall)vacuole ndash large with central positioncytoplasmmitochondriamiddle lamella ndash thin layer holding cells together contains calcium pectateplasmodesma ndash connects cytoplasm of neighbouring cellscell wall of neighbouring cellcell wall chloroplast grana just visiblesmall structures that are difficult to identifyGolgi apparatusFigure 14 Cells from the lining of the human cheek (1113088 500) each showing a centrally placed nucleus which is a typical animal cell characteristic The cells are part of a tissue known as squamous (flattened) epitheliumFigure 14 shows some actual human cells and Figure 16 shows an actual plant cell taken from a leafSAQ 11Using Figures 13 and 15 name the structures that animal and plant cells have in common those found in only plant cells and those found only in animal cellsFigure 16 Photomicrograph of a cell in a moss leaf (11130881400)nucleusnucleolus ndash deeply stainingnuclear envelopechromatin ndash deeply staining and thread-likeFigure 15 Structure of a generalised plant cell (diameter about 40 1113088m) as seen with a very high quality light microscope1 Cell structure 3Box 1A Biological drawingYou need the following equipment

bull pencil (HB) bull pencil sharpener bull eraser bull ruler bull plain paperHere are some guidelines for the quality of your drawingbull always use a pencil not a pen bull donrsquot use shading bull use clear continuous lines bull use accurate proportions and observation ndash not atextbook version For a low-power drawing (see Figure 17)bull donrsquot draw individual cells bull draw all tissues completely enclosed by lines bull draw a correct interpretation of the distribution of bull tissuesa representative portion may be drawn (eg half a transverse section)For a high-power drawingbull draw only a few representative cells bull draw the cell wall of all plant cells bull donrsquot draw the nucleus as a solid blobSome guidelines for the quality of your labellingbull label all tissues and relevant structures bull identify parts correctly bull use a ruler for label linesAnimal and plant cells have features in commonIn animals and plants each cell is surrounded by a very thin cell surface membrane which is too thin to be seen with a light microscope This is also sometimes referred to as the plasma membranebull arrange label lines neatly and ensure they donrsquot cross bull over each otherannotate your drawing if necessary (ie provideshort notes with one or more of the labels in order bull to describe or explain features of biological interest)add a scale line at the bottom of the drawing if bull appropriateuse a pencil not a penAn example of a drawing of a section through the stem of Helianthus is shown below Biological drawing is also covered in Chapter 15 page 264Figure 17 The right side of this low-power drawing shows examples of good technique while the left side shows many of the pitfalls you should avoidMany of the cell contents are colourless and transparent so they need to be stained to be seen Each cell has a nucleus which is a relatively large structure that stains intensely and is therefore very conspicuous The deeply staining material in the nucleus is called chromatin and is a massof loosely coiled threads This material collects together to form visible separate chromosomes during nuclear division44 1 Cell structure(see page 86) It contains DNA (deoxyribonucleic acid) a molecule which contains the instructions that control the activities of the cell (see Chapter 6) Within the nucleus an even more deeply staining area is visible the nucleolus which is made of loops of DNA from several chromosomes The number of nucleoli is variable one to five being

common in mammalsThe material between the nucleus and the cell surface membrane is known as cytoplasm Cytoplasm is an aqueous (watery) material varying from a fluid to a jelly-like consistency Many small structures can be seen within it These have been likened to small organs and hence are known as organelles An organelle can be defined as a functionally and structurally distinct part of a cell Organelles themselves are often surrounded by membranes so that their activities can be separated from the surrounding cytoplasm This is described as compartmentalisation Having separate compartments is essential for a structure as complex as an animal or plant cell to work efficiently Since each type of organelle has its own function the cell is said to show division of labour a sharing of the work between different specialised organellesThe most numerous organelles seen with the light microscope are usually mitochondria (singular mitochondrion) Mitochondria are only just visible but films of living cells taken with the aid of a light microscope have shown that they can move about change shape and divide They are specialised to carry out aerobic respirationThe use of special stains containing silver enabled the Golgi apparatus to be detected for the first time in 1898 by Camillo Golgi The Golgi apparatus is part of a complex internal sorting and distribution system within the cell (see page 16) It is also sometimes called the Golgi body or Golgi complexDifferences between animal and plant cellsThe only structure commonly found in animal cells which is absent from plant cells is the centriole Plant cells also differ from animal cells in possessing cell walls large permanent vacuoles and chloroplastsCentriolesUnder the light microscope the centriole appears as a small structure close to the nucleus (see Figure 13 on page 2) The centriole is involved in nuclear division (see page 92)Cell walls and plasmodesmataWith a light microscope individual plant cells are more easily seen than animal cells because they are usually larger and unlike animal cells surrounded by a cell wall outside the cell surface membrane This is relatively rigid because it contains fibres of cellulose a polysaccharide which strengthens the wall The cell wall gives the cell a definite shape It prevents the cell from bursting when water enters by osmosis allowing large pressures to develop inside the cell (see page 77) Cell walls may also be reinforced with extra cellulose or with a hard material called lignin for extra strength (see xylem on page 24) Cell walls are freely permeable allowing free movement of molecules and ions through to the cell surface membranePlant cells are linked to neighbouring cells by means of fine strands of cytoplasm called plasmodesmata (singular plasmodesma) which pass through pore-like structures in the walls of these neighbouring cells Movement through the pores is thought to be controlled by the structure of the poresVacuoles

Although animal cells may possess small vacuoles such as phagocytic vacuoles (see page 80) which are temporary structures mature plant cells often possess a large permanent central vacuole The plant vacuole is surrounded by a membrane the tonoplast which controls exchange between the vacuole and the cytoplasm The fluid in the vacuole is a solution of mineral salts sugars oxygen carbon dioxide pigments enzymes and other organic compounds including some waste productsVacuoles help to regulate the osmotic properties of cells (the flow of water inwards and outwards) as well as having a wide range of other functions For example the pigments which colour the petals of certain flowers and parts of some vegetables such as the red pigment of beetroots are sometimes located in vacuoles1 Cell structure 5ChloroplastsSome plant cells are able to carry out photosynthesis because they contain chloroplasts Chloroplasts are relatively large organelles which are green in colour due to the presence of chlorophyll At high magnifications small lsquograinsrsquo or grana (singular granum) can be seen in the chloroplasts During the process of photosynthesis light is absorbed by these grana which actually consist of stacks of membrane-bound sacs called thylakoids Starch grains may also be visible within chloroplasts Chloroplasts are found in the green parts of plants mainly in the leavesPoints to noteWe return to the differences between animal and plant cells as seen using the electron microscope on page 18Units of measurement in cell studiesIn order to measure objects in the microscopic world we need to use very small units of measurement which are unfamiliar to most people According to international agreement the International System of Units (SI units) should be used In this system the basic unit of length is the metre (symbol m) Additional units can be created in multiples of a thousand times larger or smaller using standard prefixes For example the prefix kilo means 1000 times Thus 1 kilometre 1113088 1000 metres The units of length relevant to cell studies are shown in Table 11It is difficult to imagine how small these units are but when looking down a microscope and seeing cells clearly we should not forget how amazingly small the cells actually are The smallest structure visible with the human eye is about 50ndash100 1113088m in diameter Your body contains about 60 million million cells varying in size from about 5 1113088m to 40 1113088m Try to imagine structures like mitochondria which have an average diameter of 1 1113088m The smallest cell organelles we deal with in this book ribosomes are only about 25 nm in diameter You could line up about 20 000 ribosomes across the full stop at the end of this sentenceUnit Symbolmillimetre mm micrometre 1113088m nanometre nmTable 11 Units of measurement relevant to cell studies 1113088 is the Greek letter mu 1 micrometre is a thousandth of a millimetre 1 nanometre is a thousandth of a micrometre661 Cell structure

bull bull bullbullYou can think of a plant cell as being very similar to an animal cell but with extra structures Plant cells are often larger than animal cells although cell size varies enormouslyDo not confuse the cell wall with the cell surface membrane Cell walls are relatively thick and physically strong whereas cell surface membranes are very thin Cell walls are freely permeable whereas cell surface membranes are partially permeable All cells have a cell surface membrane Vacuoles are not confined to plant cells animal cells may have small vacuoles such as phagocytic vacuoles (see page 80) although these are not usually permanent structuresFraction of a metreone thousandth 1113088 0001 1113088 11000 1113088 10-3 one millionth 1113088 0000 001 1113088 11 000 000 1113088 10-6 one thousand millionth 1113088 0000 000 001 1113088 11 000 000 000 1113088 10-9

Box 1B Measuring cellsCells and organelles can be measured with a microscope by means of an eyepiece graticule This is a transparent scale It usually has 100 divisions (see Figure 18a) The eyepiece graticule is placed in the microscope eyepiece so that it can be seen at the same time as the object to be measured as shown in Figure 18b Figure 18b shows the scale over a human cheek epithelialcell The cell lies between 40 and 60 on the scale We therefore say it measures 20 eyepiece units in diameter (the difference between 60 and 40) We will not know the actual size of the eyepiece units until the eyepiece graticule scale is calibratedTo calibrate the eyepiece graticule scale a miniature transparent ruler called a stage micrometer scale is placed on the microscope stage and is brought into focus This scale may be etched onto a glass slideor printed on a transparent film It commonly has subdivisions of 01 and 001 mm The images of the two scales can then be superimposed as shown in Figure 18cIn the eyepiece graticule shown in the figure 100 units measure 025 mm Hence the value of each eyepiece unit is0 25 00025 mm 100Or converting mm to 1113088m 0 25 1000 2 51113088m100The diameter of the cell shown superimposed on the scale in Figure 18b measures 20 eyepiece units and so its actual diameter is20times251113088m 1113088 501113088m This diameter is greater than that of many human cellsbecause the cell is a flattened epithelial cellFigure 18 Microscopical measurement Three fields of view seen using a high-power (111308840) objective lens a An eyepiece graticule scale b Superimposed images of human cheek epithelial cells and the eyepiece graticule scale c Superimposed images of the eyepiece graticule scale and the stage micrometer scaleacheek cells on a slide on the stage of the microscopebc0 1020304050607080901000 102030405060708090100eyepiece graticule scale (arbitrary units)

eyepiece graticule in the eyepiece of the microscope00stage micrometer scale (marked in 00 1mm and 01 mm divisions)10 20 30 40 50 60 70 80 90 10001 021 Cell structure 7

Electron microscopesEarlier in this chapter it was stated that by 1900 almost all the structures shown in Figures 13 and 15 (pages 2 and 3) had been discovered There followed a time of frustration for microscopists because they realised that no matter how much the design of light microscopes improved there was a limit to how much could ever be seen using lightIn order to understand the problem it is necessary to know something about the nature of light itself and to understand the difference between magnification and resolutionMagnificationMagnification is the number of times larger an image is compared with the real size of the objectWorked example 1 ndash calculating the magnification of a photograph or objectTo calculate M the magnification of a photograph or an object we can use the following methodFigure 19 shows two photographs of a section through the same plant cells The magnifications of the two photographs are the same Suppose we want to know the magnification of the plant cell in Figure 19b If we know its actual (real) length we can calculate its magnification using theThe real length of the cell is 80 1113088mmagnification observed size of the image actual sizeM I AStep 1Measure the length in mm of the cell in the photograph using a ruler You should find that it is about 60 mmStep 2Convert mm to 1113088m (It is easier if we first convert all measurements to the same units ndash in this case micrometres 1113088m)1 mm 1113088 1000 1113088m so 60mm 1113088 60 1113088 10001113088morwhere I 1113088 observed size of the image (that is what you can measure with a ruler) and A 1113088 actual size (that is the real size ndash for example the size of a cell before it is magnified)If you know two of these values you can work out the third one For example if the observed size of the image and the magnification are known you can work out the1113088 60 000 1113088m Use the equation to calculate the magnificationStep 3actual size A I If you write the formula in a triangle Mmagnification Mimage size I

as shown below and cover up the value you want to find it should be obvious how to do the right calculationactual size A 60000m80 m 750I M1113088ASome worked examples are now providedThe lsquo1113088rsquo sign in front of the number 750 means lsquotimesrsquo We say that the magnification is lsquotimes 750rsquoformula M I A88 1 Cell structurea Worked example 2 ndash calculating magnification from a scale barFigure 110 shows a lymphocyteFigure 110 A lymphocyte6 μmbFigure 19 Photographs of the same plant cells seen a with a light microscope b with an electron microscope both shown at a magnification of about 1113088 750We can calculate the magnification of the lymphocyte by simply using the scale bar All you need to do is measure the length of the scale bar and then substitute this and the length it represents into the equationStep 1Measure the scale bar Here it is 36 mmStep 2Convert mm to 1113088m 36mm 1113088 36 1113088 10001113088m 1113088 360001113088mStep 3Use the equation to calculate the magnificationmagnification M image size I actual size A36000m 6m60001 Cell structure 9Worked example 3 ndash calculating the real size of an object from its magnificationTo calculate A the real or actual size of an object we can use the following methodFigure 125 on page 19 shows a plant cell magnified 1113088 5600 One of the chloroplasts is labelled lsquochloroplastrsquo in the figure Suppose we want to know the actual length of this chloroplastStep 1Measure the observed length of the image of the chloroplast (I ) in mm using a ruler The maximum length is 36 mmStep 2Convert mm to 1113088m 30mm 1113088 30 1113088 10001113088m 1113088 300001113088mStep 3Use the equation to calculate the actual lengthactualsizeA imagesizeI magnification M30 000 m 5600 5 4 m (to one decimal place)SAQ 12a Calculate the magnification of the drawing of the animal cell in Figure 13 on page 2b Calculate the actual (real) length of the bottom chloroplast in Figure 127 on page 19Resolution

Look again at Figure 19 (page 9) Figure 19a is a light micrograph (a photograph taken with a light microscope also known as a photomicrograph) Figure 19b is an electron micrograph of the same cells taken at the same magnification (an electron micrograph is a picture taken with an electron microscope) You can see that Figure 19b the electron micrograph is much clearer This is because it has greater resolution Resolution is defined as the abilityto distinguish between two separate points If the two points cannot be resolved they will be seen as one point In practice resolution is the amount of detail that can be seen ndash the greater the resolution the greater the detailThe maximum resolution of a light microscope is 200 nm This means that if two points or objects are closer together than 200 nm they cannot be distinguished as separateIt is possible to take a photograph such as Figure 19a and to magnify (enlarge) it but we see no more detail in other words we do not improve resolution even though we often enlarge photographs because they are easier to see when larger With a microscope magnification up to the limit of resolution can reveal further detail but any further magnification increases blurring as well as the size of the imageThe electromagnetic spectrumHow is resolution linked with the nature of light One of the properties of light is that it travels in waves The length of the waves of visible light varies ranging from about 400 nm (violet light) to about 700 nm (red light) The human eye can distinguish between these different wavelengths and in the brain the differences are converted to colour differences (Colour is an invention of the brain)The whole range of different wavelengths is called the electromagnetic spectrum Visible light is only one part of this spectrum Figure 111 shows some of the parts of the electromagnetic spectrum The longer the waves the lower their frequency (all the waves travel at the same speed so imagine them passing a post shorter waves pass at higher frequency) In theory there is no limit to how short or how long the waves can be Wavelength changes with energy the greater the energy the shorter the wavelength (rather like squashing a spring)Now look at Figure 112 which shows a mitochondrion some very small cell organelles called ribosomes (see page 13) and light of 400 nm wavelength the shortest visible wavelength The mitochondrion is large enoughto interfere with the light waves However the ribosomes are far too small to have any effect on the light waves The general rule is that the limit of resolution is about one half the wavelength of the radiation used to view the specimen In other words if an object is any smaller than half the wavelength of the radiation used to view it it cannot be seen separately from nearby objects This means that the1001 1 Cell structureX-raysgamma rays01 nm400 nm violet bluev infraredmicrowaves

uv10 nmi s i b l e 1000 nmradio and TV wavesvisible light500 nm 600 nm green yellow orange105 nm 107 nm 109 nm 1011 nm 1013 nm700 nmFigure 111 Diagram of the electromagnetic spectrum (the waves are not drawn to scale) The numbers indicate the wavelengths of the different types of electromagnetic radiation Visible light is a form of electromagnetic radiationbest resolution that can be obtained using a microscope that uses visible light (a light microscope) is 200 nm since the shortest wavelength of visible light is 400 nm (violet light) In practice this corresponds to a maximum useful magnification of about 1500 times Ribosomes are approximately 25 nm in diameter and can therefore never be seen using lightIf an object is transparent it will allow light waves to pass through it and therefore will still not be visible This is why many biological structures have to be stained before they can be seenThe electron microscopeBiologists faced with the problem that they would never see anything smaller than 200 nm using a light microscope realised that the only solution would be to use radiation of a shorter wavelength than light If you study Figure 111 you will see that ultraviolet light or better still X-rays look like possible candidates Both ultraviolet and X-ray microscopes have been built the latter with little success partly because of the difficulty of focusing X-rays A much better solution is to use electrons Electrons are negatively charged particles which orbit the nucleus of an atom When a metal becomes very hot some of its electrons gain so much energy that they escape from their orbits like a rocket escaping from Earthrsquos gravity Free electrons behave like electromagnetic radiation They have a very short wavelength the greater the energy the shorter the wavelength Electrons are a very suitable form of radiation for microscopy for two major reasons Firstly their wavelength is extremely short (at least as short as that of X-rays) Secondly because they are negatively charged they can be focused easily using electromagnets (a magnet can be made to alter the path of the beam the equivalent of a glass lens bending light)Using an electron microscope a resolution of 05 nm can be obtained

400 times better than when using a light microscope ETransmission and scanning electron microscopesTwo types of electron microscope are now in common use The transmission electron microscope or TEM forwavelength 400nmstained mitochondrion of diameter 1000nm interferes with light wavesstained ribosomes of diameter 25nm do not interfere with light wavesFigure 112 A mitochondrion and some ribosomes in the path of light waves of 400 nm lengthred1 Cell structure 11

E short was the type originally developed Here the beam of electrons is passed through the specimen before being viewed Only

those electrons that are transmitted (pass through the specimen) are seen This allows us to see thin sections of specimens and thus to see inside cells In the scanning electron microscope (SEM) on the other hand the electron beam is used to scan the surfaces of structures and only the reflected beam is observedAn example of a scanning electron micrograph is shown in Figure 113 The advantage of this microscope is that surface structures can be seen Also great depth of field is obtained so that much of the specimen is in focus at the same time and a three-dimensional appearance is obtained Such a picture would be impossible to obtain with a light microscope even using the same magnification and resolution because you would have to keep focusing up and down with the objective lens to see different parts of the specimen The disadvantage of the SEM is that it cannot achieve the same resolution as a TEM Resolution is between 3 nm and 20 nmViewing specimens with the electron microscopeFigure 114 shows how an electron microscope works and Figure 115 shows one in useFigure 113 False-colour SEM of the head of a cat flea (1113088 100)

electron gun and anode which E produce a beam of electronselectron beam vacuumpathway of electronscondenser electromagnetic lens which directs the electron beam onto the specimenspecimen which is placed on a gridobjective electromagnetic lens which produces an imageprojector electromagnetic lenses which focus the magnified image onto the screenscreen or photographic plate which shows the image of the specimenFigure 114 How an electron microscope worksIt is not possible to see an electron beam so to make the image visible the electron beam has to be projected onto a fluorescent screen The areas hit by electrons shine brightly giving overall a lsquoblack and whitersquo picture The stains used to improve the contrast of biological specimens for electron microscopy contain heavy metal atoms which stop the passage of electrons The resulting picture is like an X-ray photograph with the more densely stained parts of the specimen appearing blacker lsquoFalse-colourrsquo images can be created by colouring the standard black and white image using a computerTo add to the difficulties of electron microscopy the electron beam and therefore the specimen and the fluorescent screen must be in a vacuum If electrons1221 1 Cell structure

EStructures and functions of organellesCompartmentalisation and division of labour within the cell are even more obvious with an electron microscope than with a light microscopeWe will now consider the structures and functions of some of the cell components in more detailNucleusThe nucleus (Figure 118 on page 15) is the largest cell organelle (see also page 5) It is surrounded by two membranes known as the

nuclear envelope The outer membrane of the nuclear envelope is continuous with the endoplasmic reticulum (Figure 117 on page 15) The nuclear envelope has many small pores called nuclear pores These allow and control exchange between the nucleus and the cytoplasm Examples of substances leaving the nucleus through the pores are mRNA and ribosomes for protein synthesis Examples of substances entering through the nuclear pores are proteins to help make ribosomes nucleotides ATP (aderosine triphosphate) and some hormones such as thyroid hormone T3Within the nucleus the chromosomes are in a loosely coiled state known as chromatin (except during nuclear division see Chapter 5) Chromosomes contain DNA which is organised into functional units called genes Genes control the activities of the cell and inheritance thus the nucleus controls the cellrsquos activities When a cell is aboutto divide the nucleus divides first so that each new cell will have its own nucleus (Chapters 5 and 19) Also within the nucleus the nucleolus makes ribosomes using the information in its own DNAEndoplasmic reticulum and ribosomesWhen cells were first seen with the electron microscope biologists were amazed to see so much detailed structure The existence of much of this had not been suspected This was particularly true of an extensive system of membranes running through the cytoplasm which became knownas the endoplasmic reticulum (ER) (Figure 119 on page 15 ndash see also Figures 118 on page 15 and 122 on page 17) The ER is continuous with the outer membrane of the nuclear envelope (Figure 117)There are two types of ER rough ER and smooth ER Rough ER is so called because it is covered with many tinyFigure 115 A TEM in usecollided with air molecules they would scatter making it impossible to achieve a sharp picture Also water boils at room temperature in a vacuum so all specimens must be dehydrated before being placed in the microscope This means that only dead material can be examined Great efforts are therefore made to try to preserve material in a life-like state when preparing it for the microscopeSAQ 13Explain why ribosomes are not visible using a light microscopeUltrastructure of an animal cellThe lsquofinersquo or detailed structure of a cell as revealed by the electron microscope is called its ultrastructure Figure 116 shows the appearance of typical animal cells as seen with an electron microscope and Figure 117 on page 15 is a diagram based on many other such micrographsSAQ 14Compare Figure 117 on page 15 with Figure 13 on page 2 Name the structures which can be seen with the electron microscope but not with the light microscope1 Cell structure 13lysosomeGGolgi apparatusendoplasmic reticulumglycogen granulesccell surface membrane

mmitochondriachromatinnnucleolusmmicrovillusrribosomesFFigure 116 Representative animal cells as seen with a TEM The cells are liver cells from a rat (1113088 9600) The nucleus is clearly visible in one of the cells 1441 1 Cell structurenucleusnnuclear envelopetwo centrioles close to the nucleus and at right angles to each othermitochondrion lysosomerough endoplasmic reticulumnucleoluschromatin nucleusnuclear porenuclear envelope (two membranes)microvillismooth endoplasmic reticulumFigure 117 Ultrastructure of a typical animal cell as seen with an electron microscope In reality the ER is more extensive than shown and free ribosomes may be more extensive Glycogen granules are sometimes present in the cytoplasmFigure 118 TEM of the nucleus of a cell from the pancreas of a bat (1113088 7500) The circular nucleus is surrounded by a double-layered nuclear envelope containing nuclear pores The nucleolus is more Figure 119 TEM of rough ER covered with ribosomes (black dots) darkly stained Rough ER is visible in the surrounding cytoplasm (1113088 17 000) Some free ribosomes can also be seen in the cytoplasmGolgi vesicle Golgi apparatusribosomescell surface membranecytoplasm1 Cell structure 15organelles called ribosomes These are just visible as black dots in Figures 118 and 119 on page 15 At very high magnifications they can be seen to consist of two subunits a large and a small subunit Ribosomes are the sites of protein synthesis (see pages 111ndash112) They can be found free in the cytoplasm as well as on the rough ER They are very small only about 25 nm in diameter They are made of RNA (ribonucleic acid) and protein The rough ER forms an extensive system of flattened sacs spreading in sheets throughout the cell Proteins made by the ribosomes on the rough ER enter the sacs and move through them The proteins are often processed in some way on their journey Small sacs called vesicles can break off from the ER and these can join together to form the Golgi apparatus Proteins can be exported from the cell via the Golgi apparatus (see page 80)Smooth ER so called because it lacks ribosomes has a completely different function It makes lipids and steroids such as cholesterol and the reproductive hormones oestrogen and testosteroneGolgi apparatus (Golgi body or Golgi complex)The Golgi apparatus is a stack of flattened sacs (Figure 120) This stack of sacs is sometimes referred to as theFigure 120 TEM of a Golgi apparatus A central stack of saucer-shaped sacs can be seen budding off small Golgi vesicles (green) These may form secretory vesicles whose contents can be released at the cell surface by exocytosis (see page 80)Golgi body More than one may be present in a cell The stack is constantly being formed at one end from vesicles which bud off from the ER and broken down again at the other end to form Golgi vesicles The stack of sacs with the associated vesicles is referred to as the Golgi apparatus or Golgi complex

The Golgi apparatus collects processes and sorts molecules (particularly proteins from the rough ER) ready for transport in Golgi vesicles either to other parts of the cell or out of the cell (secretion) Two examples of protein processing in the Golgi apparatus are the addition of sugars to proteins to make molecules known as glycoproteins and the removal of the first amino acid methionine from newly formed proteins to make a functioning protein In plants enzymes in the Golgi apparatus convert sugars into cell wall components Golgi vesicles are also used to make lysosomesLysosomesLysosomes (Figure 121) are spherical sacs surrounded by a single membrane and having no internal structure They are commonly 01ndash 05 1113088m in diameter They contain digestive (hydrolytic) enzymes which must be kept separateFigure 121 Lysosomes (orange) in a mouse kidney cell (1113088 55 000) They contain cell structures in the process of digestion and vesicles (green) Cytoplasm is coloured blue here1661 1 Cell structurefrom the rest of the cell to prevent damage Lysosomes are responsible for the breakdown (digestion) of unwanted structures such as old organelles or even whole cells as in mammary glands after lactation (breast feeding) In white blood cells lysosomes are used to digest bacteria (see endocytosis page 80) Enzymes are sometimes released outside the cell ndash for example in the replacement of cartilage with bone during development The headsof sperm contain a special lysosome the acrosome for digesting a path to the ovum (egg)MitochondriaMitochondria (singular mitochondrion) are usually about 1 1113088m in diameter and can be various shapes often sausage- shaped as in Figure 122 They are surrounded by two membranes (an envelope) The inner of these is foldedto form finger-like cristae which project into the interior solution or matrixThe main function of mitochondria is to carry out aerobic respiration As a result of respiration they make ATP the universal energy carrier in cells (see Chapter 16) They are also involved in the synthesis of lipids (page 37)Figure 122 Mitochondrion (orange) with its double membrane (envelope) the inner membrane is folded to form cristae (1113088 20 000) Mitochondria are the sites of aerobic cell respiration Note also the rough ERIn the 1960s it was discovered that mitochondria and chloroplasts contain ribosomes which are slightly smaller than those in the cytoplasm and are the same size as those found in bacteria The size of ribosomes is measured in lsquoS unitsrsquo which are a measure of how fast they sediment in a centrifuge Cytoplasmic ribosomes are 80S while those of bacteria mitochondria and chloroplasts are 70S It was also discovered in the 1960s that mitochondria and chloroplasts contain small circular DNA molecules also like those found in bacteria Not surprisingly it was later provedthat mitochondria and chloroplasts are in effect ancient bacteria which now live inside the larger cells typical of animals and plants (see

prokaryotic and eukaryotic cells page 18) This is known as the endosymbiont theory lsquoEndorsquo means lsquoinsidersquo and a lsquosymbiontrsquo is an organism which lives in a mutually beneficial relationship with another organism The DNA and ribosomes of mitochondria and chloroplasts are still active and responsible for the coding and synthesis of certain vital proteins but mitochondria and chloroplasts can no longer live independentlyMitochondrial ribosomes are just visible as tiny dark dots in the mitochondrial matrix in Figure 122Cell surface membraneThe cell surface membrane is extremely thin (about 7 nm) However at very high magnifications at least 1113088 100 000 it can be seen to have three layers described as a trilaminar appearance This consists of two dark lines (heavily stained) either side of a narrow pale interior (Figure 123) The membrane is partially permeable and controls exchange between the cell and its environment Membrane structure is discussed further in Chapter 4Figure 123 Cell surface membrane (1113088 250 000) At this magnification the membrane appears as two dark lines at the edge of the cellMicrovilliMicrovilli (singular microvillus) are finger-like extensions of the cell surface membrane typical of certain epithelial cells (cells covering surfaces of structures) They greatly1 Cell structure 17increase the surface area of the cell surface membrane (see Figure 117 on page 15) This is useful for example for absorption in the gut and for reabsorption in the proximal convoluted tubules of the kidney (see page 307)CentriolesThe extra resolution of the electron microscope reveals that just outside the nucleus there are really two centrioles (see Figure 124) not one as it appears under the light microscope (compare with Figure 13 on page 2) They lie close together at right-angles to each other A centriole is a hollow cylinder about 04 1113088m long formed from a ring of short microtubules tiny tubes made of a protein called tubulin These microtubules are used as a starting point for growing the spindle microtubules for nuclear division (see page 92) Centrioles are not found in plant cellsUltrastructure of a plant cellAll the structures except centrioles and microvilli so far described in animal cells are also found in plant cells The appearance of a plant cell as seen with the electron microscope is shown in Figure 125 while Figure 126 is a diagram based on many such micrographs The relatively thick cell wall and the large central vacuole are obvious as are the chloroplasts two of which are shown in detail in Figure 127 These structures and their functions have been described on pages 5 and 6 The electron microscopeFigure 124 Centrioles in transverse and longitudinal section (TS and LS) (1113088 86 000) The one on the left is seen in TS and clearly shows the nine triplets of microtubules which make up the structurereveals that chloroplasts contain 70S ribosomes and small circular

DNA molecules as do mitochondria and bacteria This has already been discussed with mitochondria above (page 17)SAQ 15Compare Figure 126 with Figure 15 on page 3 Name the structures which can be seen with the electron microscope but not with the light microscopeTwo fundamentally different types of cellAt one time it was common practice to try to classify all living organisms as either animals or plants With advances in our knowledge of living things it has become obvious that the living world is not that simple Fungi and bacteria for example are very different from animals and plants and from each other Eventually it was discovered that there are two fundamentally different types of cell The most obvious difference betweenthese types is that one possesses a nucleus and the other does notOrganisms that lack nuclei are called prokaryotes (lsquoprorsquo means before lsquokaryonrsquo means nucleus) All prokaryotes are now referred to as bacteria They are on average about 1000 to 10 000 times smaller in volume than cells with nuclei and are much simpler in structure ndash for example their DNA lies free in the cytoplasmOrganisms whose cells possess nuclei are called eukaryotes (lsquoeursquo means true) Their DNA lies inside a nucleus Eukaryotes include animals plants fungi and a group containing most of the unicellular eukaryotes known as protoctists Most biologists believe that eukaryotes evolved from prokaryotes one-and-a-half thousand million years after prokaryotes first appeared on Earth We mainly study animals and plants in this book but all eukaryotic cells have certain features in commonA generalised prokaryotic cell is shown in Figure 128 A comparison of prokaryotic and eukaryotic cells is given in Table 12 on page 211881 1 Cell structure

2 Biological molecules 29The building blocks of life 30 Monomers polymers and macromolecules 30 Carbohydrates 30 Lipids 37 Proteins 39 Water 47 End-of-chapter questions 493 Enzymes 54Enzymes reduce activation energy 57 The course of a reaction 57 Enzyme inhibitors

62 End-of-chapter questions 634 Cell membranes and transport 69Phospholipids 69 Structure of membranes 70 Transport across the cell surface membrane 73 End-of-chapter questions 815 Cell and nuclear division 86The nucleus contains chromosomes 86 The structure of chromosomes 88 Two types of nuclear division 89 Mitosis in an animal cell 90 Cancer 93 End-of-chapter questions 996 Genetic control 103The structure of DNA and RNA 103 DNA replication 105 Genes and mutations

109 DNA RNA and protein synthesis 109 End-of-chapter questions 1157 Transport in multicellular plants 118The need for transport systems in multicellular organisms 118 The transport of water

120 Transport in multicellular plants 120 Translocation 133 Differences between sieve tubes and xylem vessels 137 End-of-chapter questions 1388 The mammalian transport system 144The mammalian cardiovascular system 144 Blood plasma and tissue fluid 150 Lymph

150 Blood 151 Haemoglobin 154 Problems with oxygen transport 157 End-of-chapter questions 1609 The mammalian heart 164The cardiac cycle166 Control of the heart beat168 End-of-chapter questions 17010 Gas exchange 174Lungs 174 Trachea bronchi and bronchioles 174 Alveoli 177 End-of-chapter questions 17811 Smoking 182Tobacco smoke 182 Lung diseases 182 Cardiovascular diseases 185 Proving the links between smoking and lung disease 188 Prevention and cure of coronary heart disease 192End-of-chapter questions194Contentsiii12 Infectious diseases 197Worldwide importance of infectious diseases 197 Cholera 198 Malaria 200 Acquired immune deficiency syndrome (AIDS) 203 Tuberculosis (TB) 208 Antibiotics

211 End-of-chapter questions 21313 Immunity217Defence against disease 217 Cells of the immune system 218 Active and passive immunity 225 Vaccination 226 Problems with vaccines 227 The eradication of smallpox228 Measles 230 End-of-chapter questions 23114 Ecology 235Energy flow through organisms and ecosystems 236 Matter recycling in ecosystems 241 The nitrogen cycle 241 End-of-chapter questions 24515 Advanced practical skills 248Experiments 248 Variables and making measurements 249 Estimating uncertainty in measurement 257 Recording quantitative results 258 Constructing a line graph 259 Constructing bar charts and histograms 260 Drawing conclusions

261 Describing data 261 Making calculations from data 261 Explaining your results 263 Identifying sources of error and suggestingimprovements 263 Drawings 264 End-of-chapter questions 26516 Energy and respiration 268

The need for energy in living organisms 268 Work 268 ATP 270 Respiration 273 Anaerobic respiration 279 Respiratory substrates 280 End-of-chapter questions 28317 Photosynthesis 287An energy transfer process 287 The light-dependent reactions of photosynthesis 288 The light-independent reactions of photosynthesis 290 Leaf structure and function

290 Chloroplast structure and function 293 Factors necessary for photosynthesis 294 Trapping light energy 295 End-of-chapter questions 29718 Regulation and control 301Homeostasis 302 Excretion 302 The structure of the kidney 303 Control of water content 311 Nervous communication 314 Hormonal communication 329 Plant growth regulators336 Electrical communication in plants 339 End-of-chapter questions 34019 Inherited change 347Meiosis 347 Genetics 348 Genotype affects phenotype 351 Inheriting genes 352 Multiple alleles 354 Sex inheritance 355 Sex linkage 355 Dihybrid crosses 357 The 11130882 (chi-squared) test 359 Mutations 361 Environment and phenotype 363 End-of-chapter questions 36320 Selection and evolution 367Natural selection 368 Evolution 370 The DarwinndashWallace theory of evolution by natural selection 374 Species and speciation 375 Artificial selection 377 End-of-chapter questions 37821 Biodiversity and conservation 382The five-kingdom classification 382 Maintaining biodiversity 385 Endangered species 385 End-of-chapter questions 392ivContents22 Gene technology 395Gene technology 395 Benefits of gene technology 399 Potential hazards of gene technology 400 Social and ethical implications of genetic engineering 402 Electrophoresis 403 Cystic fibrosis 404 The genetic counsellor 407 Genetic screening 409 End-of-chapter questions 41123 Biotechnology 414Mining with microorganisms 414 Large-scale production techniques 416 Advantages of batch and continuous culture 419 How penicillin works 420 Immobilising enzymes 422 Monoclonal antibodies 424 End-of-chapter questions 42724 Crop plants 431Cereal crops 431 Maize 433 C4 plants 434 Adaptations for difficult environments 437 Crop improvement 440 End-of-chapter questions 44725 Aspects of human reproduction 451Gametogenesis 453 Human menstrual cycle 456 Birth control 457 Infertility 459 End-of-chapter questions 46426 Planning analysis and evaluation 468 Planning an investigation

468Analysis conclusionsand evaluation End-of-chapter questions4 7 3 480Appendix 1 Amino acid R groups 484 Appendix2 DNAtripletcodes 485 Glossary 486 Index 501 Acknowledgements 510CD-ROMAdvice on how to revise for and approach examinationsChapter summariesMultiple choice tests for Chapters 1ndash14Answers to self-assessment questions Answers to end-of-chapter questions

Contentsv

IntroductionThis new edition is fully updated for the 2014 syllabus to help you do well in your Cambridge International Examinations AS and A level Biology (9700) courses The book and its accompanying CD-ROM provide a self-contained resource for studying these courses with improved focus on exam preparationbull Chapters 1ndash15 provide complete coverage of the AS level syllabus This is also the first year of study for A level The AS syllabus is designed for studentsbull with O level or IGCSE Biology Chapters 16ndash26 cover all the material for the second year of study for A level This includes the relevant Core material and the Applications of Biology sectionExtension material is clearly marked with the following symbol E

and a dotted line runs down alongside the text to mark this additional contentImportant features of this new edition include the followingThe sequence of chapters mirrors the sequence of topics in the syllabus which makes it easy to navigate (Your teacher may however tackle subjects in a different order) Syllabus sections G and H are split into three and two chapters respectively for additional convenience (see table)LevelApplications of BiologySyllabus section Chapter PSelection and evolution 20 Q Biodiversity and 21conservation R Gene technology 22 S Biotechnology 23 T Crop plants 24 U Aspects of human reproduction 25Planning analysis and evaluation26LevelAS level Core syllabusA levelSyllabus section A Cell structureChapter1There are two new chapters covering practical skills Chapter 15 (AS level) and Chapter 26 (A level) Interesting information that is not required by the syllabus but will aid understanding is marked as lsquoextension materialrsquo by orange dotted barsEach chapter contains self-assessment questions (SAQs) These are to help you think about understand and remember what you have just read Each chapter ends with a set of chapter-related questions ranging from formative questions (requiring simple recall or reference to the text) to more challenging structured or essay questions requiring understanding as well as the other skills tested in examinations Some of the questions are past Cambridge examination questions so you can familiarise yourself with the style of the examination questionsBiology involves many technical terms Each time a new term is introduced it is shown in bold orange and its meaning explained The glossary contains definitions of the key terms used in the book

At the end of your course you will be tested on three sets of Assessment Objectivesbull Knowledge with understanding You are expected to know and understand all the facts and concepts listedbull in the syllabus These are all covered in this book Handling information and solving problems Questions testing these skills expect you to use your knowledge and understanding in an unfamiliar context A good knowledge and understanding of this book will enablebull you to approach new situations with confidence Experimental skills and investigations This involves practical work An examination will test your practical skills so try to do plenty of practical work Key information is provided on some practical aspects of the course in Chapters 15 and 26Additional help and guidance are available on the accompanying CD-ROMF Genetic control G Transport7 8vi IntroductionB Biological molecules 2 C Enzymes 3 D Cell membranes and transport

4 E Cell and nuclear division 56Transport in multicellular plants The mammalian transport system The mammalian heart 9H Gas exchange and smoking Gas exchange 10 Smoking 11I Infectious disease 12 J Immunity 13 K Ecology 14Advanced practical skills 15 L Energy and respiration 16 M Photosynthesis

17 N Regulation and control 18 O Inherited change 19

1 Cell structure By the end of this chapter you

should be able todescribe and interpret drawings and photographs of typical animal and plant cells as seen using the light microscope and make microscopical measurements using an eyepiece graticule and stage micrometerbe familiar with the units used in cell studies explain the meanings of and distinguish betweenthe terms resolution and magnificationdescribe and interpret drawings and photographs of typical animal and plant cells as seen using the electron microscope recognising rough and smooth endoplasmic reticulum (ER) Golgi apparatus mitochondria ribosomes lysosomes cell surface membrane centrioles nucleus (including the nuclear envelope and nucleolus) and microvilli as well asIn the early days of microscopy an English scientist Robert Hooke decided to examine thin slices of plant material He chose cork as one of his examples Looking down the microscope he was struck by the regular appearance ofthe structure and in 1665 he wrote a book containing the diagram shown in Figure 11If you examine the diagram you will see the lsquopore-likersquo regular structures that Hooke called lsquocellsrsquo Each cell appeared to be an empty box surrounded by a wall Hooke had discovered and described without realising it the fundamental unit of all living things

Although we now know that the cells of cork are dead further observations of cells in living materials were made by Hooke and other scientists However it was not until almost 200 years later that a general cell theory emerged from the work of two German scientists In 1838 Schleiden a botanist suggested that all plants are made of cells and a year later Schwann a zoologist suggested the same for animals The cell theory states that the basic unit of structure and function of all living organisms is the cell Now over 170 years later this idea is one of the most familiar and important theories in biology To it has beenthe chloroplasts cell wall large permanent vacuole tonoplast and plasmodesmata of plant cellsoutline the functions of the structures listed abovecompare the structure of typical animal and plant cellscalculate the linear magnification of and the actual sizes of specimens from drawings and photographsdescribe the structure of a prokaryotic cell and compare and contrast the structure of prokaryotic cells with that of eukaryotic cellsexplain how eukaryotic cells may be organised into tissues and organs with reference to transverse sections of stems roots and leavesdraw and label low-power plan diagrams of tissues and organsadded Virchowrsquos theory of 1855 that all cells arise from pre-existing cells by cell divisionFigure 11 Drawing of cork cells published by Robert Hooke in 16651 Cell structure 1

Why cellsA cell can be thought of as a bag in which the chemistry of life is allowed to occur partially separated from the environment outside the cell The thin membrane which surrounds all cells is essential in controlling exchange between the cell and its environment It is a very effective barrier but also allows a controlled traffic of materials across it in both directions The membrane is therefore described as partially permeable If it were freely permeable life could not exist because the chemicals of the cell would simply mix with the surrounding chemicals by diffusion (page 73)Cell biology and microscopyThe study of cells has given rise to an important branch of biology known as cell biology Cells can now be studied by many different methods but scientists began simply by looking at them using various types of microscopeThere are two fundamentally different types of microscope now in use the light microscope and the electron microscope Both use a form of radiation in order to create an image of the specimen being examined The light microscope uses light as a source of radiation while the electron microscope uses electrons for reasons which are discussed latereyepiecelight beamobjective cover slipglass slidecondenseriris diaphragmlight source pathway of lightEyepiece lens magnifies and focuses the image from the objective onto the eyeObjective lens collects light passing through the specimen and produces a magnified image

Condenser lens focuses the light onto the specimen held between the cover slip and slideCondenser iris diaphragm is closed slightly to produce a narrow beam of lightFigure 12 How the light microscope worksLight microscopyThe lsquogolden agersquo of light microscopy could be said to be the 19th century Microscopes had been available since the beginning of the 17th century but when dramatic improvements were made in the quality of glass lenses in the early 19th century interest among scientists became widespread The fascination ofthe microscopic world that opened up in biology inspired rapid progress both in microscope design and equally importantly in preparing material for examination with microscopes This branch of biology is known as cytology Figure 12 shows how the light microscope worksBy 1900 all the structures shown in Figures 13 14 and 15 except lysosomes had been discovered Figure 13 shows the structure of a generalised animal cell and Figure 15 the structure of a generalised plant cell as seen with a light microscope (A generalised cell shows all the structures that are typically found in a cell)Golgi apparatus cytoplasmcentriole ndash always found near nucleus has a role in nuclear divisionsmall structures that are difficult to identifymitochondria cell surface membranenuclear envelopechromatin ndash deeply staining and thread-likenucleolus ndash deeply stainingnucleusFigure 13 Structure of a generalised animal cell (diameter about 20 1113088m) as seen with a very high quality light microscope22 1 Cell structuretonoplast ndash membrane surrounding vacuolecell surface membrane (pressed against cell wall)vacuole ndash large with central positioncytoplasmmitochondriamiddle lamella ndash thin layer holding cells together contains calcium pectateplasmodesma ndash connects cytoplasm of neighbouring cellscell wall of neighbouring cellcell wall chloroplast grana just visiblesmall structures that are difficult to identifyGolgi apparatusFigure 14 Cells from the lining of the human cheek (1113088 500) each showing a centrally placed nucleus which is a typical animal cell characteristic The cells are part of a tissue known as squamous (flattened) epitheliumFigure 14 shows some actual human cells and Figure 16 shows an actual plant cell taken from a leafSAQ 11Using Figures 13 and 15 name the structures that animal and plant cells have in common those found in only plant cells and those found only in animal cellsFigure 16 Photomicrograph of a cell in a moss leaf (11130881400)nucleusnucleolus ndash deeply stainingnuclear envelopechromatin ndash deeply staining and thread-likeFigure 15 Structure of a generalised plant cell (diameter about 40 1113088m) as seen with a very high quality light microscope1 Cell structure 3Box 1A Biological drawingYou need the following equipment

bull pencil (HB) bull pencil sharpener bull eraser bull ruler bull plain paperHere are some guidelines for the quality of your drawingbull always use a pencil not a pen bull donrsquot use shading bull use clear continuous lines bull use accurate proportions and observation ndash not atextbook version For a low-power drawing (see Figure 17)bull donrsquot draw individual cells bull draw all tissues completely enclosed by lines bull draw a correct interpretation of the distribution of bull tissuesa representative portion may be drawn (eg half a transverse section)For a high-power drawingbull draw only a few representative cells bull draw the cell wall of all plant cells bull donrsquot draw the nucleus as a solid blobSome guidelines for the quality of your labellingbull label all tissues and relevant structures bull identify parts correctly bull use a ruler for label linesAnimal and plant cells have features in commonIn animals and plants each cell is surrounded by a very thin cell surface membrane which is too thin to be seen with a light microscope This is also sometimes referred to as the plasma membranebull arrange label lines neatly and ensure they donrsquot cross bull over each otherannotate your drawing if necessary (ie provideshort notes with one or more of the labels in order bull to describe or explain features of biological interest)add a scale line at the bottom of the drawing if bull appropriateuse a pencil not a penAn example of a drawing of a section through the stem of Helianthus is shown below Biological drawing is also covered in Chapter 15 page 264Figure 17 The right side of this low-power drawing shows examples of good technique while the left side shows many of the pitfalls you should avoidMany of the cell contents are colourless and transparent so they need to be stained to be seen Each cell has a nucleus which is a relatively large structure that stains intensely and is therefore very conspicuous The deeply staining material in the nucleus is called chromatin and is a massof loosely coiled threads This material collects together to form visible separate chromosomes during nuclear division44 1 Cell structure(see page 86) It contains DNA (deoxyribonucleic acid) a molecule which contains the instructions that control the activities of the cell (see Chapter 6) Within the nucleus an even more deeply staining area is visible the nucleolus which is made of loops of DNA from several chromosomes The number of nucleoli is variable one to five being

common in mammalsThe material between the nucleus and the cell surface membrane is known as cytoplasm Cytoplasm is an aqueous (watery) material varying from a fluid to a jelly-like consistency Many small structures can be seen within it These have been likened to small organs and hence are known as organelles An organelle can be defined as a functionally and structurally distinct part of a cell Organelles themselves are often surrounded by membranes so that their activities can be separated from the surrounding cytoplasm This is described as compartmentalisation Having separate compartments is essential for a structure as complex as an animal or plant cell to work efficiently Since each type of organelle has its own function the cell is said to show division of labour a sharing of the work between different specialised organellesThe most numerous organelles seen with the light microscope are usually mitochondria (singular mitochondrion) Mitochondria are only just visible but films of living cells taken with the aid of a light microscope have shown that they can move about change shape and divide They are specialised to carry out aerobic respirationThe use of special stains containing silver enabled the Golgi apparatus to be detected for the first time in 1898 by Camillo Golgi The Golgi apparatus is part of a complex internal sorting and distribution system within the cell (see page 16) It is also sometimes called the Golgi body or Golgi complexDifferences between animal and plant cellsThe only structure commonly found in animal cells which is absent from plant cells is the centriole Plant cells also differ from animal cells in possessing cell walls large permanent vacuoles and chloroplastsCentriolesUnder the light microscope the centriole appears as a small structure close to the nucleus (see Figure 13 on page 2) The centriole is involved in nuclear division (see page 92)Cell walls and plasmodesmataWith a light microscope individual plant cells are more easily seen than animal cells because they are usually larger and unlike animal cells surrounded by a cell wall outside the cell surface membrane This is relatively rigid because it contains fibres of cellulose a polysaccharide which strengthens the wall The cell wall gives the cell a definite shape It prevents the cell from bursting when water enters by osmosis allowing large pressures to develop inside the cell (see page 77) Cell walls may also be reinforced with extra cellulose or with a hard material called lignin for extra strength (see xylem on page 24) Cell walls are freely permeable allowing free movement of molecules and ions through to the cell surface membranePlant cells are linked to neighbouring cells by means of fine strands of cytoplasm called plasmodesmata (singular plasmodesma) which pass through pore-like structures in the walls of these neighbouring cells Movement through the pores is thought to be controlled by the structure of the poresVacuoles

Although animal cells may possess small vacuoles such as phagocytic vacuoles (see page 80) which are temporary structures mature plant cells often possess a large permanent central vacuole The plant vacuole is surrounded by a membrane the tonoplast which controls exchange between the vacuole and the cytoplasm The fluid in the vacuole is a solution of mineral salts sugars oxygen carbon dioxide pigments enzymes and other organic compounds including some waste productsVacuoles help to regulate the osmotic properties of cells (the flow of water inwards and outwards) as well as having a wide range of other functions For example the pigments which colour the petals of certain flowers and parts of some vegetables such as the red pigment of beetroots are sometimes located in vacuoles1 Cell structure 5ChloroplastsSome plant cells are able to carry out photosynthesis because they contain chloroplasts Chloroplasts are relatively large organelles which are green in colour due to the presence of chlorophyll At high magnifications small lsquograinsrsquo or grana (singular granum) can be seen in the chloroplasts During the process of photosynthesis light is absorbed by these grana which actually consist of stacks of membrane-bound sacs called thylakoids Starch grains may also be visible within chloroplasts Chloroplasts are found in the green parts of plants mainly in the leavesPoints to noteWe return to the differences between animal and plant cells as seen using the electron microscope on page 18Units of measurement in cell studiesIn order to measure objects in the microscopic world we need to use very small units of measurement which are unfamiliar to most people According to international agreement the International System of Units (SI units) should be used In this system the basic unit of length is the metre (symbol m) Additional units can be created in multiples of a thousand times larger or smaller using standard prefixes For example the prefix kilo means 1000 times Thus 1 kilometre 1113088 1000 metres The units of length relevant to cell studies are shown in Table 11It is difficult to imagine how small these units are but when looking down a microscope and seeing cells clearly we should not forget how amazingly small the cells actually are The smallest structure visible with the human eye is about 50ndash100 1113088m in diameter Your body contains about 60 million million cells varying in size from about 5 1113088m to 40 1113088m Try to imagine structures like mitochondria which have an average diameter of 1 1113088m The smallest cell organelles we deal with in this book ribosomes are only about 25 nm in diameter You could line up about 20 000 ribosomes across the full stop at the end of this sentenceUnit Symbolmillimetre mm micrometre 1113088m nanometre nmTable 11 Units of measurement relevant to cell studies 1113088 is the Greek letter mu 1 micrometre is a thousandth of a millimetre 1 nanometre is a thousandth of a micrometre661 Cell structure

bull bull bullbullYou can think of a plant cell as being very similar to an animal cell but with extra structures Plant cells are often larger than animal cells although cell size varies enormouslyDo not confuse the cell wall with the cell surface membrane Cell walls are relatively thick and physically strong whereas cell surface membranes are very thin Cell walls are freely permeable whereas cell surface membranes are partially permeable All cells have a cell surface membrane Vacuoles are not confined to plant cells animal cells may have small vacuoles such as phagocytic vacuoles (see page 80) although these are not usually permanent structuresFraction of a metreone thousandth 1113088 0001 1113088 11000 1113088 10-3 one millionth 1113088 0000 001 1113088 11 000 000 1113088 10-6 one thousand millionth 1113088 0000 000 001 1113088 11 000 000 000 1113088 10-9

Box 1B Measuring cellsCells and organelles can be measured with a microscope by means of an eyepiece graticule This is a transparent scale It usually has 100 divisions (see Figure 18a) The eyepiece graticule is placed in the microscope eyepiece so that it can be seen at the same time as the object to be measured as shown in Figure 18b Figure 18b shows the scale over a human cheek epithelialcell The cell lies between 40 and 60 on the scale We therefore say it measures 20 eyepiece units in diameter (the difference between 60 and 40) We will not know the actual size of the eyepiece units until the eyepiece graticule scale is calibratedTo calibrate the eyepiece graticule scale a miniature transparent ruler called a stage micrometer scale is placed on the microscope stage and is brought into focus This scale may be etched onto a glass slideor printed on a transparent film It commonly has subdivisions of 01 and 001 mm The images of the two scales can then be superimposed as shown in Figure 18cIn the eyepiece graticule shown in the figure 100 units measure 025 mm Hence the value of each eyepiece unit is0 25 00025 mm 100Or converting mm to 1113088m 0 25 1000 2 51113088m100The diameter of the cell shown superimposed on the scale in Figure 18b measures 20 eyepiece units and so its actual diameter is20times251113088m 1113088 501113088m This diameter is greater than that of many human cellsbecause the cell is a flattened epithelial cellFigure 18 Microscopical measurement Three fields of view seen using a high-power (111308840) objective lens a An eyepiece graticule scale b Superimposed images of human cheek epithelial cells and the eyepiece graticule scale c Superimposed images of the eyepiece graticule scale and the stage micrometer scaleacheek cells on a slide on the stage of the microscopebc0 1020304050607080901000 102030405060708090100eyepiece graticule scale (arbitrary units)

eyepiece graticule in the eyepiece of the microscope00stage micrometer scale (marked in 00 1mm and 01 mm divisions)10 20 30 40 50 60 70 80 90 10001 021 Cell structure 7

Electron microscopesEarlier in this chapter it was stated that by 1900 almost all the structures shown in Figures 13 and 15 (pages 2 and 3) had been discovered There followed a time of frustration for microscopists because they realised that no matter how much the design of light microscopes improved there was a limit to how much could ever be seen using lightIn order to understand the problem it is necessary to know something about the nature of light itself and to understand the difference between magnification and resolutionMagnificationMagnification is the number of times larger an image is compared with the real size of the objectWorked example 1 ndash calculating the magnification of a photograph or objectTo calculate M the magnification of a photograph or an object we can use the following methodFigure 19 shows two photographs of a section through the same plant cells The magnifications of the two photographs are the same Suppose we want to know the magnification of the plant cell in Figure 19b If we know its actual (real) length we can calculate its magnification using theThe real length of the cell is 80 1113088mmagnification observed size of the image actual sizeM I AStep 1Measure the length in mm of the cell in the photograph using a ruler You should find that it is about 60 mmStep 2Convert mm to 1113088m (It is easier if we first convert all measurements to the same units ndash in this case micrometres 1113088m)1 mm 1113088 1000 1113088m so 60mm 1113088 60 1113088 10001113088morwhere I 1113088 observed size of the image (that is what you can measure with a ruler) and A 1113088 actual size (that is the real size ndash for example the size of a cell before it is magnified)If you know two of these values you can work out the third one For example if the observed size of the image and the magnification are known you can work out the1113088 60 000 1113088m Use the equation to calculate the magnificationStep 3actual size A I If you write the formula in a triangle Mmagnification Mimage size I

as shown below and cover up the value you want to find it should be obvious how to do the right calculationactual size A 60000m80 m 750I M1113088ASome worked examples are now providedThe lsquo1113088rsquo sign in front of the number 750 means lsquotimesrsquo We say that the magnification is lsquotimes 750rsquoformula M I A88 1 Cell structurea Worked example 2 ndash calculating magnification from a scale barFigure 110 shows a lymphocyteFigure 110 A lymphocyte6 μmbFigure 19 Photographs of the same plant cells seen a with a light microscope b with an electron microscope both shown at a magnification of about 1113088 750We can calculate the magnification of the lymphocyte by simply using the scale bar All you need to do is measure the length of the scale bar and then substitute this and the length it represents into the equationStep 1Measure the scale bar Here it is 36 mmStep 2Convert mm to 1113088m 36mm 1113088 36 1113088 10001113088m 1113088 360001113088mStep 3Use the equation to calculate the magnificationmagnification M image size I actual size A36000m 6m60001 Cell structure 9Worked example 3 ndash calculating the real size of an object from its magnificationTo calculate A the real or actual size of an object we can use the following methodFigure 125 on page 19 shows a plant cell magnified 1113088 5600 One of the chloroplasts is labelled lsquochloroplastrsquo in the figure Suppose we want to know the actual length of this chloroplastStep 1Measure the observed length of the image of the chloroplast (I ) in mm using a ruler The maximum length is 36 mmStep 2Convert mm to 1113088m 30mm 1113088 30 1113088 10001113088m 1113088 300001113088mStep 3Use the equation to calculate the actual lengthactualsizeA imagesizeI magnification M30 000 m 5600 5 4 m (to one decimal place)SAQ 12a Calculate the magnification of the drawing of the animal cell in Figure 13 on page 2b Calculate the actual (real) length of the bottom chloroplast in Figure 127 on page 19Resolution

Look again at Figure 19 (page 9) Figure 19a is a light micrograph (a photograph taken with a light microscope also known as a photomicrograph) Figure 19b is an electron micrograph of the same cells taken at the same magnification (an electron micrograph is a picture taken with an electron microscope) You can see that Figure 19b the electron micrograph is much clearer This is because it has greater resolution Resolution is defined as the abilityto distinguish between two separate points If the two points cannot be resolved they will be seen as one point In practice resolution is the amount of detail that can be seen ndash the greater the resolution the greater the detailThe maximum resolution of a light microscope is 200 nm This means that if two points or objects are closer together than 200 nm they cannot be distinguished as separateIt is possible to take a photograph such as Figure 19a and to magnify (enlarge) it but we see no more detail in other words we do not improve resolution even though we often enlarge photographs because they are easier to see when larger With a microscope magnification up to the limit of resolution can reveal further detail but any further magnification increases blurring as well as the size of the imageThe electromagnetic spectrumHow is resolution linked with the nature of light One of the properties of light is that it travels in waves The length of the waves of visible light varies ranging from about 400 nm (violet light) to about 700 nm (red light) The human eye can distinguish between these different wavelengths and in the brain the differences are converted to colour differences (Colour is an invention of the brain)The whole range of different wavelengths is called the electromagnetic spectrum Visible light is only one part of this spectrum Figure 111 shows some of the parts of the electromagnetic spectrum The longer the waves the lower their frequency (all the waves travel at the same speed so imagine them passing a post shorter waves pass at higher frequency) In theory there is no limit to how short or how long the waves can be Wavelength changes with energy the greater the energy the shorter the wavelength (rather like squashing a spring)Now look at Figure 112 which shows a mitochondrion some very small cell organelles called ribosomes (see page 13) and light of 400 nm wavelength the shortest visible wavelength The mitochondrion is large enoughto interfere with the light waves However the ribosomes are far too small to have any effect on the light waves The general rule is that the limit of resolution is about one half the wavelength of the radiation used to view the specimen In other words if an object is any smaller than half the wavelength of the radiation used to view it it cannot be seen separately from nearby objects This means that the1001 1 Cell structureX-raysgamma rays01 nm400 nm violet bluev infraredmicrowaves

uv10 nmi s i b l e 1000 nmradio and TV wavesvisible light500 nm 600 nm green yellow orange105 nm 107 nm 109 nm 1011 nm 1013 nm700 nmFigure 111 Diagram of the electromagnetic spectrum (the waves are not drawn to scale) The numbers indicate the wavelengths of the different types of electromagnetic radiation Visible light is a form of electromagnetic radiationbest resolution that can be obtained using a microscope that uses visible light (a light microscope) is 200 nm since the shortest wavelength of visible light is 400 nm (violet light) In practice this corresponds to a maximum useful magnification of about 1500 times Ribosomes are approximately 25 nm in diameter and can therefore never be seen using lightIf an object is transparent it will allow light waves to pass through it and therefore will still not be visible This is why many biological structures have to be stained before they can be seenThe electron microscopeBiologists faced with the problem that they would never see anything smaller than 200 nm using a light microscope realised that the only solution would be to use radiation of a shorter wavelength than light If you study Figure 111 you will see that ultraviolet light or better still X-rays look like possible candidates Both ultraviolet and X-ray microscopes have been built the latter with little success partly because of the difficulty of focusing X-rays A much better solution is to use electrons Electrons are negatively charged particles which orbit the nucleus of an atom When a metal becomes very hot some of its electrons gain so much energy that they escape from their orbits like a rocket escaping from Earthrsquos gravity Free electrons behave like electromagnetic radiation They have a very short wavelength the greater the energy the shorter the wavelength Electrons are a very suitable form of radiation for microscopy for two major reasons Firstly their wavelength is extremely short (at least as short as that of X-rays) Secondly because they are negatively charged they can be focused easily using electromagnets (a magnet can be made to alter the path of the beam the equivalent of a glass lens bending light)Using an electron microscope a resolution of 05 nm can be obtained

400 times better than when using a light microscope ETransmission and scanning electron microscopesTwo types of electron microscope are now in common use The transmission electron microscope or TEM forwavelength 400nmstained mitochondrion of diameter 1000nm interferes with light wavesstained ribosomes of diameter 25nm do not interfere with light wavesFigure 112 A mitochondrion and some ribosomes in the path of light waves of 400 nm lengthred1 Cell structure 11

E short was the type originally developed Here the beam of electrons is passed through the specimen before being viewed Only

those electrons that are transmitted (pass through the specimen) are seen This allows us to see thin sections of specimens and thus to see inside cells In the scanning electron microscope (SEM) on the other hand the electron beam is used to scan the surfaces of structures and only the reflected beam is observedAn example of a scanning electron micrograph is shown in Figure 113 The advantage of this microscope is that surface structures can be seen Also great depth of field is obtained so that much of the specimen is in focus at the same time and a three-dimensional appearance is obtained Such a picture would be impossible to obtain with a light microscope even using the same magnification and resolution because you would have to keep focusing up and down with the objective lens to see different parts of the specimen The disadvantage of the SEM is that it cannot achieve the same resolution as a TEM Resolution is between 3 nm and 20 nmViewing specimens with the electron microscopeFigure 114 shows how an electron microscope works and Figure 115 shows one in useFigure 113 False-colour SEM of the head of a cat flea (1113088 100)

electron gun and anode which E produce a beam of electronselectron beam vacuumpathway of electronscondenser electromagnetic lens which directs the electron beam onto the specimenspecimen which is placed on a gridobjective electromagnetic lens which produces an imageprojector electromagnetic lenses which focus the magnified image onto the screenscreen or photographic plate which shows the image of the specimenFigure 114 How an electron microscope worksIt is not possible to see an electron beam so to make the image visible the electron beam has to be projected onto a fluorescent screen The areas hit by electrons shine brightly giving overall a lsquoblack and whitersquo picture The stains used to improve the contrast of biological specimens for electron microscopy contain heavy metal atoms which stop the passage of electrons The resulting picture is like an X-ray photograph with the more densely stained parts of the specimen appearing blacker lsquoFalse-colourrsquo images can be created by colouring the standard black and white image using a computerTo add to the difficulties of electron microscopy the electron beam and therefore the specimen and the fluorescent screen must be in a vacuum If electrons1221 1 Cell structure

EStructures and functions of organellesCompartmentalisation and division of labour within the cell are even more obvious with an electron microscope than with a light microscopeWe will now consider the structures and functions of some of the cell components in more detailNucleusThe nucleus (Figure 118 on page 15) is the largest cell organelle (see also page 5) It is surrounded by two membranes known as the

nuclear envelope The outer membrane of the nuclear envelope is continuous with the endoplasmic reticulum (Figure 117 on page 15) The nuclear envelope has many small pores called nuclear pores These allow and control exchange between the nucleus and the cytoplasm Examples of substances leaving the nucleus through the pores are mRNA and ribosomes for protein synthesis Examples of substances entering through the nuclear pores are proteins to help make ribosomes nucleotides ATP (aderosine triphosphate) and some hormones such as thyroid hormone T3Within the nucleus the chromosomes are in a loosely coiled state known as chromatin (except during nuclear division see Chapter 5) Chromosomes contain DNA which is organised into functional units called genes Genes control the activities of the cell and inheritance thus the nucleus controls the cellrsquos activities When a cell is aboutto divide the nucleus divides first so that each new cell will have its own nucleus (Chapters 5 and 19) Also within the nucleus the nucleolus makes ribosomes using the information in its own DNAEndoplasmic reticulum and ribosomesWhen cells were first seen with the electron microscope biologists were amazed to see so much detailed structure The existence of much of this had not been suspected This was particularly true of an extensive system of membranes running through the cytoplasm which became knownas the endoplasmic reticulum (ER) (Figure 119 on page 15 ndash see also Figures 118 on page 15 and 122 on page 17) The ER is continuous with the outer membrane of the nuclear envelope (Figure 117)There are two types of ER rough ER and smooth ER Rough ER is so called because it is covered with many tinyFigure 115 A TEM in usecollided with air molecules they would scatter making it impossible to achieve a sharp picture Also water boils at room temperature in a vacuum so all specimens must be dehydrated before being placed in the microscope This means that only dead material can be examined Great efforts are therefore made to try to preserve material in a life-like state when preparing it for the microscopeSAQ 13Explain why ribosomes are not visible using a light microscopeUltrastructure of an animal cellThe lsquofinersquo or detailed structure of a cell as revealed by the electron microscope is called its ultrastructure Figure 116 shows the appearance of typical animal cells as seen with an electron microscope and Figure 117 on page 15 is a diagram based on many other such micrographsSAQ 14Compare Figure 117 on page 15 with Figure 13 on page 2 Name the structures which can be seen with the electron microscope but not with the light microscope1 Cell structure 13lysosomeGGolgi apparatusendoplasmic reticulumglycogen granulesccell surface membrane

mmitochondriachromatinnnucleolusmmicrovillusrribosomesFFigure 116 Representative animal cells as seen with a TEM The cells are liver cells from a rat (1113088 9600) The nucleus is clearly visible in one of the cells 1441 1 Cell structurenucleusnnuclear envelopetwo centrioles close to the nucleus and at right angles to each othermitochondrion lysosomerough endoplasmic reticulumnucleoluschromatin nucleusnuclear porenuclear envelope (two membranes)microvillismooth endoplasmic reticulumFigure 117 Ultrastructure of a typical animal cell as seen with an electron microscope In reality the ER is more extensive than shown and free ribosomes may be more extensive Glycogen granules are sometimes present in the cytoplasmFigure 118 TEM of the nucleus of a cell from the pancreas of a bat (1113088 7500) The circular nucleus is surrounded by a double-layered nuclear envelope containing nuclear pores The nucleolus is more Figure 119 TEM of rough ER covered with ribosomes (black dots) darkly stained Rough ER is visible in the surrounding cytoplasm (1113088 17 000) Some free ribosomes can also be seen in the cytoplasmGolgi vesicle Golgi apparatusribosomescell surface membranecytoplasm1 Cell structure 15organelles called ribosomes These are just visible as black dots in Figures 118 and 119 on page 15 At very high magnifications they can be seen to consist of two subunits a large and a small subunit Ribosomes are the sites of protein synthesis (see pages 111ndash112) They can be found free in the cytoplasm as well as on the rough ER They are very small only about 25 nm in diameter They are made of RNA (ribonucleic acid) and protein The rough ER forms an extensive system of flattened sacs spreading in sheets throughout the cell Proteins made by the ribosomes on the rough ER enter the sacs and move through them The proteins are often processed in some way on their journey Small sacs called vesicles can break off from the ER and these can join together to form the Golgi apparatus Proteins can be exported from the cell via the Golgi apparatus (see page 80)Smooth ER so called because it lacks ribosomes has a completely different function It makes lipids and steroids such as cholesterol and the reproductive hormones oestrogen and testosteroneGolgi apparatus (Golgi body or Golgi complex)The Golgi apparatus is a stack of flattened sacs (Figure 120) This stack of sacs is sometimes referred to as theFigure 120 TEM of a Golgi apparatus A central stack of saucer-shaped sacs can be seen budding off small Golgi vesicles (green) These may form secretory vesicles whose contents can be released at the cell surface by exocytosis (see page 80)Golgi body More than one may be present in a cell The stack is constantly being formed at one end from vesicles which bud off from the ER and broken down again at the other end to form Golgi vesicles The stack of sacs with the associated vesicles is referred to as the Golgi apparatus or Golgi complex

The Golgi apparatus collects processes and sorts molecules (particularly proteins from the rough ER) ready for transport in Golgi vesicles either to other parts of the cell or out of the cell (secretion) Two examples of protein processing in the Golgi apparatus are the addition of sugars to proteins to make molecules known as glycoproteins and the removal of the first amino acid methionine from newly formed proteins to make a functioning protein In plants enzymes in the Golgi apparatus convert sugars into cell wall components Golgi vesicles are also used to make lysosomesLysosomesLysosomes (Figure 121) are spherical sacs surrounded by a single membrane and having no internal structure They are commonly 01ndash 05 1113088m in diameter They contain digestive (hydrolytic) enzymes which must be kept separateFigure 121 Lysosomes (orange) in a mouse kidney cell (1113088 55 000) They contain cell structures in the process of digestion and vesicles (green) Cytoplasm is coloured blue here1661 1 Cell structurefrom the rest of the cell to prevent damage Lysosomes are responsible for the breakdown (digestion) of unwanted structures such as old organelles or even whole cells as in mammary glands after lactation (breast feeding) In white blood cells lysosomes are used to digest bacteria (see endocytosis page 80) Enzymes are sometimes released outside the cell ndash for example in the replacement of cartilage with bone during development The headsof sperm contain a special lysosome the acrosome for digesting a path to the ovum (egg)MitochondriaMitochondria (singular mitochondrion) are usually about 1 1113088m in diameter and can be various shapes often sausage- shaped as in Figure 122 They are surrounded by two membranes (an envelope) The inner of these is foldedto form finger-like cristae which project into the interior solution or matrixThe main function of mitochondria is to carry out aerobic respiration As a result of respiration they make ATP the universal energy carrier in cells (see Chapter 16) They are also involved in the synthesis of lipids (page 37)Figure 122 Mitochondrion (orange) with its double membrane (envelope) the inner membrane is folded to form cristae (1113088 20 000) Mitochondria are the sites of aerobic cell respiration Note also the rough ERIn the 1960s it was discovered that mitochondria and chloroplasts contain ribosomes which are slightly smaller than those in the cytoplasm and are the same size as those found in bacteria The size of ribosomes is measured in lsquoS unitsrsquo which are a measure of how fast they sediment in a centrifuge Cytoplasmic ribosomes are 80S while those of bacteria mitochondria and chloroplasts are 70S It was also discovered in the 1960s that mitochondria and chloroplasts contain small circular DNA molecules also like those found in bacteria Not surprisingly it was later provedthat mitochondria and chloroplasts are in effect ancient bacteria which now live inside the larger cells typical of animals and plants (see

prokaryotic and eukaryotic cells page 18) This is known as the endosymbiont theory lsquoEndorsquo means lsquoinsidersquo and a lsquosymbiontrsquo is an organism which lives in a mutually beneficial relationship with another organism The DNA and ribosomes of mitochondria and chloroplasts are still active and responsible for the coding and synthesis of certain vital proteins but mitochondria and chloroplasts can no longer live independentlyMitochondrial ribosomes are just visible as tiny dark dots in the mitochondrial matrix in Figure 122Cell surface membraneThe cell surface membrane is extremely thin (about 7 nm) However at very high magnifications at least 1113088 100 000 it can be seen to have three layers described as a trilaminar appearance This consists of two dark lines (heavily stained) either side of a narrow pale interior (Figure 123) The membrane is partially permeable and controls exchange between the cell and its environment Membrane structure is discussed further in Chapter 4Figure 123 Cell surface membrane (1113088 250 000) At this magnification the membrane appears as two dark lines at the edge of the cellMicrovilliMicrovilli (singular microvillus) are finger-like extensions of the cell surface membrane typical of certain epithelial cells (cells covering surfaces of structures) They greatly1 Cell structure 17increase the surface area of the cell surface membrane (see Figure 117 on page 15) This is useful for example for absorption in the gut and for reabsorption in the proximal convoluted tubules of the kidney (see page 307)CentriolesThe extra resolution of the electron microscope reveals that just outside the nucleus there are really two centrioles (see Figure 124) not one as it appears under the light microscope (compare with Figure 13 on page 2) They lie close together at right-angles to each other A centriole is a hollow cylinder about 04 1113088m long formed from a ring of short microtubules tiny tubes made of a protein called tubulin These microtubules are used as a starting point for growing the spindle microtubules for nuclear division (see page 92) Centrioles are not found in plant cellsUltrastructure of a plant cellAll the structures except centrioles and microvilli so far described in animal cells are also found in plant cells The appearance of a plant cell as seen with the electron microscope is shown in Figure 125 while Figure 126 is a diagram based on many such micrographs The relatively thick cell wall and the large central vacuole are obvious as are the chloroplasts two of which are shown in detail in Figure 127 These structures and their functions have been described on pages 5 and 6 The electron microscopeFigure 124 Centrioles in transverse and longitudinal section (TS and LS) (1113088 86 000) The one on the left is seen in TS and clearly shows the nine triplets of microtubules which make up the structurereveals that chloroplasts contain 70S ribosomes and small circular

DNA molecules as do mitochondria and bacteria This has already been discussed with mitochondria above (page 17)SAQ 15Compare Figure 126 with Figure 15 on page 3 Name the structures which can be seen with the electron microscope but not with the light microscopeTwo fundamentally different types of cellAt one time it was common practice to try to classify all living organisms as either animals or plants With advances in our knowledge of living things it has become obvious that the living world is not that simple Fungi and bacteria for example are very different from animals and plants and from each other Eventually it was discovered that there are two fundamentally different types of cell The most obvious difference betweenthese types is that one possesses a nucleus and the other does notOrganisms that lack nuclei are called prokaryotes (lsquoprorsquo means before lsquokaryonrsquo means nucleus) All prokaryotes are now referred to as bacteria They are on average about 1000 to 10 000 times smaller in volume than cells with nuclei and are much simpler in structure ndash for example their DNA lies free in the cytoplasmOrganisms whose cells possess nuclei are called eukaryotes (lsquoeursquo means true) Their DNA lies inside a nucleus Eukaryotes include animals plants fungi and a group containing most of the unicellular eukaryotes known as protoctists Most biologists believe that eukaryotes evolved from prokaryotes one-and-a-half thousand million years after prokaryotes first appeared on Earth We mainly study animals and plants in this book but all eukaryotic cells have certain features in commonA generalised prokaryotic cell is shown in Figure 128 A comparison of prokaryotic and eukaryotic cells is given in Table 12 on page 211881 1 Cell structure

The need for energy in living organisms 268 Work 268 ATP 270 Respiration 273 Anaerobic respiration 279 Respiratory substrates 280 End-of-chapter questions 28317 Photosynthesis 287An energy transfer process 287 The light-dependent reactions of photosynthesis 288 The light-independent reactions of photosynthesis 290 Leaf structure and function

290 Chloroplast structure and function 293 Factors necessary for photosynthesis 294 Trapping light energy 295 End-of-chapter questions 29718 Regulation and control 301Homeostasis 302 Excretion 302 The structure of the kidney 303 Control of water content 311 Nervous communication 314 Hormonal communication 329 Plant growth regulators336 Electrical communication in plants 339 End-of-chapter questions 34019 Inherited change 347Meiosis 347 Genetics 348 Genotype affects phenotype 351 Inheriting genes 352 Multiple alleles 354 Sex inheritance 355 Sex linkage 355 Dihybrid crosses 357 The 11130882 (chi-squared) test 359 Mutations 361 Environment and phenotype 363 End-of-chapter questions 36320 Selection and evolution 367Natural selection 368 Evolution 370 The DarwinndashWallace theory of evolution by natural selection 374 Species and speciation 375 Artificial selection 377 End-of-chapter questions 37821 Biodiversity and conservation 382The five-kingdom classification 382 Maintaining biodiversity 385 Endangered species 385 End-of-chapter questions 392ivContents22 Gene technology 395Gene technology 395 Benefits of gene technology 399 Potential hazards of gene technology 400 Social and ethical implications of genetic engineering 402 Electrophoresis 403 Cystic fibrosis 404 The genetic counsellor 407 Genetic screening 409 End-of-chapter questions 41123 Biotechnology 414Mining with microorganisms 414 Large-scale production techniques 416 Advantages of batch and continuous culture 419 How penicillin works 420 Immobilising enzymes 422 Monoclonal antibodies 424 End-of-chapter questions 42724 Crop plants 431Cereal crops 431 Maize 433 C4 plants 434 Adaptations for difficult environments 437 Crop improvement 440 End-of-chapter questions 44725 Aspects of human reproduction 451Gametogenesis 453 Human menstrual cycle 456 Birth control 457 Infertility 459 End-of-chapter questions 46426 Planning analysis and evaluation 468 Planning an investigation

468Analysis conclusionsand evaluation End-of-chapter questions4 7 3 480Appendix 1 Amino acid R groups 484 Appendix2 DNAtripletcodes 485 Glossary 486 Index 501 Acknowledgements 510CD-ROMAdvice on how to revise for and approach examinationsChapter summariesMultiple choice tests for Chapters 1ndash14Answers to self-assessment questions Answers to end-of-chapter questions

Contentsv

IntroductionThis new edition is fully updated for the 2014 syllabus to help you do well in your Cambridge International Examinations AS and A level Biology (9700) courses The book and its accompanying CD-ROM provide a self-contained resource for studying these courses with improved focus on exam preparationbull Chapters 1ndash15 provide complete coverage of the AS level syllabus This is also the first year of study for A level The AS syllabus is designed for studentsbull with O level or IGCSE Biology Chapters 16ndash26 cover all the material for the second year of study for A level This includes the relevant Core material and the Applications of Biology sectionExtension material is clearly marked with the following symbol E

and a dotted line runs down alongside the text to mark this additional contentImportant features of this new edition include the followingThe sequence of chapters mirrors the sequence of topics in the syllabus which makes it easy to navigate (Your teacher may however tackle subjects in a different order) Syllabus sections G and H are split into three and two chapters respectively for additional convenience (see table)LevelApplications of BiologySyllabus section Chapter PSelection and evolution 20 Q Biodiversity and 21conservation R Gene technology 22 S Biotechnology 23 T Crop plants 24 U Aspects of human reproduction 25Planning analysis and evaluation26LevelAS level Core syllabusA levelSyllabus section A Cell structureChapter1There are two new chapters covering practical skills Chapter 15 (AS level) and Chapter 26 (A level) Interesting information that is not required by the syllabus but will aid understanding is marked as lsquoextension materialrsquo by orange dotted barsEach chapter contains self-assessment questions (SAQs) These are to help you think about understand and remember what you have just read Each chapter ends with a set of chapter-related questions ranging from formative questions (requiring simple recall or reference to the text) to more challenging structured or essay questions requiring understanding as well as the other skills tested in examinations Some of the questions are past Cambridge examination questions so you can familiarise yourself with the style of the examination questionsBiology involves many technical terms Each time a new term is introduced it is shown in bold orange and its meaning explained The glossary contains definitions of the key terms used in the book

At the end of your course you will be tested on three sets of Assessment Objectivesbull Knowledge with understanding You are expected to know and understand all the facts and concepts listedbull in the syllabus These are all covered in this book Handling information and solving problems Questions testing these skills expect you to use your knowledge and understanding in an unfamiliar context A good knowledge and understanding of this book will enablebull you to approach new situations with confidence Experimental skills and investigations This involves practical work An examination will test your practical skills so try to do plenty of practical work Key information is provided on some practical aspects of the course in Chapters 15 and 26Additional help and guidance are available on the accompanying CD-ROMF Genetic control G Transport7 8vi IntroductionB Biological molecules 2 C Enzymes 3 D Cell membranes and transport

4 E Cell and nuclear division 56Transport in multicellular plants The mammalian transport system The mammalian heart 9H Gas exchange and smoking Gas exchange 10 Smoking 11I Infectious disease 12 J Immunity 13 K Ecology 14Advanced practical skills 15 L Energy and respiration 16 M Photosynthesis

17 N Regulation and control 18 O Inherited change 19

1 Cell structure By the end of this chapter you

should be able todescribe and interpret drawings and photographs of typical animal and plant cells as seen using the light microscope and make microscopical measurements using an eyepiece graticule and stage micrometerbe familiar with the units used in cell studies explain the meanings of and distinguish betweenthe terms resolution and magnificationdescribe and interpret drawings and photographs of typical animal and plant cells as seen using the electron microscope recognising rough and smooth endoplasmic reticulum (ER) Golgi apparatus mitochondria ribosomes lysosomes cell surface membrane centrioles nucleus (including the nuclear envelope and nucleolus) and microvilli as well asIn the early days of microscopy an English scientist Robert Hooke decided to examine thin slices of plant material He chose cork as one of his examples Looking down the microscope he was struck by the regular appearance ofthe structure and in 1665 he wrote a book containing the diagram shown in Figure 11If you examine the diagram you will see the lsquopore-likersquo regular structures that Hooke called lsquocellsrsquo Each cell appeared to be an empty box surrounded by a wall Hooke had discovered and described without realising it the fundamental unit of all living things

Although we now know that the cells of cork are dead further observations of cells in living materials were made by Hooke and other scientists However it was not until almost 200 years later that a general cell theory emerged from the work of two German scientists In 1838 Schleiden a botanist suggested that all plants are made of cells and a year later Schwann a zoologist suggested the same for animals The cell theory states that the basic unit of structure and function of all living organisms is the cell Now over 170 years later this idea is one of the most familiar and important theories in biology To it has beenthe chloroplasts cell wall large permanent vacuole tonoplast and plasmodesmata of plant cellsoutline the functions of the structures listed abovecompare the structure of typical animal and plant cellscalculate the linear magnification of and the actual sizes of specimens from drawings and photographsdescribe the structure of a prokaryotic cell and compare and contrast the structure of prokaryotic cells with that of eukaryotic cellsexplain how eukaryotic cells may be organised into tissues and organs with reference to transverse sections of stems roots and leavesdraw and label low-power plan diagrams of tissues and organsadded Virchowrsquos theory of 1855 that all cells arise from pre-existing cells by cell divisionFigure 11 Drawing of cork cells published by Robert Hooke in 16651 Cell structure 1

Why cellsA cell can be thought of as a bag in which the chemistry of life is allowed to occur partially separated from the environment outside the cell The thin membrane which surrounds all cells is essential in controlling exchange between the cell and its environment It is a very effective barrier but also allows a controlled traffic of materials across it in both directions The membrane is therefore described as partially permeable If it were freely permeable life could not exist because the chemicals of the cell would simply mix with the surrounding chemicals by diffusion (page 73)Cell biology and microscopyThe study of cells has given rise to an important branch of biology known as cell biology Cells can now be studied by many different methods but scientists began simply by looking at them using various types of microscopeThere are two fundamentally different types of microscope now in use the light microscope and the electron microscope Both use a form of radiation in order to create an image of the specimen being examined The light microscope uses light as a source of radiation while the electron microscope uses electrons for reasons which are discussed latereyepiecelight beamobjective cover slipglass slidecondenseriris diaphragmlight source pathway of lightEyepiece lens magnifies and focuses the image from the objective onto the eyeObjective lens collects light passing through the specimen and produces a magnified image

Condenser lens focuses the light onto the specimen held between the cover slip and slideCondenser iris diaphragm is closed slightly to produce a narrow beam of lightFigure 12 How the light microscope worksLight microscopyThe lsquogolden agersquo of light microscopy could be said to be the 19th century Microscopes had been available since the beginning of the 17th century but when dramatic improvements were made in the quality of glass lenses in the early 19th century interest among scientists became widespread The fascination ofthe microscopic world that opened up in biology inspired rapid progress both in microscope design and equally importantly in preparing material for examination with microscopes This branch of biology is known as cytology Figure 12 shows how the light microscope worksBy 1900 all the structures shown in Figures 13 14 and 15 except lysosomes had been discovered Figure 13 shows the structure of a generalised animal cell and Figure 15 the structure of a generalised plant cell as seen with a light microscope (A generalised cell shows all the structures that are typically found in a cell)Golgi apparatus cytoplasmcentriole ndash always found near nucleus has a role in nuclear divisionsmall structures that are difficult to identifymitochondria cell surface membranenuclear envelopechromatin ndash deeply staining and thread-likenucleolus ndash deeply stainingnucleusFigure 13 Structure of a generalised animal cell (diameter about 20 1113088m) as seen with a very high quality light microscope22 1 Cell structuretonoplast ndash membrane surrounding vacuolecell surface membrane (pressed against cell wall)vacuole ndash large with central positioncytoplasmmitochondriamiddle lamella ndash thin layer holding cells together contains calcium pectateplasmodesma ndash connects cytoplasm of neighbouring cellscell wall of neighbouring cellcell wall chloroplast grana just visiblesmall structures that are difficult to identifyGolgi apparatusFigure 14 Cells from the lining of the human cheek (1113088 500) each showing a centrally placed nucleus which is a typical animal cell characteristic The cells are part of a tissue known as squamous (flattened) epitheliumFigure 14 shows some actual human cells and Figure 16 shows an actual plant cell taken from a leafSAQ 11Using Figures 13 and 15 name the structures that animal and plant cells have in common those found in only plant cells and those found only in animal cellsFigure 16 Photomicrograph of a cell in a moss leaf (11130881400)nucleusnucleolus ndash deeply stainingnuclear envelopechromatin ndash deeply staining and thread-likeFigure 15 Structure of a generalised plant cell (diameter about 40 1113088m) as seen with a very high quality light microscope1 Cell structure 3Box 1A Biological drawingYou need the following equipment

bull pencil (HB) bull pencil sharpener bull eraser bull ruler bull plain paperHere are some guidelines for the quality of your drawingbull always use a pencil not a pen bull donrsquot use shading bull use clear continuous lines bull use accurate proportions and observation ndash not atextbook version For a low-power drawing (see Figure 17)bull donrsquot draw individual cells bull draw all tissues completely enclosed by lines bull draw a correct interpretation of the distribution of bull tissuesa representative portion may be drawn (eg half a transverse section)For a high-power drawingbull draw only a few representative cells bull draw the cell wall of all plant cells bull donrsquot draw the nucleus as a solid blobSome guidelines for the quality of your labellingbull label all tissues and relevant structures bull identify parts correctly bull use a ruler for label linesAnimal and plant cells have features in commonIn animals and plants each cell is surrounded by a very thin cell surface membrane which is too thin to be seen with a light microscope This is also sometimes referred to as the plasma membranebull arrange label lines neatly and ensure they donrsquot cross bull over each otherannotate your drawing if necessary (ie provideshort notes with one or more of the labels in order bull to describe or explain features of biological interest)add a scale line at the bottom of the drawing if bull appropriateuse a pencil not a penAn example of a drawing of a section through the stem of Helianthus is shown below Biological drawing is also covered in Chapter 15 page 264Figure 17 The right side of this low-power drawing shows examples of good technique while the left side shows many of the pitfalls you should avoidMany of the cell contents are colourless and transparent so they need to be stained to be seen Each cell has a nucleus which is a relatively large structure that stains intensely and is therefore very conspicuous The deeply staining material in the nucleus is called chromatin and is a massof loosely coiled threads This material collects together to form visible separate chromosomes during nuclear division44 1 Cell structure(see page 86) It contains DNA (deoxyribonucleic acid) a molecule which contains the instructions that control the activities of the cell (see Chapter 6) Within the nucleus an even more deeply staining area is visible the nucleolus which is made of loops of DNA from several chromosomes The number of nucleoli is variable one to five being

common in mammalsThe material between the nucleus and the cell surface membrane is known as cytoplasm Cytoplasm is an aqueous (watery) material varying from a fluid to a jelly-like consistency Many small structures can be seen within it These have been likened to small organs and hence are known as organelles An organelle can be defined as a functionally and structurally distinct part of a cell Organelles themselves are often surrounded by membranes so that their activities can be separated from the surrounding cytoplasm This is described as compartmentalisation Having separate compartments is essential for a structure as complex as an animal or plant cell to work efficiently Since each type of organelle has its own function the cell is said to show division of labour a sharing of the work between different specialised organellesThe most numerous organelles seen with the light microscope are usually mitochondria (singular mitochondrion) Mitochondria are only just visible but films of living cells taken with the aid of a light microscope have shown that they can move about change shape and divide They are specialised to carry out aerobic respirationThe use of special stains containing silver enabled the Golgi apparatus to be detected for the first time in 1898 by Camillo Golgi The Golgi apparatus is part of a complex internal sorting and distribution system within the cell (see page 16) It is also sometimes called the Golgi body or Golgi complexDifferences between animal and plant cellsThe only structure commonly found in animal cells which is absent from plant cells is the centriole Plant cells also differ from animal cells in possessing cell walls large permanent vacuoles and chloroplastsCentriolesUnder the light microscope the centriole appears as a small structure close to the nucleus (see Figure 13 on page 2) The centriole is involved in nuclear division (see page 92)Cell walls and plasmodesmataWith a light microscope individual plant cells are more easily seen than animal cells because they are usually larger and unlike animal cells surrounded by a cell wall outside the cell surface membrane This is relatively rigid because it contains fibres of cellulose a polysaccharide which strengthens the wall The cell wall gives the cell a definite shape It prevents the cell from bursting when water enters by osmosis allowing large pressures to develop inside the cell (see page 77) Cell walls may also be reinforced with extra cellulose or with a hard material called lignin for extra strength (see xylem on page 24) Cell walls are freely permeable allowing free movement of molecules and ions through to the cell surface membranePlant cells are linked to neighbouring cells by means of fine strands of cytoplasm called plasmodesmata (singular plasmodesma) which pass through pore-like structures in the walls of these neighbouring cells Movement through the pores is thought to be controlled by the structure of the poresVacuoles

Although animal cells may possess small vacuoles such as phagocytic vacuoles (see page 80) which are temporary structures mature plant cells often possess a large permanent central vacuole The plant vacuole is surrounded by a membrane the tonoplast which controls exchange between the vacuole and the cytoplasm The fluid in the vacuole is a solution of mineral salts sugars oxygen carbon dioxide pigments enzymes and other organic compounds including some waste productsVacuoles help to regulate the osmotic properties of cells (the flow of water inwards and outwards) as well as having a wide range of other functions For example the pigments which colour the petals of certain flowers and parts of some vegetables such as the red pigment of beetroots are sometimes located in vacuoles1 Cell structure 5ChloroplastsSome plant cells are able to carry out photosynthesis because they contain chloroplasts Chloroplasts are relatively large organelles which are green in colour due to the presence of chlorophyll At high magnifications small lsquograinsrsquo or grana (singular granum) can be seen in the chloroplasts During the process of photosynthesis light is absorbed by these grana which actually consist of stacks of membrane-bound sacs called thylakoids Starch grains may also be visible within chloroplasts Chloroplasts are found in the green parts of plants mainly in the leavesPoints to noteWe return to the differences between animal and plant cells as seen using the electron microscope on page 18Units of measurement in cell studiesIn order to measure objects in the microscopic world we need to use very small units of measurement which are unfamiliar to most people According to international agreement the International System of Units (SI units) should be used In this system the basic unit of length is the metre (symbol m) Additional units can be created in multiples of a thousand times larger or smaller using standard prefixes For example the prefix kilo means 1000 times Thus 1 kilometre 1113088 1000 metres The units of length relevant to cell studies are shown in Table 11It is difficult to imagine how small these units are but when looking down a microscope and seeing cells clearly we should not forget how amazingly small the cells actually are The smallest structure visible with the human eye is about 50ndash100 1113088m in diameter Your body contains about 60 million million cells varying in size from about 5 1113088m to 40 1113088m Try to imagine structures like mitochondria which have an average diameter of 1 1113088m The smallest cell organelles we deal with in this book ribosomes are only about 25 nm in diameter You could line up about 20 000 ribosomes across the full stop at the end of this sentenceUnit Symbolmillimetre mm micrometre 1113088m nanometre nmTable 11 Units of measurement relevant to cell studies 1113088 is the Greek letter mu 1 micrometre is a thousandth of a millimetre 1 nanometre is a thousandth of a micrometre661 Cell structure

bull bull bullbullYou can think of a plant cell as being very similar to an animal cell but with extra structures Plant cells are often larger than animal cells although cell size varies enormouslyDo not confuse the cell wall with the cell surface membrane Cell walls are relatively thick and physically strong whereas cell surface membranes are very thin Cell walls are freely permeable whereas cell surface membranes are partially permeable All cells have a cell surface membrane Vacuoles are not confined to plant cells animal cells may have small vacuoles such as phagocytic vacuoles (see page 80) although these are not usually permanent structuresFraction of a metreone thousandth 1113088 0001 1113088 11000 1113088 10-3 one millionth 1113088 0000 001 1113088 11 000 000 1113088 10-6 one thousand millionth 1113088 0000 000 001 1113088 11 000 000 000 1113088 10-9

Box 1B Measuring cellsCells and organelles can be measured with a microscope by means of an eyepiece graticule This is a transparent scale It usually has 100 divisions (see Figure 18a) The eyepiece graticule is placed in the microscope eyepiece so that it can be seen at the same time as the object to be measured as shown in Figure 18b Figure 18b shows the scale over a human cheek epithelialcell The cell lies between 40 and 60 on the scale We therefore say it measures 20 eyepiece units in diameter (the difference between 60 and 40) We will not know the actual size of the eyepiece units until the eyepiece graticule scale is calibratedTo calibrate the eyepiece graticule scale a miniature transparent ruler called a stage micrometer scale is placed on the microscope stage and is brought into focus This scale may be etched onto a glass slideor printed on a transparent film It commonly has subdivisions of 01 and 001 mm The images of the two scales can then be superimposed as shown in Figure 18cIn the eyepiece graticule shown in the figure 100 units measure 025 mm Hence the value of each eyepiece unit is0 25 00025 mm 100Or converting mm to 1113088m 0 25 1000 2 51113088m100The diameter of the cell shown superimposed on the scale in Figure 18b measures 20 eyepiece units and so its actual diameter is20times251113088m 1113088 501113088m This diameter is greater than that of many human cellsbecause the cell is a flattened epithelial cellFigure 18 Microscopical measurement Three fields of view seen using a high-power (111308840) objective lens a An eyepiece graticule scale b Superimposed images of human cheek epithelial cells and the eyepiece graticule scale c Superimposed images of the eyepiece graticule scale and the stage micrometer scaleacheek cells on a slide on the stage of the microscopebc0 1020304050607080901000 102030405060708090100eyepiece graticule scale (arbitrary units)

eyepiece graticule in the eyepiece of the microscope00stage micrometer scale (marked in 00 1mm and 01 mm divisions)10 20 30 40 50 60 70 80 90 10001 021 Cell structure 7

Electron microscopesEarlier in this chapter it was stated that by 1900 almost all the structures shown in Figures 13 and 15 (pages 2 and 3) had been discovered There followed a time of frustration for microscopists because they realised that no matter how much the design of light microscopes improved there was a limit to how much could ever be seen using lightIn order to understand the problem it is necessary to know something about the nature of light itself and to understand the difference between magnification and resolutionMagnificationMagnification is the number of times larger an image is compared with the real size of the objectWorked example 1 ndash calculating the magnification of a photograph or objectTo calculate M the magnification of a photograph or an object we can use the following methodFigure 19 shows two photographs of a section through the same plant cells The magnifications of the two photographs are the same Suppose we want to know the magnification of the plant cell in Figure 19b If we know its actual (real) length we can calculate its magnification using theThe real length of the cell is 80 1113088mmagnification observed size of the image actual sizeM I AStep 1Measure the length in mm of the cell in the photograph using a ruler You should find that it is about 60 mmStep 2Convert mm to 1113088m (It is easier if we first convert all measurements to the same units ndash in this case micrometres 1113088m)1 mm 1113088 1000 1113088m so 60mm 1113088 60 1113088 10001113088morwhere I 1113088 observed size of the image (that is what you can measure with a ruler) and A 1113088 actual size (that is the real size ndash for example the size of a cell before it is magnified)If you know two of these values you can work out the third one For example if the observed size of the image and the magnification are known you can work out the1113088 60 000 1113088m Use the equation to calculate the magnificationStep 3actual size A I If you write the formula in a triangle Mmagnification Mimage size I

as shown below and cover up the value you want to find it should be obvious how to do the right calculationactual size A 60000m80 m 750I M1113088ASome worked examples are now providedThe lsquo1113088rsquo sign in front of the number 750 means lsquotimesrsquo We say that the magnification is lsquotimes 750rsquoformula M I A88 1 Cell structurea Worked example 2 ndash calculating magnification from a scale barFigure 110 shows a lymphocyteFigure 110 A lymphocyte6 μmbFigure 19 Photographs of the same plant cells seen a with a light microscope b with an electron microscope both shown at a magnification of about 1113088 750We can calculate the magnification of the lymphocyte by simply using the scale bar All you need to do is measure the length of the scale bar and then substitute this and the length it represents into the equationStep 1Measure the scale bar Here it is 36 mmStep 2Convert mm to 1113088m 36mm 1113088 36 1113088 10001113088m 1113088 360001113088mStep 3Use the equation to calculate the magnificationmagnification M image size I actual size A36000m 6m60001 Cell structure 9Worked example 3 ndash calculating the real size of an object from its magnificationTo calculate A the real or actual size of an object we can use the following methodFigure 125 on page 19 shows a plant cell magnified 1113088 5600 One of the chloroplasts is labelled lsquochloroplastrsquo in the figure Suppose we want to know the actual length of this chloroplastStep 1Measure the observed length of the image of the chloroplast (I ) in mm using a ruler The maximum length is 36 mmStep 2Convert mm to 1113088m 30mm 1113088 30 1113088 10001113088m 1113088 300001113088mStep 3Use the equation to calculate the actual lengthactualsizeA imagesizeI magnification M30 000 m 5600 5 4 m (to one decimal place)SAQ 12a Calculate the magnification of the drawing of the animal cell in Figure 13 on page 2b Calculate the actual (real) length of the bottom chloroplast in Figure 127 on page 19Resolution

Look again at Figure 19 (page 9) Figure 19a is a light micrograph (a photograph taken with a light microscope also known as a photomicrograph) Figure 19b is an electron micrograph of the same cells taken at the same magnification (an electron micrograph is a picture taken with an electron microscope) You can see that Figure 19b the electron micrograph is much clearer This is because it has greater resolution Resolution is defined as the abilityto distinguish between two separate points If the two points cannot be resolved they will be seen as one point In practice resolution is the amount of detail that can be seen ndash the greater the resolution the greater the detailThe maximum resolution of a light microscope is 200 nm This means that if two points or objects are closer together than 200 nm they cannot be distinguished as separateIt is possible to take a photograph such as Figure 19a and to magnify (enlarge) it but we see no more detail in other words we do not improve resolution even though we often enlarge photographs because they are easier to see when larger With a microscope magnification up to the limit of resolution can reveal further detail but any further magnification increases blurring as well as the size of the imageThe electromagnetic spectrumHow is resolution linked with the nature of light One of the properties of light is that it travels in waves The length of the waves of visible light varies ranging from about 400 nm (violet light) to about 700 nm (red light) The human eye can distinguish between these different wavelengths and in the brain the differences are converted to colour differences (Colour is an invention of the brain)The whole range of different wavelengths is called the electromagnetic spectrum Visible light is only one part of this spectrum Figure 111 shows some of the parts of the electromagnetic spectrum The longer the waves the lower their frequency (all the waves travel at the same speed so imagine them passing a post shorter waves pass at higher frequency) In theory there is no limit to how short or how long the waves can be Wavelength changes with energy the greater the energy the shorter the wavelength (rather like squashing a spring)Now look at Figure 112 which shows a mitochondrion some very small cell organelles called ribosomes (see page 13) and light of 400 nm wavelength the shortest visible wavelength The mitochondrion is large enoughto interfere with the light waves However the ribosomes are far too small to have any effect on the light waves The general rule is that the limit of resolution is about one half the wavelength of the radiation used to view the specimen In other words if an object is any smaller than half the wavelength of the radiation used to view it it cannot be seen separately from nearby objects This means that the1001 1 Cell structureX-raysgamma rays01 nm400 nm violet bluev infraredmicrowaves

uv10 nmi s i b l e 1000 nmradio and TV wavesvisible light500 nm 600 nm green yellow orange105 nm 107 nm 109 nm 1011 nm 1013 nm700 nmFigure 111 Diagram of the electromagnetic spectrum (the waves are not drawn to scale) The numbers indicate the wavelengths of the different types of electromagnetic radiation Visible light is a form of electromagnetic radiationbest resolution that can be obtained using a microscope that uses visible light (a light microscope) is 200 nm since the shortest wavelength of visible light is 400 nm (violet light) In practice this corresponds to a maximum useful magnification of about 1500 times Ribosomes are approximately 25 nm in diameter and can therefore never be seen using lightIf an object is transparent it will allow light waves to pass through it and therefore will still not be visible This is why many biological structures have to be stained before they can be seenThe electron microscopeBiologists faced with the problem that they would never see anything smaller than 200 nm using a light microscope realised that the only solution would be to use radiation of a shorter wavelength than light If you study Figure 111 you will see that ultraviolet light or better still X-rays look like possible candidates Both ultraviolet and X-ray microscopes have been built the latter with little success partly because of the difficulty of focusing X-rays A much better solution is to use electrons Electrons are negatively charged particles which orbit the nucleus of an atom When a metal becomes very hot some of its electrons gain so much energy that they escape from their orbits like a rocket escaping from Earthrsquos gravity Free electrons behave like electromagnetic radiation They have a very short wavelength the greater the energy the shorter the wavelength Electrons are a very suitable form of radiation for microscopy for two major reasons Firstly their wavelength is extremely short (at least as short as that of X-rays) Secondly because they are negatively charged they can be focused easily using electromagnets (a magnet can be made to alter the path of the beam the equivalent of a glass lens bending light)Using an electron microscope a resolution of 05 nm can be obtained

400 times better than when using a light microscope ETransmission and scanning electron microscopesTwo types of electron microscope are now in common use The transmission electron microscope or TEM forwavelength 400nmstained mitochondrion of diameter 1000nm interferes with light wavesstained ribosomes of diameter 25nm do not interfere with light wavesFigure 112 A mitochondrion and some ribosomes in the path of light waves of 400 nm lengthred1 Cell structure 11

E short was the type originally developed Here the beam of electrons is passed through the specimen before being viewed Only

those electrons that are transmitted (pass through the specimen) are seen This allows us to see thin sections of specimens and thus to see inside cells In the scanning electron microscope (SEM) on the other hand the electron beam is used to scan the surfaces of structures and only the reflected beam is observedAn example of a scanning electron micrograph is shown in Figure 113 The advantage of this microscope is that surface structures can be seen Also great depth of field is obtained so that much of the specimen is in focus at the same time and a three-dimensional appearance is obtained Such a picture would be impossible to obtain with a light microscope even using the same magnification and resolution because you would have to keep focusing up and down with the objective lens to see different parts of the specimen The disadvantage of the SEM is that it cannot achieve the same resolution as a TEM Resolution is between 3 nm and 20 nmViewing specimens with the electron microscopeFigure 114 shows how an electron microscope works and Figure 115 shows one in useFigure 113 False-colour SEM of the head of a cat flea (1113088 100)

electron gun and anode which E produce a beam of electronselectron beam vacuumpathway of electronscondenser electromagnetic lens which directs the electron beam onto the specimenspecimen which is placed on a gridobjective electromagnetic lens which produces an imageprojector electromagnetic lenses which focus the magnified image onto the screenscreen or photographic plate which shows the image of the specimenFigure 114 How an electron microscope worksIt is not possible to see an electron beam so to make the image visible the electron beam has to be projected onto a fluorescent screen The areas hit by electrons shine brightly giving overall a lsquoblack and whitersquo picture The stains used to improve the contrast of biological specimens for electron microscopy contain heavy metal atoms which stop the passage of electrons The resulting picture is like an X-ray photograph with the more densely stained parts of the specimen appearing blacker lsquoFalse-colourrsquo images can be created by colouring the standard black and white image using a computerTo add to the difficulties of electron microscopy the electron beam and therefore the specimen and the fluorescent screen must be in a vacuum If electrons1221 1 Cell structure

EStructures and functions of organellesCompartmentalisation and division of labour within the cell are even more obvious with an electron microscope than with a light microscopeWe will now consider the structures and functions of some of the cell components in more detailNucleusThe nucleus (Figure 118 on page 15) is the largest cell organelle (see also page 5) It is surrounded by two membranes known as the

nuclear envelope The outer membrane of the nuclear envelope is continuous with the endoplasmic reticulum (Figure 117 on page 15) The nuclear envelope has many small pores called nuclear pores These allow and control exchange between the nucleus and the cytoplasm Examples of substances leaving the nucleus through the pores are mRNA and ribosomes for protein synthesis Examples of substances entering through the nuclear pores are proteins to help make ribosomes nucleotides ATP (aderosine triphosphate) and some hormones such as thyroid hormone T3Within the nucleus the chromosomes are in a loosely coiled state known as chromatin (except during nuclear division see Chapter 5) Chromosomes contain DNA which is organised into functional units called genes Genes control the activities of the cell and inheritance thus the nucleus controls the cellrsquos activities When a cell is aboutto divide the nucleus divides first so that each new cell will have its own nucleus (Chapters 5 and 19) Also within the nucleus the nucleolus makes ribosomes using the information in its own DNAEndoplasmic reticulum and ribosomesWhen cells were first seen with the electron microscope biologists were amazed to see so much detailed structure The existence of much of this had not been suspected This was particularly true of an extensive system of membranes running through the cytoplasm which became knownas the endoplasmic reticulum (ER) (Figure 119 on page 15 ndash see also Figures 118 on page 15 and 122 on page 17) The ER is continuous with the outer membrane of the nuclear envelope (Figure 117)There are two types of ER rough ER and smooth ER Rough ER is so called because it is covered with many tinyFigure 115 A TEM in usecollided with air molecules they would scatter making it impossible to achieve a sharp picture Also water boils at room temperature in a vacuum so all specimens must be dehydrated before being placed in the microscope This means that only dead material can be examined Great efforts are therefore made to try to preserve material in a life-like state when preparing it for the microscopeSAQ 13Explain why ribosomes are not visible using a light microscopeUltrastructure of an animal cellThe lsquofinersquo or detailed structure of a cell as revealed by the electron microscope is called its ultrastructure Figure 116 shows the appearance of typical animal cells as seen with an electron microscope and Figure 117 on page 15 is a diagram based on many other such micrographsSAQ 14Compare Figure 117 on page 15 with Figure 13 on page 2 Name the structures which can be seen with the electron microscope but not with the light microscope1 Cell structure 13lysosomeGGolgi apparatusendoplasmic reticulumglycogen granulesccell surface membrane

mmitochondriachromatinnnucleolusmmicrovillusrribosomesFFigure 116 Representative animal cells as seen with a TEM The cells are liver cells from a rat (1113088 9600) The nucleus is clearly visible in one of the cells 1441 1 Cell structurenucleusnnuclear envelopetwo centrioles close to the nucleus and at right angles to each othermitochondrion lysosomerough endoplasmic reticulumnucleoluschromatin nucleusnuclear porenuclear envelope (two membranes)microvillismooth endoplasmic reticulumFigure 117 Ultrastructure of a typical animal cell as seen with an electron microscope In reality the ER is more extensive than shown and free ribosomes may be more extensive Glycogen granules are sometimes present in the cytoplasmFigure 118 TEM of the nucleus of a cell from the pancreas of a bat (1113088 7500) The circular nucleus is surrounded by a double-layered nuclear envelope containing nuclear pores The nucleolus is more Figure 119 TEM of rough ER covered with ribosomes (black dots) darkly stained Rough ER is visible in the surrounding cytoplasm (1113088 17 000) Some free ribosomes can also be seen in the cytoplasmGolgi vesicle Golgi apparatusribosomescell surface membranecytoplasm1 Cell structure 15organelles called ribosomes These are just visible as black dots in Figures 118 and 119 on page 15 At very high magnifications they can be seen to consist of two subunits a large and a small subunit Ribosomes are the sites of protein synthesis (see pages 111ndash112) They can be found free in the cytoplasm as well as on the rough ER They are very small only about 25 nm in diameter They are made of RNA (ribonucleic acid) and protein The rough ER forms an extensive system of flattened sacs spreading in sheets throughout the cell Proteins made by the ribosomes on the rough ER enter the sacs and move through them The proteins are often processed in some way on their journey Small sacs called vesicles can break off from the ER and these can join together to form the Golgi apparatus Proteins can be exported from the cell via the Golgi apparatus (see page 80)Smooth ER so called because it lacks ribosomes has a completely different function It makes lipids and steroids such as cholesterol and the reproductive hormones oestrogen and testosteroneGolgi apparatus (Golgi body or Golgi complex)The Golgi apparatus is a stack of flattened sacs (Figure 120) This stack of sacs is sometimes referred to as theFigure 120 TEM of a Golgi apparatus A central stack of saucer-shaped sacs can be seen budding off small Golgi vesicles (green) These may form secretory vesicles whose contents can be released at the cell surface by exocytosis (see page 80)Golgi body More than one may be present in a cell The stack is constantly being formed at one end from vesicles which bud off from the ER and broken down again at the other end to form Golgi vesicles The stack of sacs with the associated vesicles is referred to as the Golgi apparatus or Golgi complex

The Golgi apparatus collects processes and sorts molecules (particularly proteins from the rough ER) ready for transport in Golgi vesicles either to other parts of the cell or out of the cell (secretion) Two examples of protein processing in the Golgi apparatus are the addition of sugars to proteins to make molecules known as glycoproteins and the removal of the first amino acid methionine from newly formed proteins to make a functioning protein In plants enzymes in the Golgi apparatus convert sugars into cell wall components Golgi vesicles are also used to make lysosomesLysosomesLysosomes (Figure 121) are spherical sacs surrounded by a single membrane and having no internal structure They are commonly 01ndash 05 1113088m in diameter They contain digestive (hydrolytic) enzymes which must be kept separateFigure 121 Lysosomes (orange) in a mouse kidney cell (1113088 55 000) They contain cell structures in the process of digestion and vesicles (green) Cytoplasm is coloured blue here1661 1 Cell structurefrom the rest of the cell to prevent damage Lysosomes are responsible for the breakdown (digestion) of unwanted structures such as old organelles or even whole cells as in mammary glands after lactation (breast feeding) In white blood cells lysosomes are used to digest bacteria (see endocytosis page 80) Enzymes are sometimes released outside the cell ndash for example in the replacement of cartilage with bone during development The headsof sperm contain a special lysosome the acrosome for digesting a path to the ovum (egg)MitochondriaMitochondria (singular mitochondrion) are usually about 1 1113088m in diameter and can be various shapes often sausage- shaped as in Figure 122 They are surrounded by two membranes (an envelope) The inner of these is foldedto form finger-like cristae which project into the interior solution or matrixThe main function of mitochondria is to carry out aerobic respiration As a result of respiration they make ATP the universal energy carrier in cells (see Chapter 16) They are also involved in the synthesis of lipids (page 37)Figure 122 Mitochondrion (orange) with its double membrane (envelope) the inner membrane is folded to form cristae (1113088 20 000) Mitochondria are the sites of aerobic cell respiration Note also the rough ERIn the 1960s it was discovered that mitochondria and chloroplasts contain ribosomes which are slightly smaller than those in the cytoplasm and are the same size as those found in bacteria The size of ribosomes is measured in lsquoS unitsrsquo which are a measure of how fast they sediment in a centrifuge Cytoplasmic ribosomes are 80S while those of bacteria mitochondria and chloroplasts are 70S It was also discovered in the 1960s that mitochondria and chloroplasts contain small circular DNA molecules also like those found in bacteria Not surprisingly it was later provedthat mitochondria and chloroplasts are in effect ancient bacteria which now live inside the larger cells typical of animals and plants (see

prokaryotic and eukaryotic cells page 18) This is known as the endosymbiont theory lsquoEndorsquo means lsquoinsidersquo and a lsquosymbiontrsquo is an organism which lives in a mutually beneficial relationship with another organism The DNA and ribosomes of mitochondria and chloroplasts are still active and responsible for the coding and synthesis of certain vital proteins but mitochondria and chloroplasts can no longer live independentlyMitochondrial ribosomes are just visible as tiny dark dots in the mitochondrial matrix in Figure 122Cell surface membraneThe cell surface membrane is extremely thin (about 7 nm) However at very high magnifications at least 1113088 100 000 it can be seen to have three layers described as a trilaminar appearance This consists of two dark lines (heavily stained) either side of a narrow pale interior (Figure 123) The membrane is partially permeable and controls exchange between the cell and its environment Membrane structure is discussed further in Chapter 4Figure 123 Cell surface membrane (1113088 250 000) At this magnification the membrane appears as two dark lines at the edge of the cellMicrovilliMicrovilli (singular microvillus) are finger-like extensions of the cell surface membrane typical of certain epithelial cells (cells covering surfaces of structures) They greatly1 Cell structure 17increase the surface area of the cell surface membrane (see Figure 117 on page 15) This is useful for example for absorption in the gut and for reabsorption in the proximal convoluted tubules of the kidney (see page 307)CentriolesThe extra resolution of the electron microscope reveals that just outside the nucleus there are really two centrioles (see Figure 124) not one as it appears under the light microscope (compare with Figure 13 on page 2) They lie close together at right-angles to each other A centriole is a hollow cylinder about 04 1113088m long formed from a ring of short microtubules tiny tubes made of a protein called tubulin These microtubules are used as a starting point for growing the spindle microtubules for nuclear division (see page 92) Centrioles are not found in plant cellsUltrastructure of a plant cellAll the structures except centrioles and microvilli so far described in animal cells are also found in plant cells The appearance of a plant cell as seen with the electron microscope is shown in Figure 125 while Figure 126 is a diagram based on many such micrographs The relatively thick cell wall and the large central vacuole are obvious as are the chloroplasts two of which are shown in detail in Figure 127 These structures and their functions have been described on pages 5 and 6 The electron microscopeFigure 124 Centrioles in transverse and longitudinal section (TS and LS) (1113088 86 000) The one on the left is seen in TS and clearly shows the nine triplets of microtubules which make up the structurereveals that chloroplasts contain 70S ribosomes and small circular

DNA molecules as do mitochondria and bacteria This has already been discussed with mitochondria above (page 17)SAQ 15Compare Figure 126 with Figure 15 on page 3 Name the structures which can be seen with the electron microscope but not with the light microscopeTwo fundamentally different types of cellAt one time it was common practice to try to classify all living organisms as either animals or plants With advances in our knowledge of living things it has become obvious that the living world is not that simple Fungi and bacteria for example are very different from animals and plants and from each other Eventually it was discovered that there are two fundamentally different types of cell The most obvious difference betweenthese types is that one possesses a nucleus and the other does notOrganisms that lack nuclei are called prokaryotes (lsquoprorsquo means before lsquokaryonrsquo means nucleus) All prokaryotes are now referred to as bacteria They are on average about 1000 to 10 000 times smaller in volume than cells with nuclei and are much simpler in structure ndash for example their DNA lies free in the cytoplasmOrganisms whose cells possess nuclei are called eukaryotes (lsquoeursquo means true) Their DNA lies inside a nucleus Eukaryotes include animals plants fungi and a group containing most of the unicellular eukaryotes known as protoctists Most biologists believe that eukaryotes evolved from prokaryotes one-and-a-half thousand million years after prokaryotes first appeared on Earth We mainly study animals and plants in this book but all eukaryotic cells have certain features in commonA generalised prokaryotic cell is shown in Figure 128 A comparison of prokaryotic and eukaryotic cells is given in Table 12 on page 211881 1 Cell structure

Contentsv

IntroductionThis new edition is fully updated for the 2014 syllabus to help you do well in your Cambridge International Examinations AS and A level Biology (9700) courses The book and its accompanying CD-ROM provide a self-contained resource for studying these courses with improved focus on exam preparationbull Chapters 1ndash15 provide complete coverage of the AS level syllabus This is also the first year of study for A level The AS syllabus is designed for studentsbull with O level or IGCSE Biology Chapters 16ndash26 cover all the material for the second year of study for A level This includes the relevant Core material and the Applications of Biology sectionExtension material is clearly marked with the following symbol E

and a dotted line runs down alongside the text to mark this additional contentImportant features of this new edition include the followingThe sequence of chapters mirrors the sequence of topics in the syllabus which makes it easy to navigate (Your teacher may however tackle subjects in a different order) Syllabus sections G and H are split into three and two chapters respectively for additional convenience (see table)LevelApplications of BiologySyllabus section Chapter PSelection and evolution 20 Q Biodiversity and 21conservation R Gene technology 22 S Biotechnology 23 T Crop plants 24 U Aspects of human reproduction 25Planning analysis and evaluation26LevelAS level Core syllabusA levelSyllabus section A Cell structureChapter1There are two new chapters covering practical skills Chapter 15 (AS level) and Chapter 26 (A level) Interesting information that is not required by the syllabus but will aid understanding is marked as lsquoextension materialrsquo by orange dotted barsEach chapter contains self-assessment questions (SAQs) These are to help you think about understand and remember what you have just read Each chapter ends with a set of chapter-related questions ranging from formative questions (requiring simple recall or reference to the text) to more challenging structured or essay questions requiring understanding as well as the other skills tested in examinations Some of the questions are past Cambridge examination questions so you can familiarise yourself with the style of the examination questionsBiology involves many technical terms Each time a new term is introduced it is shown in bold orange and its meaning explained The glossary contains definitions of the key terms used in the book

At the end of your course you will be tested on three sets of Assessment Objectivesbull Knowledge with understanding You are expected to know and understand all the facts and concepts listedbull in the syllabus These are all covered in this book Handling information and solving problems Questions testing these skills expect you to use your knowledge and understanding in an unfamiliar context A good knowledge and understanding of this book will enablebull you to approach new situations with confidence Experimental skills and investigations This involves practical work An examination will test your practical skills so try to do plenty of practical work Key information is provided on some practical aspects of the course in Chapters 15 and 26Additional help and guidance are available on the accompanying CD-ROMF Genetic control G Transport7 8vi IntroductionB Biological molecules 2 C Enzymes 3 D Cell membranes and transport

4 E Cell and nuclear division 56Transport in multicellular plants The mammalian transport system The mammalian heart 9H Gas exchange and smoking Gas exchange 10 Smoking 11I Infectious disease 12 J Immunity 13 K Ecology 14Advanced practical skills 15 L Energy and respiration 16 M Photosynthesis

17 N Regulation and control 18 O Inherited change 19

1 Cell structure By the end of this chapter you

should be able todescribe and interpret drawings and photographs of typical animal and plant cells as seen using the light microscope and make microscopical measurements using an eyepiece graticule and stage micrometerbe familiar with the units used in cell studies explain the meanings of and distinguish betweenthe terms resolution and magnificationdescribe and interpret drawings and photographs of typical animal and plant cells as seen using the electron microscope recognising rough and smooth endoplasmic reticulum (ER) Golgi apparatus mitochondria ribosomes lysosomes cell surface membrane centrioles nucleus (including the nuclear envelope and nucleolus) and microvilli as well asIn the early days of microscopy an English scientist Robert Hooke decided to examine thin slices of plant material He chose cork as one of his examples Looking down the microscope he was struck by the regular appearance ofthe structure and in 1665 he wrote a book containing the diagram shown in Figure 11If you examine the diagram you will see the lsquopore-likersquo regular structures that Hooke called lsquocellsrsquo Each cell appeared to be an empty box surrounded by a wall Hooke had discovered and described without realising it the fundamental unit of all living things

Although we now know that the cells of cork are dead further observations of cells in living materials were made by Hooke and other scientists However it was not until almost 200 years later that a general cell theory emerged from the work of two German scientists In 1838 Schleiden a botanist suggested that all plants are made of cells and a year later Schwann a zoologist suggested the same for animals The cell theory states that the basic unit of structure and function of all living organisms is the cell Now over 170 years later this idea is one of the most familiar and important theories in biology To it has beenthe chloroplasts cell wall large permanent vacuole tonoplast and plasmodesmata of plant cellsoutline the functions of the structures listed abovecompare the structure of typical animal and plant cellscalculate the linear magnification of and the actual sizes of specimens from drawings and photographsdescribe the structure of a prokaryotic cell and compare and contrast the structure of prokaryotic cells with that of eukaryotic cellsexplain how eukaryotic cells may be organised into tissues and organs with reference to transverse sections of stems roots and leavesdraw and label low-power plan diagrams of tissues and organsadded Virchowrsquos theory of 1855 that all cells arise from pre-existing cells by cell divisionFigure 11 Drawing of cork cells published by Robert Hooke in 16651 Cell structure 1

Why cellsA cell can be thought of as a bag in which the chemistry of life is allowed to occur partially separated from the environment outside the cell The thin membrane which surrounds all cells is essential in controlling exchange between the cell and its environment It is a very effective barrier but also allows a controlled traffic of materials across it in both directions The membrane is therefore described as partially permeable If it were freely permeable life could not exist because the chemicals of the cell would simply mix with the surrounding chemicals by diffusion (page 73)Cell biology and microscopyThe study of cells has given rise to an important branch of biology known as cell biology Cells can now be studied by many different methods but scientists began simply by looking at them using various types of microscopeThere are two fundamentally different types of microscope now in use the light microscope and the electron microscope Both use a form of radiation in order to create an image of the specimen being examined The light microscope uses light as a source of radiation while the electron microscope uses electrons for reasons which are discussed latereyepiecelight beamobjective cover slipglass slidecondenseriris diaphragmlight source pathway of lightEyepiece lens magnifies and focuses the image from the objective onto the eyeObjective lens collects light passing through the specimen and produces a magnified image

Condenser lens focuses the light onto the specimen held between the cover slip and slideCondenser iris diaphragm is closed slightly to produce a narrow beam of lightFigure 12 How the light microscope worksLight microscopyThe lsquogolden agersquo of light microscopy could be said to be the 19th century Microscopes had been available since the beginning of the 17th century but when dramatic improvements were made in the quality of glass lenses in the early 19th century interest among scientists became widespread The fascination ofthe microscopic world that opened up in biology inspired rapid progress both in microscope design and equally importantly in preparing material for examination with microscopes This branch of biology is known as cytology Figure 12 shows how the light microscope worksBy 1900 all the structures shown in Figures 13 14 and 15 except lysosomes had been discovered Figure 13 shows the structure of a generalised animal cell and Figure 15 the structure of a generalised plant cell as seen with a light microscope (A generalised cell shows all the structures that are typically found in a cell)Golgi apparatus cytoplasmcentriole ndash always found near nucleus has a role in nuclear divisionsmall structures that are difficult to identifymitochondria cell surface membranenuclear envelopechromatin ndash deeply staining and thread-likenucleolus ndash deeply stainingnucleusFigure 13 Structure of a generalised animal cell (diameter about 20 1113088m) as seen with a very high quality light microscope22 1 Cell structuretonoplast ndash membrane surrounding vacuolecell surface membrane (pressed against cell wall)vacuole ndash large with central positioncytoplasmmitochondriamiddle lamella ndash thin layer holding cells together contains calcium pectateplasmodesma ndash connects cytoplasm of neighbouring cellscell wall of neighbouring cellcell wall chloroplast grana just visiblesmall structures that are difficult to identifyGolgi apparatusFigure 14 Cells from the lining of the human cheek (1113088 500) each showing a centrally placed nucleus which is a typical animal cell characteristic The cells are part of a tissue known as squamous (flattened) epitheliumFigure 14 shows some actual human cells and Figure 16 shows an actual plant cell taken from a leafSAQ 11Using Figures 13 and 15 name the structures that animal and plant cells have in common those found in only plant cells and those found only in animal cellsFigure 16 Photomicrograph of a cell in a moss leaf (11130881400)nucleusnucleolus ndash deeply stainingnuclear envelopechromatin ndash deeply staining and thread-likeFigure 15 Structure of a generalised plant cell (diameter about 40 1113088m) as seen with a very high quality light microscope1 Cell structure 3Box 1A Biological drawingYou need the following equipment

bull pencil (HB) bull pencil sharpener bull eraser bull ruler bull plain paperHere are some guidelines for the quality of your drawingbull always use a pencil not a pen bull donrsquot use shading bull use clear continuous lines bull use accurate proportions and observation ndash not atextbook version For a low-power drawing (see Figure 17)bull donrsquot draw individual cells bull draw all tissues completely enclosed by lines bull draw a correct interpretation of the distribution of bull tissuesa representative portion may be drawn (eg half a transverse section)For a high-power drawingbull draw only a few representative cells bull draw the cell wall of all plant cells bull donrsquot draw the nucleus as a solid blobSome guidelines for the quality of your labellingbull label all tissues and relevant structures bull identify parts correctly bull use a ruler for label linesAnimal and plant cells have features in commonIn animals and plants each cell is surrounded by a very thin cell surface membrane which is too thin to be seen with a light microscope This is also sometimes referred to as the plasma membranebull arrange label lines neatly and ensure they donrsquot cross bull over each otherannotate your drawing if necessary (ie provideshort notes with one or more of the labels in order bull to describe or explain features of biological interest)add a scale line at the bottom of the drawing if bull appropriateuse a pencil not a penAn example of a drawing of a section through the stem of Helianthus is shown below Biological drawing is also covered in Chapter 15 page 264Figure 17 The right side of this low-power drawing shows examples of good technique while the left side shows many of the pitfalls you should avoidMany of the cell contents are colourless and transparent so they need to be stained to be seen Each cell has a nucleus which is a relatively large structure that stains intensely and is therefore very conspicuous The deeply staining material in the nucleus is called chromatin and is a massof loosely coiled threads This material collects together to form visible separate chromosomes during nuclear division44 1 Cell structure(see page 86) It contains DNA (deoxyribonucleic acid) a molecule which contains the instructions that control the activities of the cell (see Chapter 6) Within the nucleus an even more deeply staining area is visible the nucleolus which is made of loops of DNA from several chromosomes The number of nucleoli is variable one to five being

common in mammalsThe material between the nucleus and the cell surface membrane is known as cytoplasm Cytoplasm is an aqueous (watery) material varying from a fluid to a jelly-like consistency Many small structures can be seen within it These have been likened to small organs and hence are known as organelles An organelle can be defined as a functionally and structurally distinct part of a cell Organelles themselves are often surrounded by membranes so that their activities can be separated from the surrounding cytoplasm This is described as compartmentalisation Having separate compartments is essential for a structure as complex as an animal or plant cell to work efficiently Since each type of organelle has its own function the cell is said to show division of labour a sharing of the work between different specialised organellesThe most numerous organelles seen with the light microscope are usually mitochondria (singular mitochondrion) Mitochondria are only just visible but films of living cells taken with the aid of a light microscope have shown that they can move about change shape and divide They are specialised to carry out aerobic respirationThe use of special stains containing silver enabled the Golgi apparatus to be detected for the first time in 1898 by Camillo Golgi The Golgi apparatus is part of a complex internal sorting and distribution system within the cell (see page 16) It is also sometimes called the Golgi body or Golgi complexDifferences between animal and plant cellsThe only structure commonly found in animal cells which is absent from plant cells is the centriole Plant cells also differ from animal cells in possessing cell walls large permanent vacuoles and chloroplastsCentriolesUnder the light microscope the centriole appears as a small structure close to the nucleus (see Figure 13 on page 2) The centriole is involved in nuclear division (see page 92)Cell walls and plasmodesmataWith a light microscope individual plant cells are more easily seen than animal cells because they are usually larger and unlike animal cells surrounded by a cell wall outside the cell surface membrane This is relatively rigid because it contains fibres of cellulose a polysaccharide which strengthens the wall The cell wall gives the cell a definite shape It prevents the cell from bursting when water enters by osmosis allowing large pressures to develop inside the cell (see page 77) Cell walls may also be reinforced with extra cellulose or with a hard material called lignin for extra strength (see xylem on page 24) Cell walls are freely permeable allowing free movement of molecules and ions through to the cell surface membranePlant cells are linked to neighbouring cells by means of fine strands of cytoplasm called plasmodesmata (singular plasmodesma) which pass through pore-like structures in the walls of these neighbouring cells Movement through the pores is thought to be controlled by the structure of the poresVacuoles

Although animal cells may possess small vacuoles such as phagocytic vacuoles (see page 80) which are temporary structures mature plant cells often possess a large permanent central vacuole The plant vacuole is surrounded by a membrane the tonoplast which controls exchange between the vacuole and the cytoplasm The fluid in the vacuole is a solution of mineral salts sugars oxygen carbon dioxide pigments enzymes and other organic compounds including some waste productsVacuoles help to regulate the osmotic properties of cells (the flow of water inwards and outwards) as well as having a wide range of other functions For example the pigments which colour the petals of certain flowers and parts of some vegetables such as the red pigment of beetroots are sometimes located in vacuoles1 Cell structure 5ChloroplastsSome plant cells are able to carry out photosynthesis because they contain chloroplasts Chloroplasts are relatively large organelles which are green in colour due to the presence of chlorophyll At high magnifications small lsquograinsrsquo or grana (singular granum) can be seen in the chloroplasts During the process of photosynthesis light is absorbed by these grana which actually consist of stacks of membrane-bound sacs called thylakoids Starch grains may also be visible within chloroplasts Chloroplasts are found in the green parts of plants mainly in the leavesPoints to noteWe return to the differences between animal and plant cells as seen using the electron microscope on page 18Units of measurement in cell studiesIn order to measure objects in the microscopic world we need to use very small units of measurement which are unfamiliar to most people According to international agreement the International System of Units (SI units) should be used In this system the basic unit of length is the metre (symbol m) Additional units can be created in multiples of a thousand times larger or smaller using standard prefixes For example the prefix kilo means 1000 times Thus 1 kilometre 1113088 1000 metres The units of length relevant to cell studies are shown in Table 11It is difficult to imagine how small these units are but when looking down a microscope and seeing cells clearly we should not forget how amazingly small the cells actually are The smallest structure visible with the human eye is about 50ndash100 1113088m in diameter Your body contains about 60 million million cells varying in size from about 5 1113088m to 40 1113088m Try to imagine structures like mitochondria which have an average diameter of 1 1113088m The smallest cell organelles we deal with in this book ribosomes are only about 25 nm in diameter You could line up about 20 000 ribosomes across the full stop at the end of this sentenceUnit Symbolmillimetre mm micrometre 1113088m nanometre nmTable 11 Units of measurement relevant to cell studies 1113088 is the Greek letter mu 1 micrometre is a thousandth of a millimetre 1 nanometre is a thousandth of a micrometre661 Cell structure

bull bull bullbullYou can think of a plant cell as being very similar to an animal cell but with extra structures Plant cells are often larger than animal cells although cell size varies enormouslyDo not confuse the cell wall with the cell surface membrane Cell walls are relatively thick and physically strong whereas cell surface membranes are very thin Cell walls are freely permeable whereas cell surface membranes are partially permeable All cells have a cell surface membrane Vacuoles are not confined to plant cells animal cells may have small vacuoles such as phagocytic vacuoles (see page 80) although these are not usually permanent structuresFraction of a metreone thousandth 1113088 0001 1113088 11000 1113088 10-3 one millionth 1113088 0000 001 1113088 11 000 000 1113088 10-6 one thousand millionth 1113088 0000 000 001 1113088 11 000 000 000 1113088 10-9

Box 1B Measuring cellsCells and organelles can be measured with a microscope by means of an eyepiece graticule This is a transparent scale It usually has 100 divisions (see Figure 18a) The eyepiece graticule is placed in the microscope eyepiece so that it can be seen at the same time as the object to be measured as shown in Figure 18b Figure 18b shows the scale over a human cheek epithelialcell The cell lies between 40 and 60 on the scale We therefore say it measures 20 eyepiece units in diameter (the difference between 60 and 40) We will not know the actual size of the eyepiece units until the eyepiece graticule scale is calibratedTo calibrate the eyepiece graticule scale a miniature transparent ruler called a stage micrometer scale is placed on the microscope stage and is brought into focus This scale may be etched onto a glass slideor printed on a transparent film It commonly has subdivisions of 01 and 001 mm The images of the two scales can then be superimposed as shown in Figure 18cIn the eyepiece graticule shown in the figure 100 units measure 025 mm Hence the value of each eyepiece unit is0 25 00025 mm 100Or converting mm to 1113088m 0 25 1000 2 51113088m100The diameter of the cell shown superimposed on the scale in Figure 18b measures 20 eyepiece units and so its actual diameter is20times251113088m 1113088 501113088m This diameter is greater than that of many human cellsbecause the cell is a flattened epithelial cellFigure 18 Microscopical measurement Three fields of view seen using a high-power (111308840) objective lens a An eyepiece graticule scale b Superimposed images of human cheek epithelial cells and the eyepiece graticule scale c Superimposed images of the eyepiece graticule scale and the stage micrometer scaleacheek cells on a slide on the stage of the microscopebc0 1020304050607080901000 102030405060708090100eyepiece graticule scale (arbitrary units)

eyepiece graticule in the eyepiece of the microscope00stage micrometer scale (marked in 00 1mm and 01 mm divisions)10 20 30 40 50 60 70 80 90 10001 021 Cell structure 7

Electron microscopesEarlier in this chapter it was stated that by 1900 almost all the structures shown in Figures 13 and 15 (pages 2 and 3) had been discovered There followed a time of frustration for microscopists because they realised that no matter how much the design of light microscopes improved there was a limit to how much could ever be seen using lightIn order to understand the problem it is necessary to know something about the nature of light itself and to understand the difference between magnification and resolutionMagnificationMagnification is the number of times larger an image is compared with the real size of the objectWorked example 1 ndash calculating the magnification of a photograph or objectTo calculate M the magnification of a photograph or an object we can use the following methodFigure 19 shows two photographs of a section through the same plant cells The magnifications of the two photographs are the same Suppose we want to know the magnification of the plant cell in Figure 19b If we know its actual (real) length we can calculate its magnification using theThe real length of the cell is 80 1113088mmagnification observed size of the image actual sizeM I AStep 1Measure the length in mm of the cell in the photograph using a ruler You should find that it is about 60 mmStep 2Convert mm to 1113088m (It is easier if we first convert all measurements to the same units ndash in this case micrometres 1113088m)1 mm 1113088 1000 1113088m so 60mm 1113088 60 1113088 10001113088morwhere I 1113088 observed size of the image (that is what you can measure with a ruler) and A 1113088 actual size (that is the real size ndash for example the size of a cell before it is magnified)If you know two of these values you can work out the third one For example if the observed size of the image and the magnification are known you can work out the1113088 60 000 1113088m Use the equation to calculate the magnificationStep 3actual size A I If you write the formula in a triangle Mmagnification Mimage size I

as shown below and cover up the value you want to find it should be obvious how to do the right calculationactual size A 60000m80 m 750I M1113088ASome worked examples are now providedThe lsquo1113088rsquo sign in front of the number 750 means lsquotimesrsquo We say that the magnification is lsquotimes 750rsquoformula M I A88 1 Cell structurea Worked example 2 ndash calculating magnification from a scale barFigure 110 shows a lymphocyteFigure 110 A lymphocyte6 μmbFigure 19 Photographs of the same plant cells seen a with a light microscope b with an electron microscope both shown at a magnification of about 1113088 750We can calculate the magnification of the lymphocyte by simply using the scale bar All you need to do is measure the length of the scale bar and then substitute this and the length it represents into the equationStep 1Measure the scale bar Here it is 36 mmStep 2Convert mm to 1113088m 36mm 1113088 36 1113088 10001113088m 1113088 360001113088mStep 3Use the equation to calculate the magnificationmagnification M image size I actual size A36000m 6m60001 Cell structure 9Worked example 3 ndash calculating the real size of an object from its magnificationTo calculate A the real or actual size of an object we can use the following methodFigure 125 on page 19 shows a plant cell magnified 1113088 5600 One of the chloroplasts is labelled lsquochloroplastrsquo in the figure Suppose we want to know the actual length of this chloroplastStep 1Measure the observed length of the image of the chloroplast (I ) in mm using a ruler The maximum length is 36 mmStep 2Convert mm to 1113088m 30mm 1113088 30 1113088 10001113088m 1113088 300001113088mStep 3Use the equation to calculate the actual lengthactualsizeA imagesizeI magnification M30 000 m 5600 5 4 m (to one decimal place)SAQ 12a Calculate the magnification of the drawing of the animal cell in Figure 13 on page 2b Calculate the actual (real) length of the bottom chloroplast in Figure 127 on page 19Resolution

Look again at Figure 19 (page 9) Figure 19a is a light micrograph (a photograph taken with a light microscope also known as a photomicrograph) Figure 19b is an electron micrograph of the same cells taken at the same magnification (an electron micrograph is a picture taken with an electron microscope) You can see that Figure 19b the electron micrograph is much clearer This is because it has greater resolution Resolution is defined as the abilityto distinguish between two separate points If the two points cannot be resolved they will be seen as one point In practice resolution is the amount of detail that can be seen ndash the greater the resolution the greater the detailThe maximum resolution of a light microscope is 200 nm This means that if two points or objects are closer together than 200 nm they cannot be distinguished as separateIt is possible to take a photograph such as Figure 19a and to magnify (enlarge) it but we see no more detail in other words we do not improve resolution even though we often enlarge photographs because they are easier to see when larger With a microscope magnification up to the limit of resolution can reveal further detail but any further magnification increases blurring as well as the size of the imageThe electromagnetic spectrumHow is resolution linked with the nature of light One of the properties of light is that it travels in waves The length of the waves of visible light varies ranging from about 400 nm (violet light) to about 700 nm (red light) The human eye can distinguish between these different wavelengths and in the brain the differences are converted to colour differences (Colour is an invention of the brain)The whole range of different wavelengths is called the electromagnetic spectrum Visible light is only one part of this spectrum Figure 111 shows some of the parts of the electromagnetic spectrum The longer the waves the lower their frequency (all the waves travel at the same speed so imagine them passing a post shorter waves pass at higher frequency) In theory there is no limit to how short or how long the waves can be Wavelength changes with energy the greater the energy the shorter the wavelength (rather like squashing a spring)Now look at Figure 112 which shows a mitochondrion some very small cell organelles called ribosomes (see page 13) and light of 400 nm wavelength the shortest visible wavelength The mitochondrion is large enoughto interfere with the light waves However the ribosomes are far too small to have any effect on the light waves The general rule is that the limit of resolution is about one half the wavelength of the radiation used to view the specimen In other words if an object is any smaller than half the wavelength of the radiation used to view it it cannot be seen separately from nearby objects This means that the1001 1 Cell structureX-raysgamma rays01 nm400 nm violet bluev infraredmicrowaves

uv10 nmi s i b l e 1000 nmradio and TV wavesvisible light500 nm 600 nm green yellow orange105 nm 107 nm 109 nm 1011 nm 1013 nm700 nmFigure 111 Diagram of the electromagnetic spectrum (the waves are not drawn to scale) The numbers indicate the wavelengths of the different types of electromagnetic radiation Visible light is a form of electromagnetic radiationbest resolution that can be obtained using a microscope that uses visible light (a light microscope) is 200 nm since the shortest wavelength of visible light is 400 nm (violet light) In practice this corresponds to a maximum useful magnification of about 1500 times Ribosomes are approximately 25 nm in diameter and can therefore never be seen using lightIf an object is transparent it will allow light waves to pass through it and therefore will still not be visible This is why many biological structures have to be stained before they can be seenThe electron microscopeBiologists faced with the problem that they would never see anything smaller than 200 nm using a light microscope realised that the only solution would be to use radiation of a shorter wavelength than light If you study Figure 111 you will see that ultraviolet light or better still X-rays look like possible candidates Both ultraviolet and X-ray microscopes have been built the latter with little success partly because of the difficulty of focusing X-rays A much better solution is to use electrons Electrons are negatively charged particles which orbit the nucleus of an atom When a metal becomes very hot some of its electrons gain so much energy that they escape from their orbits like a rocket escaping from Earthrsquos gravity Free electrons behave like electromagnetic radiation They have a very short wavelength the greater the energy the shorter the wavelength Electrons are a very suitable form of radiation for microscopy for two major reasons Firstly their wavelength is extremely short (at least as short as that of X-rays) Secondly because they are negatively charged they can be focused easily using electromagnets (a magnet can be made to alter the path of the beam the equivalent of a glass lens bending light)Using an electron microscope a resolution of 05 nm can be obtained

400 times better than when using a light microscope ETransmission and scanning electron microscopesTwo types of electron microscope are now in common use The transmission electron microscope or TEM forwavelength 400nmstained mitochondrion of diameter 1000nm interferes with light wavesstained ribosomes of diameter 25nm do not interfere with light wavesFigure 112 A mitochondrion and some ribosomes in the path of light waves of 400 nm lengthred1 Cell structure 11

E short was the type originally developed Here the beam of electrons is passed through the specimen before being viewed Only

those electrons that are transmitted (pass through the specimen) are seen This allows us to see thin sections of specimens and thus to see inside cells In the scanning electron microscope (SEM) on the other hand the electron beam is used to scan the surfaces of structures and only the reflected beam is observedAn example of a scanning electron micrograph is shown in Figure 113 The advantage of this microscope is that surface structures can be seen Also great depth of field is obtained so that much of the specimen is in focus at the same time and a three-dimensional appearance is obtained Such a picture would be impossible to obtain with a light microscope even using the same magnification and resolution because you would have to keep focusing up and down with the objective lens to see different parts of the specimen The disadvantage of the SEM is that it cannot achieve the same resolution as a TEM Resolution is between 3 nm and 20 nmViewing specimens with the electron microscopeFigure 114 shows how an electron microscope works and Figure 115 shows one in useFigure 113 False-colour SEM of the head of a cat flea (1113088 100)

electron gun and anode which E produce a beam of electronselectron beam vacuumpathway of electronscondenser electromagnetic lens which directs the electron beam onto the specimenspecimen which is placed on a gridobjective electromagnetic lens which produces an imageprojector electromagnetic lenses which focus the magnified image onto the screenscreen or photographic plate which shows the image of the specimenFigure 114 How an electron microscope worksIt is not possible to see an electron beam so to make the image visible the electron beam has to be projected onto a fluorescent screen The areas hit by electrons shine brightly giving overall a lsquoblack and whitersquo picture The stains used to improve the contrast of biological specimens for electron microscopy contain heavy metal atoms which stop the passage of electrons The resulting picture is like an X-ray photograph with the more densely stained parts of the specimen appearing blacker lsquoFalse-colourrsquo images can be created by colouring the standard black and white image using a computerTo add to the difficulties of electron microscopy the electron beam and therefore the specimen and the fluorescent screen must be in a vacuum If electrons1221 1 Cell structure

EStructures and functions of organellesCompartmentalisation and division of labour within the cell are even more obvious with an electron microscope than with a light microscopeWe will now consider the structures and functions of some of the cell components in more detailNucleusThe nucleus (Figure 118 on page 15) is the largest cell organelle (see also page 5) It is surrounded by two membranes known as the

nuclear envelope The outer membrane of the nuclear envelope is continuous with the endoplasmic reticulum (Figure 117 on page 15) The nuclear envelope has many small pores called nuclear pores These allow and control exchange between the nucleus and the cytoplasm Examples of substances leaving the nucleus through the pores are mRNA and ribosomes for protein synthesis Examples of substances entering through the nuclear pores are proteins to help make ribosomes nucleotides ATP (aderosine triphosphate) and some hormones such as thyroid hormone T3Within the nucleus the chromosomes are in a loosely coiled state known as chromatin (except during nuclear division see Chapter 5) Chromosomes contain DNA which is organised into functional units called genes Genes control the activities of the cell and inheritance thus the nucleus controls the cellrsquos activities When a cell is aboutto divide the nucleus divides first so that each new cell will have its own nucleus (Chapters 5 and 19) Also within the nucleus the nucleolus makes ribosomes using the information in its own DNAEndoplasmic reticulum and ribosomesWhen cells were first seen with the electron microscope biologists were amazed to see so much detailed structure The existence of much of this had not been suspected This was particularly true of an extensive system of membranes running through the cytoplasm which became knownas the endoplasmic reticulum (ER) (Figure 119 on page 15 ndash see also Figures 118 on page 15 and 122 on page 17) The ER is continuous with the outer membrane of the nuclear envelope (Figure 117)There are two types of ER rough ER and smooth ER Rough ER is so called because it is covered with many tinyFigure 115 A TEM in usecollided with air molecules they would scatter making it impossible to achieve a sharp picture Also water boils at room temperature in a vacuum so all specimens must be dehydrated before being placed in the microscope This means that only dead material can be examined Great efforts are therefore made to try to preserve material in a life-like state when preparing it for the microscopeSAQ 13Explain why ribosomes are not visible using a light microscopeUltrastructure of an animal cellThe lsquofinersquo or detailed structure of a cell as revealed by the electron microscope is called its ultrastructure Figure 116 shows the appearance of typical animal cells as seen with an electron microscope and Figure 117 on page 15 is a diagram based on many other such micrographsSAQ 14Compare Figure 117 on page 15 with Figure 13 on page 2 Name the structures which can be seen with the electron microscope but not with the light microscope1 Cell structure 13lysosomeGGolgi apparatusendoplasmic reticulumglycogen granulesccell surface membrane

mmitochondriachromatinnnucleolusmmicrovillusrribosomesFFigure 116 Representative animal cells as seen with a TEM The cells are liver cells from a rat (1113088 9600) The nucleus is clearly visible in one of the cells 1441 1 Cell structurenucleusnnuclear envelopetwo centrioles close to the nucleus and at right angles to each othermitochondrion lysosomerough endoplasmic reticulumnucleoluschromatin nucleusnuclear porenuclear envelope (two membranes)microvillismooth endoplasmic reticulumFigure 117 Ultrastructure of a typical animal cell as seen with an electron microscope In reality the ER is more extensive than shown and free ribosomes may be more extensive Glycogen granules are sometimes present in the cytoplasmFigure 118 TEM of the nucleus of a cell from the pancreas of a bat (1113088 7500) The circular nucleus is surrounded by a double-layered nuclear envelope containing nuclear pores The nucleolus is more Figure 119 TEM of rough ER covered with ribosomes (black dots) darkly stained Rough ER is visible in the surrounding cytoplasm (1113088 17 000) Some free ribosomes can also be seen in the cytoplasmGolgi vesicle Golgi apparatusribosomescell surface membranecytoplasm1 Cell structure 15organelles called ribosomes These are just visible as black dots in Figures 118 and 119 on page 15 At very high magnifications they can be seen to consist of two subunits a large and a small subunit Ribosomes are the sites of protein synthesis (see pages 111ndash112) They can be found free in the cytoplasm as well as on the rough ER They are very small only about 25 nm in diameter They are made of RNA (ribonucleic acid) and protein The rough ER forms an extensive system of flattened sacs spreading in sheets throughout the cell Proteins made by the ribosomes on the rough ER enter the sacs and move through them The proteins are often processed in some way on their journey Small sacs called vesicles can break off from the ER and these can join together to form the Golgi apparatus Proteins can be exported from the cell via the Golgi apparatus (see page 80)Smooth ER so called because it lacks ribosomes has a completely different function It makes lipids and steroids such as cholesterol and the reproductive hormones oestrogen and testosteroneGolgi apparatus (Golgi body or Golgi complex)The Golgi apparatus is a stack of flattened sacs (Figure 120) This stack of sacs is sometimes referred to as theFigure 120 TEM of a Golgi apparatus A central stack of saucer-shaped sacs can be seen budding off small Golgi vesicles (green) These may form secretory vesicles whose contents can be released at the cell surface by exocytosis (see page 80)Golgi body More than one may be present in a cell The stack is constantly being formed at one end from vesicles which bud off from the ER and broken down again at the other end to form Golgi vesicles The stack of sacs with the associated vesicles is referred to as the Golgi apparatus or Golgi complex

The Golgi apparatus collects processes and sorts molecules (particularly proteins from the rough ER) ready for transport in Golgi vesicles either to other parts of the cell or out of the cell (secretion) Two examples of protein processing in the Golgi apparatus are the addition of sugars to proteins to make molecules known as glycoproteins and the removal of the first amino acid methionine from newly formed proteins to make a functioning protein In plants enzymes in the Golgi apparatus convert sugars into cell wall components Golgi vesicles are also used to make lysosomesLysosomesLysosomes (Figure 121) are spherical sacs surrounded by a single membrane and having no internal structure They are commonly 01ndash 05 1113088m in diameter They contain digestive (hydrolytic) enzymes which must be kept separateFigure 121 Lysosomes (orange) in a mouse kidney cell (1113088 55 000) They contain cell structures in the process of digestion and vesicles (green) Cytoplasm is coloured blue here1661 1 Cell structurefrom the rest of the cell to prevent damage Lysosomes are responsible for the breakdown (digestion) of unwanted structures such as old organelles or even whole cells as in mammary glands after lactation (breast feeding) In white blood cells lysosomes are used to digest bacteria (see endocytosis page 80) Enzymes are sometimes released outside the cell ndash for example in the replacement of cartilage with bone during development The headsof sperm contain a special lysosome the acrosome for digesting a path to the ovum (egg)MitochondriaMitochondria (singular mitochondrion) are usually about 1 1113088m in diameter and can be various shapes often sausage- shaped as in Figure 122 They are surrounded by two membranes (an envelope) The inner of these is foldedto form finger-like cristae which project into the interior solution or matrixThe main function of mitochondria is to carry out aerobic respiration As a result of respiration they make ATP the universal energy carrier in cells (see Chapter 16) They are also involved in the synthesis of lipids (page 37)Figure 122 Mitochondrion (orange) with its double membrane (envelope) the inner membrane is folded to form cristae (1113088 20 000) Mitochondria are the sites of aerobic cell respiration Note also the rough ERIn the 1960s it was discovered that mitochondria and chloroplasts contain ribosomes which are slightly smaller than those in the cytoplasm and are the same size as those found in bacteria The size of ribosomes is measured in lsquoS unitsrsquo which are a measure of how fast they sediment in a centrifuge Cytoplasmic ribosomes are 80S while those of bacteria mitochondria and chloroplasts are 70S It was also discovered in the 1960s that mitochondria and chloroplasts contain small circular DNA molecules also like those found in bacteria Not surprisingly it was later provedthat mitochondria and chloroplasts are in effect ancient bacteria which now live inside the larger cells typical of animals and plants (see

prokaryotic and eukaryotic cells page 18) This is known as the endosymbiont theory lsquoEndorsquo means lsquoinsidersquo and a lsquosymbiontrsquo is an organism which lives in a mutually beneficial relationship with another organism The DNA and ribosomes of mitochondria and chloroplasts are still active and responsible for the coding and synthesis of certain vital proteins but mitochondria and chloroplasts can no longer live independentlyMitochondrial ribosomes are just visible as tiny dark dots in the mitochondrial matrix in Figure 122Cell surface membraneThe cell surface membrane is extremely thin (about 7 nm) However at very high magnifications at least 1113088 100 000 it can be seen to have three layers described as a trilaminar appearance This consists of two dark lines (heavily stained) either side of a narrow pale interior (Figure 123) The membrane is partially permeable and controls exchange between the cell and its environment Membrane structure is discussed further in Chapter 4Figure 123 Cell surface membrane (1113088 250 000) At this magnification the membrane appears as two dark lines at the edge of the cellMicrovilliMicrovilli (singular microvillus) are finger-like extensions of the cell surface membrane typical of certain epithelial cells (cells covering surfaces of structures) They greatly1 Cell structure 17increase the surface area of the cell surface membrane (see Figure 117 on page 15) This is useful for example for absorption in the gut and for reabsorption in the proximal convoluted tubules of the kidney (see page 307)CentriolesThe extra resolution of the electron microscope reveals that just outside the nucleus there are really two centrioles (see Figure 124) not one as it appears under the light microscope (compare with Figure 13 on page 2) They lie close together at right-angles to each other A centriole is a hollow cylinder about 04 1113088m long formed from a ring of short microtubules tiny tubes made of a protein called tubulin These microtubules are used as a starting point for growing the spindle microtubules for nuclear division (see page 92) Centrioles are not found in plant cellsUltrastructure of a plant cellAll the structures except centrioles and microvilli so far described in animal cells are also found in plant cells The appearance of a plant cell as seen with the electron microscope is shown in Figure 125 while Figure 126 is a diagram based on many such micrographs The relatively thick cell wall and the large central vacuole are obvious as are the chloroplasts two of which are shown in detail in Figure 127 These structures and their functions have been described on pages 5 and 6 The electron microscopeFigure 124 Centrioles in transverse and longitudinal section (TS and LS) (1113088 86 000) The one on the left is seen in TS and clearly shows the nine triplets of microtubules which make up the structurereveals that chloroplasts contain 70S ribosomes and small circular

DNA molecules as do mitochondria and bacteria This has already been discussed with mitochondria above (page 17)SAQ 15Compare Figure 126 with Figure 15 on page 3 Name the structures which can be seen with the electron microscope but not with the light microscopeTwo fundamentally different types of cellAt one time it was common practice to try to classify all living organisms as either animals or plants With advances in our knowledge of living things it has become obvious that the living world is not that simple Fungi and bacteria for example are very different from animals and plants and from each other Eventually it was discovered that there are two fundamentally different types of cell The most obvious difference betweenthese types is that one possesses a nucleus and the other does notOrganisms that lack nuclei are called prokaryotes (lsquoprorsquo means before lsquokaryonrsquo means nucleus) All prokaryotes are now referred to as bacteria They are on average about 1000 to 10 000 times smaller in volume than cells with nuclei and are much simpler in structure ndash for example their DNA lies free in the cytoplasmOrganisms whose cells possess nuclei are called eukaryotes (lsquoeursquo means true) Their DNA lies inside a nucleus Eukaryotes include animals plants fungi and a group containing most of the unicellular eukaryotes known as protoctists Most biologists believe that eukaryotes evolved from prokaryotes one-and-a-half thousand million years after prokaryotes first appeared on Earth We mainly study animals and plants in this book but all eukaryotic cells have certain features in commonA generalised prokaryotic cell is shown in Figure 128 A comparison of prokaryotic and eukaryotic cells is given in Table 12 on page 211881 1 Cell structure

At the end of your course you will be tested on three sets of Assessment Objectivesbull Knowledge with understanding You are expected to know and understand all the facts and concepts listedbull in the syllabus These are all covered in this book Handling information and solving problems Questions testing these skills expect you to use your knowledge and understanding in an unfamiliar context A good knowledge and understanding of this book will enablebull you to approach new situations with confidence Experimental skills and investigations This involves practical work An examination will test your practical skills so try to do plenty of practical work Key information is provided on some practical aspects of the course in Chapters 15 and 26Additional help and guidance are available on the accompanying CD-ROMF Genetic control G Transport7 8vi IntroductionB Biological molecules 2 C Enzymes 3 D Cell membranes and transport

4 E Cell and nuclear division 56Transport in multicellular plants The mammalian transport system The mammalian heart 9H Gas exchange and smoking Gas exchange 10 Smoking 11I Infectious disease 12 J Immunity 13 K Ecology 14Advanced practical skills 15 L Energy and respiration 16 M Photosynthesis

17 N Regulation and control 18 O Inherited change 19

1 Cell structure By the end of this chapter you

should be able todescribe and interpret drawings and photographs of typical animal and plant cells as seen using the light microscope and make microscopical measurements using an eyepiece graticule and stage micrometerbe familiar with the units used in cell studies explain the meanings of and distinguish betweenthe terms resolution and magnificationdescribe and interpret drawings and photographs of typical animal and plant cells as seen using the electron microscope recognising rough and smooth endoplasmic reticulum (ER) Golgi apparatus mitochondria ribosomes lysosomes cell surface membrane centrioles nucleus (including the nuclear envelope and nucleolus) and microvilli as well asIn the early days of microscopy an English scientist Robert Hooke decided to examine thin slices of plant material He chose cork as one of his examples Looking down the microscope he was struck by the regular appearance ofthe structure and in 1665 he wrote a book containing the diagram shown in Figure 11If you examine the diagram you will see the lsquopore-likersquo regular structures that Hooke called lsquocellsrsquo Each cell appeared to be an empty box surrounded by a wall Hooke had discovered and described without realising it the fundamental unit of all living things

Although we now know that the cells of cork are dead further observations of cells in living materials were made by Hooke and other scientists However it was not until almost 200 years later that a general cell theory emerged from the work of two German scientists In 1838 Schleiden a botanist suggested that all plants are made of cells and a year later Schwann a zoologist suggested the same for animals The cell theory states that the basic unit of structure and function of all living organisms is the cell Now over 170 years later this idea is one of the most familiar and important theories in biology To it has beenthe chloroplasts cell wall large permanent vacuole tonoplast and plasmodesmata of plant cellsoutline the functions of the structures listed abovecompare the structure of typical animal and plant cellscalculate the linear magnification of and the actual sizes of specimens from drawings and photographsdescribe the structure of a prokaryotic cell and compare and contrast the structure of prokaryotic cells with that of eukaryotic cellsexplain how eukaryotic cells may be organised into tissues and organs with reference to transverse sections of stems roots and leavesdraw and label low-power plan diagrams of tissues and organsadded Virchowrsquos theory of 1855 that all cells arise from pre-existing cells by cell divisionFigure 11 Drawing of cork cells published by Robert Hooke in 16651 Cell structure 1

Why cellsA cell can be thought of as a bag in which the chemistry of life is allowed to occur partially separated from the environment outside the cell The thin membrane which surrounds all cells is essential in controlling exchange between the cell and its environment It is a very effective barrier but also allows a controlled traffic of materials across it in both directions The membrane is therefore described as partially permeable If it were freely permeable life could not exist because the chemicals of the cell would simply mix with the surrounding chemicals by diffusion (page 73)Cell biology and microscopyThe study of cells has given rise to an important branch of biology known as cell biology Cells can now be studied by many different methods but scientists began simply by looking at them using various types of microscopeThere are two fundamentally different types of microscope now in use the light microscope and the electron microscope Both use a form of radiation in order to create an image of the specimen being examined The light microscope uses light as a source of radiation while the electron microscope uses electrons for reasons which are discussed latereyepiecelight beamobjective cover slipglass slidecondenseriris diaphragmlight source pathway of lightEyepiece lens magnifies and focuses the image from the objective onto the eyeObjective lens collects light passing through the specimen and produces a magnified image

Condenser lens focuses the light onto the specimen held between the cover slip and slideCondenser iris diaphragm is closed slightly to produce a narrow beam of lightFigure 12 How the light microscope worksLight microscopyThe lsquogolden agersquo of light microscopy could be said to be the 19th century Microscopes had been available since the beginning of the 17th century but when dramatic improvements were made in the quality of glass lenses in the early 19th century interest among scientists became widespread The fascination ofthe microscopic world that opened up in biology inspired rapid progress both in microscope design and equally importantly in preparing material for examination with microscopes This branch of biology is known as cytology Figure 12 shows how the light microscope worksBy 1900 all the structures shown in Figures 13 14 and 15 except lysosomes had been discovered Figure 13 shows the structure of a generalised animal cell and Figure 15 the structure of a generalised plant cell as seen with a light microscope (A generalised cell shows all the structures that are typically found in a cell)Golgi apparatus cytoplasmcentriole ndash always found near nucleus has a role in nuclear divisionsmall structures that are difficult to identifymitochondria cell surface membranenuclear envelopechromatin ndash deeply staining and thread-likenucleolus ndash deeply stainingnucleusFigure 13 Structure of a generalised animal cell (diameter about 20 1113088m) as seen with a very high quality light microscope22 1 Cell structuretonoplast ndash membrane surrounding vacuolecell surface membrane (pressed against cell wall)vacuole ndash large with central positioncytoplasmmitochondriamiddle lamella ndash thin layer holding cells together contains calcium pectateplasmodesma ndash connects cytoplasm of neighbouring cellscell wall of neighbouring cellcell wall chloroplast grana just visiblesmall structures that are difficult to identifyGolgi apparatusFigure 14 Cells from the lining of the human cheek (1113088 500) each showing a centrally placed nucleus which is a typical animal cell characteristic The cells are part of a tissue known as squamous (flattened) epitheliumFigure 14 shows some actual human cells and Figure 16 shows an actual plant cell taken from a leafSAQ 11Using Figures 13 and 15 name the structures that animal and plant cells have in common those found in only plant cells and those found only in animal cellsFigure 16 Photomicrograph of a cell in a moss leaf (11130881400)nucleusnucleolus ndash deeply stainingnuclear envelopechromatin ndash deeply staining and thread-likeFigure 15 Structure of a generalised plant cell (diameter about 40 1113088m) as seen with a very high quality light microscope1 Cell structure 3Box 1A Biological drawingYou need the following equipment

bull pencil (HB) bull pencil sharpener bull eraser bull ruler bull plain paperHere are some guidelines for the quality of your drawingbull always use a pencil not a pen bull donrsquot use shading bull use clear continuous lines bull use accurate proportions and observation ndash not atextbook version For a low-power drawing (see Figure 17)bull donrsquot draw individual cells bull draw all tissues completely enclosed by lines bull draw a correct interpretation of the distribution of bull tissuesa representative portion may be drawn (eg half a transverse section)For a high-power drawingbull draw only a few representative cells bull draw the cell wall of all plant cells bull donrsquot draw the nucleus as a solid blobSome guidelines for the quality of your labellingbull label all tissues and relevant structures bull identify parts correctly bull use a ruler for label linesAnimal and plant cells have features in commonIn animals and plants each cell is surrounded by a very thin cell surface membrane which is too thin to be seen with a light microscope This is also sometimes referred to as the plasma membranebull arrange label lines neatly and ensure they donrsquot cross bull over each otherannotate your drawing if necessary (ie provideshort notes with one or more of the labels in order bull to describe or explain features of biological interest)add a scale line at the bottom of the drawing if bull appropriateuse a pencil not a penAn example of a drawing of a section through the stem of Helianthus is shown below Biological drawing is also covered in Chapter 15 page 264Figure 17 The right side of this low-power drawing shows examples of good technique while the left side shows many of the pitfalls you should avoidMany of the cell contents are colourless and transparent so they need to be stained to be seen Each cell has a nucleus which is a relatively large structure that stains intensely and is therefore very conspicuous The deeply staining material in the nucleus is called chromatin and is a massof loosely coiled threads This material collects together to form visible separate chromosomes during nuclear division44 1 Cell structure(see page 86) It contains DNA (deoxyribonucleic acid) a molecule which contains the instructions that control the activities of the cell (see Chapter 6) Within the nucleus an even more deeply staining area is visible the nucleolus which is made of loops of DNA from several chromosomes The number of nucleoli is variable one to five being

common in mammalsThe material between the nucleus and the cell surface membrane is known as cytoplasm Cytoplasm is an aqueous (watery) material varying from a fluid to a jelly-like consistency Many small structures can be seen within it These have been likened to small organs and hence are known as organelles An organelle can be defined as a functionally and structurally distinct part of a cell Organelles themselves are often surrounded by membranes so that their activities can be separated from the surrounding cytoplasm This is described as compartmentalisation Having separate compartments is essential for a structure as complex as an animal or plant cell to work efficiently Since each type of organelle has its own function the cell is said to show division of labour a sharing of the work between different specialised organellesThe most numerous organelles seen with the light microscope are usually mitochondria (singular mitochondrion) Mitochondria are only just visible but films of living cells taken with the aid of a light microscope have shown that they can move about change shape and divide They are specialised to carry out aerobic respirationThe use of special stains containing silver enabled the Golgi apparatus to be detected for the first time in 1898 by Camillo Golgi The Golgi apparatus is part of a complex internal sorting and distribution system within the cell (see page 16) It is also sometimes called the Golgi body or Golgi complexDifferences between animal and plant cellsThe only structure commonly found in animal cells which is absent from plant cells is the centriole Plant cells also differ from animal cells in possessing cell walls large permanent vacuoles and chloroplastsCentriolesUnder the light microscope the centriole appears as a small structure close to the nucleus (see Figure 13 on page 2) The centriole is involved in nuclear division (see page 92)Cell walls and plasmodesmataWith a light microscope individual plant cells are more easily seen than animal cells because they are usually larger and unlike animal cells surrounded by a cell wall outside the cell surface membrane This is relatively rigid because it contains fibres of cellulose a polysaccharide which strengthens the wall The cell wall gives the cell a definite shape It prevents the cell from bursting when water enters by osmosis allowing large pressures to develop inside the cell (see page 77) Cell walls may also be reinforced with extra cellulose or with a hard material called lignin for extra strength (see xylem on page 24) Cell walls are freely permeable allowing free movement of molecules and ions through to the cell surface membranePlant cells are linked to neighbouring cells by means of fine strands of cytoplasm called plasmodesmata (singular plasmodesma) which pass through pore-like structures in the walls of these neighbouring cells Movement through the pores is thought to be controlled by the structure of the poresVacuoles

Although animal cells may possess small vacuoles such as phagocytic vacuoles (see page 80) which are temporary structures mature plant cells often possess a large permanent central vacuole The plant vacuole is surrounded by a membrane the tonoplast which controls exchange between the vacuole and the cytoplasm The fluid in the vacuole is a solution of mineral salts sugars oxygen carbon dioxide pigments enzymes and other organic compounds including some waste productsVacuoles help to regulate the osmotic properties of cells (the flow of water inwards and outwards) as well as having a wide range of other functions For example the pigments which colour the petals of certain flowers and parts of some vegetables such as the red pigment of beetroots are sometimes located in vacuoles1 Cell structure 5ChloroplastsSome plant cells are able to carry out photosynthesis because they contain chloroplasts Chloroplasts are relatively large organelles which are green in colour due to the presence of chlorophyll At high magnifications small lsquograinsrsquo or grana (singular granum) can be seen in the chloroplasts During the process of photosynthesis light is absorbed by these grana which actually consist of stacks of membrane-bound sacs called thylakoids Starch grains may also be visible within chloroplasts Chloroplasts are found in the green parts of plants mainly in the leavesPoints to noteWe return to the differences between animal and plant cells as seen using the electron microscope on page 18Units of measurement in cell studiesIn order to measure objects in the microscopic world we need to use very small units of measurement which are unfamiliar to most people According to international agreement the International System of Units (SI units) should be used In this system the basic unit of length is the metre (symbol m) Additional units can be created in multiples of a thousand times larger or smaller using standard prefixes For example the prefix kilo means 1000 times Thus 1 kilometre 1113088 1000 metres The units of length relevant to cell studies are shown in Table 11It is difficult to imagine how small these units are but when looking down a microscope and seeing cells clearly we should not forget how amazingly small the cells actually are The smallest structure visible with the human eye is about 50ndash100 1113088m in diameter Your body contains about 60 million million cells varying in size from about 5 1113088m to 40 1113088m Try to imagine structures like mitochondria which have an average diameter of 1 1113088m The smallest cell organelles we deal with in this book ribosomes are only about 25 nm in diameter You could line up about 20 000 ribosomes across the full stop at the end of this sentenceUnit Symbolmillimetre mm micrometre 1113088m nanometre nmTable 11 Units of measurement relevant to cell studies 1113088 is the Greek letter mu 1 micrometre is a thousandth of a millimetre 1 nanometre is a thousandth of a micrometre661 Cell structure

bull bull bullbullYou can think of a plant cell as being very similar to an animal cell but with extra structures Plant cells are often larger than animal cells although cell size varies enormouslyDo not confuse the cell wall with the cell surface membrane Cell walls are relatively thick and physically strong whereas cell surface membranes are very thin Cell walls are freely permeable whereas cell surface membranes are partially permeable All cells have a cell surface membrane Vacuoles are not confined to plant cells animal cells may have small vacuoles such as phagocytic vacuoles (see page 80) although these are not usually permanent structuresFraction of a metreone thousandth 1113088 0001 1113088 11000 1113088 10-3 one millionth 1113088 0000 001 1113088 11 000 000 1113088 10-6 one thousand millionth 1113088 0000 000 001 1113088 11 000 000 000 1113088 10-9

Box 1B Measuring cellsCells and organelles can be measured with a microscope by means of an eyepiece graticule This is a transparent scale It usually has 100 divisions (see Figure 18a) The eyepiece graticule is placed in the microscope eyepiece so that it can be seen at the same time as the object to be measured as shown in Figure 18b Figure 18b shows the scale over a human cheek epithelialcell The cell lies between 40 and 60 on the scale We therefore say it measures 20 eyepiece units in diameter (the difference between 60 and 40) We will not know the actual size of the eyepiece units until the eyepiece graticule scale is calibratedTo calibrate the eyepiece graticule scale a miniature transparent ruler called a stage micrometer scale is placed on the microscope stage and is brought into focus This scale may be etched onto a glass slideor printed on a transparent film It commonly has subdivisions of 01 and 001 mm The images of the two scales can then be superimposed as shown in Figure 18cIn the eyepiece graticule shown in the figure 100 units measure 025 mm Hence the value of each eyepiece unit is0 25 00025 mm 100Or converting mm to 1113088m 0 25 1000 2 51113088m100The diameter of the cell shown superimposed on the scale in Figure 18b measures 20 eyepiece units and so its actual diameter is20times251113088m 1113088 501113088m This diameter is greater than that of many human cellsbecause the cell is a flattened epithelial cellFigure 18 Microscopical measurement Three fields of view seen using a high-power (111308840) objective lens a An eyepiece graticule scale b Superimposed images of human cheek epithelial cells and the eyepiece graticule scale c Superimposed images of the eyepiece graticule scale and the stage micrometer scaleacheek cells on a slide on the stage of the microscopebc0 1020304050607080901000 102030405060708090100eyepiece graticule scale (arbitrary units)

eyepiece graticule in the eyepiece of the microscope00stage micrometer scale (marked in 00 1mm and 01 mm divisions)10 20 30 40 50 60 70 80 90 10001 021 Cell structure 7

Electron microscopesEarlier in this chapter it was stated that by 1900 almost all the structures shown in Figures 13 and 15 (pages 2 and 3) had been discovered There followed a time of frustration for microscopists because they realised that no matter how much the design of light microscopes improved there was a limit to how much could ever be seen using lightIn order to understand the problem it is necessary to know something about the nature of light itself and to understand the difference between magnification and resolutionMagnificationMagnification is the number of times larger an image is compared with the real size of the objectWorked example 1 ndash calculating the magnification of a photograph or objectTo calculate M the magnification of a photograph or an object we can use the following methodFigure 19 shows two photographs of a section through the same plant cells The magnifications of the two photographs are the same Suppose we want to know the magnification of the plant cell in Figure 19b If we know its actual (real) length we can calculate its magnification using theThe real length of the cell is 80 1113088mmagnification observed size of the image actual sizeM I AStep 1Measure the length in mm of the cell in the photograph using a ruler You should find that it is about 60 mmStep 2Convert mm to 1113088m (It is easier if we first convert all measurements to the same units ndash in this case micrometres 1113088m)1 mm 1113088 1000 1113088m so 60mm 1113088 60 1113088 10001113088morwhere I 1113088 observed size of the image (that is what you can measure with a ruler) and A 1113088 actual size (that is the real size ndash for example the size of a cell before it is magnified)If you know two of these values you can work out the third one For example if the observed size of the image and the magnification are known you can work out the1113088 60 000 1113088m Use the equation to calculate the magnificationStep 3actual size A I If you write the formula in a triangle Mmagnification Mimage size I

as shown below and cover up the value you want to find it should be obvious how to do the right calculationactual size A 60000m80 m 750I M1113088ASome worked examples are now providedThe lsquo1113088rsquo sign in front of the number 750 means lsquotimesrsquo We say that the magnification is lsquotimes 750rsquoformula M I A88 1 Cell structurea Worked example 2 ndash calculating magnification from a scale barFigure 110 shows a lymphocyteFigure 110 A lymphocyte6 μmbFigure 19 Photographs of the same plant cells seen a with a light microscope b with an electron microscope both shown at a magnification of about 1113088 750We can calculate the magnification of the lymphocyte by simply using the scale bar All you need to do is measure the length of the scale bar and then substitute this and the length it represents into the equationStep 1Measure the scale bar Here it is 36 mmStep 2Convert mm to 1113088m 36mm 1113088 36 1113088 10001113088m 1113088 360001113088mStep 3Use the equation to calculate the magnificationmagnification M image size I actual size A36000m 6m60001 Cell structure 9Worked example 3 ndash calculating the real size of an object from its magnificationTo calculate A the real or actual size of an object we can use the following methodFigure 125 on page 19 shows a plant cell magnified 1113088 5600 One of the chloroplasts is labelled lsquochloroplastrsquo in the figure Suppose we want to know the actual length of this chloroplastStep 1Measure the observed length of the image of the chloroplast (I ) in mm using a ruler The maximum length is 36 mmStep 2Convert mm to 1113088m 30mm 1113088 30 1113088 10001113088m 1113088 300001113088mStep 3Use the equation to calculate the actual lengthactualsizeA imagesizeI magnification M30 000 m 5600 5 4 m (to one decimal place)SAQ 12a Calculate the magnification of the drawing of the animal cell in Figure 13 on page 2b Calculate the actual (real) length of the bottom chloroplast in Figure 127 on page 19Resolution

Look again at Figure 19 (page 9) Figure 19a is a light micrograph (a photograph taken with a light microscope also known as a photomicrograph) Figure 19b is an electron micrograph of the same cells taken at the same magnification (an electron micrograph is a picture taken with an electron microscope) You can see that Figure 19b the electron micrograph is much clearer This is because it has greater resolution Resolution is defined as the abilityto distinguish between two separate points If the two points cannot be resolved they will be seen as one point In practice resolution is the amount of detail that can be seen ndash the greater the resolution the greater the detailThe maximum resolution of a light microscope is 200 nm This means that if two points or objects are closer together than 200 nm they cannot be distinguished as separateIt is possible to take a photograph such as Figure 19a and to magnify (enlarge) it but we see no more detail in other words we do not improve resolution even though we often enlarge photographs because they are easier to see when larger With a microscope magnification up to the limit of resolution can reveal further detail but any further magnification increases blurring as well as the size of the imageThe electromagnetic spectrumHow is resolution linked with the nature of light One of the properties of light is that it travels in waves The length of the waves of visible light varies ranging from about 400 nm (violet light) to about 700 nm (red light) The human eye can distinguish between these different wavelengths and in the brain the differences are converted to colour differences (Colour is an invention of the brain)The whole range of different wavelengths is called the electromagnetic spectrum Visible light is only one part of this spectrum Figure 111 shows some of the parts of the electromagnetic spectrum The longer the waves the lower their frequency (all the waves travel at the same speed so imagine them passing a post shorter waves pass at higher frequency) In theory there is no limit to how short or how long the waves can be Wavelength changes with energy the greater the energy the shorter the wavelength (rather like squashing a spring)Now look at Figure 112 which shows a mitochondrion some very small cell organelles called ribosomes (see page 13) and light of 400 nm wavelength the shortest visible wavelength The mitochondrion is large enoughto interfere with the light waves However the ribosomes are far too small to have any effect on the light waves The general rule is that the limit of resolution is about one half the wavelength of the radiation used to view the specimen In other words if an object is any smaller than half the wavelength of the radiation used to view it it cannot be seen separately from nearby objects This means that the1001 1 Cell structureX-raysgamma rays01 nm400 nm violet bluev infraredmicrowaves

uv10 nmi s i b l e 1000 nmradio and TV wavesvisible light500 nm 600 nm green yellow orange105 nm 107 nm 109 nm 1011 nm 1013 nm700 nmFigure 111 Diagram of the electromagnetic spectrum (the waves are not drawn to scale) The numbers indicate the wavelengths of the different types of electromagnetic radiation Visible light is a form of electromagnetic radiationbest resolution that can be obtained using a microscope that uses visible light (a light microscope) is 200 nm since the shortest wavelength of visible light is 400 nm (violet light) In practice this corresponds to a maximum useful magnification of about 1500 times Ribosomes are approximately 25 nm in diameter and can therefore never be seen using lightIf an object is transparent it will allow light waves to pass through it and therefore will still not be visible This is why many biological structures have to be stained before they can be seenThe electron microscopeBiologists faced with the problem that they would never see anything smaller than 200 nm using a light microscope realised that the only solution would be to use radiation of a shorter wavelength than light If you study Figure 111 you will see that ultraviolet light or better still X-rays look like possible candidates Both ultraviolet and X-ray microscopes have been built the latter with little success partly because of the difficulty of focusing X-rays A much better solution is to use electrons Electrons are negatively charged particles which orbit the nucleus of an atom When a metal becomes very hot some of its electrons gain so much energy that they escape from their orbits like a rocket escaping from Earthrsquos gravity Free electrons behave like electromagnetic radiation They have a very short wavelength the greater the energy the shorter the wavelength Electrons are a very suitable form of radiation for microscopy for two major reasons Firstly their wavelength is extremely short (at least as short as that of X-rays) Secondly because they are negatively charged they can be focused easily using electromagnets (a magnet can be made to alter the path of the beam the equivalent of a glass lens bending light)Using an electron microscope a resolution of 05 nm can be obtained

400 times better than when using a light microscope ETransmission and scanning electron microscopesTwo types of electron microscope are now in common use The transmission electron microscope or TEM forwavelength 400nmstained mitochondrion of diameter 1000nm interferes with light wavesstained ribosomes of diameter 25nm do not interfere with light wavesFigure 112 A mitochondrion and some ribosomes in the path of light waves of 400 nm lengthred1 Cell structure 11

E short was the type originally developed Here the beam of electrons is passed through the specimen before being viewed Only

those electrons that are transmitted (pass through the specimen) are seen This allows us to see thin sections of specimens and thus to see inside cells In the scanning electron microscope (SEM) on the other hand the electron beam is used to scan the surfaces of structures and only the reflected beam is observedAn example of a scanning electron micrograph is shown in Figure 113 The advantage of this microscope is that surface structures can be seen Also great depth of field is obtained so that much of the specimen is in focus at the same time and a three-dimensional appearance is obtained Such a picture would be impossible to obtain with a light microscope even using the same magnification and resolution because you would have to keep focusing up and down with the objective lens to see different parts of the specimen The disadvantage of the SEM is that it cannot achieve the same resolution as a TEM Resolution is between 3 nm and 20 nmViewing specimens with the electron microscopeFigure 114 shows how an electron microscope works and Figure 115 shows one in useFigure 113 False-colour SEM of the head of a cat flea (1113088 100)

electron gun and anode which E produce a beam of electronselectron beam vacuumpathway of electronscondenser electromagnetic lens which directs the electron beam onto the specimenspecimen which is placed on a gridobjective electromagnetic lens which produces an imageprojector electromagnetic lenses which focus the magnified image onto the screenscreen or photographic plate which shows the image of the specimenFigure 114 How an electron microscope worksIt is not possible to see an electron beam so to make the image visible the electron beam has to be projected onto a fluorescent screen The areas hit by electrons shine brightly giving overall a lsquoblack and whitersquo picture The stains used to improve the contrast of biological specimens for electron microscopy contain heavy metal atoms which stop the passage of electrons The resulting picture is like an X-ray photograph with the more densely stained parts of the specimen appearing blacker lsquoFalse-colourrsquo images can be created by colouring the standard black and white image using a computerTo add to the difficulties of electron microscopy the electron beam and therefore the specimen and the fluorescent screen must be in a vacuum If electrons1221 1 Cell structure

EStructures and functions of organellesCompartmentalisation and division of labour within the cell are even more obvious with an electron microscope than with a light microscopeWe will now consider the structures and functions of some of the cell components in more detailNucleusThe nucleus (Figure 118 on page 15) is the largest cell organelle (see also page 5) It is surrounded by two membranes known as the

nuclear envelope The outer membrane of the nuclear envelope is continuous with the endoplasmic reticulum (Figure 117 on page 15) The nuclear envelope has many small pores called nuclear pores These allow and control exchange between the nucleus and the cytoplasm Examples of substances leaving the nucleus through the pores are mRNA and ribosomes for protein synthesis Examples of substances entering through the nuclear pores are proteins to help make ribosomes nucleotides ATP (aderosine triphosphate) and some hormones such as thyroid hormone T3Within the nucleus the chromosomes are in a loosely coiled state known as chromatin (except during nuclear division see Chapter 5) Chromosomes contain DNA which is organised into functional units called genes Genes control the activities of the cell and inheritance thus the nucleus controls the cellrsquos activities When a cell is aboutto divide the nucleus divides first so that each new cell will have its own nucleus (Chapters 5 and 19) Also within the nucleus the nucleolus makes ribosomes using the information in its own DNAEndoplasmic reticulum and ribosomesWhen cells were first seen with the electron microscope biologists were amazed to see so much detailed structure The existence of much of this had not been suspected This was particularly true of an extensive system of membranes running through the cytoplasm which became knownas the endoplasmic reticulum (ER) (Figure 119 on page 15 ndash see also Figures 118 on page 15 and 122 on page 17) The ER is continuous with the outer membrane of the nuclear envelope (Figure 117)There are two types of ER rough ER and smooth ER Rough ER is so called because it is covered with many tinyFigure 115 A TEM in usecollided with air molecules they would scatter making it impossible to achieve a sharp picture Also water boils at room temperature in a vacuum so all specimens must be dehydrated before being placed in the microscope This means that only dead material can be examined Great efforts are therefore made to try to preserve material in a life-like state when preparing it for the microscopeSAQ 13Explain why ribosomes are not visible using a light microscopeUltrastructure of an animal cellThe lsquofinersquo or detailed structure of a cell as revealed by the electron microscope is called its ultrastructure Figure 116 shows the appearance of typical animal cells as seen with an electron microscope and Figure 117 on page 15 is a diagram based on many other such micrographsSAQ 14Compare Figure 117 on page 15 with Figure 13 on page 2 Name the structures which can be seen with the electron microscope but not with the light microscope1 Cell structure 13lysosomeGGolgi apparatusendoplasmic reticulumglycogen granulesccell surface membrane

mmitochondriachromatinnnucleolusmmicrovillusrribosomesFFigure 116 Representative animal cells as seen with a TEM The cells are liver cells from a rat (1113088 9600) The nucleus is clearly visible in one of the cells 1441 1 Cell structurenucleusnnuclear envelopetwo centrioles close to the nucleus and at right angles to each othermitochondrion lysosomerough endoplasmic reticulumnucleoluschromatin nucleusnuclear porenuclear envelope (two membranes)microvillismooth endoplasmic reticulumFigure 117 Ultrastructure of a typical animal cell as seen with an electron microscope In reality the ER is more extensive than shown and free ribosomes may be more extensive Glycogen granules are sometimes present in the cytoplasmFigure 118 TEM of the nucleus of a cell from the pancreas of a bat (1113088 7500) The circular nucleus is surrounded by a double-layered nuclear envelope containing nuclear pores The nucleolus is more Figure 119 TEM of rough ER covered with ribosomes (black dots) darkly stained Rough ER is visible in the surrounding cytoplasm (1113088 17 000) Some free ribosomes can also be seen in the cytoplasmGolgi vesicle Golgi apparatusribosomescell surface membranecytoplasm1 Cell structure 15organelles called ribosomes These are just visible as black dots in Figures 118 and 119 on page 15 At very high magnifications they can be seen to consist of two subunits a large and a small subunit Ribosomes are the sites of protein synthesis (see pages 111ndash112) They can be found free in the cytoplasm as well as on the rough ER They are very small only about 25 nm in diameter They are made of RNA (ribonucleic acid) and protein The rough ER forms an extensive system of flattened sacs spreading in sheets throughout the cell Proteins made by the ribosomes on the rough ER enter the sacs and move through them The proteins are often processed in some way on their journey Small sacs called vesicles can break off from the ER and these can join together to form the Golgi apparatus Proteins can be exported from the cell via the Golgi apparatus (see page 80)Smooth ER so called because it lacks ribosomes has a completely different function It makes lipids and steroids such as cholesterol and the reproductive hormones oestrogen and testosteroneGolgi apparatus (Golgi body or Golgi complex)The Golgi apparatus is a stack of flattened sacs (Figure 120) This stack of sacs is sometimes referred to as theFigure 120 TEM of a Golgi apparatus A central stack of saucer-shaped sacs can be seen budding off small Golgi vesicles (green) These may form secretory vesicles whose contents can be released at the cell surface by exocytosis (see page 80)Golgi body More than one may be present in a cell The stack is constantly being formed at one end from vesicles which bud off from the ER and broken down again at the other end to form Golgi vesicles The stack of sacs with the associated vesicles is referred to as the Golgi apparatus or Golgi complex

The Golgi apparatus collects processes and sorts molecules (particularly proteins from the rough ER) ready for transport in Golgi vesicles either to other parts of the cell or out of the cell (secretion) Two examples of protein processing in the Golgi apparatus are the addition of sugars to proteins to make molecules known as glycoproteins and the removal of the first amino acid methionine from newly formed proteins to make a functioning protein In plants enzymes in the Golgi apparatus convert sugars into cell wall components Golgi vesicles are also used to make lysosomesLysosomesLysosomes (Figure 121) are spherical sacs surrounded by a single membrane and having no internal structure They are commonly 01ndash 05 1113088m in diameter They contain digestive (hydrolytic) enzymes which must be kept separateFigure 121 Lysosomes (orange) in a mouse kidney cell (1113088 55 000) They contain cell structures in the process of digestion and vesicles (green) Cytoplasm is coloured blue here1661 1 Cell structurefrom the rest of the cell to prevent damage Lysosomes are responsible for the breakdown (digestion) of unwanted structures such as old organelles or even whole cells as in mammary glands after lactation (breast feeding) In white blood cells lysosomes are used to digest bacteria (see endocytosis page 80) Enzymes are sometimes released outside the cell ndash for example in the replacement of cartilage with bone during development The headsof sperm contain a special lysosome the acrosome for digesting a path to the ovum (egg)MitochondriaMitochondria (singular mitochondrion) are usually about 1 1113088m in diameter and can be various shapes often sausage- shaped as in Figure 122 They are surrounded by two membranes (an envelope) The inner of these is foldedto form finger-like cristae which project into the interior solution or matrixThe main function of mitochondria is to carry out aerobic respiration As a result of respiration they make ATP the universal energy carrier in cells (see Chapter 16) They are also involved in the synthesis of lipids (page 37)Figure 122 Mitochondrion (orange) with its double membrane (envelope) the inner membrane is folded to form cristae (1113088 20 000) Mitochondria are the sites of aerobic cell respiration Note also the rough ERIn the 1960s it was discovered that mitochondria and chloroplasts contain ribosomes which are slightly smaller than those in the cytoplasm and are the same size as those found in bacteria The size of ribosomes is measured in lsquoS unitsrsquo which are a measure of how fast they sediment in a centrifuge Cytoplasmic ribosomes are 80S while those of bacteria mitochondria and chloroplasts are 70S It was also discovered in the 1960s that mitochondria and chloroplasts contain small circular DNA molecules also like those found in bacteria Not surprisingly it was later provedthat mitochondria and chloroplasts are in effect ancient bacteria which now live inside the larger cells typical of animals and plants (see

prokaryotic and eukaryotic cells page 18) This is known as the endosymbiont theory lsquoEndorsquo means lsquoinsidersquo and a lsquosymbiontrsquo is an organism which lives in a mutually beneficial relationship with another organism The DNA and ribosomes of mitochondria and chloroplasts are still active and responsible for the coding and synthesis of certain vital proteins but mitochondria and chloroplasts can no longer live independentlyMitochondrial ribosomes are just visible as tiny dark dots in the mitochondrial matrix in Figure 122Cell surface membraneThe cell surface membrane is extremely thin (about 7 nm) However at very high magnifications at least 1113088 100 000 it can be seen to have three layers described as a trilaminar appearance This consists of two dark lines (heavily stained) either side of a narrow pale interior (Figure 123) The membrane is partially permeable and controls exchange between the cell and its environment Membrane structure is discussed further in Chapter 4Figure 123 Cell surface membrane (1113088 250 000) At this magnification the membrane appears as two dark lines at the edge of the cellMicrovilliMicrovilli (singular microvillus) are finger-like extensions of the cell surface membrane typical of certain epithelial cells (cells covering surfaces of structures) They greatly1 Cell structure 17increase the surface area of the cell surface membrane (see Figure 117 on page 15) This is useful for example for absorption in the gut and for reabsorption in the proximal convoluted tubules of the kidney (see page 307)CentriolesThe extra resolution of the electron microscope reveals that just outside the nucleus there are really two centrioles (see Figure 124) not one as it appears under the light microscope (compare with Figure 13 on page 2) They lie close together at right-angles to each other A centriole is a hollow cylinder about 04 1113088m long formed from a ring of short microtubules tiny tubes made of a protein called tubulin These microtubules are used as a starting point for growing the spindle microtubules for nuclear division (see page 92) Centrioles are not found in plant cellsUltrastructure of a plant cellAll the structures except centrioles and microvilli so far described in animal cells are also found in plant cells The appearance of a plant cell as seen with the electron microscope is shown in Figure 125 while Figure 126 is a diagram based on many such micrographs The relatively thick cell wall and the large central vacuole are obvious as are the chloroplasts two of which are shown in detail in Figure 127 These structures and their functions have been described on pages 5 and 6 The electron microscopeFigure 124 Centrioles in transverse and longitudinal section (TS and LS) (1113088 86 000) The one on the left is seen in TS and clearly shows the nine triplets of microtubules which make up the structurereveals that chloroplasts contain 70S ribosomes and small circular

DNA molecules as do mitochondria and bacteria This has already been discussed with mitochondria above (page 17)SAQ 15Compare Figure 126 with Figure 15 on page 3 Name the structures which can be seen with the electron microscope but not with the light microscopeTwo fundamentally different types of cellAt one time it was common practice to try to classify all living organisms as either animals or plants With advances in our knowledge of living things it has become obvious that the living world is not that simple Fungi and bacteria for example are very different from animals and plants and from each other Eventually it was discovered that there are two fundamentally different types of cell The most obvious difference betweenthese types is that one possesses a nucleus and the other does notOrganisms that lack nuclei are called prokaryotes (lsquoprorsquo means before lsquokaryonrsquo means nucleus) All prokaryotes are now referred to as bacteria They are on average about 1000 to 10 000 times smaller in volume than cells with nuclei and are much simpler in structure ndash for example their DNA lies free in the cytoplasmOrganisms whose cells possess nuclei are called eukaryotes (lsquoeursquo means true) Their DNA lies inside a nucleus Eukaryotes include animals plants fungi and a group containing most of the unicellular eukaryotes known as protoctists Most biologists believe that eukaryotes evolved from prokaryotes one-and-a-half thousand million years after prokaryotes first appeared on Earth We mainly study animals and plants in this book but all eukaryotic cells have certain features in commonA generalised prokaryotic cell is shown in Figure 128 A comparison of prokaryotic and eukaryotic cells is given in Table 12 on page 211881 1 Cell structure

Although we now know that the cells of cork are dead further observations of cells in living materials were made by Hooke and other scientists However it was not until almost 200 years later that a general cell theory emerged from the work of two German scientists In 1838 Schleiden a botanist suggested that all plants are made of cells and a year later Schwann a zoologist suggested the same for animals The cell theory states that the basic unit of structure and function of all living organisms is the cell Now over 170 years later this idea is one of the most familiar and important theories in biology To it has beenthe chloroplasts cell wall large permanent vacuole tonoplast and plasmodesmata of plant cellsoutline the functions of the structures listed abovecompare the structure of typical animal and plant cellscalculate the linear magnification of and the actual sizes of specimens from drawings and photographsdescribe the structure of a prokaryotic cell and compare and contrast the structure of prokaryotic cells with that of eukaryotic cellsexplain how eukaryotic cells may be organised into tissues and organs with reference to transverse sections of stems roots and leavesdraw and label low-power plan diagrams of tissues and organsadded Virchowrsquos theory of 1855 that all cells arise from pre-existing cells by cell divisionFigure 11 Drawing of cork cells published by Robert Hooke in 16651 Cell structure 1

Why cellsA cell can be thought of as a bag in which the chemistry of life is allowed to occur partially separated from the environment outside the cell The thin membrane which surrounds all cells is essential in controlling exchange between the cell and its environment It is a very effective barrier but also allows a controlled traffic of materials across it in both directions The membrane is therefore described as partially permeable If it were freely permeable life could not exist because the chemicals of the cell would simply mix with the surrounding chemicals by diffusion (page 73)Cell biology and microscopyThe study of cells has given rise to an important branch of biology known as cell biology Cells can now be studied by many different methods but scientists began simply by looking at them using various types of microscopeThere are two fundamentally different types of microscope now in use the light microscope and the electron microscope Both use a form of radiation in order to create an image of the specimen being examined The light microscope uses light as a source of radiation while the electron microscope uses electrons for reasons which are discussed latereyepiecelight beamobjective cover slipglass slidecondenseriris diaphragmlight source pathway of lightEyepiece lens magnifies and focuses the image from the objective onto the eyeObjective lens collects light passing through the specimen and produces a magnified image

Condenser lens focuses the light onto the specimen held between the cover slip and slideCondenser iris diaphragm is closed slightly to produce a narrow beam of lightFigure 12 How the light microscope worksLight microscopyThe lsquogolden agersquo of light microscopy could be said to be the 19th century Microscopes had been available since the beginning of the 17th century but when dramatic improvements were made in the quality of glass lenses in the early 19th century interest among scientists became widespread The fascination ofthe microscopic world that opened up in biology inspired rapid progress both in microscope design and equally importantly in preparing material for examination with microscopes This branch of biology is known as cytology Figure 12 shows how the light microscope worksBy 1900 all the structures shown in Figures 13 14 and 15 except lysosomes had been discovered Figure 13 shows the structure of a generalised animal cell and Figure 15 the structure of a generalised plant cell as seen with a light microscope (A generalised cell shows all the structures that are typically found in a cell)Golgi apparatus cytoplasmcentriole ndash always found near nucleus has a role in nuclear divisionsmall structures that are difficult to identifymitochondria cell surface membranenuclear envelopechromatin ndash deeply staining and thread-likenucleolus ndash deeply stainingnucleusFigure 13 Structure of a generalised animal cell (diameter about 20 1113088m) as seen with a very high quality light microscope22 1 Cell structuretonoplast ndash membrane surrounding vacuolecell surface membrane (pressed against cell wall)vacuole ndash large with central positioncytoplasmmitochondriamiddle lamella ndash thin layer holding cells together contains calcium pectateplasmodesma ndash connects cytoplasm of neighbouring cellscell wall of neighbouring cellcell wall chloroplast grana just visiblesmall structures that are difficult to identifyGolgi apparatusFigure 14 Cells from the lining of the human cheek (1113088 500) each showing a centrally placed nucleus which is a typical animal cell characteristic The cells are part of a tissue known as squamous (flattened) epitheliumFigure 14 shows some actual human cells and Figure 16 shows an actual plant cell taken from a leafSAQ 11Using Figures 13 and 15 name the structures that animal and plant cells have in common those found in only plant cells and those found only in animal cellsFigure 16 Photomicrograph of a cell in a moss leaf (11130881400)nucleusnucleolus ndash deeply stainingnuclear envelopechromatin ndash deeply staining and thread-likeFigure 15 Structure of a generalised plant cell (diameter about 40 1113088m) as seen with a very high quality light microscope1 Cell structure 3Box 1A Biological drawingYou need the following equipment

bull pencil (HB) bull pencil sharpener bull eraser bull ruler bull plain paperHere are some guidelines for the quality of your drawingbull always use a pencil not a pen bull donrsquot use shading bull use clear continuous lines bull use accurate proportions and observation ndash not atextbook version For a low-power drawing (see Figure 17)bull donrsquot draw individual cells bull draw all tissues completely enclosed by lines bull draw a correct interpretation of the distribution of bull tissuesa representative portion may be drawn (eg half a transverse section)For a high-power drawingbull draw only a few representative cells bull draw the cell wall of all plant cells bull donrsquot draw the nucleus as a solid blobSome guidelines for the quality of your labellingbull label all tissues and relevant structures bull identify parts correctly bull use a ruler for label linesAnimal and plant cells have features in commonIn animals and plants each cell is surrounded by a very thin cell surface membrane which is too thin to be seen with a light microscope This is also sometimes referred to as the plasma membranebull arrange label lines neatly and ensure they donrsquot cross bull over each otherannotate your drawing if necessary (ie provideshort notes with one or more of the labels in order bull to describe or explain features of biological interest)add a scale line at the bottom of the drawing if bull appropriateuse a pencil not a penAn example of a drawing of a section through the stem of Helianthus is shown below Biological drawing is also covered in Chapter 15 page 264Figure 17 The right side of this low-power drawing shows examples of good technique while the left side shows many of the pitfalls you should avoidMany of the cell contents are colourless and transparent so they need to be stained to be seen Each cell has a nucleus which is a relatively large structure that stains intensely and is therefore very conspicuous The deeply staining material in the nucleus is called chromatin and is a massof loosely coiled threads This material collects together to form visible separate chromosomes during nuclear division44 1 Cell structure(see page 86) It contains DNA (deoxyribonucleic acid) a molecule which contains the instructions that control the activities of the cell (see Chapter 6) Within the nucleus an even more deeply staining area is visible the nucleolus which is made of loops of DNA from several chromosomes The number of nucleoli is variable one to five being

common in mammalsThe material between the nucleus and the cell surface membrane is known as cytoplasm Cytoplasm is an aqueous (watery) material varying from a fluid to a jelly-like consistency Many small structures can be seen within it These have been likened to small organs and hence are known as organelles An organelle can be defined as a functionally and structurally distinct part of a cell Organelles themselves are often surrounded by membranes so that their activities can be separated from the surrounding cytoplasm This is described as compartmentalisation Having separate compartments is essential for a structure as complex as an animal or plant cell to work efficiently Since each type of organelle has its own function the cell is said to show division of labour a sharing of the work between different specialised organellesThe most numerous organelles seen with the light microscope are usually mitochondria (singular mitochondrion) Mitochondria are only just visible but films of living cells taken with the aid of a light microscope have shown that they can move about change shape and divide They are specialised to carry out aerobic respirationThe use of special stains containing silver enabled the Golgi apparatus to be detected for the first time in 1898 by Camillo Golgi The Golgi apparatus is part of a complex internal sorting and distribution system within the cell (see page 16) It is also sometimes called the Golgi body or Golgi complexDifferences between animal and plant cellsThe only structure commonly found in animal cells which is absent from plant cells is the centriole Plant cells also differ from animal cells in possessing cell walls large permanent vacuoles and chloroplastsCentriolesUnder the light microscope the centriole appears as a small structure close to the nucleus (see Figure 13 on page 2) The centriole is involved in nuclear division (see page 92)Cell walls and plasmodesmataWith a light microscope individual plant cells are more easily seen than animal cells because they are usually larger and unlike animal cells surrounded by a cell wall outside the cell surface membrane This is relatively rigid because it contains fibres of cellulose a polysaccharide which strengthens the wall The cell wall gives the cell a definite shape It prevents the cell from bursting when water enters by osmosis allowing large pressures to develop inside the cell (see page 77) Cell walls may also be reinforced with extra cellulose or with a hard material called lignin for extra strength (see xylem on page 24) Cell walls are freely permeable allowing free movement of molecules and ions through to the cell surface membranePlant cells are linked to neighbouring cells by means of fine strands of cytoplasm called plasmodesmata (singular plasmodesma) which pass through pore-like structures in the walls of these neighbouring cells Movement through the pores is thought to be controlled by the structure of the poresVacuoles

Although animal cells may possess small vacuoles such as phagocytic vacuoles (see page 80) which are temporary structures mature plant cells often possess a large permanent central vacuole The plant vacuole is surrounded by a membrane the tonoplast which controls exchange between the vacuole and the cytoplasm The fluid in the vacuole is a solution of mineral salts sugars oxygen carbon dioxide pigments enzymes and other organic compounds including some waste productsVacuoles help to regulate the osmotic properties of cells (the flow of water inwards and outwards) as well as having a wide range of other functions For example the pigments which colour the petals of certain flowers and parts of some vegetables such as the red pigment of beetroots are sometimes located in vacuoles1 Cell structure 5ChloroplastsSome plant cells are able to carry out photosynthesis because they contain chloroplasts Chloroplasts are relatively large organelles which are green in colour due to the presence of chlorophyll At high magnifications small lsquograinsrsquo or grana (singular granum) can be seen in the chloroplasts During the process of photosynthesis light is absorbed by these grana which actually consist of stacks of membrane-bound sacs called thylakoids Starch grains may also be visible within chloroplasts Chloroplasts are found in the green parts of plants mainly in the leavesPoints to noteWe return to the differences between animal and plant cells as seen using the electron microscope on page 18Units of measurement in cell studiesIn order to measure objects in the microscopic world we need to use very small units of measurement which are unfamiliar to most people According to international agreement the International System of Units (SI units) should be used In this system the basic unit of length is the metre (symbol m) Additional units can be created in multiples of a thousand times larger or smaller using standard prefixes For example the prefix kilo means 1000 times Thus 1 kilometre 1113088 1000 metres The units of length relevant to cell studies are shown in Table 11It is difficult to imagine how small these units are but when looking down a microscope and seeing cells clearly we should not forget how amazingly small the cells actually are The smallest structure visible with the human eye is about 50ndash100 1113088m in diameter Your body contains about 60 million million cells varying in size from about 5 1113088m to 40 1113088m Try to imagine structures like mitochondria which have an average diameter of 1 1113088m The smallest cell organelles we deal with in this book ribosomes are only about 25 nm in diameter You could line up about 20 000 ribosomes across the full stop at the end of this sentenceUnit Symbolmillimetre mm micrometre 1113088m nanometre nmTable 11 Units of measurement relevant to cell studies 1113088 is the Greek letter mu 1 micrometre is a thousandth of a millimetre 1 nanometre is a thousandth of a micrometre661 Cell structure

bull bull bullbullYou can think of a plant cell as being very similar to an animal cell but with extra structures Plant cells are often larger than animal cells although cell size varies enormouslyDo not confuse the cell wall with the cell surface membrane Cell walls are relatively thick and physically strong whereas cell surface membranes are very thin Cell walls are freely permeable whereas cell surface membranes are partially permeable All cells have a cell surface membrane Vacuoles are not confined to plant cells animal cells may have small vacuoles such as phagocytic vacuoles (see page 80) although these are not usually permanent structuresFraction of a metreone thousandth 1113088 0001 1113088 11000 1113088 10-3 one millionth 1113088 0000 001 1113088 11 000 000 1113088 10-6 one thousand millionth 1113088 0000 000 001 1113088 11 000 000 000 1113088 10-9

Box 1B Measuring cellsCells and organelles can be measured with a microscope by means of an eyepiece graticule This is a transparent scale It usually has 100 divisions (see Figure 18a) The eyepiece graticule is placed in the microscope eyepiece so that it can be seen at the same time as the object to be measured as shown in Figure 18b Figure 18b shows the scale over a human cheek epithelialcell The cell lies between 40 and 60 on the scale We therefore say it measures 20 eyepiece units in diameter (the difference between 60 and 40) We will not know the actual size of the eyepiece units until the eyepiece graticule scale is calibratedTo calibrate the eyepiece graticule scale a miniature transparent ruler called a stage micrometer scale is placed on the microscope stage and is brought into focus This scale may be etched onto a glass slideor printed on a transparent film It commonly has subdivisions of 01 and 001 mm The images of the two scales can then be superimposed as shown in Figure 18cIn the eyepiece graticule shown in the figure 100 units measure 025 mm Hence the value of each eyepiece unit is0 25 00025 mm 100Or converting mm to 1113088m 0 25 1000 2 51113088m100The diameter of the cell shown superimposed on the scale in Figure 18b measures 20 eyepiece units and so its actual diameter is20times251113088m 1113088 501113088m This diameter is greater than that of many human cellsbecause the cell is a flattened epithelial cellFigure 18 Microscopical measurement Three fields of view seen using a high-power (111308840) objective lens a An eyepiece graticule scale b Superimposed images of human cheek epithelial cells and the eyepiece graticule scale c Superimposed images of the eyepiece graticule scale and the stage micrometer scaleacheek cells on a slide on the stage of the microscopebc0 1020304050607080901000 102030405060708090100eyepiece graticule scale (arbitrary units)

eyepiece graticule in the eyepiece of the microscope00stage micrometer scale (marked in 00 1mm and 01 mm divisions)10 20 30 40 50 60 70 80 90 10001 021 Cell structure 7

Electron microscopesEarlier in this chapter it was stated that by 1900 almost all the structures shown in Figures 13 and 15 (pages 2 and 3) had been discovered There followed a time of frustration for microscopists because they realised that no matter how much the design of light microscopes improved there was a limit to how much could ever be seen using lightIn order to understand the problem it is necessary to know something about the nature of light itself and to understand the difference between magnification and resolutionMagnificationMagnification is the number of times larger an image is compared with the real size of the objectWorked example 1 ndash calculating the magnification of a photograph or objectTo calculate M the magnification of a photograph or an object we can use the following methodFigure 19 shows two photographs of a section through the same plant cells The magnifications of the two photographs are the same Suppose we want to know the magnification of the plant cell in Figure 19b If we know its actual (real) length we can calculate its magnification using theThe real length of the cell is 80 1113088mmagnification observed size of the image actual sizeM I AStep 1Measure the length in mm of the cell in the photograph using a ruler You should find that it is about 60 mmStep 2Convert mm to 1113088m (It is easier if we first convert all measurements to the same units ndash in this case micrometres 1113088m)1 mm 1113088 1000 1113088m so 60mm 1113088 60 1113088 10001113088morwhere I 1113088 observed size of the image (that is what you can measure with a ruler) and A 1113088 actual size (that is the real size ndash for example the size of a cell before it is magnified)If you know two of these values you can work out the third one For example if the observed size of the image and the magnification are known you can work out the1113088 60 000 1113088m Use the equation to calculate the magnificationStep 3actual size A I If you write the formula in a triangle Mmagnification Mimage size I

as shown below and cover up the value you want to find it should be obvious how to do the right calculationactual size A 60000m80 m 750I M1113088ASome worked examples are now providedThe lsquo1113088rsquo sign in front of the number 750 means lsquotimesrsquo We say that the magnification is lsquotimes 750rsquoformula M I A88 1 Cell structurea Worked example 2 ndash calculating magnification from a scale barFigure 110 shows a lymphocyteFigure 110 A lymphocyte6 μmbFigure 19 Photographs of the same plant cells seen a with a light microscope b with an electron microscope both shown at a magnification of about 1113088 750We can calculate the magnification of the lymphocyte by simply using the scale bar All you need to do is measure the length of the scale bar and then substitute this and the length it represents into the equationStep 1Measure the scale bar Here it is 36 mmStep 2Convert mm to 1113088m 36mm 1113088 36 1113088 10001113088m 1113088 360001113088mStep 3Use the equation to calculate the magnificationmagnification M image size I actual size A36000m 6m60001 Cell structure 9Worked example 3 ndash calculating the real size of an object from its magnificationTo calculate A the real or actual size of an object we can use the following methodFigure 125 on page 19 shows a plant cell magnified 1113088 5600 One of the chloroplasts is labelled lsquochloroplastrsquo in the figure Suppose we want to know the actual length of this chloroplastStep 1Measure the observed length of the image of the chloroplast (I ) in mm using a ruler The maximum length is 36 mmStep 2Convert mm to 1113088m 30mm 1113088 30 1113088 10001113088m 1113088 300001113088mStep 3Use the equation to calculate the actual lengthactualsizeA imagesizeI magnification M30 000 m 5600 5 4 m (to one decimal place)SAQ 12a Calculate the magnification of the drawing of the animal cell in Figure 13 on page 2b Calculate the actual (real) length of the bottom chloroplast in Figure 127 on page 19Resolution

Look again at Figure 19 (page 9) Figure 19a is a light micrograph (a photograph taken with a light microscope also known as a photomicrograph) Figure 19b is an electron micrograph of the same cells taken at the same magnification (an electron micrograph is a picture taken with an electron microscope) You can see that Figure 19b the electron micrograph is much clearer This is because it has greater resolution Resolution is defined as the abilityto distinguish between two separate points If the two points cannot be resolved they will be seen as one point In practice resolution is the amount of detail that can be seen ndash the greater the resolution the greater the detailThe maximum resolution of a light microscope is 200 nm This means that if two points or objects are closer together than 200 nm they cannot be distinguished as separateIt is possible to take a photograph such as Figure 19a and to magnify (enlarge) it but we see no more detail in other words we do not improve resolution even though we often enlarge photographs because they are easier to see when larger With a microscope magnification up to the limit of resolution can reveal further detail but any further magnification increases blurring as well as the size of the imageThe electromagnetic spectrumHow is resolution linked with the nature of light One of the properties of light is that it travels in waves The length of the waves of visible light varies ranging from about 400 nm (violet light) to about 700 nm (red light) The human eye can distinguish between these different wavelengths and in the brain the differences are converted to colour differences (Colour is an invention of the brain)The whole range of different wavelengths is called the electromagnetic spectrum Visible light is only one part of this spectrum Figure 111 shows some of the parts of the electromagnetic spectrum The longer the waves the lower their frequency (all the waves travel at the same speed so imagine them passing a post shorter waves pass at higher frequency) In theory there is no limit to how short or how long the waves can be Wavelength changes with energy the greater the energy the shorter the wavelength (rather like squashing a spring)Now look at Figure 112 which shows a mitochondrion some very small cell organelles called ribosomes (see page 13) and light of 400 nm wavelength the shortest visible wavelength The mitochondrion is large enoughto interfere with the light waves However the ribosomes are far too small to have any effect on the light waves The general rule is that the limit of resolution is about one half the wavelength of the radiation used to view the specimen In other words if an object is any smaller than half the wavelength of the radiation used to view it it cannot be seen separately from nearby objects This means that the1001 1 Cell structureX-raysgamma rays01 nm400 nm violet bluev infraredmicrowaves

uv10 nmi s i b l e 1000 nmradio and TV wavesvisible light500 nm 600 nm green yellow orange105 nm 107 nm 109 nm 1011 nm 1013 nm700 nmFigure 111 Diagram of the electromagnetic spectrum (the waves are not drawn to scale) The numbers indicate the wavelengths of the different types of electromagnetic radiation Visible light is a form of electromagnetic radiationbest resolution that can be obtained using a microscope that uses visible light (a light microscope) is 200 nm since the shortest wavelength of visible light is 400 nm (violet light) In practice this corresponds to a maximum useful magnification of about 1500 times Ribosomes are approximately 25 nm in diameter and can therefore never be seen using lightIf an object is transparent it will allow light waves to pass through it and therefore will still not be visible This is why many biological structures have to be stained before they can be seenThe electron microscopeBiologists faced with the problem that they would never see anything smaller than 200 nm using a light microscope realised that the only solution would be to use radiation of a shorter wavelength than light If you study Figure 111 you will see that ultraviolet light or better still X-rays look like possible candidates Both ultraviolet and X-ray microscopes have been built the latter with little success partly because of the difficulty of focusing X-rays A much better solution is to use electrons Electrons are negatively charged particles which orbit the nucleus of an atom When a metal becomes very hot some of its electrons gain so much energy that they escape from their orbits like a rocket escaping from Earthrsquos gravity Free electrons behave like electromagnetic radiation They have a very short wavelength the greater the energy the shorter the wavelength Electrons are a very suitable form of radiation for microscopy for two major reasons Firstly their wavelength is extremely short (at least as short as that of X-rays) Secondly because they are negatively charged they can be focused easily using electromagnets (a magnet can be made to alter the path of the beam the equivalent of a glass lens bending light)Using an electron microscope a resolution of 05 nm can be obtained

400 times better than when using a light microscope ETransmission and scanning electron microscopesTwo types of electron microscope are now in common use The transmission electron microscope or TEM forwavelength 400nmstained mitochondrion of diameter 1000nm interferes with light wavesstained ribosomes of diameter 25nm do not interfere with light wavesFigure 112 A mitochondrion and some ribosomes in the path of light waves of 400 nm lengthred1 Cell structure 11

E short was the type originally developed Here the beam of electrons is passed through the specimen before being viewed Only

those electrons that are transmitted (pass through the specimen) are seen This allows us to see thin sections of specimens and thus to see inside cells In the scanning electron microscope (SEM) on the other hand the electron beam is used to scan the surfaces of structures and only the reflected beam is observedAn example of a scanning electron micrograph is shown in Figure 113 The advantage of this microscope is that surface structures can be seen Also great depth of field is obtained so that much of the specimen is in focus at the same time and a three-dimensional appearance is obtained Such a picture would be impossible to obtain with a light microscope even using the same magnification and resolution because you would have to keep focusing up and down with the objective lens to see different parts of the specimen The disadvantage of the SEM is that it cannot achieve the same resolution as a TEM Resolution is between 3 nm and 20 nmViewing specimens with the electron microscopeFigure 114 shows how an electron microscope works and Figure 115 shows one in useFigure 113 False-colour SEM of the head of a cat flea (1113088 100)

electron gun and anode which E produce a beam of electronselectron beam vacuumpathway of electronscondenser electromagnetic lens which directs the electron beam onto the specimenspecimen which is placed on a gridobjective electromagnetic lens which produces an imageprojector electromagnetic lenses which focus the magnified image onto the screenscreen or photographic plate which shows the image of the specimenFigure 114 How an electron microscope worksIt is not possible to see an electron beam so to make the image visible the electron beam has to be projected onto a fluorescent screen The areas hit by electrons shine brightly giving overall a lsquoblack and whitersquo picture The stains used to improve the contrast of biological specimens for electron microscopy contain heavy metal atoms which stop the passage of electrons The resulting picture is like an X-ray photograph with the more densely stained parts of the specimen appearing blacker lsquoFalse-colourrsquo images can be created by colouring the standard black and white image using a computerTo add to the difficulties of electron microscopy the electron beam and therefore the specimen and the fluorescent screen must be in a vacuum If electrons1221 1 Cell structure

EStructures and functions of organellesCompartmentalisation and division of labour within the cell are even more obvious with an electron microscope than with a light microscopeWe will now consider the structures and functions of some of the cell components in more detailNucleusThe nucleus (Figure 118 on page 15) is the largest cell organelle (see also page 5) It is surrounded by two membranes known as the

nuclear envelope The outer membrane of the nuclear envelope is continuous with the endoplasmic reticulum (Figure 117 on page 15) The nuclear envelope has many small pores called nuclear pores These allow and control exchange between the nucleus and the cytoplasm Examples of substances leaving the nucleus through the pores are mRNA and ribosomes for protein synthesis Examples of substances entering through the nuclear pores are proteins to help make ribosomes nucleotides ATP (aderosine triphosphate) and some hormones such as thyroid hormone T3Within the nucleus the chromosomes are in a loosely coiled state known as chromatin (except during nuclear division see Chapter 5) Chromosomes contain DNA which is organised into functional units called genes Genes control the activities of the cell and inheritance thus the nucleus controls the cellrsquos activities When a cell is aboutto divide the nucleus divides first so that each new cell will have its own nucleus (Chapters 5 and 19) Also within the nucleus the nucleolus makes ribosomes using the information in its own DNAEndoplasmic reticulum and ribosomesWhen cells were first seen with the electron microscope biologists were amazed to see so much detailed structure The existence of much of this had not been suspected This was particularly true of an extensive system of membranes running through the cytoplasm which became knownas the endoplasmic reticulum (ER) (Figure 119 on page 15 ndash see also Figures 118 on page 15 and 122 on page 17) The ER is continuous with the outer membrane of the nuclear envelope (Figure 117)There are two types of ER rough ER and smooth ER Rough ER is so called because it is covered with many tinyFigure 115 A TEM in usecollided with air molecules they would scatter making it impossible to achieve a sharp picture Also water boils at room temperature in a vacuum so all specimens must be dehydrated before being placed in the microscope This means that only dead material can be examined Great efforts are therefore made to try to preserve material in a life-like state when preparing it for the microscopeSAQ 13Explain why ribosomes are not visible using a light microscopeUltrastructure of an animal cellThe lsquofinersquo or detailed structure of a cell as revealed by the electron microscope is called its ultrastructure Figure 116 shows the appearance of typical animal cells as seen with an electron microscope and Figure 117 on page 15 is a diagram based on many other such micrographsSAQ 14Compare Figure 117 on page 15 with Figure 13 on page 2 Name the structures which can be seen with the electron microscope but not with the light microscope1 Cell structure 13lysosomeGGolgi apparatusendoplasmic reticulumglycogen granulesccell surface membrane

mmitochondriachromatinnnucleolusmmicrovillusrribosomesFFigure 116 Representative animal cells as seen with a TEM The cells are liver cells from a rat (1113088 9600) The nucleus is clearly visible in one of the cells 1441 1 Cell structurenucleusnnuclear envelopetwo centrioles close to the nucleus and at right angles to each othermitochondrion lysosomerough endoplasmic reticulumnucleoluschromatin nucleusnuclear porenuclear envelope (two membranes)microvillismooth endoplasmic reticulumFigure 117 Ultrastructure of a typical animal cell as seen with an electron microscope In reality the ER is more extensive than shown and free ribosomes may be more extensive Glycogen granules are sometimes present in the cytoplasmFigure 118 TEM of the nucleus of a cell from the pancreas of a bat (1113088 7500) The circular nucleus is surrounded by a double-layered nuclear envelope containing nuclear pores The nucleolus is more Figure 119 TEM of rough ER covered with ribosomes (black dots) darkly stained Rough ER is visible in the surrounding cytoplasm (1113088 17 000) Some free ribosomes can also be seen in the cytoplasmGolgi vesicle Golgi apparatusribosomescell surface membranecytoplasm1 Cell structure 15organelles called ribosomes These are just visible as black dots in Figures 118 and 119 on page 15 At very high magnifications they can be seen to consist of two subunits a large and a small subunit Ribosomes are the sites of protein synthesis (see pages 111ndash112) They can be found free in the cytoplasm as well as on the rough ER They are very small only about 25 nm in diameter They are made of RNA (ribonucleic acid) and protein The rough ER forms an extensive system of flattened sacs spreading in sheets throughout the cell Proteins made by the ribosomes on the rough ER enter the sacs and move through them The proteins are often processed in some way on their journey Small sacs called vesicles can break off from the ER and these can join together to form the Golgi apparatus Proteins can be exported from the cell via the Golgi apparatus (see page 80)Smooth ER so called because it lacks ribosomes has a completely different function It makes lipids and steroids such as cholesterol and the reproductive hormones oestrogen and testosteroneGolgi apparatus (Golgi body or Golgi complex)The Golgi apparatus is a stack of flattened sacs (Figure 120) This stack of sacs is sometimes referred to as theFigure 120 TEM of a Golgi apparatus A central stack of saucer-shaped sacs can be seen budding off small Golgi vesicles (green) These may form secretory vesicles whose contents can be released at the cell surface by exocytosis (see page 80)Golgi body More than one may be present in a cell The stack is constantly being formed at one end from vesicles which bud off from the ER and broken down again at the other end to form Golgi vesicles The stack of sacs with the associated vesicles is referred to as the Golgi apparatus or Golgi complex

The Golgi apparatus collects processes and sorts molecules (particularly proteins from the rough ER) ready for transport in Golgi vesicles either to other parts of the cell or out of the cell (secretion) Two examples of protein processing in the Golgi apparatus are the addition of sugars to proteins to make molecules known as glycoproteins and the removal of the first amino acid methionine from newly formed proteins to make a functioning protein In plants enzymes in the Golgi apparatus convert sugars into cell wall components Golgi vesicles are also used to make lysosomesLysosomesLysosomes (Figure 121) are spherical sacs surrounded by a single membrane and having no internal structure They are commonly 01ndash 05 1113088m in diameter They contain digestive (hydrolytic) enzymes which must be kept separateFigure 121 Lysosomes (orange) in a mouse kidney cell (1113088 55 000) They contain cell structures in the process of digestion and vesicles (green) Cytoplasm is coloured blue here1661 1 Cell structurefrom the rest of the cell to prevent damage Lysosomes are responsible for the breakdown (digestion) of unwanted structures such as old organelles or even whole cells as in mammary glands after lactation (breast feeding) In white blood cells lysosomes are used to digest bacteria (see endocytosis page 80) Enzymes are sometimes released outside the cell ndash for example in the replacement of cartilage with bone during development The headsof sperm contain a special lysosome the acrosome for digesting a path to the ovum (egg)MitochondriaMitochondria (singular mitochondrion) are usually about 1 1113088m in diameter and can be various shapes often sausage- shaped as in Figure 122 They are surrounded by two membranes (an envelope) The inner of these is foldedto form finger-like cristae which project into the interior solution or matrixThe main function of mitochondria is to carry out aerobic respiration As a result of respiration they make ATP the universal energy carrier in cells (see Chapter 16) They are also involved in the synthesis of lipids (page 37)Figure 122 Mitochondrion (orange) with its double membrane (envelope) the inner membrane is folded to form cristae (1113088 20 000) Mitochondria are the sites of aerobic cell respiration Note also the rough ERIn the 1960s it was discovered that mitochondria and chloroplasts contain ribosomes which are slightly smaller than those in the cytoplasm and are the same size as those found in bacteria The size of ribosomes is measured in lsquoS unitsrsquo which are a measure of how fast they sediment in a centrifuge Cytoplasmic ribosomes are 80S while those of bacteria mitochondria and chloroplasts are 70S It was also discovered in the 1960s that mitochondria and chloroplasts contain small circular DNA molecules also like those found in bacteria Not surprisingly it was later provedthat mitochondria and chloroplasts are in effect ancient bacteria which now live inside the larger cells typical of animals and plants (see

prokaryotic and eukaryotic cells page 18) This is known as the endosymbiont theory lsquoEndorsquo means lsquoinsidersquo and a lsquosymbiontrsquo is an organism which lives in a mutually beneficial relationship with another organism The DNA and ribosomes of mitochondria and chloroplasts are still active and responsible for the coding and synthesis of certain vital proteins but mitochondria and chloroplasts can no longer live independentlyMitochondrial ribosomes are just visible as tiny dark dots in the mitochondrial matrix in Figure 122Cell surface membraneThe cell surface membrane is extremely thin (about 7 nm) However at very high magnifications at least 1113088 100 000 it can be seen to have three layers described as a trilaminar appearance This consists of two dark lines (heavily stained) either side of a narrow pale interior (Figure 123) The membrane is partially permeable and controls exchange between the cell and its environment Membrane structure is discussed further in Chapter 4Figure 123 Cell surface membrane (1113088 250 000) At this magnification the membrane appears as two dark lines at the edge of the cellMicrovilliMicrovilli (singular microvillus) are finger-like extensions of the cell surface membrane typical of certain epithelial cells (cells covering surfaces of structures) They greatly1 Cell structure 17increase the surface area of the cell surface membrane (see Figure 117 on page 15) This is useful for example for absorption in the gut and for reabsorption in the proximal convoluted tubules of the kidney (see page 307)CentriolesThe extra resolution of the electron microscope reveals that just outside the nucleus there are really two centrioles (see Figure 124) not one as it appears under the light microscope (compare with Figure 13 on page 2) They lie close together at right-angles to each other A centriole is a hollow cylinder about 04 1113088m long formed from a ring of short microtubules tiny tubes made of a protein called tubulin These microtubules are used as a starting point for growing the spindle microtubules for nuclear division (see page 92) Centrioles are not found in plant cellsUltrastructure of a plant cellAll the structures except centrioles and microvilli so far described in animal cells are also found in plant cells The appearance of a plant cell as seen with the electron microscope is shown in Figure 125 while Figure 126 is a diagram based on many such micrographs The relatively thick cell wall and the large central vacuole are obvious as are the chloroplasts two of which are shown in detail in Figure 127 These structures and their functions have been described on pages 5 and 6 The electron microscopeFigure 124 Centrioles in transverse and longitudinal section (TS and LS) (1113088 86 000) The one on the left is seen in TS and clearly shows the nine triplets of microtubules which make up the structurereveals that chloroplasts contain 70S ribosomes and small circular

DNA molecules as do mitochondria and bacteria This has already been discussed with mitochondria above (page 17)SAQ 15Compare Figure 126 with Figure 15 on page 3 Name the structures which can be seen with the electron microscope but not with the light microscopeTwo fundamentally different types of cellAt one time it was common practice to try to classify all living organisms as either animals or plants With advances in our knowledge of living things it has become obvious that the living world is not that simple Fungi and bacteria for example are very different from animals and plants and from each other Eventually it was discovered that there are two fundamentally different types of cell The most obvious difference betweenthese types is that one possesses a nucleus and the other does notOrganisms that lack nuclei are called prokaryotes (lsquoprorsquo means before lsquokaryonrsquo means nucleus) All prokaryotes are now referred to as bacteria They are on average about 1000 to 10 000 times smaller in volume than cells with nuclei and are much simpler in structure ndash for example their DNA lies free in the cytoplasmOrganisms whose cells possess nuclei are called eukaryotes (lsquoeursquo means true) Their DNA lies inside a nucleus Eukaryotes include animals plants fungi and a group containing most of the unicellular eukaryotes known as protoctists Most biologists believe that eukaryotes evolved from prokaryotes one-and-a-half thousand million years after prokaryotes first appeared on Earth We mainly study animals and plants in this book but all eukaryotic cells have certain features in commonA generalised prokaryotic cell is shown in Figure 128 A comparison of prokaryotic and eukaryotic cells is given in Table 12 on page 211881 1 Cell structure

Condenser lens focuses the light onto the specimen held between the cover slip and slideCondenser iris diaphragm is closed slightly to produce a narrow beam of lightFigure 12 How the light microscope worksLight microscopyThe lsquogolden agersquo of light microscopy could be said to be the 19th century Microscopes had been available since the beginning of the 17th century but when dramatic improvements were made in the quality of glass lenses in the early 19th century interest among scientists became widespread The fascination ofthe microscopic world that opened up in biology inspired rapid progress both in microscope design and equally importantly in preparing material for examination with microscopes This branch of biology is known as cytology Figure 12 shows how the light microscope worksBy 1900 all the structures shown in Figures 13 14 and 15 except lysosomes had been discovered Figure 13 shows the structure of a generalised animal cell and Figure 15 the structure of a generalised plant cell as seen with a light microscope (A generalised cell shows all the structures that are typically found in a cell)Golgi apparatus cytoplasmcentriole ndash always found near nucleus has a role in nuclear divisionsmall structures that are difficult to identifymitochondria cell surface membranenuclear envelopechromatin ndash deeply staining and thread-likenucleolus ndash deeply stainingnucleusFigure 13 Structure of a generalised animal cell (diameter about 20 1113088m) as seen with a very high quality light microscope22 1 Cell structuretonoplast ndash membrane surrounding vacuolecell surface membrane (pressed against cell wall)vacuole ndash large with central positioncytoplasmmitochondriamiddle lamella ndash thin layer holding cells together contains calcium pectateplasmodesma ndash connects cytoplasm of neighbouring cellscell wall of neighbouring cellcell wall chloroplast grana just visiblesmall structures that are difficult to identifyGolgi apparatusFigure 14 Cells from the lining of the human cheek (1113088 500) each showing a centrally placed nucleus which is a typical animal cell characteristic The cells are part of a tissue known as squamous (flattened) epitheliumFigure 14 shows some actual human cells and Figure 16 shows an actual plant cell taken from a leafSAQ 11Using Figures 13 and 15 name the structures that animal and plant cells have in common those found in only plant cells and those found only in animal cellsFigure 16 Photomicrograph of a cell in a moss leaf (11130881400)nucleusnucleolus ndash deeply stainingnuclear envelopechromatin ndash deeply staining and thread-likeFigure 15 Structure of a generalised plant cell (diameter about 40 1113088m) as seen with a very high quality light microscope1 Cell structure 3Box 1A Biological drawingYou need the following equipment

bull pencil (HB) bull pencil sharpener bull eraser bull ruler bull plain paperHere are some guidelines for the quality of your drawingbull always use a pencil not a pen bull donrsquot use shading bull use clear continuous lines bull use accurate proportions and observation ndash not atextbook version For a low-power drawing (see Figure 17)bull donrsquot draw individual cells bull draw all tissues completely enclosed by lines bull draw a correct interpretation of the distribution of bull tissuesa representative portion may be drawn (eg half a transverse section)For a high-power drawingbull draw only a few representative cells bull draw the cell wall of all plant cells bull donrsquot draw the nucleus as a solid blobSome guidelines for the quality of your labellingbull label all tissues and relevant structures bull identify parts correctly bull use a ruler for label linesAnimal and plant cells have features in commonIn animals and plants each cell is surrounded by a very thin cell surface membrane which is too thin to be seen with a light microscope This is also sometimes referred to as the plasma membranebull arrange label lines neatly and ensure they donrsquot cross bull over each otherannotate your drawing if necessary (ie provideshort notes with one or more of the labels in order bull to describe or explain features of biological interest)add a scale line at the bottom of the drawing if bull appropriateuse a pencil not a penAn example of a drawing of a section through the stem of Helianthus is shown below Biological drawing is also covered in Chapter 15 page 264Figure 17 The right side of this low-power drawing shows examples of good technique while the left side shows many of the pitfalls you should avoidMany of the cell contents are colourless and transparent so they need to be stained to be seen Each cell has a nucleus which is a relatively large structure that stains intensely and is therefore very conspicuous The deeply staining material in the nucleus is called chromatin and is a massof loosely coiled threads This material collects together to form visible separate chromosomes during nuclear division44 1 Cell structure(see page 86) It contains DNA (deoxyribonucleic acid) a molecule which contains the instructions that control the activities of the cell (see Chapter 6) Within the nucleus an even more deeply staining area is visible the nucleolus which is made of loops of DNA from several chromosomes The number of nucleoli is variable one to five being

common in mammalsThe material between the nucleus and the cell surface membrane is known as cytoplasm Cytoplasm is an aqueous (watery) material varying from a fluid to a jelly-like consistency Many small structures can be seen within it These have been likened to small organs and hence are known as organelles An organelle can be defined as a functionally and structurally distinct part of a cell Organelles themselves are often surrounded by membranes so that their activities can be separated from the surrounding cytoplasm This is described as compartmentalisation Having separate compartments is essential for a structure as complex as an animal or plant cell to work efficiently Since each type of organelle has its own function the cell is said to show division of labour a sharing of the work between different specialised organellesThe most numerous organelles seen with the light microscope are usually mitochondria (singular mitochondrion) Mitochondria are only just visible but films of living cells taken with the aid of a light microscope have shown that they can move about change shape and divide They are specialised to carry out aerobic respirationThe use of special stains containing silver enabled the Golgi apparatus to be detected for the first time in 1898 by Camillo Golgi The Golgi apparatus is part of a complex internal sorting and distribution system within the cell (see page 16) It is also sometimes called the Golgi body or Golgi complexDifferences between animal and plant cellsThe only structure commonly found in animal cells which is absent from plant cells is the centriole Plant cells also differ from animal cells in possessing cell walls large permanent vacuoles and chloroplastsCentriolesUnder the light microscope the centriole appears as a small structure close to the nucleus (see Figure 13 on page 2) The centriole is involved in nuclear division (see page 92)Cell walls and plasmodesmataWith a light microscope individual plant cells are more easily seen than animal cells because they are usually larger and unlike animal cells surrounded by a cell wall outside the cell surface membrane This is relatively rigid because it contains fibres of cellulose a polysaccharide which strengthens the wall The cell wall gives the cell a definite shape It prevents the cell from bursting when water enters by osmosis allowing large pressures to develop inside the cell (see page 77) Cell walls may also be reinforced with extra cellulose or with a hard material called lignin for extra strength (see xylem on page 24) Cell walls are freely permeable allowing free movement of molecules and ions through to the cell surface membranePlant cells are linked to neighbouring cells by means of fine strands of cytoplasm called plasmodesmata (singular plasmodesma) which pass through pore-like structures in the walls of these neighbouring cells Movement through the pores is thought to be controlled by the structure of the poresVacuoles

Although animal cells may possess small vacuoles such as phagocytic vacuoles (see page 80) which are temporary structures mature plant cells often possess a large permanent central vacuole The plant vacuole is surrounded by a membrane the tonoplast which controls exchange between the vacuole and the cytoplasm The fluid in the vacuole is a solution of mineral salts sugars oxygen carbon dioxide pigments enzymes and other organic compounds including some waste productsVacuoles help to regulate the osmotic properties of cells (the flow of water inwards and outwards) as well as having a wide range of other functions For example the pigments which colour the petals of certain flowers and parts of some vegetables such as the red pigment of beetroots are sometimes located in vacuoles1 Cell structure 5ChloroplastsSome plant cells are able to carry out photosynthesis because they contain chloroplasts Chloroplasts are relatively large organelles which are green in colour due to the presence of chlorophyll At high magnifications small lsquograinsrsquo or grana (singular granum) can be seen in the chloroplasts During the process of photosynthesis light is absorbed by these grana which actually consist of stacks of membrane-bound sacs called thylakoids Starch grains may also be visible within chloroplasts Chloroplasts are found in the green parts of plants mainly in the leavesPoints to noteWe return to the differences between animal and plant cells as seen using the electron microscope on page 18Units of measurement in cell studiesIn order to measure objects in the microscopic world we need to use very small units of measurement which are unfamiliar to most people According to international agreement the International System of Units (SI units) should be used In this system the basic unit of length is the metre (symbol m) Additional units can be created in multiples of a thousand times larger or smaller using standard prefixes For example the prefix kilo means 1000 times Thus 1 kilometre 1113088 1000 metres The units of length relevant to cell studies are shown in Table 11It is difficult to imagine how small these units are but when looking down a microscope and seeing cells clearly we should not forget how amazingly small the cells actually are The smallest structure visible with the human eye is about 50ndash100 1113088m in diameter Your body contains about 60 million million cells varying in size from about 5 1113088m to 40 1113088m Try to imagine structures like mitochondria which have an average diameter of 1 1113088m The smallest cell organelles we deal with in this book ribosomes are only about 25 nm in diameter You could line up about 20 000 ribosomes across the full stop at the end of this sentenceUnit Symbolmillimetre mm micrometre 1113088m nanometre nmTable 11 Units of measurement relevant to cell studies 1113088 is the Greek letter mu 1 micrometre is a thousandth of a millimetre 1 nanometre is a thousandth of a micrometre661 Cell structure

bull bull bullbullYou can think of a plant cell as being very similar to an animal cell but with extra structures Plant cells are often larger than animal cells although cell size varies enormouslyDo not confuse the cell wall with the cell surface membrane Cell walls are relatively thick and physically strong whereas cell surface membranes are very thin Cell walls are freely permeable whereas cell surface membranes are partially permeable All cells have a cell surface membrane Vacuoles are not confined to plant cells animal cells may have small vacuoles such as phagocytic vacuoles (see page 80) although these are not usually permanent structuresFraction of a metreone thousandth 1113088 0001 1113088 11000 1113088 10-3 one millionth 1113088 0000 001 1113088 11 000 000 1113088 10-6 one thousand millionth 1113088 0000 000 001 1113088 11 000 000 000 1113088 10-9

Box 1B Measuring cellsCells and organelles can be measured with a microscope by means of an eyepiece graticule This is a transparent scale It usually has 100 divisions (see Figure 18a) The eyepiece graticule is placed in the microscope eyepiece so that it can be seen at the same time as the object to be measured as shown in Figure 18b Figure 18b shows the scale over a human cheek epithelialcell The cell lies between 40 and 60 on the scale We therefore say it measures 20 eyepiece units in diameter (the difference between 60 and 40) We will not know the actual size of the eyepiece units until the eyepiece graticule scale is calibratedTo calibrate the eyepiece graticule scale a miniature transparent ruler called a stage micrometer scale is placed on the microscope stage and is brought into focus This scale may be etched onto a glass slideor printed on a transparent film It commonly has subdivisions of 01 and 001 mm The images of the two scales can then be superimposed as shown in Figure 18cIn the eyepiece graticule shown in the figure 100 units measure 025 mm Hence the value of each eyepiece unit is0 25 00025 mm 100Or converting mm to 1113088m 0 25 1000 2 51113088m100The diameter of the cell shown superimposed on the scale in Figure 18b measures 20 eyepiece units and so its actual diameter is20times251113088m 1113088 501113088m This diameter is greater than that of many human cellsbecause the cell is a flattened epithelial cellFigure 18 Microscopical measurement Three fields of view seen using a high-power (111308840) objective lens a An eyepiece graticule scale b Superimposed images of human cheek epithelial cells and the eyepiece graticule scale c Superimposed images of the eyepiece graticule scale and the stage micrometer scaleacheek cells on a slide on the stage of the microscopebc0 1020304050607080901000 102030405060708090100eyepiece graticule scale (arbitrary units)

eyepiece graticule in the eyepiece of the microscope00stage micrometer scale (marked in 00 1mm and 01 mm divisions)10 20 30 40 50 60 70 80 90 10001 021 Cell structure 7

Electron microscopesEarlier in this chapter it was stated that by 1900 almost all the structures shown in Figures 13 and 15 (pages 2 and 3) had been discovered There followed a time of frustration for microscopists because they realised that no matter how much the design of light microscopes improved there was a limit to how much could ever be seen using lightIn order to understand the problem it is necessary to know something about the nature of light itself and to understand the difference between magnification and resolutionMagnificationMagnification is the number of times larger an image is compared with the real size of the objectWorked example 1 ndash calculating the magnification of a photograph or objectTo calculate M the magnification of a photograph or an object we can use the following methodFigure 19 shows two photographs of a section through the same plant cells The magnifications of the two photographs are the same Suppose we want to know the magnification of the plant cell in Figure 19b If we know its actual (real) length we can calculate its magnification using theThe real length of the cell is 80 1113088mmagnification observed size of the image actual sizeM I AStep 1Measure the length in mm of the cell in the photograph using a ruler You should find that it is about 60 mmStep 2Convert mm to 1113088m (It is easier if we first convert all measurements to the same units ndash in this case micrometres 1113088m)1 mm 1113088 1000 1113088m so 60mm 1113088 60 1113088 10001113088morwhere I 1113088 observed size of the image (that is what you can measure with a ruler) and A 1113088 actual size (that is the real size ndash for example the size of a cell before it is magnified)If you know two of these values you can work out the third one For example if the observed size of the image and the magnification are known you can work out the1113088 60 000 1113088m Use the equation to calculate the magnificationStep 3actual size A I If you write the formula in a triangle Mmagnification Mimage size I

as shown below and cover up the value you want to find it should be obvious how to do the right calculationactual size A 60000m80 m 750I M1113088ASome worked examples are now providedThe lsquo1113088rsquo sign in front of the number 750 means lsquotimesrsquo We say that the magnification is lsquotimes 750rsquoformula M I A88 1 Cell structurea Worked example 2 ndash calculating magnification from a scale barFigure 110 shows a lymphocyteFigure 110 A lymphocyte6 μmbFigure 19 Photographs of the same plant cells seen a with a light microscope b with an electron microscope both shown at a magnification of about 1113088 750We can calculate the magnification of the lymphocyte by simply using the scale bar All you need to do is measure the length of the scale bar and then substitute this and the length it represents into the equationStep 1Measure the scale bar Here it is 36 mmStep 2Convert mm to 1113088m 36mm 1113088 36 1113088 10001113088m 1113088 360001113088mStep 3Use the equation to calculate the magnificationmagnification M image size I actual size A36000m 6m60001 Cell structure 9Worked example 3 ndash calculating the real size of an object from its magnificationTo calculate A the real or actual size of an object we can use the following methodFigure 125 on page 19 shows a plant cell magnified 1113088 5600 One of the chloroplasts is labelled lsquochloroplastrsquo in the figure Suppose we want to know the actual length of this chloroplastStep 1Measure the observed length of the image of the chloroplast (I ) in mm using a ruler The maximum length is 36 mmStep 2Convert mm to 1113088m 30mm 1113088 30 1113088 10001113088m 1113088 300001113088mStep 3Use the equation to calculate the actual lengthactualsizeA imagesizeI magnification M30 000 m 5600 5 4 m (to one decimal place)SAQ 12a Calculate the magnification of the drawing of the animal cell in Figure 13 on page 2b Calculate the actual (real) length of the bottom chloroplast in Figure 127 on page 19Resolution

Look again at Figure 19 (page 9) Figure 19a is a light micrograph (a photograph taken with a light microscope also known as a photomicrograph) Figure 19b is an electron micrograph of the same cells taken at the same magnification (an electron micrograph is a picture taken with an electron microscope) You can see that Figure 19b the electron micrograph is much clearer This is because it has greater resolution Resolution is defined as the abilityto distinguish between two separate points If the two points cannot be resolved they will be seen as one point In practice resolution is the amount of detail that can be seen ndash the greater the resolution the greater the detailThe maximum resolution of a light microscope is 200 nm This means that if two points or objects are closer together than 200 nm they cannot be distinguished as separateIt is possible to take a photograph such as Figure 19a and to magnify (enlarge) it but we see no more detail in other words we do not improve resolution even though we often enlarge photographs because they are easier to see when larger With a microscope magnification up to the limit of resolution can reveal further detail but any further magnification increases blurring as well as the size of the imageThe electromagnetic spectrumHow is resolution linked with the nature of light One of the properties of light is that it travels in waves The length of the waves of visible light varies ranging from about 400 nm (violet light) to about 700 nm (red light) The human eye can distinguish between these different wavelengths and in the brain the differences are converted to colour differences (Colour is an invention of the brain)The whole range of different wavelengths is called the electromagnetic spectrum Visible light is only one part of this spectrum Figure 111 shows some of the parts of the electromagnetic spectrum The longer the waves the lower their frequency (all the waves travel at the same speed so imagine them passing a post shorter waves pass at higher frequency) In theory there is no limit to how short or how long the waves can be Wavelength changes with energy the greater the energy the shorter the wavelength (rather like squashing a spring)Now look at Figure 112 which shows a mitochondrion some very small cell organelles called ribosomes (see page 13) and light of 400 nm wavelength the shortest visible wavelength The mitochondrion is large enoughto interfere with the light waves However the ribosomes are far too small to have any effect on the light waves The general rule is that the limit of resolution is about one half the wavelength of the radiation used to view the specimen In other words if an object is any smaller than half the wavelength of the radiation used to view it it cannot be seen separately from nearby objects This means that the1001 1 Cell structureX-raysgamma rays01 nm400 nm violet bluev infraredmicrowaves

uv10 nmi s i b l e 1000 nmradio and TV wavesvisible light500 nm 600 nm green yellow orange105 nm 107 nm 109 nm 1011 nm 1013 nm700 nmFigure 111 Diagram of the electromagnetic spectrum (the waves are not drawn to scale) The numbers indicate the wavelengths of the different types of electromagnetic radiation Visible light is a form of electromagnetic radiationbest resolution that can be obtained using a microscope that uses visible light (a light microscope) is 200 nm since the shortest wavelength of visible light is 400 nm (violet light) In practice this corresponds to a maximum useful magnification of about 1500 times Ribosomes are approximately 25 nm in diameter and can therefore never be seen using lightIf an object is transparent it will allow light waves to pass through it and therefore will still not be visible This is why many biological structures have to be stained before they can be seenThe electron microscopeBiologists faced with the problem that they would never see anything smaller than 200 nm using a light microscope realised that the only solution would be to use radiation of a shorter wavelength than light If you study Figure 111 you will see that ultraviolet light or better still X-rays look like possible candidates Both ultraviolet and X-ray microscopes have been built the latter with little success partly because of the difficulty of focusing X-rays A much better solution is to use electrons Electrons are negatively charged particles which orbit the nucleus of an atom When a metal becomes very hot some of its electrons gain so much energy that they escape from their orbits like a rocket escaping from Earthrsquos gravity Free electrons behave like electromagnetic radiation They have a very short wavelength the greater the energy the shorter the wavelength Electrons are a very suitable form of radiation for microscopy for two major reasons Firstly their wavelength is extremely short (at least as short as that of X-rays) Secondly because they are negatively charged they can be focused easily using electromagnets (a magnet can be made to alter the path of the beam the equivalent of a glass lens bending light)Using an electron microscope a resolution of 05 nm can be obtained

400 times better than when using a light microscope ETransmission and scanning electron microscopesTwo types of electron microscope are now in common use The transmission electron microscope or TEM forwavelength 400nmstained mitochondrion of diameter 1000nm interferes with light wavesstained ribosomes of diameter 25nm do not interfere with light wavesFigure 112 A mitochondrion and some ribosomes in the path of light waves of 400 nm lengthred1 Cell structure 11

E short was the type originally developed Here the beam of electrons is passed through the specimen before being viewed Only

those electrons that are transmitted (pass through the specimen) are seen This allows us to see thin sections of specimens and thus to see inside cells In the scanning electron microscope (SEM) on the other hand the electron beam is used to scan the surfaces of structures and only the reflected beam is observedAn example of a scanning electron micrograph is shown in Figure 113 The advantage of this microscope is that surface structures can be seen Also great depth of field is obtained so that much of the specimen is in focus at the same time and a three-dimensional appearance is obtained Such a picture would be impossible to obtain with a light microscope even using the same magnification and resolution because you would have to keep focusing up and down with the objective lens to see different parts of the specimen The disadvantage of the SEM is that it cannot achieve the same resolution as a TEM Resolution is between 3 nm and 20 nmViewing specimens with the electron microscopeFigure 114 shows how an electron microscope works and Figure 115 shows one in useFigure 113 False-colour SEM of the head of a cat flea (1113088 100)

electron gun and anode which E produce a beam of electronselectron beam vacuumpathway of electronscondenser electromagnetic lens which directs the electron beam onto the specimenspecimen which is placed on a gridobjective electromagnetic lens which produces an imageprojector electromagnetic lenses which focus the magnified image onto the screenscreen or photographic plate which shows the image of the specimenFigure 114 How an electron microscope worksIt is not possible to see an electron beam so to make the image visible the electron beam has to be projected onto a fluorescent screen The areas hit by electrons shine brightly giving overall a lsquoblack and whitersquo picture The stains used to improve the contrast of biological specimens for electron microscopy contain heavy metal atoms which stop the passage of electrons The resulting picture is like an X-ray photograph with the more densely stained parts of the specimen appearing blacker lsquoFalse-colourrsquo images can be created by colouring the standard black and white image using a computerTo add to the difficulties of electron microscopy the electron beam and therefore the specimen and the fluorescent screen must be in a vacuum If electrons1221 1 Cell structure

EStructures and functions of organellesCompartmentalisation and division of labour within the cell are even more obvious with an electron microscope than with a light microscopeWe will now consider the structures and functions of some of the cell components in more detailNucleusThe nucleus (Figure 118 on page 15) is the largest cell organelle (see also page 5) It is surrounded by two membranes known as the

nuclear envelope The outer membrane of the nuclear envelope is continuous with the endoplasmic reticulum (Figure 117 on page 15) The nuclear envelope has many small pores called nuclear pores These allow and control exchange between the nucleus and the cytoplasm Examples of substances leaving the nucleus through the pores are mRNA and ribosomes for protein synthesis Examples of substances entering through the nuclear pores are proteins to help make ribosomes nucleotides ATP (aderosine triphosphate) and some hormones such as thyroid hormone T3Within the nucleus the chromosomes are in a loosely coiled state known as chromatin (except during nuclear division see Chapter 5) Chromosomes contain DNA which is organised into functional units called genes Genes control the activities of the cell and inheritance thus the nucleus controls the cellrsquos activities When a cell is aboutto divide the nucleus divides first so that each new cell will have its own nucleus (Chapters 5 and 19) Also within the nucleus the nucleolus makes ribosomes using the information in its own DNAEndoplasmic reticulum and ribosomesWhen cells were first seen with the electron microscope biologists were amazed to see so much detailed structure The existence of much of this had not been suspected This was particularly true of an extensive system of membranes running through the cytoplasm which became knownas the endoplasmic reticulum (ER) (Figure 119 on page 15 ndash see also Figures 118 on page 15 and 122 on page 17) The ER is continuous with the outer membrane of the nuclear envelope (Figure 117)There are two types of ER rough ER and smooth ER Rough ER is so called because it is covered with many tinyFigure 115 A TEM in usecollided with air molecules they would scatter making it impossible to achieve a sharp picture Also water boils at room temperature in a vacuum so all specimens must be dehydrated before being placed in the microscope This means that only dead material can be examined Great efforts are therefore made to try to preserve material in a life-like state when preparing it for the microscopeSAQ 13Explain why ribosomes are not visible using a light microscopeUltrastructure of an animal cellThe lsquofinersquo or detailed structure of a cell as revealed by the electron microscope is called its ultrastructure Figure 116 shows the appearance of typical animal cells as seen with an electron microscope and Figure 117 on page 15 is a diagram based on many other such micrographsSAQ 14Compare Figure 117 on page 15 with Figure 13 on page 2 Name the structures which can be seen with the electron microscope but not with the light microscope1 Cell structure 13lysosomeGGolgi apparatusendoplasmic reticulumglycogen granulesccell surface membrane

mmitochondriachromatinnnucleolusmmicrovillusrribosomesFFigure 116 Representative animal cells as seen with a TEM The cells are liver cells from a rat (1113088 9600) The nucleus is clearly visible in one of the cells 1441 1 Cell structurenucleusnnuclear envelopetwo centrioles close to the nucleus and at right angles to each othermitochondrion lysosomerough endoplasmic reticulumnucleoluschromatin nucleusnuclear porenuclear envelope (two membranes)microvillismooth endoplasmic reticulumFigure 117 Ultrastructure of a typical animal cell as seen with an electron microscope In reality the ER is more extensive than shown and free ribosomes may be more extensive Glycogen granules are sometimes present in the cytoplasmFigure 118 TEM of the nucleus of a cell from the pancreas of a bat (1113088 7500) The circular nucleus is surrounded by a double-layered nuclear envelope containing nuclear pores The nucleolus is more Figure 119 TEM of rough ER covered with ribosomes (black dots) darkly stained Rough ER is visible in the surrounding cytoplasm (1113088 17 000) Some free ribosomes can also be seen in the cytoplasmGolgi vesicle Golgi apparatusribosomescell surface membranecytoplasm1 Cell structure 15organelles called ribosomes These are just visible as black dots in Figures 118 and 119 on page 15 At very high magnifications they can be seen to consist of two subunits a large and a small subunit Ribosomes are the sites of protein synthesis (see pages 111ndash112) They can be found free in the cytoplasm as well as on the rough ER They are very small only about 25 nm in diameter They are made of RNA (ribonucleic acid) and protein The rough ER forms an extensive system of flattened sacs spreading in sheets throughout the cell Proteins made by the ribosomes on the rough ER enter the sacs and move through them The proteins are often processed in some way on their journey Small sacs called vesicles can break off from the ER and these can join together to form the Golgi apparatus Proteins can be exported from the cell via the Golgi apparatus (see page 80)Smooth ER so called because it lacks ribosomes has a completely different function It makes lipids and steroids such as cholesterol and the reproductive hormones oestrogen and testosteroneGolgi apparatus (Golgi body or Golgi complex)The Golgi apparatus is a stack of flattened sacs (Figure 120) This stack of sacs is sometimes referred to as theFigure 120 TEM of a Golgi apparatus A central stack of saucer-shaped sacs can be seen budding off small Golgi vesicles (green) These may form secretory vesicles whose contents can be released at the cell surface by exocytosis (see page 80)Golgi body More than one may be present in a cell The stack is constantly being formed at one end from vesicles which bud off from the ER and broken down again at the other end to form Golgi vesicles The stack of sacs with the associated vesicles is referred to as the Golgi apparatus or Golgi complex

The Golgi apparatus collects processes and sorts molecules (particularly proteins from the rough ER) ready for transport in Golgi vesicles either to other parts of the cell or out of the cell (secretion) Two examples of protein processing in the Golgi apparatus are the addition of sugars to proteins to make molecules known as glycoproteins and the removal of the first amino acid methionine from newly formed proteins to make a functioning protein In plants enzymes in the Golgi apparatus convert sugars into cell wall components Golgi vesicles are also used to make lysosomesLysosomesLysosomes (Figure 121) are spherical sacs surrounded by a single membrane and having no internal structure They are commonly 01ndash 05 1113088m in diameter They contain digestive (hydrolytic) enzymes which must be kept separateFigure 121 Lysosomes (orange) in a mouse kidney cell (1113088 55 000) They contain cell structures in the process of digestion and vesicles (green) Cytoplasm is coloured blue here1661 1 Cell structurefrom the rest of the cell to prevent damage Lysosomes are responsible for the breakdown (digestion) of unwanted structures such as old organelles or even whole cells as in mammary glands after lactation (breast feeding) In white blood cells lysosomes are used to digest bacteria (see endocytosis page 80) Enzymes are sometimes released outside the cell ndash for example in the replacement of cartilage with bone during development The headsof sperm contain a special lysosome the acrosome for digesting a path to the ovum (egg)MitochondriaMitochondria (singular mitochondrion) are usually about 1 1113088m in diameter and can be various shapes often sausage- shaped as in Figure 122 They are surrounded by two membranes (an envelope) The inner of these is foldedto form finger-like cristae which project into the interior solution or matrixThe main function of mitochondria is to carry out aerobic respiration As a result of respiration they make ATP the universal energy carrier in cells (see Chapter 16) They are also involved in the synthesis of lipids (page 37)Figure 122 Mitochondrion (orange) with its double membrane (envelope) the inner membrane is folded to form cristae (1113088 20 000) Mitochondria are the sites of aerobic cell respiration Note also the rough ERIn the 1960s it was discovered that mitochondria and chloroplasts contain ribosomes which are slightly smaller than those in the cytoplasm and are the same size as those found in bacteria The size of ribosomes is measured in lsquoS unitsrsquo which are a measure of how fast they sediment in a centrifuge Cytoplasmic ribosomes are 80S while those of bacteria mitochondria and chloroplasts are 70S It was also discovered in the 1960s that mitochondria and chloroplasts contain small circular DNA molecules also like those found in bacteria Not surprisingly it was later provedthat mitochondria and chloroplasts are in effect ancient bacteria which now live inside the larger cells typical of animals and plants (see

prokaryotic and eukaryotic cells page 18) This is known as the endosymbiont theory lsquoEndorsquo means lsquoinsidersquo and a lsquosymbiontrsquo is an organism which lives in a mutually beneficial relationship with another organism The DNA and ribosomes of mitochondria and chloroplasts are still active and responsible for the coding and synthesis of certain vital proteins but mitochondria and chloroplasts can no longer live independentlyMitochondrial ribosomes are just visible as tiny dark dots in the mitochondrial matrix in Figure 122Cell surface membraneThe cell surface membrane is extremely thin (about 7 nm) However at very high magnifications at least 1113088 100 000 it can be seen to have three layers described as a trilaminar appearance This consists of two dark lines (heavily stained) either side of a narrow pale interior (Figure 123) The membrane is partially permeable and controls exchange between the cell and its environment Membrane structure is discussed further in Chapter 4Figure 123 Cell surface membrane (1113088 250 000) At this magnification the membrane appears as two dark lines at the edge of the cellMicrovilliMicrovilli (singular microvillus) are finger-like extensions of the cell surface membrane typical of certain epithelial cells (cells covering surfaces of structures) They greatly1 Cell structure 17increase the surface area of the cell surface membrane (see Figure 117 on page 15) This is useful for example for absorption in the gut and for reabsorption in the proximal convoluted tubules of the kidney (see page 307)CentriolesThe extra resolution of the electron microscope reveals that just outside the nucleus there are really two centrioles (see Figure 124) not one as it appears under the light microscope (compare with Figure 13 on page 2) They lie close together at right-angles to each other A centriole is a hollow cylinder about 04 1113088m long formed from a ring of short microtubules tiny tubes made of a protein called tubulin These microtubules are used as a starting point for growing the spindle microtubules for nuclear division (see page 92) Centrioles are not found in plant cellsUltrastructure of a plant cellAll the structures except centrioles and microvilli so far described in animal cells are also found in plant cells The appearance of a plant cell as seen with the electron microscope is shown in Figure 125 while Figure 126 is a diagram based on many such micrographs The relatively thick cell wall and the large central vacuole are obvious as are the chloroplasts two of which are shown in detail in Figure 127 These structures and their functions have been described on pages 5 and 6 The electron microscopeFigure 124 Centrioles in transverse and longitudinal section (TS and LS) (1113088 86 000) The one on the left is seen in TS and clearly shows the nine triplets of microtubules which make up the structurereveals that chloroplasts contain 70S ribosomes and small circular

DNA molecules as do mitochondria and bacteria This has already been discussed with mitochondria above (page 17)SAQ 15Compare Figure 126 with Figure 15 on page 3 Name the structures which can be seen with the electron microscope but not with the light microscopeTwo fundamentally different types of cellAt one time it was common practice to try to classify all living organisms as either animals or plants With advances in our knowledge of living things it has become obvious that the living world is not that simple Fungi and bacteria for example are very different from animals and plants and from each other Eventually it was discovered that there are two fundamentally different types of cell The most obvious difference betweenthese types is that one possesses a nucleus and the other does notOrganisms that lack nuclei are called prokaryotes (lsquoprorsquo means before lsquokaryonrsquo means nucleus) All prokaryotes are now referred to as bacteria They are on average about 1000 to 10 000 times smaller in volume than cells with nuclei and are much simpler in structure ndash for example their DNA lies free in the cytoplasmOrganisms whose cells possess nuclei are called eukaryotes (lsquoeursquo means true) Their DNA lies inside a nucleus Eukaryotes include animals plants fungi and a group containing most of the unicellular eukaryotes known as protoctists Most biologists believe that eukaryotes evolved from prokaryotes one-and-a-half thousand million years after prokaryotes first appeared on Earth We mainly study animals and plants in this book but all eukaryotic cells have certain features in commonA generalised prokaryotic cell is shown in Figure 128 A comparison of prokaryotic and eukaryotic cells is given in Table 12 on page 211881 1 Cell structure

bull pencil (HB) bull pencil sharpener bull eraser bull ruler bull plain paperHere are some guidelines for the quality of your drawingbull always use a pencil not a pen bull donrsquot use shading bull use clear continuous lines bull use accurate proportions and observation ndash not atextbook version For a low-power drawing (see Figure 17)bull donrsquot draw individual cells bull draw all tissues completely enclosed by lines bull draw a correct interpretation of the distribution of bull tissuesa representative portion may be drawn (eg half a transverse section)For a high-power drawingbull draw only a few representative cells bull draw the cell wall of all plant cells bull donrsquot draw the nucleus as a solid blobSome guidelines for the quality of your labellingbull label all tissues and relevant structures bull identify parts correctly bull use a ruler for label linesAnimal and plant cells have features in commonIn animals and plants each cell is surrounded by a very thin cell surface membrane which is too thin to be seen with a light microscope This is also sometimes referred to as the plasma membranebull arrange label lines neatly and ensure they donrsquot cross bull over each otherannotate your drawing if necessary (ie provideshort notes with one or more of the labels in order bull to describe or explain features of biological interest)add a scale line at the bottom of the drawing if bull appropriateuse a pencil not a penAn example of a drawing of a section through the stem of Helianthus is shown below Biological drawing is also covered in Chapter 15 page 264Figure 17 The right side of this low-power drawing shows examples of good technique while the left side shows many of the pitfalls you should avoidMany of the cell contents are colourless and transparent so they need to be stained to be seen Each cell has a nucleus which is a relatively large structure that stains intensely and is therefore very conspicuous The deeply staining material in the nucleus is called chromatin and is a massof loosely coiled threads This material collects together to form visible separate chromosomes during nuclear division44 1 Cell structure(see page 86) It contains DNA (deoxyribonucleic acid) a molecule which contains the instructions that control the activities of the cell (see Chapter 6) Within the nucleus an even more deeply staining area is visible the nucleolus which is made of loops of DNA from several chromosomes The number of nucleoli is variable one to five being

common in mammalsThe material between the nucleus and the cell surface membrane is known as cytoplasm Cytoplasm is an aqueous (watery) material varying from a fluid to a jelly-like consistency Many small structures can be seen within it These have been likened to small organs and hence are known as organelles An organelle can be defined as a functionally and structurally distinct part of a cell Organelles themselves are often surrounded by membranes so that their activities can be separated from the surrounding cytoplasm This is described as compartmentalisation Having separate compartments is essential for a structure as complex as an animal or plant cell to work efficiently Since each type of organelle has its own function the cell is said to show division of labour a sharing of the work between different specialised organellesThe most numerous organelles seen with the light microscope are usually mitochondria (singular mitochondrion) Mitochondria are only just visible but films of living cells taken with the aid of a light microscope have shown that they can move about change shape and divide They are specialised to carry out aerobic respirationThe use of special stains containing silver enabled the Golgi apparatus to be detected for the first time in 1898 by Camillo Golgi The Golgi apparatus is part of a complex internal sorting and distribution system within the cell (see page 16) It is also sometimes called the Golgi body or Golgi complexDifferences between animal and plant cellsThe only structure commonly found in animal cells which is absent from plant cells is the centriole Plant cells also differ from animal cells in possessing cell walls large permanent vacuoles and chloroplastsCentriolesUnder the light microscope the centriole appears as a small structure close to the nucleus (see Figure 13 on page 2) The centriole is involved in nuclear division (see page 92)Cell walls and plasmodesmataWith a light microscope individual plant cells are more easily seen than animal cells because they are usually larger and unlike animal cells surrounded by a cell wall outside the cell surface membrane This is relatively rigid because it contains fibres of cellulose a polysaccharide which strengthens the wall The cell wall gives the cell a definite shape It prevents the cell from bursting when water enters by osmosis allowing large pressures to develop inside the cell (see page 77) Cell walls may also be reinforced with extra cellulose or with a hard material called lignin for extra strength (see xylem on page 24) Cell walls are freely permeable allowing free movement of molecules and ions through to the cell surface membranePlant cells are linked to neighbouring cells by means of fine strands of cytoplasm called plasmodesmata (singular plasmodesma) which pass through pore-like structures in the walls of these neighbouring cells Movement through the pores is thought to be controlled by the structure of the poresVacuoles

Although animal cells may possess small vacuoles such as phagocytic vacuoles (see page 80) which are temporary structures mature plant cells often possess a large permanent central vacuole The plant vacuole is surrounded by a membrane the tonoplast which controls exchange between the vacuole and the cytoplasm The fluid in the vacuole is a solution of mineral salts sugars oxygen carbon dioxide pigments enzymes and other organic compounds including some waste productsVacuoles help to regulate the osmotic properties of cells (the flow of water inwards and outwards) as well as having a wide range of other functions For example the pigments which colour the petals of certain flowers and parts of some vegetables such as the red pigment of beetroots are sometimes located in vacuoles1 Cell structure 5ChloroplastsSome plant cells are able to carry out photosynthesis because they contain chloroplasts Chloroplasts are relatively large organelles which are green in colour due to the presence of chlorophyll At high magnifications small lsquograinsrsquo or grana (singular granum) can be seen in the chloroplasts During the process of photosynthesis light is absorbed by these grana which actually consist of stacks of membrane-bound sacs called thylakoids Starch grains may also be visible within chloroplasts Chloroplasts are found in the green parts of plants mainly in the leavesPoints to noteWe return to the differences between animal and plant cells as seen using the electron microscope on page 18Units of measurement in cell studiesIn order to measure objects in the microscopic world we need to use very small units of measurement which are unfamiliar to most people According to international agreement the International System of Units (SI units) should be used In this system the basic unit of length is the metre (symbol m) Additional units can be created in multiples of a thousand times larger or smaller using standard prefixes For example the prefix kilo means 1000 times Thus 1 kilometre 1113088 1000 metres The units of length relevant to cell studies are shown in Table 11It is difficult to imagine how small these units are but when looking down a microscope and seeing cells clearly we should not forget how amazingly small the cells actually are The smallest structure visible with the human eye is about 50ndash100 1113088m in diameter Your body contains about 60 million million cells varying in size from about 5 1113088m to 40 1113088m Try to imagine structures like mitochondria which have an average diameter of 1 1113088m The smallest cell organelles we deal with in this book ribosomes are only about 25 nm in diameter You could line up about 20 000 ribosomes across the full stop at the end of this sentenceUnit Symbolmillimetre mm micrometre 1113088m nanometre nmTable 11 Units of measurement relevant to cell studies 1113088 is the Greek letter mu 1 micrometre is a thousandth of a millimetre 1 nanometre is a thousandth of a micrometre661 Cell structure

bull bull bullbullYou can think of a plant cell as being very similar to an animal cell but with extra structures Plant cells are often larger than animal cells although cell size varies enormouslyDo not confuse the cell wall with the cell surface membrane Cell walls are relatively thick and physically strong whereas cell surface membranes are very thin Cell walls are freely permeable whereas cell surface membranes are partially permeable All cells have a cell surface membrane Vacuoles are not confined to plant cells animal cells may have small vacuoles such as phagocytic vacuoles (see page 80) although these are not usually permanent structuresFraction of a metreone thousandth 1113088 0001 1113088 11000 1113088 10-3 one millionth 1113088 0000 001 1113088 11 000 000 1113088 10-6 one thousand millionth 1113088 0000 000 001 1113088 11 000 000 000 1113088 10-9

Box 1B Measuring cellsCells and organelles can be measured with a microscope by means of an eyepiece graticule This is a transparent scale It usually has 100 divisions (see Figure 18a) The eyepiece graticule is placed in the microscope eyepiece so that it can be seen at the same time as the object to be measured as shown in Figure 18b Figure 18b shows the scale over a human cheek epithelialcell The cell lies between 40 and 60 on the scale We therefore say it measures 20 eyepiece units in diameter (the difference between 60 and 40) We will not know the actual size of the eyepiece units until the eyepiece graticule scale is calibratedTo calibrate the eyepiece graticule scale a miniature transparent ruler called a stage micrometer scale is placed on the microscope stage and is brought into focus This scale may be etched onto a glass slideor printed on a transparent film It commonly has subdivisions of 01 and 001 mm The images of the two scales can then be superimposed as shown in Figure 18cIn the eyepiece graticule shown in the figure 100 units measure 025 mm Hence the value of each eyepiece unit is0 25 00025 mm 100Or converting mm to 1113088m 0 25 1000 2 51113088m100The diameter of the cell shown superimposed on the scale in Figure 18b measures 20 eyepiece units and so its actual diameter is20times251113088m 1113088 501113088m This diameter is greater than that of many human cellsbecause the cell is a flattened epithelial cellFigure 18 Microscopical measurement Three fields of view seen using a high-power (111308840) objective lens a An eyepiece graticule scale b Superimposed images of human cheek epithelial cells and the eyepiece graticule scale c Superimposed images of the eyepiece graticule scale and the stage micrometer scaleacheek cells on a slide on the stage of the microscopebc0 1020304050607080901000 102030405060708090100eyepiece graticule scale (arbitrary units)

eyepiece graticule in the eyepiece of the microscope00stage micrometer scale (marked in 00 1mm and 01 mm divisions)10 20 30 40 50 60 70 80 90 10001 021 Cell structure 7

Electron microscopesEarlier in this chapter it was stated that by 1900 almost all the structures shown in Figures 13 and 15 (pages 2 and 3) had been discovered There followed a time of frustration for microscopists because they realised that no matter how much the design of light microscopes improved there was a limit to how much could ever be seen using lightIn order to understand the problem it is necessary to know something about the nature of light itself and to understand the difference between magnification and resolutionMagnificationMagnification is the number of times larger an image is compared with the real size of the objectWorked example 1 ndash calculating the magnification of a photograph or objectTo calculate M the magnification of a photograph or an object we can use the following methodFigure 19 shows two photographs of a section through the same plant cells The magnifications of the two photographs are the same Suppose we want to know the magnification of the plant cell in Figure 19b If we know its actual (real) length we can calculate its magnification using theThe real length of the cell is 80 1113088mmagnification observed size of the image actual sizeM I AStep 1Measure the length in mm of the cell in the photograph using a ruler You should find that it is about 60 mmStep 2Convert mm to 1113088m (It is easier if we first convert all measurements to the same units ndash in this case micrometres 1113088m)1 mm 1113088 1000 1113088m so 60mm 1113088 60 1113088 10001113088morwhere I 1113088 observed size of the image (that is what you can measure with a ruler) and A 1113088 actual size (that is the real size ndash for example the size of a cell before it is magnified)If you know two of these values you can work out the third one For example if the observed size of the image and the magnification are known you can work out the1113088 60 000 1113088m Use the equation to calculate the magnificationStep 3actual size A I If you write the formula in a triangle Mmagnification Mimage size I

as shown below and cover up the value you want to find it should be obvious how to do the right calculationactual size A 60000m80 m 750I M1113088ASome worked examples are now providedThe lsquo1113088rsquo sign in front of the number 750 means lsquotimesrsquo We say that the magnification is lsquotimes 750rsquoformula M I A88 1 Cell structurea Worked example 2 ndash calculating magnification from a scale barFigure 110 shows a lymphocyteFigure 110 A lymphocyte6 μmbFigure 19 Photographs of the same plant cells seen a with a light microscope b with an electron microscope both shown at a magnification of about 1113088 750We can calculate the magnification of the lymphocyte by simply using the scale bar All you need to do is measure the length of the scale bar and then substitute this and the length it represents into the equationStep 1Measure the scale bar Here it is 36 mmStep 2Convert mm to 1113088m 36mm 1113088 36 1113088 10001113088m 1113088 360001113088mStep 3Use the equation to calculate the magnificationmagnification M image size I actual size A36000m 6m60001 Cell structure 9Worked example 3 ndash calculating the real size of an object from its magnificationTo calculate A the real or actual size of an object we can use the following methodFigure 125 on page 19 shows a plant cell magnified 1113088 5600 One of the chloroplasts is labelled lsquochloroplastrsquo in the figure Suppose we want to know the actual length of this chloroplastStep 1Measure the observed length of the image of the chloroplast (I ) in mm using a ruler The maximum length is 36 mmStep 2Convert mm to 1113088m 30mm 1113088 30 1113088 10001113088m 1113088 300001113088mStep 3Use the equation to calculate the actual lengthactualsizeA imagesizeI magnification M30 000 m 5600 5 4 m (to one decimal place)SAQ 12a Calculate the magnification of the drawing of the animal cell in Figure 13 on page 2b Calculate the actual (real) length of the bottom chloroplast in Figure 127 on page 19Resolution

Look again at Figure 19 (page 9) Figure 19a is a light micrograph (a photograph taken with a light microscope also known as a photomicrograph) Figure 19b is an electron micrograph of the same cells taken at the same magnification (an electron micrograph is a picture taken with an electron microscope) You can see that Figure 19b the electron micrograph is much clearer This is because it has greater resolution Resolution is defined as the abilityto distinguish between two separate points If the two points cannot be resolved they will be seen as one point In practice resolution is the amount of detail that can be seen ndash the greater the resolution the greater the detailThe maximum resolution of a light microscope is 200 nm This means that if two points or objects are closer together than 200 nm they cannot be distinguished as separateIt is possible to take a photograph such as Figure 19a and to magnify (enlarge) it but we see no more detail in other words we do not improve resolution even though we often enlarge photographs because they are easier to see when larger With a microscope magnification up to the limit of resolution can reveal further detail but any further magnification increases blurring as well as the size of the imageThe electromagnetic spectrumHow is resolution linked with the nature of light One of the properties of light is that it travels in waves The length of the waves of visible light varies ranging from about 400 nm (violet light) to about 700 nm (red light) The human eye can distinguish between these different wavelengths and in the brain the differences are converted to colour differences (Colour is an invention of the brain)The whole range of different wavelengths is called the electromagnetic spectrum Visible light is only one part of this spectrum Figure 111 shows some of the parts of the electromagnetic spectrum The longer the waves the lower their frequency (all the waves travel at the same speed so imagine them passing a post shorter waves pass at higher frequency) In theory there is no limit to how short or how long the waves can be Wavelength changes with energy the greater the energy the shorter the wavelength (rather like squashing a spring)Now look at Figure 112 which shows a mitochondrion some very small cell organelles called ribosomes (see page 13) and light of 400 nm wavelength the shortest visible wavelength The mitochondrion is large enoughto interfere with the light waves However the ribosomes are far too small to have any effect on the light waves The general rule is that the limit of resolution is about one half the wavelength of the radiation used to view the specimen In other words if an object is any smaller than half the wavelength of the radiation used to view it it cannot be seen separately from nearby objects This means that the1001 1 Cell structureX-raysgamma rays01 nm400 nm violet bluev infraredmicrowaves

uv10 nmi s i b l e 1000 nmradio and TV wavesvisible light500 nm 600 nm green yellow orange105 nm 107 nm 109 nm 1011 nm 1013 nm700 nmFigure 111 Diagram of the electromagnetic spectrum (the waves are not drawn to scale) The numbers indicate the wavelengths of the different types of electromagnetic radiation Visible light is a form of electromagnetic radiationbest resolution that can be obtained using a microscope that uses visible light (a light microscope) is 200 nm since the shortest wavelength of visible light is 400 nm (violet light) In practice this corresponds to a maximum useful magnification of about 1500 times Ribosomes are approximately 25 nm in diameter and can therefore never be seen using lightIf an object is transparent it will allow light waves to pass through it and therefore will still not be visible This is why many biological structures have to be stained before they can be seenThe electron microscopeBiologists faced with the problem that they would never see anything smaller than 200 nm using a light microscope realised that the only solution would be to use radiation of a shorter wavelength than light If you study Figure 111 you will see that ultraviolet light or better still X-rays look like possible candidates Both ultraviolet and X-ray microscopes have been built the latter with little success partly because of the difficulty of focusing X-rays A much better solution is to use electrons Electrons are negatively charged particles which orbit the nucleus of an atom When a metal becomes very hot some of its electrons gain so much energy that they escape from their orbits like a rocket escaping from Earthrsquos gravity Free electrons behave like electromagnetic radiation They have a very short wavelength the greater the energy the shorter the wavelength Electrons are a very suitable form of radiation for microscopy for two major reasons Firstly their wavelength is extremely short (at least as short as that of X-rays) Secondly because they are negatively charged they can be focused easily using electromagnets (a magnet can be made to alter the path of the beam the equivalent of a glass lens bending light)Using an electron microscope a resolution of 05 nm can be obtained

400 times better than when using a light microscope ETransmission and scanning electron microscopesTwo types of electron microscope are now in common use The transmission electron microscope or TEM forwavelength 400nmstained mitochondrion of diameter 1000nm interferes with light wavesstained ribosomes of diameter 25nm do not interfere with light wavesFigure 112 A mitochondrion and some ribosomes in the path of light waves of 400 nm lengthred1 Cell structure 11

E short was the type originally developed Here the beam of electrons is passed through the specimen before being viewed Only

those electrons that are transmitted (pass through the specimen) are seen This allows us to see thin sections of specimens and thus to see inside cells In the scanning electron microscope (SEM) on the other hand the electron beam is used to scan the surfaces of structures and only the reflected beam is observedAn example of a scanning electron micrograph is shown in Figure 113 The advantage of this microscope is that surface structures can be seen Also great depth of field is obtained so that much of the specimen is in focus at the same time and a three-dimensional appearance is obtained Such a picture would be impossible to obtain with a light microscope even using the same magnification and resolution because you would have to keep focusing up and down with the objective lens to see different parts of the specimen The disadvantage of the SEM is that it cannot achieve the same resolution as a TEM Resolution is between 3 nm and 20 nmViewing specimens with the electron microscopeFigure 114 shows how an electron microscope works and Figure 115 shows one in useFigure 113 False-colour SEM of the head of a cat flea (1113088 100)

electron gun and anode which E produce a beam of electronselectron beam vacuumpathway of electronscondenser electromagnetic lens which directs the electron beam onto the specimenspecimen which is placed on a gridobjective electromagnetic lens which produces an imageprojector electromagnetic lenses which focus the magnified image onto the screenscreen or photographic plate which shows the image of the specimenFigure 114 How an electron microscope worksIt is not possible to see an electron beam so to make the image visible the electron beam has to be projected onto a fluorescent screen The areas hit by electrons shine brightly giving overall a lsquoblack and whitersquo picture The stains used to improve the contrast of biological specimens for electron microscopy contain heavy metal atoms which stop the passage of electrons The resulting picture is like an X-ray photograph with the more densely stained parts of the specimen appearing blacker lsquoFalse-colourrsquo images can be created by colouring the standard black and white image using a computerTo add to the difficulties of electron microscopy the electron beam and therefore the specimen and the fluorescent screen must be in a vacuum If electrons1221 1 Cell structure

EStructures and functions of organellesCompartmentalisation and division of labour within the cell are even more obvious with an electron microscope than with a light microscopeWe will now consider the structures and functions of some of the cell components in more detailNucleusThe nucleus (Figure 118 on page 15) is the largest cell organelle (see also page 5) It is surrounded by two membranes known as the

nuclear envelope The outer membrane of the nuclear envelope is continuous with the endoplasmic reticulum (Figure 117 on page 15) The nuclear envelope has many small pores called nuclear pores These allow and control exchange between the nucleus and the cytoplasm Examples of substances leaving the nucleus through the pores are mRNA and ribosomes for protein synthesis Examples of substances entering through the nuclear pores are proteins to help make ribosomes nucleotides ATP (aderosine triphosphate) and some hormones such as thyroid hormone T3Within the nucleus the chromosomes are in a loosely coiled state known as chromatin (except during nuclear division see Chapter 5) Chromosomes contain DNA which is organised into functional units called genes Genes control the activities of the cell and inheritance thus the nucleus controls the cellrsquos activities When a cell is aboutto divide the nucleus divides first so that each new cell will have its own nucleus (Chapters 5 and 19) Also within the nucleus the nucleolus makes ribosomes using the information in its own DNAEndoplasmic reticulum and ribosomesWhen cells were first seen with the electron microscope biologists were amazed to see so much detailed structure The existence of much of this had not been suspected This was particularly true of an extensive system of membranes running through the cytoplasm which became knownas the endoplasmic reticulum (ER) (Figure 119 on page 15 ndash see also Figures 118 on page 15 and 122 on page 17) The ER is continuous with the outer membrane of the nuclear envelope (Figure 117)There are two types of ER rough ER and smooth ER Rough ER is so called because it is covered with many tinyFigure 115 A TEM in usecollided with air molecules they would scatter making it impossible to achieve a sharp picture Also water boils at room temperature in a vacuum so all specimens must be dehydrated before being placed in the microscope This means that only dead material can be examined Great efforts are therefore made to try to preserve material in a life-like state when preparing it for the microscopeSAQ 13Explain why ribosomes are not visible using a light microscopeUltrastructure of an animal cellThe lsquofinersquo or detailed structure of a cell as revealed by the electron microscope is called its ultrastructure Figure 116 shows the appearance of typical animal cells as seen with an electron microscope and Figure 117 on page 15 is a diagram based on many other such micrographsSAQ 14Compare Figure 117 on page 15 with Figure 13 on page 2 Name the structures which can be seen with the electron microscope but not with the light microscope1 Cell structure 13lysosomeGGolgi apparatusendoplasmic reticulumglycogen granulesccell surface membrane

mmitochondriachromatinnnucleolusmmicrovillusrribosomesFFigure 116 Representative animal cells as seen with a TEM The cells are liver cells from a rat (1113088 9600) The nucleus is clearly visible in one of the cells 1441 1 Cell structurenucleusnnuclear envelopetwo centrioles close to the nucleus and at right angles to each othermitochondrion lysosomerough endoplasmic reticulumnucleoluschromatin nucleusnuclear porenuclear envelope (two membranes)microvillismooth endoplasmic reticulumFigure 117 Ultrastructure of a typical animal cell as seen with an electron microscope In reality the ER is more extensive than shown and free ribosomes may be more extensive Glycogen granules are sometimes present in the cytoplasmFigure 118 TEM of the nucleus of a cell from the pancreas of a bat (1113088 7500) The circular nucleus is surrounded by a double-layered nuclear envelope containing nuclear pores The nucleolus is more Figure 119 TEM of rough ER covered with ribosomes (black dots) darkly stained Rough ER is visible in the surrounding cytoplasm (1113088 17 000) Some free ribosomes can also be seen in the cytoplasmGolgi vesicle Golgi apparatusribosomescell surface membranecytoplasm1 Cell structure 15organelles called ribosomes These are just visible as black dots in Figures 118 and 119 on page 15 At very high magnifications they can be seen to consist of two subunits a large and a small subunit Ribosomes are the sites of protein synthesis (see pages 111ndash112) They can be found free in the cytoplasm as well as on the rough ER They are very small only about 25 nm in diameter They are made of RNA (ribonucleic acid) and protein The rough ER forms an extensive system of flattened sacs spreading in sheets throughout the cell Proteins made by the ribosomes on the rough ER enter the sacs and move through them The proteins are often processed in some way on their journey Small sacs called vesicles can break off from the ER and these can join together to form the Golgi apparatus Proteins can be exported from the cell via the Golgi apparatus (see page 80)Smooth ER so called because it lacks ribosomes has a completely different function It makes lipids and steroids such as cholesterol and the reproductive hormones oestrogen and testosteroneGolgi apparatus (Golgi body or Golgi complex)The Golgi apparatus is a stack of flattened sacs (Figure 120) This stack of sacs is sometimes referred to as theFigure 120 TEM of a Golgi apparatus A central stack of saucer-shaped sacs can be seen budding off small Golgi vesicles (green) These may form secretory vesicles whose contents can be released at the cell surface by exocytosis (see page 80)Golgi body More than one may be present in a cell The stack is constantly being formed at one end from vesicles which bud off from the ER and broken down again at the other end to form Golgi vesicles The stack of sacs with the associated vesicles is referred to as the Golgi apparatus or Golgi complex

The Golgi apparatus collects processes and sorts molecules (particularly proteins from the rough ER) ready for transport in Golgi vesicles either to other parts of the cell or out of the cell (secretion) Two examples of protein processing in the Golgi apparatus are the addition of sugars to proteins to make molecules known as glycoproteins and the removal of the first amino acid methionine from newly formed proteins to make a functioning protein In plants enzymes in the Golgi apparatus convert sugars into cell wall components Golgi vesicles are also used to make lysosomesLysosomesLysosomes (Figure 121) are spherical sacs surrounded by a single membrane and having no internal structure They are commonly 01ndash 05 1113088m in diameter They contain digestive (hydrolytic) enzymes which must be kept separateFigure 121 Lysosomes (orange) in a mouse kidney cell (1113088 55 000) They contain cell structures in the process of digestion and vesicles (green) Cytoplasm is coloured blue here1661 1 Cell structurefrom the rest of the cell to prevent damage Lysosomes are responsible for the breakdown (digestion) of unwanted structures such as old organelles or even whole cells as in mammary glands after lactation (breast feeding) In white blood cells lysosomes are used to digest bacteria (see endocytosis page 80) Enzymes are sometimes released outside the cell ndash for example in the replacement of cartilage with bone during development The headsof sperm contain a special lysosome the acrosome for digesting a path to the ovum (egg)MitochondriaMitochondria (singular mitochondrion) are usually about 1 1113088m in diameter and can be various shapes often sausage- shaped as in Figure 122 They are surrounded by two membranes (an envelope) The inner of these is foldedto form finger-like cristae which project into the interior solution or matrixThe main function of mitochondria is to carry out aerobic respiration As a result of respiration they make ATP the universal energy carrier in cells (see Chapter 16) They are also involved in the synthesis of lipids (page 37)Figure 122 Mitochondrion (orange) with its double membrane (envelope) the inner membrane is folded to form cristae (1113088 20 000) Mitochondria are the sites of aerobic cell respiration Note also the rough ERIn the 1960s it was discovered that mitochondria and chloroplasts contain ribosomes which are slightly smaller than those in the cytoplasm and are the same size as those found in bacteria The size of ribosomes is measured in lsquoS unitsrsquo which are a measure of how fast they sediment in a centrifuge Cytoplasmic ribosomes are 80S while those of bacteria mitochondria and chloroplasts are 70S It was also discovered in the 1960s that mitochondria and chloroplasts contain small circular DNA molecules also like those found in bacteria Not surprisingly it was later provedthat mitochondria and chloroplasts are in effect ancient bacteria which now live inside the larger cells typical of animals and plants (see

prokaryotic and eukaryotic cells page 18) This is known as the endosymbiont theory lsquoEndorsquo means lsquoinsidersquo and a lsquosymbiontrsquo is an organism which lives in a mutually beneficial relationship with another organism The DNA and ribosomes of mitochondria and chloroplasts are still active and responsible for the coding and synthesis of certain vital proteins but mitochondria and chloroplasts can no longer live independentlyMitochondrial ribosomes are just visible as tiny dark dots in the mitochondrial matrix in Figure 122Cell surface membraneThe cell surface membrane is extremely thin (about 7 nm) However at very high magnifications at least 1113088 100 000 it can be seen to have three layers described as a trilaminar appearance This consists of two dark lines (heavily stained) either side of a narrow pale interior (Figure 123) The membrane is partially permeable and controls exchange between the cell and its environment Membrane structure is discussed further in Chapter 4Figure 123 Cell surface membrane (1113088 250 000) At this magnification the membrane appears as two dark lines at the edge of the cellMicrovilliMicrovilli (singular microvillus) are finger-like extensions of the cell surface membrane typical of certain epithelial cells (cells covering surfaces of structures) They greatly1 Cell structure 17increase the surface area of the cell surface membrane (see Figure 117 on page 15) This is useful for example for absorption in the gut and for reabsorption in the proximal convoluted tubules of the kidney (see page 307)CentriolesThe extra resolution of the electron microscope reveals that just outside the nucleus there are really two centrioles (see Figure 124) not one as it appears under the light microscope (compare with Figure 13 on page 2) They lie close together at right-angles to each other A centriole is a hollow cylinder about 04 1113088m long formed from a ring of short microtubules tiny tubes made of a protein called tubulin These microtubules are used as a starting point for growing the spindle microtubules for nuclear division (see page 92) Centrioles are not found in plant cellsUltrastructure of a plant cellAll the structures except centrioles and microvilli so far described in animal cells are also found in plant cells The appearance of a plant cell as seen with the electron microscope is shown in Figure 125 while Figure 126 is a diagram based on many such micrographs The relatively thick cell wall and the large central vacuole are obvious as are the chloroplasts two of which are shown in detail in Figure 127 These structures and their functions have been described on pages 5 and 6 The electron microscopeFigure 124 Centrioles in transverse and longitudinal section (TS and LS) (1113088 86 000) The one on the left is seen in TS and clearly shows the nine triplets of microtubules which make up the structurereveals that chloroplasts contain 70S ribosomes and small circular

DNA molecules as do mitochondria and bacteria This has already been discussed with mitochondria above (page 17)SAQ 15Compare Figure 126 with Figure 15 on page 3 Name the structures which can be seen with the electron microscope but not with the light microscopeTwo fundamentally different types of cellAt one time it was common practice to try to classify all living organisms as either animals or plants With advances in our knowledge of living things it has become obvious that the living world is not that simple Fungi and bacteria for example are very different from animals and plants and from each other Eventually it was discovered that there are two fundamentally different types of cell The most obvious difference betweenthese types is that one possesses a nucleus and the other does notOrganisms that lack nuclei are called prokaryotes (lsquoprorsquo means before lsquokaryonrsquo means nucleus) All prokaryotes are now referred to as bacteria They are on average about 1000 to 10 000 times smaller in volume than cells with nuclei and are much simpler in structure ndash for example their DNA lies free in the cytoplasmOrganisms whose cells possess nuclei are called eukaryotes (lsquoeursquo means true) Their DNA lies inside a nucleus Eukaryotes include animals plants fungi and a group containing most of the unicellular eukaryotes known as protoctists Most biologists believe that eukaryotes evolved from prokaryotes one-and-a-half thousand million years after prokaryotes first appeared on Earth We mainly study animals and plants in this book but all eukaryotic cells have certain features in commonA generalised prokaryotic cell is shown in Figure 128 A comparison of prokaryotic and eukaryotic cells is given in Table 12 on page 211881 1 Cell structure

common in mammalsThe material between the nucleus and the cell surface membrane is known as cytoplasm Cytoplasm is an aqueous (watery) material varying from a fluid to a jelly-like consistency Many small structures can be seen within it These have been likened to small organs and hence are known as organelles An organelle can be defined as a functionally and structurally distinct part of a cell Organelles themselves are often surrounded by membranes so that their activities can be separated from the surrounding cytoplasm This is described as compartmentalisation Having separate compartments is essential for a structure as complex as an animal or plant cell to work efficiently Since each type of organelle has its own function the cell is said to show division of labour a sharing of the work between different specialised organellesThe most numerous organelles seen with the light microscope are usually mitochondria (singular mitochondrion) Mitochondria are only just visible but films of living cells taken with the aid of a light microscope have shown that they can move about change shape and divide They are specialised to carry out aerobic respirationThe use of special stains containing silver enabled the Golgi apparatus to be detected for the first time in 1898 by Camillo Golgi The Golgi apparatus is part of a complex internal sorting and distribution system within the cell (see page 16) It is also sometimes called the Golgi body or Golgi complexDifferences between animal and plant cellsThe only structure commonly found in animal cells which is absent from plant cells is the centriole Plant cells also differ from animal cells in possessing cell walls large permanent vacuoles and chloroplastsCentriolesUnder the light microscope the centriole appears as a small structure close to the nucleus (see Figure 13 on page 2) The centriole is involved in nuclear division (see page 92)Cell walls and plasmodesmataWith a light microscope individual plant cells are more easily seen than animal cells because they are usually larger and unlike animal cells surrounded by a cell wall outside the cell surface membrane This is relatively rigid because it contains fibres of cellulose a polysaccharide which strengthens the wall The cell wall gives the cell a definite shape It prevents the cell from bursting when water enters by osmosis allowing large pressures to develop inside the cell (see page 77) Cell walls may also be reinforced with extra cellulose or with a hard material called lignin for extra strength (see xylem on page 24) Cell walls are freely permeable allowing free movement of molecules and ions through to the cell surface membranePlant cells are linked to neighbouring cells by means of fine strands of cytoplasm called plasmodesmata (singular plasmodesma) which pass through pore-like structures in the walls of these neighbouring cells Movement through the pores is thought to be controlled by the structure of the poresVacuoles

Although animal cells may possess small vacuoles such as phagocytic vacuoles (see page 80) which are temporary structures mature plant cells often possess a large permanent central vacuole The plant vacuole is surrounded by a membrane the tonoplast which controls exchange between the vacuole and the cytoplasm The fluid in the vacuole is a solution of mineral salts sugars oxygen carbon dioxide pigments enzymes and other organic compounds including some waste productsVacuoles help to regulate the osmotic properties of cells (the flow of water inwards and outwards) as well as having a wide range of other functions For example the pigments which colour the petals of certain flowers and parts of some vegetables such as the red pigment of beetroots are sometimes located in vacuoles1 Cell structure 5ChloroplastsSome plant cells are able to carry out photosynthesis because they contain chloroplasts Chloroplasts are relatively large organelles which are green in colour due to the presence of chlorophyll At high magnifications small lsquograinsrsquo or grana (singular granum) can be seen in the chloroplasts During the process of photosynthesis light is absorbed by these grana which actually consist of stacks of membrane-bound sacs called thylakoids Starch grains may also be visible within chloroplasts Chloroplasts are found in the green parts of plants mainly in the leavesPoints to noteWe return to the differences between animal and plant cells as seen using the electron microscope on page 18Units of measurement in cell studiesIn order to measure objects in the microscopic world we need to use very small units of measurement which are unfamiliar to most people According to international agreement the International System of Units (SI units) should be used In this system the basic unit of length is the metre (symbol m) Additional units can be created in multiples of a thousand times larger or smaller using standard prefixes For example the prefix kilo means 1000 times Thus 1 kilometre 1113088 1000 metres The units of length relevant to cell studies are shown in Table 11It is difficult to imagine how small these units are but when looking down a microscope and seeing cells clearly we should not forget how amazingly small the cells actually are The smallest structure visible with the human eye is about 50ndash100 1113088m in diameter Your body contains about 60 million million cells varying in size from about 5 1113088m to 40 1113088m Try to imagine structures like mitochondria which have an average diameter of 1 1113088m The smallest cell organelles we deal with in this book ribosomes are only about 25 nm in diameter You could line up about 20 000 ribosomes across the full stop at the end of this sentenceUnit Symbolmillimetre mm micrometre 1113088m nanometre nmTable 11 Units of measurement relevant to cell studies 1113088 is the Greek letter mu 1 micrometre is a thousandth of a millimetre 1 nanometre is a thousandth of a micrometre661 Cell structure

bull bull bullbullYou can think of a plant cell as being very similar to an animal cell but with extra structures Plant cells are often larger than animal cells although cell size varies enormouslyDo not confuse the cell wall with the cell surface membrane Cell walls are relatively thick and physically strong whereas cell surface membranes are very thin Cell walls are freely permeable whereas cell surface membranes are partially permeable All cells have a cell surface membrane Vacuoles are not confined to plant cells animal cells may have small vacuoles such as phagocytic vacuoles (see page 80) although these are not usually permanent structuresFraction of a metreone thousandth 1113088 0001 1113088 11000 1113088 10-3 one millionth 1113088 0000 001 1113088 11 000 000 1113088 10-6 one thousand millionth 1113088 0000 000 001 1113088 11 000 000 000 1113088 10-9

Box 1B Measuring cellsCells and organelles can be measured with a microscope by means of an eyepiece graticule This is a transparent scale It usually has 100 divisions (see Figure 18a) The eyepiece graticule is placed in the microscope eyepiece so that it can be seen at the same time as the object to be measured as shown in Figure 18b Figure 18b shows the scale over a human cheek epithelialcell The cell lies between 40 and 60 on the scale We therefore say it measures 20 eyepiece units in diameter (the difference between 60 and 40) We will not know the actual size of the eyepiece units until the eyepiece graticule scale is calibratedTo calibrate the eyepiece graticule scale a miniature transparent ruler called a stage micrometer scale is placed on the microscope stage and is brought into focus This scale may be etched onto a glass slideor printed on a transparent film It commonly has subdivisions of 01 and 001 mm The images of the two scales can then be superimposed as shown in Figure 18cIn the eyepiece graticule shown in the figure 100 units measure 025 mm Hence the value of each eyepiece unit is0 25 00025 mm 100Or converting mm to 1113088m 0 25 1000 2 51113088m100The diameter of the cell shown superimposed on the scale in Figure 18b measures 20 eyepiece units and so its actual diameter is20times251113088m 1113088 501113088m This diameter is greater than that of many human cellsbecause the cell is a flattened epithelial cellFigure 18 Microscopical measurement Three fields of view seen using a high-power (111308840) objective lens a An eyepiece graticule scale b Superimposed images of human cheek epithelial cells and the eyepiece graticule scale c Superimposed images of the eyepiece graticule scale and the stage micrometer scaleacheek cells on a slide on the stage of the microscopebc0 1020304050607080901000 102030405060708090100eyepiece graticule scale (arbitrary units)

eyepiece graticule in the eyepiece of the microscope00stage micrometer scale (marked in 00 1mm and 01 mm divisions)10 20 30 40 50 60 70 80 90 10001 021 Cell structure 7

Electron microscopesEarlier in this chapter it was stated that by 1900 almost all the structures shown in Figures 13 and 15 (pages 2 and 3) had been discovered There followed a time of frustration for microscopists because they realised that no matter how much the design of light microscopes improved there was a limit to how much could ever be seen using lightIn order to understand the problem it is necessary to know something about the nature of light itself and to understand the difference between magnification and resolutionMagnificationMagnification is the number of times larger an image is compared with the real size of the objectWorked example 1 ndash calculating the magnification of a photograph or objectTo calculate M the magnification of a photograph or an object we can use the following methodFigure 19 shows two photographs of a section through the same plant cells The magnifications of the two photographs are the same Suppose we want to know the magnification of the plant cell in Figure 19b If we know its actual (real) length we can calculate its magnification using theThe real length of the cell is 80 1113088mmagnification observed size of the image actual sizeM I AStep 1Measure the length in mm of the cell in the photograph using a ruler You should find that it is about 60 mmStep 2Convert mm to 1113088m (It is easier if we first convert all measurements to the same units ndash in this case micrometres 1113088m)1 mm 1113088 1000 1113088m so 60mm 1113088 60 1113088 10001113088morwhere I 1113088 observed size of the image (that is what you can measure with a ruler) and A 1113088 actual size (that is the real size ndash for example the size of a cell before it is magnified)If you know two of these values you can work out the third one For example if the observed size of the image and the magnification are known you can work out the1113088 60 000 1113088m Use the equation to calculate the magnificationStep 3actual size A I If you write the formula in a triangle Mmagnification Mimage size I

as shown below and cover up the value you want to find it should be obvious how to do the right calculationactual size A 60000m80 m 750I M1113088ASome worked examples are now providedThe lsquo1113088rsquo sign in front of the number 750 means lsquotimesrsquo We say that the magnification is lsquotimes 750rsquoformula M I A88 1 Cell structurea Worked example 2 ndash calculating magnification from a scale barFigure 110 shows a lymphocyteFigure 110 A lymphocyte6 μmbFigure 19 Photographs of the same plant cells seen a with a light microscope b with an electron microscope both shown at a magnification of about 1113088 750We can calculate the magnification of the lymphocyte by simply using the scale bar All you need to do is measure the length of the scale bar and then substitute this and the length it represents into the equationStep 1Measure the scale bar Here it is 36 mmStep 2Convert mm to 1113088m 36mm 1113088 36 1113088 10001113088m 1113088 360001113088mStep 3Use the equation to calculate the magnificationmagnification M image size I actual size A36000m 6m60001 Cell structure 9Worked example 3 ndash calculating the real size of an object from its magnificationTo calculate A the real or actual size of an object we can use the following methodFigure 125 on page 19 shows a plant cell magnified 1113088 5600 One of the chloroplasts is labelled lsquochloroplastrsquo in the figure Suppose we want to know the actual length of this chloroplastStep 1Measure the observed length of the image of the chloroplast (I ) in mm using a ruler The maximum length is 36 mmStep 2Convert mm to 1113088m 30mm 1113088 30 1113088 10001113088m 1113088 300001113088mStep 3Use the equation to calculate the actual lengthactualsizeA imagesizeI magnification M30 000 m 5600 5 4 m (to one decimal place)SAQ 12a Calculate the magnification of the drawing of the animal cell in Figure 13 on page 2b Calculate the actual (real) length of the bottom chloroplast in Figure 127 on page 19Resolution

Look again at Figure 19 (page 9) Figure 19a is a light micrograph (a photograph taken with a light microscope also known as a photomicrograph) Figure 19b is an electron micrograph of the same cells taken at the same magnification (an electron micrograph is a picture taken with an electron microscope) You can see that Figure 19b the electron micrograph is much clearer This is because it has greater resolution Resolution is defined as the abilityto distinguish between two separate points If the two points cannot be resolved they will be seen as one point In practice resolution is the amount of detail that can be seen ndash the greater the resolution the greater the detailThe maximum resolution of a light microscope is 200 nm This means that if two points or objects are closer together than 200 nm they cannot be distinguished as separateIt is possible to take a photograph such as Figure 19a and to magnify (enlarge) it but we see no more detail in other words we do not improve resolution even though we often enlarge photographs because they are easier to see when larger With a microscope magnification up to the limit of resolution can reveal further detail but any further magnification increases blurring as well as the size of the imageThe electromagnetic spectrumHow is resolution linked with the nature of light One of the properties of light is that it travels in waves The length of the waves of visible light varies ranging from about 400 nm (violet light) to about 700 nm (red light) The human eye can distinguish between these different wavelengths and in the brain the differences are converted to colour differences (Colour is an invention of the brain)The whole range of different wavelengths is called the electromagnetic spectrum Visible light is only one part of this spectrum Figure 111 shows some of the parts of the electromagnetic spectrum The longer the waves the lower their frequency (all the waves travel at the same speed so imagine them passing a post shorter waves pass at higher frequency) In theory there is no limit to how short or how long the waves can be Wavelength changes with energy the greater the energy the shorter the wavelength (rather like squashing a spring)Now look at Figure 112 which shows a mitochondrion some very small cell organelles called ribosomes (see page 13) and light of 400 nm wavelength the shortest visible wavelength The mitochondrion is large enoughto interfere with the light waves However the ribosomes are far too small to have any effect on the light waves The general rule is that the limit of resolution is about one half the wavelength of the radiation used to view the specimen In other words if an object is any smaller than half the wavelength of the radiation used to view it it cannot be seen separately from nearby objects This means that the1001 1 Cell structureX-raysgamma rays01 nm400 nm violet bluev infraredmicrowaves

uv10 nmi s i b l e 1000 nmradio and TV wavesvisible light500 nm 600 nm green yellow orange105 nm 107 nm 109 nm 1011 nm 1013 nm700 nmFigure 111 Diagram of the electromagnetic spectrum (the waves are not drawn to scale) The numbers indicate the wavelengths of the different types of electromagnetic radiation Visible light is a form of electromagnetic radiationbest resolution that can be obtained using a microscope that uses visible light (a light microscope) is 200 nm since the shortest wavelength of visible light is 400 nm (violet light) In practice this corresponds to a maximum useful magnification of about 1500 times Ribosomes are approximately 25 nm in diameter and can therefore never be seen using lightIf an object is transparent it will allow light waves to pass through it and therefore will still not be visible This is why many biological structures have to be stained before they can be seenThe electron microscopeBiologists faced with the problem that they would never see anything smaller than 200 nm using a light microscope realised that the only solution would be to use radiation of a shorter wavelength than light If you study Figure 111 you will see that ultraviolet light or better still X-rays look like possible candidates Both ultraviolet and X-ray microscopes have been built the latter with little success partly because of the difficulty of focusing X-rays A much better solution is to use electrons Electrons are negatively charged particles which orbit the nucleus of an atom When a metal becomes very hot some of its electrons gain so much energy that they escape from their orbits like a rocket escaping from Earthrsquos gravity Free electrons behave like electromagnetic radiation They have a very short wavelength the greater the energy the shorter the wavelength Electrons are a very suitable form of radiation for microscopy for two major reasons Firstly their wavelength is extremely short (at least as short as that of X-rays) Secondly because they are negatively charged they can be focused easily using electromagnets (a magnet can be made to alter the path of the beam the equivalent of a glass lens bending light)Using an electron microscope a resolution of 05 nm can be obtained

400 times better than when using a light microscope ETransmission and scanning electron microscopesTwo types of electron microscope are now in common use The transmission electron microscope or TEM forwavelength 400nmstained mitochondrion of diameter 1000nm interferes with light wavesstained ribosomes of diameter 25nm do not interfere with light wavesFigure 112 A mitochondrion and some ribosomes in the path of light waves of 400 nm lengthred1 Cell structure 11

E short was the type originally developed Here the beam of electrons is passed through the specimen before being viewed Only

those electrons that are transmitted (pass through the specimen) are seen This allows us to see thin sections of specimens and thus to see inside cells In the scanning electron microscope (SEM) on the other hand the electron beam is used to scan the surfaces of structures and only the reflected beam is observedAn example of a scanning electron micrograph is shown in Figure 113 The advantage of this microscope is that surface structures can be seen Also great depth of field is obtained so that much of the specimen is in focus at the same time and a three-dimensional appearance is obtained Such a picture would be impossible to obtain with a light microscope even using the same magnification and resolution because you would have to keep focusing up and down with the objective lens to see different parts of the specimen The disadvantage of the SEM is that it cannot achieve the same resolution as a TEM Resolution is between 3 nm and 20 nmViewing specimens with the electron microscopeFigure 114 shows how an electron microscope works and Figure 115 shows one in useFigure 113 False-colour SEM of the head of a cat flea (1113088 100)

electron gun and anode which E produce a beam of electronselectron beam vacuumpathway of electronscondenser electromagnetic lens which directs the electron beam onto the specimenspecimen which is placed on a gridobjective electromagnetic lens which produces an imageprojector electromagnetic lenses which focus the magnified image onto the screenscreen or photographic plate which shows the image of the specimenFigure 114 How an electron microscope worksIt is not possible to see an electron beam so to make the image visible the electron beam has to be projected onto a fluorescent screen The areas hit by electrons shine brightly giving overall a lsquoblack and whitersquo picture The stains used to improve the contrast of biological specimens for electron microscopy contain heavy metal atoms which stop the passage of electrons The resulting picture is like an X-ray photograph with the more densely stained parts of the specimen appearing blacker lsquoFalse-colourrsquo images can be created by colouring the standard black and white image using a computerTo add to the difficulties of electron microscopy the electron beam and therefore the specimen and the fluorescent screen must be in a vacuum If electrons1221 1 Cell structure

EStructures and functions of organellesCompartmentalisation and division of labour within the cell are even more obvious with an electron microscope than with a light microscopeWe will now consider the structures and functions of some of the cell components in more detailNucleusThe nucleus (Figure 118 on page 15) is the largest cell organelle (see also page 5) It is surrounded by two membranes known as the

nuclear envelope The outer membrane of the nuclear envelope is continuous with the endoplasmic reticulum (Figure 117 on page 15) The nuclear envelope has many small pores called nuclear pores These allow and control exchange between the nucleus and the cytoplasm Examples of substances leaving the nucleus through the pores are mRNA and ribosomes for protein synthesis Examples of substances entering through the nuclear pores are proteins to help make ribosomes nucleotides ATP (aderosine triphosphate) and some hormones such as thyroid hormone T3Within the nucleus the chromosomes are in a loosely coiled state known as chromatin (except during nuclear division see Chapter 5) Chromosomes contain DNA which is organised into functional units called genes Genes control the activities of the cell and inheritance thus the nucleus controls the cellrsquos activities When a cell is aboutto divide the nucleus divides first so that each new cell will have its own nucleus (Chapters 5 and 19) Also within the nucleus the nucleolus makes ribosomes using the information in its own DNAEndoplasmic reticulum and ribosomesWhen cells were first seen with the electron microscope biologists were amazed to see so much detailed structure The existence of much of this had not been suspected This was particularly true of an extensive system of membranes running through the cytoplasm which became knownas the endoplasmic reticulum (ER) (Figure 119 on page 15 ndash see also Figures 118 on page 15 and 122 on page 17) The ER is continuous with the outer membrane of the nuclear envelope (Figure 117)There are two types of ER rough ER and smooth ER Rough ER is so called because it is covered with many tinyFigure 115 A TEM in usecollided with air molecules they would scatter making it impossible to achieve a sharp picture Also water boils at room temperature in a vacuum so all specimens must be dehydrated before being placed in the microscope This means that only dead material can be examined Great efforts are therefore made to try to preserve material in a life-like state when preparing it for the microscopeSAQ 13Explain why ribosomes are not visible using a light microscopeUltrastructure of an animal cellThe lsquofinersquo or detailed structure of a cell as revealed by the electron microscope is called its ultrastructure Figure 116 shows the appearance of typical animal cells as seen with an electron microscope and Figure 117 on page 15 is a diagram based on many other such micrographsSAQ 14Compare Figure 117 on page 15 with Figure 13 on page 2 Name the structures which can be seen with the electron microscope but not with the light microscope1 Cell structure 13lysosomeGGolgi apparatusendoplasmic reticulumglycogen granulesccell surface membrane

mmitochondriachromatinnnucleolusmmicrovillusrribosomesFFigure 116 Representative animal cells as seen with a TEM The cells are liver cells from a rat (1113088 9600) The nucleus is clearly visible in one of the cells 1441 1 Cell structurenucleusnnuclear envelopetwo centrioles close to the nucleus and at right angles to each othermitochondrion lysosomerough endoplasmic reticulumnucleoluschromatin nucleusnuclear porenuclear envelope (two membranes)microvillismooth endoplasmic reticulumFigure 117 Ultrastructure of a typical animal cell as seen with an electron microscope In reality the ER is more extensive than shown and free ribosomes may be more extensive Glycogen granules are sometimes present in the cytoplasmFigure 118 TEM of the nucleus of a cell from the pancreas of a bat (1113088 7500) The circular nucleus is surrounded by a double-layered nuclear envelope containing nuclear pores The nucleolus is more Figure 119 TEM of rough ER covered with ribosomes (black dots) darkly stained Rough ER is visible in the surrounding cytoplasm (1113088 17 000) Some free ribosomes can also be seen in the cytoplasmGolgi vesicle Golgi apparatusribosomescell surface membranecytoplasm1 Cell structure 15organelles called ribosomes These are just visible as black dots in Figures 118 and 119 on page 15 At very high magnifications they can be seen to consist of two subunits a large and a small subunit Ribosomes are the sites of protein synthesis (see pages 111ndash112) They can be found free in the cytoplasm as well as on the rough ER They are very small only about 25 nm in diameter They are made of RNA (ribonucleic acid) and protein The rough ER forms an extensive system of flattened sacs spreading in sheets throughout the cell Proteins made by the ribosomes on the rough ER enter the sacs and move through them The proteins are often processed in some way on their journey Small sacs called vesicles can break off from the ER and these can join together to form the Golgi apparatus Proteins can be exported from the cell via the Golgi apparatus (see page 80)Smooth ER so called because it lacks ribosomes has a completely different function It makes lipids and steroids such as cholesterol and the reproductive hormones oestrogen and testosteroneGolgi apparatus (Golgi body or Golgi complex)The Golgi apparatus is a stack of flattened sacs (Figure 120) This stack of sacs is sometimes referred to as theFigure 120 TEM of a Golgi apparatus A central stack of saucer-shaped sacs can be seen budding off small Golgi vesicles (green) These may form secretory vesicles whose contents can be released at the cell surface by exocytosis (see page 80)Golgi body More than one may be present in a cell The stack is constantly being formed at one end from vesicles which bud off from the ER and broken down again at the other end to form Golgi vesicles The stack of sacs with the associated vesicles is referred to as the Golgi apparatus or Golgi complex

The Golgi apparatus collects processes and sorts molecules (particularly proteins from the rough ER) ready for transport in Golgi vesicles either to other parts of the cell or out of the cell (secretion) Two examples of protein processing in the Golgi apparatus are the addition of sugars to proteins to make molecules known as glycoproteins and the removal of the first amino acid methionine from newly formed proteins to make a functioning protein In plants enzymes in the Golgi apparatus convert sugars into cell wall components Golgi vesicles are also used to make lysosomesLysosomesLysosomes (Figure 121) are spherical sacs surrounded by a single membrane and having no internal structure They are commonly 01ndash 05 1113088m in diameter They contain digestive (hydrolytic) enzymes which must be kept separateFigure 121 Lysosomes (orange) in a mouse kidney cell (1113088 55 000) They contain cell structures in the process of digestion and vesicles (green) Cytoplasm is coloured blue here1661 1 Cell structurefrom the rest of the cell to prevent damage Lysosomes are responsible for the breakdown (digestion) of unwanted structures such as old organelles or even whole cells as in mammary glands after lactation (breast feeding) In white blood cells lysosomes are used to digest bacteria (see endocytosis page 80) Enzymes are sometimes released outside the cell ndash for example in the replacement of cartilage with bone during development The headsof sperm contain a special lysosome the acrosome for digesting a path to the ovum (egg)MitochondriaMitochondria (singular mitochondrion) are usually about 1 1113088m in diameter and can be various shapes often sausage- shaped as in Figure 122 They are surrounded by two membranes (an envelope) The inner of these is foldedto form finger-like cristae which project into the interior solution or matrixThe main function of mitochondria is to carry out aerobic respiration As a result of respiration they make ATP the universal energy carrier in cells (see Chapter 16) They are also involved in the synthesis of lipids (page 37)Figure 122 Mitochondrion (orange) with its double membrane (envelope) the inner membrane is folded to form cristae (1113088 20 000) Mitochondria are the sites of aerobic cell respiration Note also the rough ERIn the 1960s it was discovered that mitochondria and chloroplasts contain ribosomes which are slightly smaller than those in the cytoplasm and are the same size as those found in bacteria The size of ribosomes is measured in lsquoS unitsrsquo which are a measure of how fast they sediment in a centrifuge Cytoplasmic ribosomes are 80S while those of bacteria mitochondria and chloroplasts are 70S It was also discovered in the 1960s that mitochondria and chloroplasts contain small circular DNA molecules also like those found in bacteria Not surprisingly it was later provedthat mitochondria and chloroplasts are in effect ancient bacteria which now live inside the larger cells typical of animals and plants (see

prokaryotic and eukaryotic cells page 18) This is known as the endosymbiont theory lsquoEndorsquo means lsquoinsidersquo and a lsquosymbiontrsquo is an organism which lives in a mutually beneficial relationship with another organism The DNA and ribosomes of mitochondria and chloroplasts are still active and responsible for the coding and synthesis of certain vital proteins but mitochondria and chloroplasts can no longer live independentlyMitochondrial ribosomes are just visible as tiny dark dots in the mitochondrial matrix in Figure 122Cell surface membraneThe cell surface membrane is extremely thin (about 7 nm) However at very high magnifications at least 1113088 100 000 it can be seen to have three layers described as a trilaminar appearance This consists of two dark lines (heavily stained) either side of a narrow pale interior (Figure 123) The membrane is partially permeable and controls exchange between the cell and its environment Membrane structure is discussed further in Chapter 4Figure 123 Cell surface membrane (1113088 250 000) At this magnification the membrane appears as two dark lines at the edge of the cellMicrovilliMicrovilli (singular microvillus) are finger-like extensions of the cell surface membrane typical of certain epithelial cells (cells covering surfaces of structures) They greatly1 Cell structure 17increase the surface area of the cell surface membrane (see Figure 117 on page 15) This is useful for example for absorption in the gut and for reabsorption in the proximal convoluted tubules of the kidney (see page 307)CentriolesThe extra resolution of the electron microscope reveals that just outside the nucleus there are really two centrioles (see Figure 124) not one as it appears under the light microscope (compare with Figure 13 on page 2) They lie close together at right-angles to each other A centriole is a hollow cylinder about 04 1113088m long formed from a ring of short microtubules tiny tubes made of a protein called tubulin These microtubules are used as a starting point for growing the spindle microtubules for nuclear division (see page 92) Centrioles are not found in plant cellsUltrastructure of a plant cellAll the structures except centrioles and microvilli so far described in animal cells are also found in plant cells The appearance of a plant cell as seen with the electron microscope is shown in Figure 125 while Figure 126 is a diagram based on many such micrographs The relatively thick cell wall and the large central vacuole are obvious as are the chloroplasts two of which are shown in detail in Figure 127 These structures and their functions have been described on pages 5 and 6 The electron microscopeFigure 124 Centrioles in transverse and longitudinal section (TS and LS) (1113088 86 000) The one on the left is seen in TS and clearly shows the nine triplets of microtubules which make up the structurereveals that chloroplasts contain 70S ribosomes and small circular

DNA molecules as do mitochondria and bacteria This has already been discussed with mitochondria above (page 17)SAQ 15Compare Figure 126 with Figure 15 on page 3 Name the structures which can be seen with the electron microscope but not with the light microscopeTwo fundamentally different types of cellAt one time it was common practice to try to classify all living organisms as either animals or plants With advances in our knowledge of living things it has become obvious that the living world is not that simple Fungi and bacteria for example are very different from animals and plants and from each other Eventually it was discovered that there are two fundamentally different types of cell The most obvious difference betweenthese types is that one possesses a nucleus and the other does notOrganisms that lack nuclei are called prokaryotes (lsquoprorsquo means before lsquokaryonrsquo means nucleus) All prokaryotes are now referred to as bacteria They are on average about 1000 to 10 000 times smaller in volume than cells with nuclei and are much simpler in structure ndash for example their DNA lies free in the cytoplasmOrganisms whose cells possess nuclei are called eukaryotes (lsquoeursquo means true) Their DNA lies inside a nucleus Eukaryotes include animals plants fungi and a group containing most of the unicellular eukaryotes known as protoctists Most biologists believe that eukaryotes evolved from prokaryotes one-and-a-half thousand million years after prokaryotes first appeared on Earth We mainly study animals and plants in this book but all eukaryotic cells have certain features in commonA generalised prokaryotic cell is shown in Figure 128 A comparison of prokaryotic and eukaryotic cells is given in Table 12 on page 211881 1 Cell structure

Although animal cells may possess small vacuoles such as phagocytic vacuoles (see page 80) which are temporary structures mature plant cells often possess a large permanent central vacuole The plant vacuole is surrounded by a membrane the tonoplast which controls exchange between the vacuole and the cytoplasm The fluid in the vacuole is a solution of mineral salts sugars oxygen carbon dioxide pigments enzymes and other organic compounds including some waste productsVacuoles help to regulate the osmotic properties of cells (the flow of water inwards and outwards) as well as having a wide range of other functions For example the pigments which colour the petals of certain flowers and parts of some vegetables such as the red pigment of beetroots are sometimes located in vacuoles1 Cell structure 5ChloroplastsSome plant cells are able to carry out photosynthesis because they contain chloroplasts Chloroplasts are relatively large organelles which are green in colour due to the presence of chlorophyll At high magnifications small lsquograinsrsquo or grana (singular granum) can be seen in the chloroplasts During the process of photosynthesis light is absorbed by these grana which actually consist of stacks of membrane-bound sacs called thylakoids Starch grains may also be visible within chloroplasts Chloroplasts are found in the green parts of plants mainly in the leavesPoints to noteWe return to the differences between animal and plant cells as seen using the electron microscope on page 18Units of measurement in cell studiesIn order to measure objects in the microscopic world we need to use very small units of measurement which are unfamiliar to most people According to international agreement the International System of Units (SI units) should be used In this system the basic unit of length is the metre (symbol m) Additional units can be created in multiples of a thousand times larger or smaller using standard prefixes For example the prefix kilo means 1000 times Thus 1 kilometre 1113088 1000 metres The units of length relevant to cell studies are shown in Table 11It is difficult to imagine how small these units are but when looking down a microscope and seeing cells clearly we should not forget how amazingly small the cells actually are The smallest structure visible with the human eye is about 50ndash100 1113088m in diameter Your body contains about 60 million million cells varying in size from about 5 1113088m to 40 1113088m Try to imagine structures like mitochondria which have an average diameter of 1 1113088m The smallest cell organelles we deal with in this book ribosomes are only about 25 nm in diameter You could line up about 20 000 ribosomes across the full stop at the end of this sentenceUnit Symbolmillimetre mm micrometre 1113088m nanometre nmTable 11 Units of measurement relevant to cell studies 1113088 is the Greek letter mu 1 micrometre is a thousandth of a millimetre 1 nanometre is a thousandth of a micrometre661 Cell structure

bull bull bullbullYou can think of a plant cell as being very similar to an animal cell but with extra structures Plant cells are often larger than animal cells although cell size varies enormouslyDo not confuse the cell wall with the cell surface membrane Cell walls are relatively thick and physically strong whereas cell surface membranes are very thin Cell walls are freely permeable whereas cell surface membranes are partially permeable All cells have a cell surface membrane Vacuoles are not confined to plant cells animal cells may have small vacuoles such as phagocytic vacuoles (see page 80) although these are not usually permanent structuresFraction of a metreone thousandth 1113088 0001 1113088 11000 1113088 10-3 one millionth 1113088 0000 001 1113088 11 000 000 1113088 10-6 one thousand millionth 1113088 0000 000 001 1113088 11 000 000 000 1113088 10-9

Box 1B Measuring cellsCells and organelles can be measured with a microscope by means of an eyepiece graticule This is a transparent scale It usually has 100 divisions (see Figure 18a) The eyepiece graticule is placed in the microscope eyepiece so that it can be seen at the same time as the object to be measured as shown in Figure 18b Figure 18b shows the scale over a human cheek epithelialcell The cell lies between 40 and 60 on the scale We therefore say it measures 20 eyepiece units in diameter (the difference between 60 and 40) We will not know the actual size of the eyepiece units until the eyepiece graticule scale is calibratedTo calibrate the eyepiece graticule scale a miniature transparent ruler called a stage micrometer scale is placed on the microscope stage and is brought into focus This scale may be etched onto a glass slideor printed on a transparent film It commonly has subdivisions of 01 and 001 mm The images of the two scales can then be superimposed as shown in Figure 18cIn the eyepiece graticule shown in the figure 100 units measure 025 mm Hence the value of each eyepiece unit is0 25 00025 mm 100Or converting mm to 1113088m 0 25 1000 2 51113088m100The diameter of the cell shown superimposed on the scale in Figure 18b measures 20 eyepiece units and so its actual diameter is20times251113088m 1113088 501113088m This diameter is greater than that of many human cellsbecause the cell is a flattened epithelial cellFigure 18 Microscopical measurement Three fields of view seen using a high-power (111308840) objective lens a An eyepiece graticule scale b Superimposed images of human cheek epithelial cells and the eyepiece graticule scale c Superimposed images of the eyepiece graticule scale and the stage micrometer scaleacheek cells on a slide on the stage of the microscopebc0 1020304050607080901000 102030405060708090100eyepiece graticule scale (arbitrary units)

eyepiece graticule in the eyepiece of the microscope00stage micrometer scale (marked in 00 1mm and 01 mm divisions)10 20 30 40 50 60 70 80 90 10001 021 Cell structure 7

Electron microscopesEarlier in this chapter it was stated that by 1900 almost all the structures shown in Figures 13 and 15 (pages 2 and 3) had been discovered There followed a time of frustration for microscopists because they realised that no matter how much the design of light microscopes improved there was a limit to how much could ever be seen using lightIn order to understand the problem it is necessary to know something about the nature of light itself and to understand the difference between magnification and resolutionMagnificationMagnification is the number of times larger an image is compared with the real size of the objectWorked example 1 ndash calculating the magnification of a photograph or objectTo calculate M the magnification of a photograph or an object we can use the following methodFigure 19 shows two photographs of a section through the same plant cells The magnifications of the two photographs are the same Suppose we want to know the magnification of the plant cell in Figure 19b If we know its actual (real) length we can calculate its magnification using theThe real length of the cell is 80 1113088mmagnification observed size of the image actual sizeM I AStep 1Measure the length in mm of the cell in the photograph using a ruler You should find that it is about 60 mmStep 2Convert mm to 1113088m (It is easier if we first convert all measurements to the same units ndash in this case micrometres 1113088m)1 mm 1113088 1000 1113088m so 60mm 1113088 60 1113088 10001113088morwhere I 1113088 observed size of the image (that is what you can measure with a ruler) and A 1113088 actual size (that is the real size ndash for example the size of a cell before it is magnified)If you know two of these values you can work out the third one For example if the observed size of the image and the magnification are known you can work out the1113088 60 000 1113088m Use the equation to calculate the magnificationStep 3actual size A I If you write the formula in a triangle Mmagnification Mimage size I

as shown below and cover up the value you want to find it should be obvious how to do the right calculationactual size A 60000m80 m 750I M1113088ASome worked examples are now providedThe lsquo1113088rsquo sign in front of the number 750 means lsquotimesrsquo We say that the magnification is lsquotimes 750rsquoformula M I A88 1 Cell structurea Worked example 2 ndash calculating magnification from a scale barFigure 110 shows a lymphocyteFigure 110 A lymphocyte6 μmbFigure 19 Photographs of the same plant cells seen a with a light microscope b with an electron microscope both shown at a magnification of about 1113088 750We can calculate the magnification of the lymphocyte by simply using the scale bar All you need to do is measure the length of the scale bar and then substitute this and the length it represents into the equationStep 1Measure the scale bar Here it is 36 mmStep 2Convert mm to 1113088m 36mm 1113088 36 1113088 10001113088m 1113088 360001113088mStep 3Use the equation to calculate the magnificationmagnification M image size I actual size A36000m 6m60001 Cell structure 9Worked example 3 ndash calculating the real size of an object from its magnificationTo calculate A the real or actual size of an object we can use the following methodFigure 125 on page 19 shows a plant cell magnified 1113088 5600 One of the chloroplasts is labelled lsquochloroplastrsquo in the figure Suppose we want to know the actual length of this chloroplastStep 1Measure the observed length of the image of the chloroplast (I ) in mm using a ruler The maximum length is 36 mmStep 2Convert mm to 1113088m 30mm 1113088 30 1113088 10001113088m 1113088 300001113088mStep 3Use the equation to calculate the actual lengthactualsizeA imagesizeI magnification M30 000 m 5600 5 4 m (to one decimal place)SAQ 12a Calculate the magnification of the drawing of the animal cell in Figure 13 on page 2b Calculate the actual (real) length of the bottom chloroplast in Figure 127 on page 19Resolution

Look again at Figure 19 (page 9) Figure 19a is a light micrograph (a photograph taken with a light microscope also known as a photomicrograph) Figure 19b is an electron micrograph of the same cells taken at the same magnification (an electron micrograph is a picture taken with an electron microscope) You can see that Figure 19b the electron micrograph is much clearer This is because it has greater resolution Resolution is defined as the abilityto distinguish between two separate points If the two points cannot be resolved they will be seen as one point In practice resolution is the amount of detail that can be seen ndash the greater the resolution the greater the detailThe maximum resolution of a light microscope is 200 nm This means that if two points or objects are closer together than 200 nm they cannot be distinguished as separateIt is possible to take a photograph such as Figure 19a and to magnify (enlarge) it but we see no more detail in other words we do not improve resolution even though we often enlarge photographs because they are easier to see when larger With a microscope magnification up to the limit of resolution can reveal further detail but any further magnification increases blurring as well as the size of the imageThe electromagnetic spectrumHow is resolution linked with the nature of light One of the properties of light is that it travels in waves The length of the waves of visible light varies ranging from about 400 nm (violet light) to about 700 nm (red light) The human eye can distinguish between these different wavelengths and in the brain the differences are converted to colour differences (Colour is an invention of the brain)The whole range of different wavelengths is called the electromagnetic spectrum Visible light is only one part of this spectrum Figure 111 shows some of the parts of the electromagnetic spectrum The longer the waves the lower their frequency (all the waves travel at the same speed so imagine them passing a post shorter waves pass at higher frequency) In theory there is no limit to how short or how long the waves can be Wavelength changes with energy the greater the energy the shorter the wavelength (rather like squashing a spring)Now look at Figure 112 which shows a mitochondrion some very small cell organelles called ribosomes (see page 13) and light of 400 nm wavelength the shortest visible wavelength The mitochondrion is large enoughto interfere with the light waves However the ribosomes are far too small to have any effect on the light waves The general rule is that the limit of resolution is about one half the wavelength of the radiation used to view the specimen In other words if an object is any smaller than half the wavelength of the radiation used to view it it cannot be seen separately from nearby objects This means that the1001 1 Cell structureX-raysgamma rays01 nm400 nm violet bluev infraredmicrowaves

uv10 nmi s i b l e 1000 nmradio and TV wavesvisible light500 nm 600 nm green yellow orange105 nm 107 nm 109 nm 1011 nm 1013 nm700 nmFigure 111 Diagram of the electromagnetic spectrum (the waves are not drawn to scale) The numbers indicate the wavelengths of the different types of electromagnetic radiation Visible light is a form of electromagnetic radiationbest resolution that can be obtained using a microscope that uses visible light (a light microscope) is 200 nm since the shortest wavelength of visible light is 400 nm (violet light) In practice this corresponds to a maximum useful magnification of about 1500 times Ribosomes are approximately 25 nm in diameter and can therefore never be seen using lightIf an object is transparent it will allow light waves to pass through it and therefore will still not be visible This is why many biological structures have to be stained before they can be seenThe electron microscopeBiologists faced with the problem that they would never see anything smaller than 200 nm using a light microscope realised that the only solution would be to use radiation of a shorter wavelength than light If you study Figure 111 you will see that ultraviolet light or better still X-rays look like possible candidates Both ultraviolet and X-ray microscopes have been built the latter with little success partly because of the difficulty of focusing X-rays A much better solution is to use electrons Electrons are negatively charged particles which orbit the nucleus of an atom When a metal becomes very hot some of its electrons gain so much energy that they escape from their orbits like a rocket escaping from Earthrsquos gravity Free electrons behave like electromagnetic radiation They have a very short wavelength the greater the energy the shorter the wavelength Electrons are a very suitable form of radiation for microscopy for two major reasons Firstly their wavelength is extremely short (at least as short as that of X-rays) Secondly because they are negatively charged they can be focused easily using electromagnets (a magnet can be made to alter the path of the beam the equivalent of a glass lens bending light)Using an electron microscope a resolution of 05 nm can be obtained

400 times better than when using a light microscope ETransmission and scanning electron microscopesTwo types of electron microscope are now in common use The transmission electron microscope or TEM forwavelength 400nmstained mitochondrion of diameter 1000nm interferes with light wavesstained ribosomes of diameter 25nm do not interfere with light wavesFigure 112 A mitochondrion and some ribosomes in the path of light waves of 400 nm lengthred1 Cell structure 11

E short was the type originally developed Here the beam of electrons is passed through the specimen before being viewed Only

those electrons that are transmitted (pass through the specimen) are seen This allows us to see thin sections of specimens and thus to see inside cells In the scanning electron microscope (SEM) on the other hand the electron beam is used to scan the surfaces of structures and only the reflected beam is observedAn example of a scanning electron micrograph is shown in Figure 113 The advantage of this microscope is that surface structures can be seen Also great depth of field is obtained so that much of the specimen is in focus at the same time and a three-dimensional appearance is obtained Such a picture would be impossible to obtain with a light microscope even using the same magnification and resolution because you would have to keep focusing up and down with the objective lens to see different parts of the specimen The disadvantage of the SEM is that it cannot achieve the same resolution as a TEM Resolution is between 3 nm and 20 nmViewing specimens with the electron microscopeFigure 114 shows how an electron microscope works and Figure 115 shows one in useFigure 113 False-colour SEM of the head of a cat flea (1113088 100)

electron gun and anode which E produce a beam of electronselectron beam vacuumpathway of electronscondenser electromagnetic lens which directs the electron beam onto the specimenspecimen which is placed on a gridobjective electromagnetic lens which produces an imageprojector electromagnetic lenses which focus the magnified image onto the screenscreen or photographic plate which shows the image of the specimenFigure 114 How an electron microscope worksIt is not possible to see an electron beam so to make the image visible the electron beam has to be projected onto a fluorescent screen The areas hit by electrons shine brightly giving overall a lsquoblack and whitersquo picture The stains used to improve the contrast of biological specimens for electron microscopy contain heavy metal atoms which stop the passage of electrons The resulting picture is like an X-ray photograph with the more densely stained parts of the specimen appearing blacker lsquoFalse-colourrsquo images can be created by colouring the standard black and white image using a computerTo add to the difficulties of electron microscopy the electron beam and therefore the specimen and the fluorescent screen must be in a vacuum If electrons1221 1 Cell structure

EStructures and functions of organellesCompartmentalisation and division of labour within the cell are even more obvious with an electron microscope than with a light microscopeWe will now consider the structures and functions of some of the cell components in more detailNucleusThe nucleus (Figure 118 on page 15) is the largest cell organelle (see also page 5) It is surrounded by two membranes known as the

nuclear envelope The outer membrane of the nuclear envelope is continuous with the endoplasmic reticulum (Figure 117 on page 15) The nuclear envelope has many small pores called nuclear pores These allow and control exchange between the nucleus and the cytoplasm Examples of substances leaving the nucleus through the pores are mRNA and ribosomes for protein synthesis Examples of substances entering through the nuclear pores are proteins to help make ribosomes nucleotides ATP (aderosine triphosphate) and some hormones such as thyroid hormone T3Within the nucleus the chromosomes are in a loosely coiled state known as chromatin (except during nuclear division see Chapter 5) Chromosomes contain DNA which is organised into functional units called genes Genes control the activities of the cell and inheritance thus the nucleus controls the cellrsquos activities When a cell is aboutto divide the nucleus divides first so that each new cell will have its own nucleus (Chapters 5 and 19) Also within the nucleus the nucleolus makes ribosomes using the information in its own DNAEndoplasmic reticulum and ribosomesWhen cells were first seen with the electron microscope biologists were amazed to see so much detailed structure The existence of much of this had not been suspected This was particularly true of an extensive system of membranes running through the cytoplasm which became knownas the endoplasmic reticulum (ER) (Figure 119 on page 15 ndash see also Figures 118 on page 15 and 122 on page 17) The ER is continuous with the outer membrane of the nuclear envelope (Figure 117)There are two types of ER rough ER and smooth ER Rough ER is so called because it is covered with many tinyFigure 115 A TEM in usecollided with air molecules they would scatter making it impossible to achieve a sharp picture Also water boils at room temperature in a vacuum so all specimens must be dehydrated before being placed in the microscope This means that only dead material can be examined Great efforts are therefore made to try to preserve material in a life-like state when preparing it for the microscopeSAQ 13Explain why ribosomes are not visible using a light microscopeUltrastructure of an animal cellThe lsquofinersquo or detailed structure of a cell as revealed by the electron microscope is called its ultrastructure Figure 116 shows the appearance of typical animal cells as seen with an electron microscope and Figure 117 on page 15 is a diagram based on many other such micrographsSAQ 14Compare Figure 117 on page 15 with Figure 13 on page 2 Name the structures which can be seen with the electron microscope but not with the light microscope1 Cell structure 13lysosomeGGolgi apparatusendoplasmic reticulumglycogen granulesccell surface membrane

mmitochondriachromatinnnucleolusmmicrovillusrribosomesFFigure 116 Representative animal cells as seen with a TEM The cells are liver cells from a rat (1113088 9600) The nucleus is clearly visible in one of the cells 1441 1 Cell structurenucleusnnuclear envelopetwo centrioles close to the nucleus and at right angles to each othermitochondrion lysosomerough endoplasmic reticulumnucleoluschromatin nucleusnuclear porenuclear envelope (two membranes)microvillismooth endoplasmic reticulumFigure 117 Ultrastructure of a typical animal cell as seen with an electron microscope In reality the ER is more extensive than shown and free ribosomes may be more extensive Glycogen granules are sometimes present in the cytoplasmFigure 118 TEM of the nucleus of a cell from the pancreas of a bat (1113088 7500) The circular nucleus is surrounded by a double-layered nuclear envelope containing nuclear pores The nucleolus is more Figure 119 TEM of rough ER covered with ribosomes (black dots) darkly stained Rough ER is visible in the surrounding cytoplasm (1113088 17 000) Some free ribosomes can also be seen in the cytoplasmGolgi vesicle Golgi apparatusribosomescell surface membranecytoplasm1 Cell structure 15organelles called ribosomes These are just visible as black dots in Figures 118 and 119 on page 15 At very high magnifications they can be seen to consist of two subunits a large and a small subunit Ribosomes are the sites of protein synthesis (see pages 111ndash112) They can be found free in the cytoplasm as well as on the rough ER They are very small only about 25 nm in diameter They are made of RNA (ribonucleic acid) and protein The rough ER forms an extensive system of flattened sacs spreading in sheets throughout the cell Proteins made by the ribosomes on the rough ER enter the sacs and move through them The proteins are often processed in some way on their journey Small sacs called vesicles can break off from the ER and these can join together to form the Golgi apparatus Proteins can be exported from the cell via the Golgi apparatus (see page 80)Smooth ER so called because it lacks ribosomes has a completely different function It makes lipids and steroids such as cholesterol and the reproductive hormones oestrogen and testosteroneGolgi apparatus (Golgi body or Golgi complex)The Golgi apparatus is a stack of flattened sacs (Figure 120) This stack of sacs is sometimes referred to as theFigure 120 TEM of a Golgi apparatus A central stack of saucer-shaped sacs can be seen budding off small Golgi vesicles (green) These may form secretory vesicles whose contents can be released at the cell surface by exocytosis (see page 80)Golgi body More than one may be present in a cell The stack is constantly being formed at one end from vesicles which bud off from the ER and broken down again at the other end to form Golgi vesicles The stack of sacs with the associated vesicles is referred to as the Golgi apparatus or Golgi complex

The Golgi apparatus collects processes and sorts molecules (particularly proteins from the rough ER) ready for transport in Golgi vesicles either to other parts of the cell or out of the cell (secretion) Two examples of protein processing in the Golgi apparatus are the addition of sugars to proteins to make molecules known as glycoproteins and the removal of the first amino acid methionine from newly formed proteins to make a functioning protein In plants enzymes in the Golgi apparatus convert sugars into cell wall components Golgi vesicles are also used to make lysosomesLysosomesLysosomes (Figure 121) are spherical sacs surrounded by a single membrane and having no internal structure They are commonly 01ndash 05 1113088m in diameter They contain digestive (hydrolytic) enzymes which must be kept separateFigure 121 Lysosomes (orange) in a mouse kidney cell (1113088 55 000) They contain cell structures in the process of digestion and vesicles (green) Cytoplasm is coloured blue here1661 1 Cell structurefrom the rest of the cell to prevent damage Lysosomes are responsible for the breakdown (digestion) of unwanted structures such as old organelles or even whole cells as in mammary glands after lactation (breast feeding) In white blood cells lysosomes are used to digest bacteria (see endocytosis page 80) Enzymes are sometimes released outside the cell ndash for example in the replacement of cartilage with bone during development The headsof sperm contain a special lysosome the acrosome for digesting a path to the ovum (egg)MitochondriaMitochondria (singular mitochondrion) are usually about 1 1113088m in diameter and can be various shapes often sausage- shaped as in Figure 122 They are surrounded by two membranes (an envelope) The inner of these is foldedto form finger-like cristae which project into the interior solution or matrixThe main function of mitochondria is to carry out aerobic respiration As a result of respiration they make ATP the universal energy carrier in cells (see Chapter 16) They are also involved in the synthesis of lipids (page 37)Figure 122 Mitochondrion (orange) with its double membrane (envelope) the inner membrane is folded to form cristae (1113088 20 000) Mitochondria are the sites of aerobic cell respiration Note also the rough ERIn the 1960s it was discovered that mitochondria and chloroplasts contain ribosomes which are slightly smaller than those in the cytoplasm and are the same size as those found in bacteria The size of ribosomes is measured in lsquoS unitsrsquo which are a measure of how fast they sediment in a centrifuge Cytoplasmic ribosomes are 80S while those of bacteria mitochondria and chloroplasts are 70S It was also discovered in the 1960s that mitochondria and chloroplasts contain small circular DNA molecules also like those found in bacteria Not surprisingly it was later provedthat mitochondria and chloroplasts are in effect ancient bacteria which now live inside the larger cells typical of animals and plants (see

prokaryotic and eukaryotic cells page 18) This is known as the endosymbiont theory lsquoEndorsquo means lsquoinsidersquo and a lsquosymbiontrsquo is an organism which lives in a mutually beneficial relationship with another organism The DNA and ribosomes of mitochondria and chloroplasts are still active and responsible for the coding and synthesis of certain vital proteins but mitochondria and chloroplasts can no longer live independentlyMitochondrial ribosomes are just visible as tiny dark dots in the mitochondrial matrix in Figure 122Cell surface membraneThe cell surface membrane is extremely thin (about 7 nm) However at very high magnifications at least 1113088 100 000 it can be seen to have three layers described as a trilaminar appearance This consists of two dark lines (heavily stained) either side of a narrow pale interior (Figure 123) The membrane is partially permeable and controls exchange between the cell and its environment Membrane structure is discussed further in Chapter 4Figure 123 Cell surface membrane (1113088 250 000) At this magnification the membrane appears as two dark lines at the edge of the cellMicrovilliMicrovilli (singular microvillus) are finger-like extensions of the cell surface membrane typical of certain epithelial cells (cells covering surfaces of structures) They greatly1 Cell structure 17increase the surface area of the cell surface membrane (see Figure 117 on page 15) This is useful for example for absorption in the gut and for reabsorption in the proximal convoluted tubules of the kidney (see page 307)CentriolesThe extra resolution of the electron microscope reveals that just outside the nucleus there are really two centrioles (see Figure 124) not one as it appears under the light microscope (compare with Figure 13 on page 2) They lie close together at right-angles to each other A centriole is a hollow cylinder about 04 1113088m long formed from a ring of short microtubules tiny tubes made of a protein called tubulin These microtubules are used as a starting point for growing the spindle microtubules for nuclear division (see page 92) Centrioles are not found in plant cellsUltrastructure of a plant cellAll the structures except centrioles and microvilli so far described in animal cells are also found in plant cells The appearance of a plant cell as seen with the electron microscope is shown in Figure 125 while Figure 126 is a diagram based on many such micrographs The relatively thick cell wall and the large central vacuole are obvious as are the chloroplasts two of which are shown in detail in Figure 127 These structures and their functions have been described on pages 5 and 6 The electron microscopeFigure 124 Centrioles in transverse and longitudinal section (TS and LS) (1113088 86 000) The one on the left is seen in TS and clearly shows the nine triplets of microtubules which make up the structurereveals that chloroplasts contain 70S ribosomes and small circular

DNA molecules as do mitochondria and bacteria This has already been discussed with mitochondria above (page 17)SAQ 15Compare Figure 126 with Figure 15 on page 3 Name the structures which can be seen with the electron microscope but not with the light microscopeTwo fundamentally different types of cellAt one time it was common practice to try to classify all living organisms as either animals or plants With advances in our knowledge of living things it has become obvious that the living world is not that simple Fungi and bacteria for example are very different from animals and plants and from each other Eventually it was discovered that there are two fundamentally different types of cell The most obvious difference betweenthese types is that one possesses a nucleus and the other does notOrganisms that lack nuclei are called prokaryotes (lsquoprorsquo means before lsquokaryonrsquo means nucleus) All prokaryotes are now referred to as bacteria They are on average about 1000 to 10 000 times smaller in volume than cells with nuclei and are much simpler in structure ndash for example their DNA lies free in the cytoplasmOrganisms whose cells possess nuclei are called eukaryotes (lsquoeursquo means true) Their DNA lies inside a nucleus Eukaryotes include animals plants fungi and a group containing most of the unicellular eukaryotes known as protoctists Most biologists believe that eukaryotes evolved from prokaryotes one-and-a-half thousand million years after prokaryotes first appeared on Earth We mainly study animals and plants in this book but all eukaryotic cells have certain features in commonA generalised prokaryotic cell is shown in Figure 128 A comparison of prokaryotic and eukaryotic cells is given in Table 12 on page 211881 1 Cell structure

bull bull bullbullYou can think of a plant cell as being very similar to an animal cell but with extra structures Plant cells are often larger than animal cells although cell size varies enormouslyDo not confuse the cell wall with the cell surface membrane Cell walls are relatively thick and physically strong whereas cell surface membranes are very thin Cell walls are freely permeable whereas cell surface membranes are partially permeable All cells have a cell surface membrane Vacuoles are not confined to plant cells animal cells may have small vacuoles such as phagocytic vacuoles (see page 80) although these are not usually permanent structuresFraction of a metreone thousandth 1113088 0001 1113088 11000 1113088 10-3 one millionth 1113088 0000 001 1113088 11 000 000 1113088 10-6 one thousand millionth 1113088 0000 000 001 1113088 11 000 000 000 1113088 10-9

Box 1B Measuring cellsCells and organelles can be measured with a microscope by means of an eyepiece graticule This is a transparent scale It usually has 100 divisions (see Figure 18a) The eyepiece graticule is placed in the microscope eyepiece so that it can be seen at the same time as the object to be measured as shown in Figure 18b Figure 18b shows the scale over a human cheek epithelialcell The cell lies between 40 and 60 on the scale We therefore say it measures 20 eyepiece units in diameter (the difference between 60 and 40) We will not know the actual size of the eyepiece units until the eyepiece graticule scale is calibratedTo calibrate the eyepiece graticule scale a miniature transparent ruler called a stage micrometer scale is placed on the microscope stage and is brought into focus This scale may be etched onto a glass slideor printed on a transparent film It commonly has subdivisions of 01 and 001 mm The images of the two scales can then be superimposed as shown in Figure 18cIn the eyepiece graticule shown in the figure 100 units measure 025 mm Hence the value of each eyepiece unit is0 25 00025 mm 100Or converting mm to 1113088m 0 25 1000 2 51113088m100The diameter of the cell shown superimposed on the scale in Figure 18b measures 20 eyepiece units and so its actual diameter is20times251113088m 1113088 501113088m This diameter is greater than that of many human cellsbecause the cell is a flattened epithelial cellFigure 18 Microscopical measurement Three fields of view seen using a high-power (111308840) objective lens a An eyepiece graticule scale b Superimposed images of human cheek epithelial cells and the eyepiece graticule scale c Superimposed images of the eyepiece graticule scale and the stage micrometer scaleacheek cells on a slide on the stage of the microscopebc0 1020304050607080901000 102030405060708090100eyepiece graticule scale (arbitrary units)

eyepiece graticule in the eyepiece of the microscope00stage micrometer scale (marked in 00 1mm and 01 mm divisions)10 20 30 40 50 60 70 80 90 10001 021 Cell structure 7

Electron microscopesEarlier in this chapter it was stated that by 1900 almost all the structures shown in Figures 13 and 15 (pages 2 and 3) had been discovered There followed a time of frustration for microscopists because they realised that no matter how much the design of light microscopes improved there was a limit to how much could ever be seen using lightIn order to understand the problem it is necessary to know something about the nature of light itself and to understand the difference between magnification and resolutionMagnificationMagnification is the number of times larger an image is compared with the real size of the objectWorked example 1 ndash calculating the magnification of a photograph or objectTo calculate M the magnification of a photograph or an object we can use the following methodFigure 19 shows two photographs of a section through the same plant cells The magnifications of the two photographs are the same Suppose we want to know the magnification of the plant cell in Figure 19b If we know its actual (real) length we can calculate its magnification using theThe real length of the cell is 80 1113088mmagnification observed size of the image actual sizeM I AStep 1Measure the length in mm of the cell in the photograph using a ruler You should find that it is about 60 mmStep 2Convert mm to 1113088m (It is easier if we first convert all measurements to the same units ndash in this case micrometres 1113088m)1 mm 1113088 1000 1113088m so 60mm 1113088 60 1113088 10001113088morwhere I 1113088 observed size of the image (that is what you can measure with a ruler) and A 1113088 actual size (that is the real size ndash for example the size of a cell before it is magnified)If you know two of these values you can work out the third one For example if the observed size of the image and the magnification are known you can work out the1113088 60 000 1113088m Use the equation to calculate the magnificationStep 3actual size A I If you write the formula in a triangle Mmagnification Mimage size I

as shown below and cover up the value you want to find it should be obvious how to do the right calculationactual size A 60000m80 m 750I M1113088ASome worked examples are now providedThe lsquo1113088rsquo sign in front of the number 750 means lsquotimesrsquo We say that the magnification is lsquotimes 750rsquoformula M I A88 1 Cell structurea Worked example 2 ndash calculating magnification from a scale barFigure 110 shows a lymphocyteFigure 110 A lymphocyte6 μmbFigure 19 Photographs of the same plant cells seen a with a light microscope b with an electron microscope both shown at a magnification of about 1113088 750We can calculate the magnification of the lymphocyte by simply using the scale bar All you need to do is measure the length of the scale bar and then substitute this and the length it represents into the equationStep 1Measure the scale bar Here it is 36 mmStep 2Convert mm to 1113088m 36mm 1113088 36 1113088 10001113088m 1113088 360001113088mStep 3Use the equation to calculate the magnificationmagnification M image size I actual size A36000m 6m60001 Cell structure 9Worked example 3 ndash calculating the real size of an object from its magnificationTo calculate A the real or actual size of an object we can use the following methodFigure 125 on page 19 shows a plant cell magnified 1113088 5600 One of the chloroplasts is labelled lsquochloroplastrsquo in the figure Suppose we want to know the actual length of this chloroplastStep 1Measure the observed length of the image of the chloroplast (I ) in mm using a ruler The maximum length is 36 mmStep 2Convert mm to 1113088m 30mm 1113088 30 1113088 10001113088m 1113088 300001113088mStep 3Use the equation to calculate the actual lengthactualsizeA imagesizeI magnification M30 000 m 5600 5 4 m (to one decimal place)SAQ 12a Calculate the magnification of the drawing of the animal cell in Figure 13 on page 2b Calculate the actual (real) length of the bottom chloroplast in Figure 127 on page 19Resolution

Look again at Figure 19 (page 9) Figure 19a is a light micrograph (a photograph taken with a light microscope also known as a photomicrograph) Figure 19b is an electron micrograph of the same cells taken at the same magnification (an electron micrograph is a picture taken with an electron microscope) You can see that Figure 19b the electron micrograph is much clearer This is because it has greater resolution Resolution is defined as the abilityto distinguish between two separate points If the two points cannot be resolved they will be seen as one point In practice resolution is the amount of detail that can be seen ndash the greater the resolution the greater the detailThe maximum resolution of a light microscope is 200 nm This means that if two points or objects are closer together than 200 nm they cannot be distinguished as separateIt is possible to take a photograph such as Figure 19a and to magnify (enlarge) it but we see no more detail in other words we do not improve resolution even though we often enlarge photographs because they are easier to see when larger With a microscope magnification up to the limit of resolution can reveal further detail but any further magnification increases blurring as well as the size of the imageThe electromagnetic spectrumHow is resolution linked with the nature of light One of the properties of light is that it travels in waves The length of the waves of visible light varies ranging from about 400 nm (violet light) to about 700 nm (red light) The human eye can distinguish between these different wavelengths and in the brain the differences are converted to colour differences (Colour is an invention of the brain)The whole range of different wavelengths is called the electromagnetic spectrum Visible light is only one part of this spectrum Figure 111 shows some of the parts of the electromagnetic spectrum The longer the waves the lower their frequency (all the waves travel at the same speed so imagine them passing a post shorter waves pass at higher frequency) In theory there is no limit to how short or how long the waves can be Wavelength changes with energy the greater the energy the shorter the wavelength (rather like squashing a spring)Now look at Figure 112 which shows a mitochondrion some very small cell organelles called ribosomes (see page 13) and light of 400 nm wavelength the shortest visible wavelength The mitochondrion is large enoughto interfere with the light waves However the ribosomes are far too small to have any effect on the light waves The general rule is that the limit of resolution is about one half the wavelength of the radiation used to view the specimen In other words if an object is any smaller than half the wavelength of the radiation used to view it it cannot be seen separately from nearby objects This means that the1001 1 Cell structureX-raysgamma rays01 nm400 nm violet bluev infraredmicrowaves

uv10 nmi s i b l e 1000 nmradio and TV wavesvisible light500 nm 600 nm green yellow orange105 nm 107 nm 109 nm 1011 nm 1013 nm700 nmFigure 111 Diagram of the electromagnetic spectrum (the waves are not drawn to scale) The numbers indicate the wavelengths of the different types of electromagnetic radiation Visible light is a form of electromagnetic radiationbest resolution that can be obtained using a microscope that uses visible light (a light microscope) is 200 nm since the shortest wavelength of visible light is 400 nm (violet light) In practice this corresponds to a maximum useful magnification of about 1500 times Ribosomes are approximately 25 nm in diameter and can therefore never be seen using lightIf an object is transparent it will allow light waves to pass through it and therefore will still not be visible This is why many biological structures have to be stained before they can be seenThe electron microscopeBiologists faced with the problem that they would never see anything smaller than 200 nm using a light microscope realised that the only solution would be to use radiation of a shorter wavelength than light If you study Figure 111 you will see that ultraviolet light or better still X-rays look like possible candidates Both ultraviolet and X-ray microscopes have been built the latter with little success partly because of the difficulty of focusing X-rays A much better solution is to use electrons Electrons are negatively charged particles which orbit the nucleus of an atom When a metal becomes very hot some of its electrons gain so much energy that they escape from their orbits like a rocket escaping from Earthrsquos gravity Free electrons behave like electromagnetic radiation They have a very short wavelength the greater the energy the shorter the wavelength Electrons are a very suitable form of radiation for microscopy for two major reasons Firstly their wavelength is extremely short (at least as short as that of X-rays) Secondly because they are negatively charged they can be focused easily using electromagnets (a magnet can be made to alter the path of the beam the equivalent of a glass lens bending light)Using an electron microscope a resolution of 05 nm can be obtained

400 times better than when using a light microscope ETransmission and scanning electron microscopesTwo types of electron microscope are now in common use The transmission electron microscope or TEM forwavelength 400nmstained mitochondrion of diameter 1000nm interferes with light wavesstained ribosomes of diameter 25nm do not interfere with light wavesFigure 112 A mitochondrion and some ribosomes in the path of light waves of 400 nm lengthred1 Cell structure 11

E short was the type originally developed Here the beam of electrons is passed through the specimen before being viewed Only

those electrons that are transmitted (pass through the specimen) are seen This allows us to see thin sections of specimens and thus to see inside cells In the scanning electron microscope (SEM) on the other hand the electron beam is used to scan the surfaces of structures and only the reflected beam is observedAn example of a scanning electron micrograph is shown in Figure 113 The advantage of this microscope is that surface structures can be seen Also great depth of field is obtained so that much of the specimen is in focus at the same time and a three-dimensional appearance is obtained Such a picture would be impossible to obtain with a light microscope even using the same magnification and resolution because you would have to keep focusing up and down with the objective lens to see different parts of the specimen The disadvantage of the SEM is that it cannot achieve the same resolution as a TEM Resolution is between 3 nm and 20 nmViewing specimens with the electron microscopeFigure 114 shows how an electron microscope works and Figure 115 shows one in useFigure 113 False-colour SEM of the head of a cat flea (1113088 100)

electron gun and anode which E produce a beam of electronselectron beam vacuumpathway of electronscondenser electromagnetic lens which directs the electron beam onto the specimenspecimen which is placed on a gridobjective electromagnetic lens which produces an imageprojector electromagnetic lenses which focus the magnified image onto the screenscreen or photographic plate which shows the image of the specimenFigure 114 How an electron microscope worksIt is not possible to see an electron beam so to make the image visible the electron beam has to be projected onto a fluorescent screen The areas hit by electrons shine brightly giving overall a lsquoblack and whitersquo picture The stains used to improve the contrast of biological specimens for electron microscopy contain heavy metal atoms which stop the passage of electrons The resulting picture is like an X-ray photograph with the more densely stained parts of the specimen appearing blacker lsquoFalse-colourrsquo images can be created by colouring the standard black and white image using a computerTo add to the difficulties of electron microscopy the electron beam and therefore the specimen and the fluorescent screen must be in a vacuum If electrons1221 1 Cell structure

EStructures and functions of organellesCompartmentalisation and division of labour within the cell are even more obvious with an electron microscope than with a light microscopeWe will now consider the structures and functions of some of the cell components in more detailNucleusThe nucleus (Figure 118 on page 15) is the largest cell organelle (see also page 5) It is surrounded by two membranes known as the

nuclear envelope The outer membrane of the nuclear envelope is continuous with the endoplasmic reticulum (Figure 117 on page 15) The nuclear envelope has many small pores called nuclear pores These allow and control exchange between the nucleus and the cytoplasm Examples of substances leaving the nucleus through the pores are mRNA and ribosomes for protein synthesis Examples of substances entering through the nuclear pores are proteins to help make ribosomes nucleotides ATP (aderosine triphosphate) and some hormones such as thyroid hormone T3Within the nucleus the chromosomes are in a loosely coiled state known as chromatin (except during nuclear division see Chapter 5) Chromosomes contain DNA which is organised into functional units called genes Genes control the activities of the cell and inheritance thus the nucleus controls the cellrsquos activities When a cell is aboutto divide the nucleus divides first so that each new cell will have its own nucleus (Chapters 5 and 19) Also within the nucleus the nucleolus makes ribosomes using the information in its own DNAEndoplasmic reticulum and ribosomesWhen cells were first seen with the electron microscope biologists were amazed to see so much detailed structure The existence of much of this had not been suspected This was particularly true of an extensive system of membranes running through the cytoplasm which became knownas the endoplasmic reticulum (ER) (Figure 119 on page 15 ndash see also Figures 118 on page 15 and 122 on page 17) The ER is continuous with the outer membrane of the nuclear envelope (Figure 117)There are two types of ER rough ER and smooth ER Rough ER is so called because it is covered with many tinyFigure 115 A TEM in usecollided with air molecules they would scatter making it impossible to achieve a sharp picture Also water boils at room temperature in a vacuum so all specimens must be dehydrated before being placed in the microscope This means that only dead material can be examined Great efforts are therefore made to try to preserve material in a life-like state when preparing it for the microscopeSAQ 13Explain why ribosomes are not visible using a light microscopeUltrastructure of an animal cellThe lsquofinersquo or detailed structure of a cell as revealed by the electron microscope is called its ultrastructure Figure 116 shows the appearance of typical animal cells as seen with an electron microscope and Figure 117 on page 15 is a diagram based on many other such micrographsSAQ 14Compare Figure 117 on page 15 with Figure 13 on page 2 Name the structures which can be seen with the electron microscope but not with the light microscope1 Cell structure 13lysosomeGGolgi apparatusendoplasmic reticulumglycogen granulesccell surface membrane

mmitochondriachromatinnnucleolusmmicrovillusrribosomesFFigure 116 Representative animal cells as seen with a TEM The cells are liver cells from a rat (1113088 9600) The nucleus is clearly visible in one of the cells 1441 1 Cell structurenucleusnnuclear envelopetwo centrioles close to the nucleus and at right angles to each othermitochondrion lysosomerough endoplasmic reticulumnucleoluschromatin nucleusnuclear porenuclear envelope (two membranes)microvillismooth endoplasmic reticulumFigure 117 Ultrastructure of a typical animal cell as seen with an electron microscope In reality the ER is more extensive than shown and free ribosomes may be more extensive Glycogen granules are sometimes present in the cytoplasmFigure 118 TEM of the nucleus of a cell from the pancreas of a bat (1113088 7500) The circular nucleus is surrounded by a double-layered nuclear envelope containing nuclear pores The nucleolus is more Figure 119 TEM of rough ER covered with ribosomes (black dots) darkly stained Rough ER is visible in the surrounding cytoplasm (1113088 17 000) Some free ribosomes can also be seen in the cytoplasmGolgi vesicle Golgi apparatusribosomescell surface membranecytoplasm1 Cell structure 15organelles called ribosomes These are just visible as black dots in Figures 118 and 119 on page 15 At very high magnifications they can be seen to consist of two subunits a large and a small subunit Ribosomes are the sites of protein synthesis (see pages 111ndash112) They can be found free in the cytoplasm as well as on the rough ER They are very small only about 25 nm in diameter They are made of RNA (ribonucleic acid) and protein The rough ER forms an extensive system of flattened sacs spreading in sheets throughout the cell Proteins made by the ribosomes on the rough ER enter the sacs and move through them The proteins are often processed in some way on their journey Small sacs called vesicles can break off from the ER and these can join together to form the Golgi apparatus Proteins can be exported from the cell via the Golgi apparatus (see page 80)Smooth ER so called because it lacks ribosomes has a completely different function It makes lipids and steroids such as cholesterol and the reproductive hormones oestrogen and testosteroneGolgi apparatus (Golgi body or Golgi complex)The Golgi apparatus is a stack of flattened sacs (Figure 120) This stack of sacs is sometimes referred to as theFigure 120 TEM of a Golgi apparatus A central stack of saucer-shaped sacs can be seen budding off small Golgi vesicles (green) These may form secretory vesicles whose contents can be released at the cell surface by exocytosis (see page 80)Golgi body More than one may be present in a cell The stack is constantly being formed at one end from vesicles which bud off from the ER and broken down again at the other end to form Golgi vesicles The stack of sacs with the associated vesicles is referred to as the Golgi apparatus or Golgi complex

The Golgi apparatus collects processes and sorts molecules (particularly proteins from the rough ER) ready for transport in Golgi vesicles either to other parts of the cell or out of the cell (secretion) Two examples of protein processing in the Golgi apparatus are the addition of sugars to proteins to make molecules known as glycoproteins and the removal of the first amino acid methionine from newly formed proteins to make a functioning protein In plants enzymes in the Golgi apparatus convert sugars into cell wall components Golgi vesicles are also used to make lysosomesLysosomesLysosomes (Figure 121) are spherical sacs surrounded by a single membrane and having no internal structure They are commonly 01ndash 05 1113088m in diameter They contain digestive (hydrolytic) enzymes which must be kept separateFigure 121 Lysosomes (orange) in a mouse kidney cell (1113088 55 000) They contain cell structures in the process of digestion and vesicles (green) Cytoplasm is coloured blue here1661 1 Cell structurefrom the rest of the cell to prevent damage Lysosomes are responsible for the breakdown (digestion) of unwanted structures such as old organelles or even whole cells as in mammary glands after lactation (breast feeding) In white blood cells lysosomes are used to digest bacteria (see endocytosis page 80) Enzymes are sometimes released outside the cell ndash for example in the replacement of cartilage with bone during development The headsof sperm contain a special lysosome the acrosome for digesting a path to the ovum (egg)MitochondriaMitochondria (singular mitochondrion) are usually about 1 1113088m in diameter and can be various shapes often sausage- shaped as in Figure 122 They are surrounded by two membranes (an envelope) The inner of these is foldedto form finger-like cristae which project into the interior solution or matrixThe main function of mitochondria is to carry out aerobic respiration As a result of respiration they make ATP the universal energy carrier in cells (see Chapter 16) They are also involved in the synthesis of lipids (page 37)Figure 122 Mitochondrion (orange) with its double membrane (envelope) the inner membrane is folded to form cristae (1113088 20 000) Mitochondria are the sites of aerobic cell respiration Note also the rough ERIn the 1960s it was discovered that mitochondria and chloroplasts contain ribosomes which are slightly smaller than those in the cytoplasm and are the same size as those found in bacteria The size of ribosomes is measured in lsquoS unitsrsquo which are a measure of how fast they sediment in a centrifuge Cytoplasmic ribosomes are 80S while those of bacteria mitochondria and chloroplasts are 70S It was also discovered in the 1960s that mitochondria and chloroplasts contain small circular DNA molecules also like those found in bacteria Not surprisingly it was later provedthat mitochondria and chloroplasts are in effect ancient bacteria which now live inside the larger cells typical of animals and plants (see

prokaryotic and eukaryotic cells page 18) This is known as the endosymbiont theory lsquoEndorsquo means lsquoinsidersquo and a lsquosymbiontrsquo is an organism which lives in a mutually beneficial relationship with another organism The DNA and ribosomes of mitochondria and chloroplasts are still active and responsible for the coding and synthesis of certain vital proteins but mitochondria and chloroplasts can no longer live independentlyMitochondrial ribosomes are just visible as tiny dark dots in the mitochondrial matrix in Figure 122Cell surface membraneThe cell surface membrane is extremely thin (about 7 nm) However at very high magnifications at least 1113088 100 000 it can be seen to have three layers described as a trilaminar appearance This consists of two dark lines (heavily stained) either side of a narrow pale interior (Figure 123) The membrane is partially permeable and controls exchange between the cell and its environment Membrane structure is discussed further in Chapter 4Figure 123 Cell surface membrane (1113088 250 000) At this magnification the membrane appears as two dark lines at the edge of the cellMicrovilliMicrovilli (singular microvillus) are finger-like extensions of the cell surface membrane typical of certain epithelial cells (cells covering surfaces of structures) They greatly1 Cell structure 17increase the surface area of the cell surface membrane (see Figure 117 on page 15) This is useful for example for absorption in the gut and for reabsorption in the proximal convoluted tubules of the kidney (see page 307)CentriolesThe extra resolution of the electron microscope reveals that just outside the nucleus there are really two centrioles (see Figure 124) not one as it appears under the light microscope (compare with Figure 13 on page 2) They lie close together at right-angles to each other A centriole is a hollow cylinder about 04 1113088m long formed from a ring of short microtubules tiny tubes made of a protein called tubulin These microtubules are used as a starting point for growing the spindle microtubules for nuclear division (see page 92) Centrioles are not found in plant cellsUltrastructure of a plant cellAll the structures except centrioles and microvilli so far described in animal cells are also found in plant cells The appearance of a plant cell as seen with the electron microscope is shown in Figure 125 while Figure 126 is a diagram based on many such micrographs The relatively thick cell wall and the large central vacuole are obvious as are the chloroplasts two of which are shown in detail in Figure 127 These structures and their functions have been described on pages 5 and 6 The electron microscopeFigure 124 Centrioles in transverse and longitudinal section (TS and LS) (1113088 86 000) The one on the left is seen in TS and clearly shows the nine triplets of microtubules which make up the structurereveals that chloroplasts contain 70S ribosomes and small circular

DNA molecules as do mitochondria and bacteria This has already been discussed with mitochondria above (page 17)SAQ 15Compare Figure 126 with Figure 15 on page 3 Name the structures which can be seen with the electron microscope but not with the light microscopeTwo fundamentally different types of cellAt one time it was common practice to try to classify all living organisms as either animals or plants With advances in our knowledge of living things it has become obvious that the living world is not that simple Fungi and bacteria for example are very different from animals and plants and from each other Eventually it was discovered that there are two fundamentally different types of cell The most obvious difference betweenthese types is that one possesses a nucleus and the other does notOrganisms that lack nuclei are called prokaryotes (lsquoprorsquo means before lsquokaryonrsquo means nucleus) All prokaryotes are now referred to as bacteria They are on average about 1000 to 10 000 times smaller in volume than cells with nuclei and are much simpler in structure ndash for example their DNA lies free in the cytoplasmOrganisms whose cells possess nuclei are called eukaryotes (lsquoeursquo means true) Their DNA lies inside a nucleus Eukaryotes include animals plants fungi and a group containing most of the unicellular eukaryotes known as protoctists Most biologists believe that eukaryotes evolved from prokaryotes one-and-a-half thousand million years after prokaryotes first appeared on Earth We mainly study animals and plants in this book but all eukaryotic cells have certain features in commonA generalised prokaryotic cell is shown in Figure 128 A comparison of prokaryotic and eukaryotic cells is given in Table 12 on page 211881 1 Cell structure

eyepiece graticule in the eyepiece of the microscope00stage micrometer scale (marked in 00 1mm and 01 mm divisions)10 20 30 40 50 60 70 80 90 10001 021 Cell structure 7

Electron microscopesEarlier in this chapter it was stated that by 1900 almost all the structures shown in Figures 13 and 15 (pages 2 and 3) had been discovered There followed a time of frustration for microscopists because they realised that no matter how much the design of light microscopes improved there was a limit to how much could ever be seen using lightIn order to understand the problem it is necessary to know something about the nature of light itself and to understand the difference between magnification and resolutionMagnificationMagnification is the number of times larger an image is compared with the real size of the objectWorked example 1 ndash calculating the magnification of a photograph or objectTo calculate M the magnification of a photograph or an object we can use the following methodFigure 19 shows two photographs of a section through the same plant cells The magnifications of the two photographs are the same Suppose we want to know the magnification of the plant cell in Figure 19b If we know its actual (real) length we can calculate its magnification using theThe real length of the cell is 80 1113088mmagnification observed size of the image actual sizeM I AStep 1Measure the length in mm of the cell in the photograph using a ruler You should find that it is about 60 mmStep 2Convert mm to 1113088m (It is easier if we first convert all measurements to the same units ndash in this case micrometres 1113088m)1 mm 1113088 1000 1113088m so 60mm 1113088 60 1113088 10001113088morwhere I 1113088 observed size of the image (that is what you can measure with a ruler) and A 1113088 actual size (that is the real size ndash for example the size of a cell before it is magnified)If you know two of these values you can work out the third one For example if the observed size of the image and the magnification are known you can work out the1113088 60 000 1113088m Use the equation to calculate the magnificationStep 3actual size A I If you write the formula in a triangle Mmagnification Mimage size I

as shown below and cover up the value you want to find it should be obvious how to do the right calculationactual size A 60000m80 m 750I M1113088ASome worked examples are now providedThe lsquo1113088rsquo sign in front of the number 750 means lsquotimesrsquo We say that the magnification is lsquotimes 750rsquoformula M I A88 1 Cell structurea Worked example 2 ndash calculating magnification from a scale barFigure 110 shows a lymphocyteFigure 110 A lymphocyte6 μmbFigure 19 Photographs of the same plant cells seen a with a light microscope b with an electron microscope both shown at a magnification of about 1113088 750We can calculate the magnification of the lymphocyte by simply using the scale bar All you need to do is measure the length of the scale bar and then substitute this and the length it represents into the equationStep 1Measure the scale bar Here it is 36 mmStep 2Convert mm to 1113088m 36mm 1113088 36 1113088 10001113088m 1113088 360001113088mStep 3Use the equation to calculate the magnificationmagnification M image size I actual size A36000m 6m60001 Cell structure 9Worked example 3 ndash calculating the real size of an object from its magnificationTo calculate A the real or actual size of an object we can use the following methodFigure 125 on page 19 shows a plant cell magnified 1113088 5600 One of the chloroplasts is labelled lsquochloroplastrsquo in the figure Suppose we want to know the actual length of this chloroplastStep 1Measure the observed length of the image of the chloroplast (I ) in mm using a ruler The maximum length is 36 mmStep 2Convert mm to 1113088m 30mm 1113088 30 1113088 10001113088m 1113088 300001113088mStep 3Use the equation to calculate the actual lengthactualsizeA imagesizeI magnification M30 000 m 5600 5 4 m (to one decimal place)SAQ 12a Calculate the magnification of the drawing of the animal cell in Figure 13 on page 2b Calculate the actual (real) length of the bottom chloroplast in Figure 127 on page 19Resolution

Look again at Figure 19 (page 9) Figure 19a is a light micrograph (a photograph taken with a light microscope also known as a photomicrograph) Figure 19b is an electron micrograph of the same cells taken at the same magnification (an electron micrograph is a picture taken with an electron microscope) You can see that Figure 19b the electron micrograph is much clearer This is because it has greater resolution Resolution is defined as the abilityto distinguish between two separate points If the two points cannot be resolved they will be seen as one point In practice resolution is the amount of detail that can be seen ndash the greater the resolution the greater the detailThe maximum resolution of a light microscope is 200 nm This means that if two points or objects are closer together than 200 nm they cannot be distinguished as separateIt is possible to take a photograph such as Figure 19a and to magnify (enlarge) it but we see no more detail in other words we do not improve resolution even though we often enlarge photographs because they are easier to see when larger With a microscope magnification up to the limit of resolution can reveal further detail but any further magnification increases blurring as well as the size of the imageThe electromagnetic spectrumHow is resolution linked with the nature of light One of the properties of light is that it travels in waves The length of the waves of visible light varies ranging from about 400 nm (violet light) to about 700 nm (red light) The human eye can distinguish between these different wavelengths and in the brain the differences are converted to colour differences (Colour is an invention of the brain)The whole range of different wavelengths is called the electromagnetic spectrum Visible light is only one part of this spectrum Figure 111 shows some of the parts of the electromagnetic spectrum The longer the waves the lower their frequency (all the waves travel at the same speed so imagine them passing a post shorter waves pass at higher frequency) In theory there is no limit to how short or how long the waves can be Wavelength changes with energy the greater the energy the shorter the wavelength (rather like squashing a spring)Now look at Figure 112 which shows a mitochondrion some very small cell organelles called ribosomes (see page 13) and light of 400 nm wavelength the shortest visible wavelength The mitochondrion is large enoughto interfere with the light waves However the ribosomes are far too small to have any effect on the light waves The general rule is that the limit of resolution is about one half the wavelength of the radiation used to view the specimen In other words if an object is any smaller than half the wavelength of the radiation used to view it it cannot be seen separately from nearby objects This means that the1001 1 Cell structureX-raysgamma rays01 nm400 nm violet bluev infraredmicrowaves

uv10 nmi s i b l e 1000 nmradio and TV wavesvisible light500 nm 600 nm green yellow orange105 nm 107 nm 109 nm 1011 nm 1013 nm700 nmFigure 111 Diagram of the electromagnetic spectrum (the waves are not drawn to scale) The numbers indicate the wavelengths of the different types of electromagnetic radiation Visible light is a form of electromagnetic radiationbest resolution that can be obtained using a microscope that uses visible light (a light microscope) is 200 nm since the shortest wavelength of visible light is 400 nm (violet light) In practice this corresponds to a maximum useful magnification of about 1500 times Ribosomes are approximately 25 nm in diameter and can therefore never be seen using lightIf an object is transparent it will allow light waves to pass through it and therefore will still not be visible This is why many biological structures have to be stained before they can be seenThe electron microscopeBiologists faced with the problem that they would never see anything smaller than 200 nm using a light microscope realised that the only solution would be to use radiation of a shorter wavelength than light If you study Figure 111 you will see that ultraviolet light or better still X-rays look like possible candidates Both ultraviolet and X-ray microscopes have been built the latter with little success partly because of the difficulty of focusing X-rays A much better solution is to use electrons Electrons are negatively charged particles which orbit the nucleus of an atom When a metal becomes very hot some of its electrons gain so much energy that they escape from their orbits like a rocket escaping from Earthrsquos gravity Free electrons behave like electromagnetic radiation They have a very short wavelength the greater the energy the shorter the wavelength Electrons are a very suitable form of radiation for microscopy for two major reasons Firstly their wavelength is extremely short (at least as short as that of X-rays) Secondly because they are negatively charged they can be focused easily using electromagnets (a magnet can be made to alter the path of the beam the equivalent of a glass lens bending light)Using an electron microscope a resolution of 05 nm can be obtained

400 times better than when using a light microscope ETransmission and scanning electron microscopesTwo types of electron microscope are now in common use The transmission electron microscope or TEM forwavelength 400nmstained mitochondrion of diameter 1000nm interferes with light wavesstained ribosomes of diameter 25nm do not interfere with light wavesFigure 112 A mitochondrion and some ribosomes in the path of light waves of 400 nm lengthred1 Cell structure 11

E short was the type originally developed Here the beam of electrons is passed through the specimen before being viewed Only

those electrons that are transmitted (pass through the specimen) are seen This allows us to see thin sections of specimens and thus to see inside cells In the scanning electron microscope (SEM) on the other hand the electron beam is used to scan the surfaces of structures and only the reflected beam is observedAn example of a scanning electron micrograph is shown in Figure 113 The advantage of this microscope is that surface structures can be seen Also great depth of field is obtained so that much of the specimen is in focus at the same time and a three-dimensional appearance is obtained Such a picture would be impossible to obtain with a light microscope even using the same magnification and resolution because you would have to keep focusing up and down with the objective lens to see different parts of the specimen The disadvantage of the SEM is that it cannot achieve the same resolution as a TEM Resolution is between 3 nm and 20 nmViewing specimens with the electron microscopeFigure 114 shows how an electron microscope works and Figure 115 shows one in useFigure 113 False-colour SEM of the head of a cat flea (1113088 100)

electron gun and anode which E produce a beam of electronselectron beam vacuumpathway of electronscondenser electromagnetic lens which directs the electron beam onto the specimenspecimen which is placed on a gridobjective electromagnetic lens which produces an imageprojector electromagnetic lenses which focus the magnified image onto the screenscreen or photographic plate which shows the image of the specimenFigure 114 How an electron microscope worksIt is not possible to see an electron beam so to make the image visible the electron beam has to be projected onto a fluorescent screen The areas hit by electrons shine brightly giving overall a lsquoblack and whitersquo picture The stains used to improve the contrast of biological specimens for electron microscopy contain heavy metal atoms which stop the passage of electrons The resulting picture is like an X-ray photograph with the more densely stained parts of the specimen appearing blacker lsquoFalse-colourrsquo images can be created by colouring the standard black and white image using a computerTo add to the difficulties of electron microscopy the electron beam and therefore the specimen and the fluorescent screen must be in a vacuum If electrons1221 1 Cell structure

EStructures and functions of organellesCompartmentalisation and division of labour within the cell are even more obvious with an electron microscope than with a light microscopeWe will now consider the structures and functions of some of the cell components in more detailNucleusThe nucleus (Figure 118 on page 15) is the largest cell organelle (see also page 5) It is surrounded by two membranes known as the

nuclear envelope The outer membrane of the nuclear envelope is continuous with the endoplasmic reticulum (Figure 117 on page 15) The nuclear envelope has many small pores called nuclear pores These allow and control exchange between the nucleus and the cytoplasm Examples of substances leaving the nucleus through the pores are mRNA and ribosomes for protein synthesis Examples of substances entering through the nuclear pores are proteins to help make ribosomes nucleotides ATP (aderosine triphosphate) and some hormones such as thyroid hormone T3Within the nucleus the chromosomes are in a loosely coiled state known as chromatin (except during nuclear division see Chapter 5) Chromosomes contain DNA which is organised into functional units called genes Genes control the activities of the cell and inheritance thus the nucleus controls the cellrsquos activities When a cell is aboutto divide the nucleus divides first so that each new cell will have its own nucleus (Chapters 5 and 19) Also within the nucleus the nucleolus makes ribosomes using the information in its own DNAEndoplasmic reticulum and ribosomesWhen cells were first seen with the electron microscope biologists were amazed to see so much detailed structure The existence of much of this had not been suspected This was particularly true of an extensive system of membranes running through the cytoplasm which became knownas the endoplasmic reticulum (ER) (Figure 119 on page 15 ndash see also Figures 118 on page 15 and 122 on page 17) The ER is continuous with the outer membrane of the nuclear envelope (Figure 117)There are two types of ER rough ER and smooth ER Rough ER is so called because it is covered with many tinyFigure 115 A TEM in usecollided with air molecules they would scatter making it impossible to achieve a sharp picture Also water boils at room temperature in a vacuum so all specimens must be dehydrated before being placed in the microscope This means that only dead material can be examined Great efforts are therefore made to try to preserve material in a life-like state when preparing it for the microscopeSAQ 13Explain why ribosomes are not visible using a light microscopeUltrastructure of an animal cellThe lsquofinersquo or detailed structure of a cell as revealed by the electron microscope is called its ultrastructure Figure 116 shows the appearance of typical animal cells as seen with an electron microscope and Figure 117 on page 15 is a diagram based on many other such micrographsSAQ 14Compare Figure 117 on page 15 with Figure 13 on page 2 Name the structures which can be seen with the electron microscope but not with the light microscope1 Cell structure 13lysosomeGGolgi apparatusendoplasmic reticulumglycogen granulesccell surface membrane

mmitochondriachromatinnnucleolusmmicrovillusrribosomesFFigure 116 Representative animal cells as seen with a TEM The cells are liver cells from a rat (1113088 9600) The nucleus is clearly visible in one of the cells 1441 1 Cell structurenucleusnnuclear envelopetwo centrioles close to the nucleus and at right angles to each othermitochondrion lysosomerough endoplasmic reticulumnucleoluschromatin nucleusnuclear porenuclear envelope (two membranes)microvillismooth endoplasmic reticulumFigure 117 Ultrastructure of a typical animal cell as seen with an electron microscope In reality the ER is more extensive than shown and free ribosomes may be more extensive Glycogen granules are sometimes present in the cytoplasmFigure 118 TEM of the nucleus of a cell from the pancreas of a bat (1113088 7500) The circular nucleus is surrounded by a double-layered nuclear envelope containing nuclear pores The nucleolus is more Figure 119 TEM of rough ER covered with ribosomes (black dots) darkly stained Rough ER is visible in the surrounding cytoplasm (1113088 17 000) Some free ribosomes can also be seen in the cytoplasmGolgi vesicle Golgi apparatusribosomescell surface membranecytoplasm1 Cell structure 15organelles called ribosomes These are just visible as black dots in Figures 118 and 119 on page 15 At very high magnifications they can be seen to consist of two subunits a large and a small subunit Ribosomes are the sites of protein synthesis (see pages 111ndash112) They can be found free in the cytoplasm as well as on the rough ER They are very small only about 25 nm in diameter They are made of RNA (ribonucleic acid) and protein The rough ER forms an extensive system of flattened sacs spreading in sheets throughout the cell Proteins made by the ribosomes on the rough ER enter the sacs and move through them The proteins are often processed in some way on their journey Small sacs called vesicles can break off from the ER and these can join together to form the Golgi apparatus Proteins can be exported from the cell via the Golgi apparatus (see page 80)Smooth ER so called because it lacks ribosomes has a completely different function It makes lipids and steroids such as cholesterol and the reproductive hormones oestrogen and testosteroneGolgi apparatus (Golgi body or Golgi complex)The Golgi apparatus is a stack of flattened sacs (Figure 120) This stack of sacs is sometimes referred to as theFigure 120 TEM of a Golgi apparatus A central stack of saucer-shaped sacs can be seen budding off small Golgi vesicles (green) These may form secretory vesicles whose contents can be released at the cell surface by exocytosis (see page 80)Golgi body More than one may be present in a cell The stack is constantly being formed at one end from vesicles which bud off from the ER and broken down again at the other end to form Golgi vesicles The stack of sacs with the associated vesicles is referred to as the Golgi apparatus or Golgi complex

The Golgi apparatus collects processes and sorts molecules (particularly proteins from the rough ER) ready for transport in Golgi vesicles either to other parts of the cell or out of the cell (secretion) Two examples of protein processing in the Golgi apparatus are the addition of sugars to proteins to make molecules known as glycoproteins and the removal of the first amino acid methionine from newly formed proteins to make a functioning protein In plants enzymes in the Golgi apparatus convert sugars into cell wall components Golgi vesicles are also used to make lysosomesLysosomesLysosomes (Figure 121) are spherical sacs surrounded by a single membrane and having no internal structure They are commonly 01ndash 05 1113088m in diameter They contain digestive (hydrolytic) enzymes which must be kept separateFigure 121 Lysosomes (orange) in a mouse kidney cell (1113088 55 000) They contain cell structures in the process of digestion and vesicles (green) Cytoplasm is coloured blue here1661 1 Cell structurefrom the rest of the cell to prevent damage Lysosomes are responsible for the breakdown (digestion) of unwanted structures such as old organelles or even whole cells as in mammary glands after lactation (breast feeding) In white blood cells lysosomes are used to digest bacteria (see endocytosis page 80) Enzymes are sometimes released outside the cell ndash for example in the replacement of cartilage with bone during development The headsof sperm contain a special lysosome the acrosome for digesting a path to the ovum (egg)MitochondriaMitochondria (singular mitochondrion) are usually about 1 1113088m in diameter and can be various shapes often sausage- shaped as in Figure 122 They are surrounded by two membranes (an envelope) The inner of these is foldedto form finger-like cristae which project into the interior solution or matrixThe main function of mitochondria is to carry out aerobic respiration As a result of respiration they make ATP the universal energy carrier in cells (see Chapter 16) They are also involved in the synthesis of lipids (page 37)Figure 122 Mitochondrion (orange) with its double membrane (envelope) the inner membrane is folded to form cristae (1113088 20 000) Mitochondria are the sites of aerobic cell respiration Note also the rough ERIn the 1960s it was discovered that mitochondria and chloroplasts contain ribosomes which are slightly smaller than those in the cytoplasm and are the same size as those found in bacteria The size of ribosomes is measured in lsquoS unitsrsquo which are a measure of how fast they sediment in a centrifuge Cytoplasmic ribosomes are 80S while those of bacteria mitochondria and chloroplasts are 70S It was also discovered in the 1960s that mitochondria and chloroplasts contain small circular DNA molecules also like those found in bacteria Not surprisingly it was later provedthat mitochondria and chloroplasts are in effect ancient bacteria which now live inside the larger cells typical of animals and plants (see

prokaryotic and eukaryotic cells page 18) This is known as the endosymbiont theory lsquoEndorsquo means lsquoinsidersquo and a lsquosymbiontrsquo is an organism which lives in a mutually beneficial relationship with another organism The DNA and ribosomes of mitochondria and chloroplasts are still active and responsible for the coding and synthesis of certain vital proteins but mitochondria and chloroplasts can no longer live independentlyMitochondrial ribosomes are just visible as tiny dark dots in the mitochondrial matrix in Figure 122Cell surface membraneThe cell surface membrane is extremely thin (about 7 nm) However at very high magnifications at least 1113088 100 000 it can be seen to have three layers described as a trilaminar appearance This consists of two dark lines (heavily stained) either side of a narrow pale interior (Figure 123) The membrane is partially permeable and controls exchange between the cell and its environment Membrane structure is discussed further in Chapter 4Figure 123 Cell surface membrane (1113088 250 000) At this magnification the membrane appears as two dark lines at the edge of the cellMicrovilliMicrovilli (singular microvillus) are finger-like extensions of the cell surface membrane typical of certain epithelial cells (cells covering surfaces of structures) They greatly1 Cell structure 17increase the surface area of the cell surface membrane (see Figure 117 on page 15) This is useful for example for absorption in the gut and for reabsorption in the proximal convoluted tubules of the kidney (see page 307)CentriolesThe extra resolution of the electron microscope reveals that just outside the nucleus there are really two centrioles (see Figure 124) not one as it appears under the light microscope (compare with Figure 13 on page 2) They lie close together at right-angles to each other A centriole is a hollow cylinder about 04 1113088m long formed from a ring of short microtubules tiny tubes made of a protein called tubulin These microtubules are used as a starting point for growing the spindle microtubules for nuclear division (see page 92) Centrioles are not found in plant cellsUltrastructure of a plant cellAll the structures except centrioles and microvilli so far described in animal cells are also found in plant cells The appearance of a plant cell as seen with the electron microscope is shown in Figure 125 while Figure 126 is a diagram based on many such micrographs The relatively thick cell wall and the large central vacuole are obvious as are the chloroplasts two of which are shown in detail in Figure 127 These structures and their functions have been described on pages 5 and 6 The electron microscopeFigure 124 Centrioles in transverse and longitudinal section (TS and LS) (1113088 86 000) The one on the left is seen in TS and clearly shows the nine triplets of microtubules which make up the structurereveals that chloroplasts contain 70S ribosomes and small circular

DNA molecules as do mitochondria and bacteria This has already been discussed with mitochondria above (page 17)SAQ 15Compare Figure 126 with Figure 15 on page 3 Name the structures which can be seen with the electron microscope but not with the light microscopeTwo fundamentally different types of cellAt one time it was common practice to try to classify all living organisms as either animals or plants With advances in our knowledge of living things it has become obvious that the living world is not that simple Fungi and bacteria for example are very different from animals and plants and from each other Eventually it was discovered that there are two fundamentally different types of cell The most obvious difference betweenthese types is that one possesses a nucleus and the other does notOrganisms that lack nuclei are called prokaryotes (lsquoprorsquo means before lsquokaryonrsquo means nucleus) All prokaryotes are now referred to as bacteria They are on average about 1000 to 10 000 times smaller in volume than cells with nuclei and are much simpler in structure ndash for example their DNA lies free in the cytoplasmOrganisms whose cells possess nuclei are called eukaryotes (lsquoeursquo means true) Their DNA lies inside a nucleus Eukaryotes include animals plants fungi and a group containing most of the unicellular eukaryotes known as protoctists Most biologists believe that eukaryotes evolved from prokaryotes one-and-a-half thousand million years after prokaryotes first appeared on Earth We mainly study animals and plants in this book but all eukaryotic cells have certain features in commonA generalised prokaryotic cell is shown in Figure 128 A comparison of prokaryotic and eukaryotic cells is given in Table 12 on page 211881 1 Cell structure

as shown below and cover up the value you want to find it should be obvious how to do the right calculationactual size A 60000m80 m 750I M1113088ASome worked examples are now providedThe lsquo1113088rsquo sign in front of the number 750 means lsquotimesrsquo We say that the magnification is lsquotimes 750rsquoformula M I A88 1 Cell structurea Worked example 2 ndash calculating magnification from a scale barFigure 110 shows a lymphocyteFigure 110 A lymphocyte6 μmbFigure 19 Photographs of the same plant cells seen a with a light microscope b with an electron microscope both shown at a magnification of about 1113088 750We can calculate the magnification of the lymphocyte by simply using the scale bar All you need to do is measure the length of the scale bar and then substitute this and the length it represents into the equationStep 1Measure the scale bar Here it is 36 mmStep 2Convert mm to 1113088m 36mm 1113088 36 1113088 10001113088m 1113088 360001113088mStep 3Use the equation to calculate the magnificationmagnification M image size I actual size A36000m 6m60001 Cell structure 9Worked example 3 ndash calculating the real size of an object from its magnificationTo calculate A the real or actual size of an object we can use the following methodFigure 125 on page 19 shows a plant cell magnified 1113088 5600 One of the chloroplasts is labelled lsquochloroplastrsquo in the figure Suppose we want to know the actual length of this chloroplastStep 1Measure the observed length of the image of the chloroplast (I ) in mm using a ruler The maximum length is 36 mmStep 2Convert mm to 1113088m 30mm 1113088 30 1113088 10001113088m 1113088 300001113088mStep 3Use the equation to calculate the actual lengthactualsizeA imagesizeI magnification M30 000 m 5600 5 4 m (to one decimal place)SAQ 12a Calculate the magnification of the drawing of the animal cell in Figure 13 on page 2b Calculate the actual (real) length of the bottom chloroplast in Figure 127 on page 19Resolution

Look again at Figure 19 (page 9) Figure 19a is a light micrograph (a photograph taken with a light microscope also known as a photomicrograph) Figure 19b is an electron micrograph of the same cells taken at the same magnification (an electron micrograph is a picture taken with an electron microscope) You can see that Figure 19b the electron micrograph is much clearer This is because it has greater resolution Resolution is defined as the abilityto distinguish between two separate points If the two points cannot be resolved they will be seen as one point In practice resolution is the amount of detail that can be seen ndash the greater the resolution the greater the detailThe maximum resolution of a light microscope is 200 nm This means that if two points or objects are closer together than 200 nm they cannot be distinguished as separateIt is possible to take a photograph such as Figure 19a and to magnify (enlarge) it but we see no more detail in other words we do not improve resolution even though we often enlarge photographs because they are easier to see when larger With a microscope magnification up to the limit of resolution can reveal further detail but any further magnification increases blurring as well as the size of the imageThe electromagnetic spectrumHow is resolution linked with the nature of light One of the properties of light is that it travels in waves The length of the waves of visible light varies ranging from about 400 nm (violet light) to about 700 nm (red light) The human eye can distinguish between these different wavelengths and in the brain the differences are converted to colour differences (Colour is an invention of the brain)The whole range of different wavelengths is called the electromagnetic spectrum Visible light is only one part of this spectrum Figure 111 shows some of the parts of the electromagnetic spectrum The longer the waves the lower their frequency (all the waves travel at the same speed so imagine them passing a post shorter waves pass at higher frequency) In theory there is no limit to how short or how long the waves can be Wavelength changes with energy the greater the energy the shorter the wavelength (rather like squashing a spring)Now look at Figure 112 which shows a mitochondrion some very small cell organelles called ribosomes (see page 13) and light of 400 nm wavelength the shortest visible wavelength The mitochondrion is large enoughto interfere with the light waves However the ribosomes are far too small to have any effect on the light waves The general rule is that the limit of resolution is about one half the wavelength of the radiation used to view the specimen In other words if an object is any smaller than half the wavelength of the radiation used to view it it cannot be seen separately from nearby objects This means that the1001 1 Cell structureX-raysgamma rays01 nm400 nm violet bluev infraredmicrowaves

uv10 nmi s i b l e 1000 nmradio and TV wavesvisible light500 nm 600 nm green yellow orange105 nm 107 nm 109 nm 1011 nm 1013 nm700 nmFigure 111 Diagram of the electromagnetic spectrum (the waves are not drawn to scale) The numbers indicate the wavelengths of the different types of electromagnetic radiation Visible light is a form of electromagnetic radiationbest resolution that can be obtained using a microscope that uses visible light (a light microscope) is 200 nm since the shortest wavelength of visible light is 400 nm (violet light) In practice this corresponds to a maximum useful magnification of about 1500 times Ribosomes are approximately 25 nm in diameter and can therefore never be seen using lightIf an object is transparent it will allow light waves to pass through it and therefore will still not be visible This is why many biological structures have to be stained before they can be seenThe electron microscopeBiologists faced with the problem that they would never see anything smaller than 200 nm using a light microscope realised that the only solution would be to use radiation of a shorter wavelength than light If you study Figure 111 you will see that ultraviolet light or better still X-rays look like possible candidates Both ultraviolet and X-ray microscopes have been built the latter with little success partly because of the difficulty of focusing X-rays A much better solution is to use electrons Electrons are negatively charged particles which orbit the nucleus of an atom When a metal becomes very hot some of its electrons gain so much energy that they escape from their orbits like a rocket escaping from Earthrsquos gravity Free electrons behave like electromagnetic radiation They have a very short wavelength the greater the energy the shorter the wavelength Electrons are a very suitable form of radiation for microscopy for two major reasons Firstly their wavelength is extremely short (at least as short as that of X-rays) Secondly because they are negatively charged they can be focused easily using electromagnets (a magnet can be made to alter the path of the beam the equivalent of a glass lens bending light)Using an electron microscope a resolution of 05 nm can be obtained

400 times better than when using a light microscope ETransmission and scanning electron microscopesTwo types of electron microscope are now in common use The transmission electron microscope or TEM forwavelength 400nmstained mitochondrion of diameter 1000nm interferes with light wavesstained ribosomes of diameter 25nm do not interfere with light wavesFigure 112 A mitochondrion and some ribosomes in the path of light waves of 400 nm lengthred1 Cell structure 11

E short was the type originally developed Here the beam of electrons is passed through the specimen before being viewed Only

those electrons that are transmitted (pass through the specimen) are seen This allows us to see thin sections of specimens and thus to see inside cells In the scanning electron microscope (SEM) on the other hand the electron beam is used to scan the surfaces of structures and only the reflected beam is observedAn example of a scanning electron micrograph is shown in Figure 113 The advantage of this microscope is that surface structures can be seen Also great depth of field is obtained so that much of the specimen is in focus at the same time and a three-dimensional appearance is obtained Such a picture would be impossible to obtain with a light microscope even using the same magnification and resolution because you would have to keep focusing up and down with the objective lens to see different parts of the specimen The disadvantage of the SEM is that it cannot achieve the same resolution as a TEM Resolution is between 3 nm and 20 nmViewing specimens with the electron microscopeFigure 114 shows how an electron microscope works and Figure 115 shows one in useFigure 113 False-colour SEM of the head of a cat flea (1113088 100)

electron gun and anode which E produce a beam of electronselectron beam vacuumpathway of electronscondenser electromagnetic lens which directs the electron beam onto the specimenspecimen which is placed on a gridobjective electromagnetic lens which produces an imageprojector electromagnetic lenses which focus the magnified image onto the screenscreen or photographic plate which shows the image of the specimenFigure 114 How an electron microscope worksIt is not possible to see an electron beam so to make the image visible the electron beam has to be projected onto a fluorescent screen The areas hit by electrons shine brightly giving overall a lsquoblack and whitersquo picture The stains used to improve the contrast of biological specimens for electron microscopy contain heavy metal atoms which stop the passage of electrons The resulting picture is like an X-ray photograph with the more densely stained parts of the specimen appearing blacker lsquoFalse-colourrsquo images can be created by colouring the standard black and white image using a computerTo add to the difficulties of electron microscopy the electron beam and therefore the specimen and the fluorescent screen must be in a vacuum If electrons1221 1 Cell structure

EStructures and functions of organellesCompartmentalisation and division of labour within the cell are even more obvious with an electron microscope than with a light microscopeWe will now consider the structures and functions of some of the cell components in more detailNucleusThe nucleus (Figure 118 on page 15) is the largest cell organelle (see also page 5) It is surrounded by two membranes known as the

nuclear envelope The outer membrane of the nuclear envelope is continuous with the endoplasmic reticulum (Figure 117 on page 15) The nuclear envelope has many small pores called nuclear pores These allow and control exchange between the nucleus and the cytoplasm Examples of substances leaving the nucleus through the pores are mRNA and ribosomes for protein synthesis Examples of substances entering through the nuclear pores are proteins to help make ribosomes nucleotides ATP (aderosine triphosphate) and some hormones such as thyroid hormone T3Within the nucleus the chromosomes are in a loosely coiled state known as chromatin (except during nuclear division see Chapter 5) Chromosomes contain DNA which is organised into functional units called genes Genes control the activities of the cell and inheritance thus the nucleus controls the cellrsquos activities When a cell is aboutto divide the nucleus divides first so that each new cell will have its own nucleus (Chapters 5 and 19) Also within the nucleus the nucleolus makes ribosomes using the information in its own DNAEndoplasmic reticulum and ribosomesWhen cells were first seen with the electron microscope biologists were amazed to see so much detailed structure The existence of much of this had not been suspected This was particularly true of an extensive system of membranes running through the cytoplasm which became knownas the endoplasmic reticulum (ER) (Figure 119 on page 15 ndash see also Figures 118 on page 15 and 122 on page 17) The ER is continuous with the outer membrane of the nuclear envelope (Figure 117)There are two types of ER rough ER and smooth ER Rough ER is so called because it is covered with many tinyFigure 115 A TEM in usecollided with air molecules they would scatter making it impossible to achieve a sharp picture Also water boils at room temperature in a vacuum so all specimens must be dehydrated before being placed in the microscope This means that only dead material can be examined Great efforts are therefore made to try to preserve material in a life-like state when preparing it for the microscopeSAQ 13Explain why ribosomes are not visible using a light microscopeUltrastructure of an animal cellThe lsquofinersquo or detailed structure of a cell as revealed by the electron microscope is called its ultrastructure Figure 116 shows the appearance of typical animal cells as seen with an electron microscope and Figure 117 on page 15 is a diagram based on many other such micrographsSAQ 14Compare Figure 117 on page 15 with Figure 13 on page 2 Name the structures which can be seen with the electron microscope but not with the light microscope1 Cell structure 13lysosomeGGolgi apparatusendoplasmic reticulumglycogen granulesccell surface membrane

mmitochondriachromatinnnucleolusmmicrovillusrribosomesFFigure 116 Representative animal cells as seen with a TEM The cells are liver cells from a rat (1113088 9600) The nucleus is clearly visible in one of the cells 1441 1 Cell structurenucleusnnuclear envelopetwo centrioles close to the nucleus and at right angles to each othermitochondrion lysosomerough endoplasmic reticulumnucleoluschromatin nucleusnuclear porenuclear envelope (two membranes)microvillismooth endoplasmic reticulumFigure 117 Ultrastructure of a typical animal cell as seen with an electron microscope In reality the ER is more extensive than shown and free ribosomes may be more extensive Glycogen granules are sometimes present in the cytoplasmFigure 118 TEM of the nucleus of a cell from the pancreas of a bat (1113088 7500) The circular nucleus is surrounded by a double-layered nuclear envelope containing nuclear pores The nucleolus is more Figure 119 TEM of rough ER covered with ribosomes (black dots) darkly stained Rough ER is visible in the surrounding cytoplasm (1113088 17 000) Some free ribosomes can also be seen in the cytoplasmGolgi vesicle Golgi apparatusribosomescell surface membranecytoplasm1 Cell structure 15organelles called ribosomes These are just visible as black dots in Figures 118 and 119 on page 15 At very high magnifications they can be seen to consist of two subunits a large and a small subunit Ribosomes are the sites of protein synthesis (see pages 111ndash112) They can be found free in the cytoplasm as well as on the rough ER They are very small only about 25 nm in diameter They are made of RNA (ribonucleic acid) and protein The rough ER forms an extensive system of flattened sacs spreading in sheets throughout the cell Proteins made by the ribosomes on the rough ER enter the sacs and move through them The proteins are often processed in some way on their journey Small sacs called vesicles can break off from the ER and these can join together to form the Golgi apparatus Proteins can be exported from the cell via the Golgi apparatus (see page 80)Smooth ER so called because it lacks ribosomes has a completely different function It makes lipids and steroids such as cholesterol and the reproductive hormones oestrogen and testosteroneGolgi apparatus (Golgi body or Golgi complex)The Golgi apparatus is a stack of flattened sacs (Figure 120) This stack of sacs is sometimes referred to as theFigure 120 TEM of a Golgi apparatus A central stack of saucer-shaped sacs can be seen budding off small Golgi vesicles (green) These may form secretory vesicles whose contents can be released at the cell surface by exocytosis (see page 80)Golgi body More than one may be present in a cell The stack is constantly being formed at one end from vesicles which bud off from the ER and broken down again at the other end to form Golgi vesicles The stack of sacs with the associated vesicles is referred to as the Golgi apparatus or Golgi complex

The Golgi apparatus collects processes and sorts molecules (particularly proteins from the rough ER) ready for transport in Golgi vesicles either to other parts of the cell or out of the cell (secretion) Two examples of protein processing in the Golgi apparatus are the addition of sugars to proteins to make molecules known as glycoproteins and the removal of the first amino acid methionine from newly formed proteins to make a functioning protein In plants enzymes in the Golgi apparatus convert sugars into cell wall components Golgi vesicles are also used to make lysosomesLysosomesLysosomes (Figure 121) are spherical sacs surrounded by a single membrane and having no internal structure They are commonly 01ndash 05 1113088m in diameter They contain digestive (hydrolytic) enzymes which must be kept separateFigure 121 Lysosomes (orange) in a mouse kidney cell (1113088 55 000) They contain cell structures in the process of digestion and vesicles (green) Cytoplasm is coloured blue here1661 1 Cell structurefrom the rest of the cell to prevent damage Lysosomes are responsible for the breakdown (digestion) of unwanted structures such as old organelles or even whole cells as in mammary glands after lactation (breast feeding) In white blood cells lysosomes are used to digest bacteria (see endocytosis page 80) Enzymes are sometimes released outside the cell ndash for example in the replacement of cartilage with bone during development The headsof sperm contain a special lysosome the acrosome for digesting a path to the ovum (egg)MitochondriaMitochondria (singular mitochondrion) are usually about 1 1113088m in diameter and can be various shapes often sausage- shaped as in Figure 122 They are surrounded by two membranes (an envelope) The inner of these is foldedto form finger-like cristae which project into the interior solution or matrixThe main function of mitochondria is to carry out aerobic respiration As a result of respiration they make ATP the universal energy carrier in cells (see Chapter 16) They are also involved in the synthesis of lipids (page 37)Figure 122 Mitochondrion (orange) with its double membrane (envelope) the inner membrane is folded to form cristae (1113088 20 000) Mitochondria are the sites of aerobic cell respiration Note also the rough ERIn the 1960s it was discovered that mitochondria and chloroplasts contain ribosomes which are slightly smaller than those in the cytoplasm and are the same size as those found in bacteria The size of ribosomes is measured in lsquoS unitsrsquo which are a measure of how fast they sediment in a centrifuge Cytoplasmic ribosomes are 80S while those of bacteria mitochondria and chloroplasts are 70S It was also discovered in the 1960s that mitochondria and chloroplasts contain small circular DNA molecules also like those found in bacteria Not surprisingly it was later provedthat mitochondria and chloroplasts are in effect ancient bacteria which now live inside the larger cells typical of animals and plants (see

prokaryotic and eukaryotic cells page 18) This is known as the endosymbiont theory lsquoEndorsquo means lsquoinsidersquo and a lsquosymbiontrsquo is an organism which lives in a mutually beneficial relationship with another organism The DNA and ribosomes of mitochondria and chloroplasts are still active and responsible for the coding and synthesis of certain vital proteins but mitochondria and chloroplasts can no longer live independentlyMitochondrial ribosomes are just visible as tiny dark dots in the mitochondrial matrix in Figure 122Cell surface membraneThe cell surface membrane is extremely thin (about 7 nm) However at very high magnifications at least 1113088 100 000 it can be seen to have three layers described as a trilaminar appearance This consists of two dark lines (heavily stained) either side of a narrow pale interior (Figure 123) The membrane is partially permeable and controls exchange between the cell and its environment Membrane structure is discussed further in Chapter 4Figure 123 Cell surface membrane (1113088 250 000) At this magnification the membrane appears as two dark lines at the edge of the cellMicrovilliMicrovilli (singular microvillus) are finger-like extensions of the cell surface membrane typical of certain epithelial cells (cells covering surfaces of structures) They greatly1 Cell structure 17increase the surface area of the cell surface membrane (see Figure 117 on page 15) This is useful for example for absorption in the gut and for reabsorption in the proximal convoluted tubules of the kidney (see page 307)CentriolesThe extra resolution of the electron microscope reveals that just outside the nucleus there are really two centrioles (see Figure 124) not one as it appears under the light microscope (compare with Figure 13 on page 2) They lie close together at right-angles to each other A centriole is a hollow cylinder about 04 1113088m long formed from a ring of short microtubules tiny tubes made of a protein called tubulin These microtubules are used as a starting point for growing the spindle microtubules for nuclear division (see page 92) Centrioles are not found in plant cellsUltrastructure of a plant cellAll the structures except centrioles and microvilli so far described in animal cells are also found in plant cells The appearance of a plant cell as seen with the electron microscope is shown in Figure 125 while Figure 126 is a diagram based on many such micrographs The relatively thick cell wall and the large central vacuole are obvious as are the chloroplasts two of which are shown in detail in Figure 127 These structures and their functions have been described on pages 5 and 6 The electron microscopeFigure 124 Centrioles in transverse and longitudinal section (TS and LS) (1113088 86 000) The one on the left is seen in TS and clearly shows the nine triplets of microtubules which make up the structurereveals that chloroplasts contain 70S ribosomes and small circular

DNA molecules as do mitochondria and bacteria This has already been discussed with mitochondria above (page 17)SAQ 15Compare Figure 126 with Figure 15 on page 3 Name the structures which can be seen with the electron microscope but not with the light microscopeTwo fundamentally different types of cellAt one time it was common practice to try to classify all living organisms as either animals or plants With advances in our knowledge of living things it has become obvious that the living world is not that simple Fungi and bacteria for example are very different from animals and plants and from each other Eventually it was discovered that there are two fundamentally different types of cell The most obvious difference betweenthese types is that one possesses a nucleus and the other does notOrganisms that lack nuclei are called prokaryotes (lsquoprorsquo means before lsquokaryonrsquo means nucleus) All prokaryotes are now referred to as bacteria They are on average about 1000 to 10 000 times smaller in volume than cells with nuclei and are much simpler in structure ndash for example their DNA lies free in the cytoplasmOrganisms whose cells possess nuclei are called eukaryotes (lsquoeursquo means true) Their DNA lies inside a nucleus Eukaryotes include animals plants fungi and a group containing most of the unicellular eukaryotes known as protoctists Most biologists believe that eukaryotes evolved from prokaryotes one-and-a-half thousand million years after prokaryotes first appeared on Earth We mainly study animals and plants in this book but all eukaryotic cells have certain features in commonA generalised prokaryotic cell is shown in Figure 128 A comparison of prokaryotic and eukaryotic cells is given in Table 12 on page 211881 1 Cell structure

Look again at Figure 19 (page 9) Figure 19a is a light micrograph (a photograph taken with a light microscope also known as a photomicrograph) Figure 19b is an electron micrograph of the same cells taken at the same magnification (an electron micrograph is a picture taken with an electron microscope) You can see that Figure 19b the electron micrograph is much clearer This is because it has greater resolution Resolution is defined as the abilityto distinguish between two separate points If the two points cannot be resolved they will be seen as one point In practice resolution is the amount of detail that can be seen ndash the greater the resolution the greater the detailThe maximum resolution of a light microscope is 200 nm This means that if two points or objects are closer together than 200 nm they cannot be distinguished as separateIt is possible to take a photograph such as Figure 19a and to magnify (enlarge) it but we see no more detail in other words we do not improve resolution even though we often enlarge photographs because they are easier to see when larger With a microscope magnification up to the limit of resolution can reveal further detail but any further magnification increases blurring as well as the size of the imageThe electromagnetic spectrumHow is resolution linked with the nature of light One of the properties of light is that it travels in waves The length of the waves of visible light varies ranging from about 400 nm (violet light) to about 700 nm (red light) The human eye can distinguish between these different wavelengths and in the brain the differences are converted to colour differences (Colour is an invention of the brain)The whole range of different wavelengths is called the electromagnetic spectrum Visible light is only one part of this spectrum Figure 111 shows some of the parts of the electromagnetic spectrum The longer the waves the lower their frequency (all the waves travel at the same speed so imagine them passing a post shorter waves pass at higher frequency) In theory there is no limit to how short or how long the waves can be Wavelength changes with energy the greater the energy the shorter the wavelength (rather like squashing a spring)Now look at Figure 112 which shows a mitochondrion some very small cell organelles called ribosomes (see page 13) and light of 400 nm wavelength the shortest visible wavelength The mitochondrion is large enoughto interfere with the light waves However the ribosomes are far too small to have any effect on the light waves The general rule is that the limit of resolution is about one half the wavelength of the radiation used to view the specimen In other words if an object is any smaller than half the wavelength of the radiation used to view it it cannot be seen separately from nearby objects This means that the1001 1 Cell structureX-raysgamma rays01 nm400 nm violet bluev infraredmicrowaves

uv10 nmi s i b l e 1000 nmradio and TV wavesvisible light500 nm 600 nm green yellow orange105 nm 107 nm 109 nm 1011 nm 1013 nm700 nmFigure 111 Diagram of the electromagnetic spectrum (the waves are not drawn to scale) The numbers indicate the wavelengths of the different types of electromagnetic radiation Visible light is a form of electromagnetic radiationbest resolution that can be obtained using a microscope that uses visible light (a light microscope) is 200 nm since the shortest wavelength of visible light is 400 nm (violet light) In practice this corresponds to a maximum useful magnification of about 1500 times Ribosomes are approximately 25 nm in diameter and can therefore never be seen using lightIf an object is transparent it will allow light waves to pass through it and therefore will still not be visible This is why many biological structures have to be stained before they can be seenThe electron microscopeBiologists faced with the problem that they would never see anything smaller than 200 nm using a light microscope realised that the only solution would be to use radiation of a shorter wavelength than light If you study Figure 111 you will see that ultraviolet light or better still X-rays look like possible candidates Both ultraviolet and X-ray microscopes have been built the latter with little success partly because of the difficulty of focusing X-rays A much better solution is to use electrons Electrons are negatively charged particles which orbit the nucleus of an atom When a metal becomes very hot some of its electrons gain so much energy that they escape from their orbits like a rocket escaping from Earthrsquos gravity Free electrons behave like electromagnetic radiation They have a very short wavelength the greater the energy the shorter the wavelength Electrons are a very suitable form of radiation for microscopy for two major reasons Firstly their wavelength is extremely short (at least as short as that of X-rays) Secondly because they are negatively charged they can be focused easily using electromagnets (a magnet can be made to alter the path of the beam the equivalent of a glass lens bending light)Using an electron microscope a resolution of 05 nm can be obtained

400 times better than when using a light microscope ETransmission and scanning electron microscopesTwo types of electron microscope are now in common use The transmission electron microscope or TEM forwavelength 400nmstained mitochondrion of diameter 1000nm interferes with light wavesstained ribosomes of diameter 25nm do not interfere with light wavesFigure 112 A mitochondrion and some ribosomes in the path of light waves of 400 nm lengthred1 Cell structure 11

E short was the type originally developed Here the beam of electrons is passed through the specimen before being viewed Only

those electrons that are transmitted (pass through the specimen) are seen This allows us to see thin sections of specimens and thus to see inside cells In the scanning electron microscope (SEM) on the other hand the electron beam is used to scan the surfaces of structures and only the reflected beam is observedAn example of a scanning electron micrograph is shown in Figure 113 The advantage of this microscope is that surface structures can be seen Also great depth of field is obtained so that much of the specimen is in focus at the same time and a three-dimensional appearance is obtained Such a picture would be impossible to obtain with a light microscope even using the same magnification and resolution because you would have to keep focusing up and down with the objective lens to see different parts of the specimen The disadvantage of the SEM is that it cannot achieve the same resolution as a TEM Resolution is between 3 nm and 20 nmViewing specimens with the electron microscopeFigure 114 shows how an electron microscope works and Figure 115 shows one in useFigure 113 False-colour SEM of the head of a cat flea (1113088 100)

electron gun and anode which E produce a beam of electronselectron beam vacuumpathway of electronscondenser electromagnetic lens which directs the electron beam onto the specimenspecimen which is placed on a gridobjective electromagnetic lens which produces an imageprojector electromagnetic lenses which focus the magnified image onto the screenscreen or photographic plate which shows the image of the specimenFigure 114 How an electron microscope worksIt is not possible to see an electron beam so to make the image visible the electron beam has to be projected onto a fluorescent screen The areas hit by electrons shine brightly giving overall a lsquoblack and whitersquo picture The stains used to improve the contrast of biological specimens for electron microscopy contain heavy metal atoms which stop the passage of electrons The resulting picture is like an X-ray photograph with the more densely stained parts of the specimen appearing blacker lsquoFalse-colourrsquo images can be created by colouring the standard black and white image using a computerTo add to the difficulties of electron microscopy the electron beam and therefore the specimen and the fluorescent screen must be in a vacuum If electrons1221 1 Cell structure

EStructures and functions of organellesCompartmentalisation and division of labour within the cell are even more obvious with an electron microscope than with a light microscopeWe will now consider the structures and functions of some of the cell components in more detailNucleusThe nucleus (Figure 118 on page 15) is the largest cell organelle (see also page 5) It is surrounded by two membranes known as the

nuclear envelope The outer membrane of the nuclear envelope is continuous with the endoplasmic reticulum (Figure 117 on page 15) The nuclear envelope has many small pores called nuclear pores These allow and control exchange between the nucleus and the cytoplasm Examples of substances leaving the nucleus through the pores are mRNA and ribosomes for protein synthesis Examples of substances entering through the nuclear pores are proteins to help make ribosomes nucleotides ATP (aderosine triphosphate) and some hormones such as thyroid hormone T3Within the nucleus the chromosomes are in a loosely coiled state known as chromatin (except during nuclear division see Chapter 5) Chromosomes contain DNA which is organised into functional units called genes Genes control the activities of the cell and inheritance thus the nucleus controls the cellrsquos activities When a cell is aboutto divide the nucleus divides first so that each new cell will have its own nucleus (Chapters 5 and 19) Also within the nucleus the nucleolus makes ribosomes using the information in its own DNAEndoplasmic reticulum and ribosomesWhen cells were first seen with the electron microscope biologists were amazed to see so much detailed structure The existence of much of this had not been suspected This was particularly true of an extensive system of membranes running through the cytoplasm which became knownas the endoplasmic reticulum (ER) (Figure 119 on page 15 ndash see also Figures 118 on page 15 and 122 on page 17) The ER is continuous with the outer membrane of the nuclear envelope (Figure 117)There are two types of ER rough ER and smooth ER Rough ER is so called because it is covered with many tinyFigure 115 A TEM in usecollided with air molecules they would scatter making it impossible to achieve a sharp picture Also water boils at room temperature in a vacuum so all specimens must be dehydrated before being placed in the microscope This means that only dead material can be examined Great efforts are therefore made to try to preserve material in a life-like state when preparing it for the microscopeSAQ 13Explain why ribosomes are not visible using a light microscopeUltrastructure of an animal cellThe lsquofinersquo or detailed structure of a cell as revealed by the electron microscope is called its ultrastructure Figure 116 shows the appearance of typical animal cells as seen with an electron microscope and Figure 117 on page 15 is a diagram based on many other such micrographsSAQ 14Compare Figure 117 on page 15 with Figure 13 on page 2 Name the structures which can be seen with the electron microscope but not with the light microscope1 Cell structure 13lysosomeGGolgi apparatusendoplasmic reticulumglycogen granulesccell surface membrane

mmitochondriachromatinnnucleolusmmicrovillusrribosomesFFigure 116 Representative animal cells as seen with a TEM The cells are liver cells from a rat (1113088 9600) The nucleus is clearly visible in one of the cells 1441 1 Cell structurenucleusnnuclear envelopetwo centrioles close to the nucleus and at right angles to each othermitochondrion lysosomerough endoplasmic reticulumnucleoluschromatin nucleusnuclear porenuclear envelope (two membranes)microvillismooth endoplasmic reticulumFigure 117 Ultrastructure of a typical animal cell as seen with an electron microscope In reality the ER is more extensive than shown and free ribosomes may be more extensive Glycogen granules are sometimes present in the cytoplasmFigure 118 TEM of the nucleus of a cell from the pancreas of a bat (1113088 7500) The circular nucleus is surrounded by a double-layered nuclear envelope containing nuclear pores The nucleolus is more Figure 119 TEM of rough ER covered with ribosomes (black dots) darkly stained Rough ER is visible in the surrounding cytoplasm (1113088 17 000) Some free ribosomes can also be seen in the cytoplasmGolgi vesicle Golgi apparatusribosomescell surface membranecytoplasm1 Cell structure 15organelles called ribosomes These are just visible as black dots in Figures 118 and 119 on page 15 At very high magnifications they can be seen to consist of two subunits a large and a small subunit Ribosomes are the sites of protein synthesis (see pages 111ndash112) They can be found free in the cytoplasm as well as on the rough ER They are very small only about 25 nm in diameter They are made of RNA (ribonucleic acid) and protein The rough ER forms an extensive system of flattened sacs spreading in sheets throughout the cell Proteins made by the ribosomes on the rough ER enter the sacs and move through them The proteins are often processed in some way on their journey Small sacs called vesicles can break off from the ER and these can join together to form the Golgi apparatus Proteins can be exported from the cell via the Golgi apparatus (see page 80)Smooth ER so called because it lacks ribosomes has a completely different function It makes lipids and steroids such as cholesterol and the reproductive hormones oestrogen and testosteroneGolgi apparatus (Golgi body or Golgi complex)The Golgi apparatus is a stack of flattened sacs (Figure 120) This stack of sacs is sometimes referred to as theFigure 120 TEM of a Golgi apparatus A central stack of saucer-shaped sacs can be seen budding off small Golgi vesicles (green) These may form secretory vesicles whose contents can be released at the cell surface by exocytosis (see page 80)Golgi body More than one may be present in a cell The stack is constantly being formed at one end from vesicles which bud off from the ER and broken down again at the other end to form Golgi vesicles The stack of sacs with the associated vesicles is referred to as the Golgi apparatus or Golgi complex

The Golgi apparatus collects processes and sorts molecules (particularly proteins from the rough ER) ready for transport in Golgi vesicles either to other parts of the cell or out of the cell (secretion) Two examples of protein processing in the Golgi apparatus are the addition of sugars to proteins to make molecules known as glycoproteins and the removal of the first amino acid methionine from newly formed proteins to make a functioning protein In plants enzymes in the Golgi apparatus convert sugars into cell wall components Golgi vesicles are also used to make lysosomesLysosomesLysosomes (Figure 121) are spherical sacs surrounded by a single membrane and having no internal structure They are commonly 01ndash 05 1113088m in diameter They contain digestive (hydrolytic) enzymes which must be kept separateFigure 121 Lysosomes (orange) in a mouse kidney cell (1113088 55 000) They contain cell structures in the process of digestion and vesicles (green) Cytoplasm is coloured blue here1661 1 Cell structurefrom the rest of the cell to prevent damage Lysosomes are responsible for the breakdown (digestion) of unwanted structures such as old organelles or even whole cells as in mammary glands after lactation (breast feeding) In white blood cells lysosomes are used to digest bacteria (see endocytosis page 80) Enzymes are sometimes released outside the cell ndash for example in the replacement of cartilage with bone during development The headsof sperm contain a special lysosome the acrosome for digesting a path to the ovum (egg)MitochondriaMitochondria (singular mitochondrion) are usually about 1 1113088m in diameter and can be various shapes often sausage- shaped as in Figure 122 They are surrounded by two membranes (an envelope) The inner of these is foldedto form finger-like cristae which project into the interior solution or matrixThe main function of mitochondria is to carry out aerobic respiration As a result of respiration they make ATP the universal energy carrier in cells (see Chapter 16) They are also involved in the synthesis of lipids (page 37)Figure 122 Mitochondrion (orange) with its double membrane (envelope) the inner membrane is folded to form cristae (1113088 20 000) Mitochondria are the sites of aerobic cell respiration Note also the rough ERIn the 1960s it was discovered that mitochondria and chloroplasts contain ribosomes which are slightly smaller than those in the cytoplasm and are the same size as those found in bacteria The size of ribosomes is measured in lsquoS unitsrsquo which are a measure of how fast they sediment in a centrifuge Cytoplasmic ribosomes are 80S while those of bacteria mitochondria and chloroplasts are 70S It was also discovered in the 1960s that mitochondria and chloroplasts contain small circular DNA molecules also like those found in bacteria Not surprisingly it was later provedthat mitochondria and chloroplasts are in effect ancient bacteria which now live inside the larger cells typical of animals and plants (see

prokaryotic and eukaryotic cells page 18) This is known as the endosymbiont theory lsquoEndorsquo means lsquoinsidersquo and a lsquosymbiontrsquo is an organism which lives in a mutually beneficial relationship with another organism The DNA and ribosomes of mitochondria and chloroplasts are still active and responsible for the coding and synthesis of certain vital proteins but mitochondria and chloroplasts can no longer live independentlyMitochondrial ribosomes are just visible as tiny dark dots in the mitochondrial matrix in Figure 122Cell surface membraneThe cell surface membrane is extremely thin (about 7 nm) However at very high magnifications at least 1113088 100 000 it can be seen to have three layers described as a trilaminar appearance This consists of two dark lines (heavily stained) either side of a narrow pale interior (Figure 123) The membrane is partially permeable and controls exchange between the cell and its environment Membrane structure is discussed further in Chapter 4Figure 123 Cell surface membrane (1113088 250 000) At this magnification the membrane appears as two dark lines at the edge of the cellMicrovilliMicrovilli (singular microvillus) are finger-like extensions of the cell surface membrane typical of certain epithelial cells (cells covering surfaces of structures) They greatly1 Cell structure 17increase the surface area of the cell surface membrane (see Figure 117 on page 15) This is useful for example for absorption in the gut and for reabsorption in the proximal convoluted tubules of the kidney (see page 307)CentriolesThe extra resolution of the electron microscope reveals that just outside the nucleus there are really two centrioles (see Figure 124) not one as it appears under the light microscope (compare with Figure 13 on page 2) They lie close together at right-angles to each other A centriole is a hollow cylinder about 04 1113088m long formed from a ring of short microtubules tiny tubes made of a protein called tubulin These microtubules are used as a starting point for growing the spindle microtubules for nuclear division (see page 92) Centrioles are not found in plant cellsUltrastructure of a plant cellAll the structures except centrioles and microvilli so far described in animal cells are also found in plant cells The appearance of a plant cell as seen with the electron microscope is shown in Figure 125 while Figure 126 is a diagram based on many such micrographs The relatively thick cell wall and the large central vacuole are obvious as are the chloroplasts two of which are shown in detail in Figure 127 These structures and their functions have been described on pages 5 and 6 The electron microscopeFigure 124 Centrioles in transverse and longitudinal section (TS and LS) (1113088 86 000) The one on the left is seen in TS and clearly shows the nine triplets of microtubules which make up the structurereveals that chloroplasts contain 70S ribosomes and small circular

DNA molecules as do mitochondria and bacteria This has already been discussed with mitochondria above (page 17)SAQ 15Compare Figure 126 with Figure 15 on page 3 Name the structures which can be seen with the electron microscope but not with the light microscopeTwo fundamentally different types of cellAt one time it was common practice to try to classify all living organisms as either animals or plants With advances in our knowledge of living things it has become obvious that the living world is not that simple Fungi and bacteria for example are very different from animals and plants and from each other Eventually it was discovered that there are two fundamentally different types of cell The most obvious difference betweenthese types is that one possesses a nucleus and the other does notOrganisms that lack nuclei are called prokaryotes (lsquoprorsquo means before lsquokaryonrsquo means nucleus) All prokaryotes are now referred to as bacteria They are on average about 1000 to 10 000 times smaller in volume than cells with nuclei and are much simpler in structure ndash for example their DNA lies free in the cytoplasmOrganisms whose cells possess nuclei are called eukaryotes (lsquoeursquo means true) Their DNA lies inside a nucleus Eukaryotes include animals plants fungi and a group containing most of the unicellular eukaryotes known as protoctists Most biologists believe that eukaryotes evolved from prokaryotes one-and-a-half thousand million years after prokaryotes first appeared on Earth We mainly study animals and plants in this book but all eukaryotic cells have certain features in commonA generalised prokaryotic cell is shown in Figure 128 A comparison of prokaryotic and eukaryotic cells is given in Table 12 on page 211881 1 Cell structure

uv10 nmi s i b l e 1000 nmradio and TV wavesvisible light500 nm 600 nm green yellow orange105 nm 107 nm 109 nm 1011 nm 1013 nm700 nmFigure 111 Diagram of the electromagnetic spectrum (the waves are not drawn to scale) The numbers indicate the wavelengths of the different types of electromagnetic radiation Visible light is a form of electromagnetic radiationbest resolution that can be obtained using a microscope that uses visible light (a light microscope) is 200 nm since the shortest wavelength of visible light is 400 nm (violet light) In practice this corresponds to a maximum useful magnification of about 1500 times Ribosomes are approximately 25 nm in diameter and can therefore never be seen using lightIf an object is transparent it will allow light waves to pass through it and therefore will still not be visible This is why many biological structures have to be stained before they can be seenThe electron microscopeBiologists faced with the problem that they would never see anything smaller than 200 nm using a light microscope realised that the only solution would be to use radiation of a shorter wavelength than light If you study Figure 111 you will see that ultraviolet light or better still X-rays look like possible candidates Both ultraviolet and X-ray microscopes have been built the latter with little success partly because of the difficulty of focusing X-rays A much better solution is to use electrons Electrons are negatively charged particles which orbit the nucleus of an atom When a metal becomes very hot some of its electrons gain so much energy that they escape from their orbits like a rocket escaping from Earthrsquos gravity Free electrons behave like electromagnetic radiation They have a very short wavelength the greater the energy the shorter the wavelength Electrons are a very suitable form of radiation for microscopy for two major reasons Firstly their wavelength is extremely short (at least as short as that of X-rays) Secondly because they are negatively charged they can be focused easily using electromagnets (a magnet can be made to alter the path of the beam the equivalent of a glass lens bending light)Using an electron microscope a resolution of 05 nm can be obtained

400 times better than when using a light microscope ETransmission and scanning electron microscopesTwo types of electron microscope are now in common use The transmission electron microscope or TEM forwavelength 400nmstained mitochondrion of diameter 1000nm interferes with light wavesstained ribosomes of diameter 25nm do not interfere with light wavesFigure 112 A mitochondrion and some ribosomes in the path of light waves of 400 nm lengthred1 Cell structure 11

E short was the type originally developed Here the beam of electrons is passed through the specimen before being viewed Only

those electrons that are transmitted (pass through the specimen) are seen This allows us to see thin sections of specimens and thus to see inside cells In the scanning electron microscope (SEM) on the other hand the electron beam is used to scan the surfaces of structures and only the reflected beam is observedAn example of a scanning electron micrograph is shown in Figure 113 The advantage of this microscope is that surface structures can be seen Also great depth of field is obtained so that much of the specimen is in focus at the same time and a three-dimensional appearance is obtained Such a picture would be impossible to obtain with a light microscope even using the same magnification and resolution because you would have to keep focusing up and down with the objective lens to see different parts of the specimen The disadvantage of the SEM is that it cannot achieve the same resolution as a TEM Resolution is between 3 nm and 20 nmViewing specimens with the electron microscopeFigure 114 shows how an electron microscope works and Figure 115 shows one in useFigure 113 False-colour SEM of the head of a cat flea (1113088 100)

electron gun and anode which E produce a beam of electronselectron beam vacuumpathway of electronscondenser electromagnetic lens which directs the electron beam onto the specimenspecimen which is placed on a gridobjective electromagnetic lens which produces an imageprojector electromagnetic lenses which focus the magnified image onto the screenscreen or photographic plate which shows the image of the specimenFigure 114 How an electron microscope worksIt is not possible to see an electron beam so to make the image visible the electron beam has to be projected onto a fluorescent screen The areas hit by electrons shine brightly giving overall a lsquoblack and whitersquo picture The stains used to improve the contrast of biological specimens for electron microscopy contain heavy metal atoms which stop the passage of electrons The resulting picture is like an X-ray photograph with the more densely stained parts of the specimen appearing blacker lsquoFalse-colourrsquo images can be created by colouring the standard black and white image using a computerTo add to the difficulties of electron microscopy the electron beam and therefore the specimen and the fluorescent screen must be in a vacuum If electrons1221 1 Cell structure

EStructures and functions of organellesCompartmentalisation and division of labour within the cell are even more obvious with an electron microscope than with a light microscopeWe will now consider the structures and functions of some of the cell components in more detailNucleusThe nucleus (Figure 118 on page 15) is the largest cell organelle (see also page 5) It is surrounded by two membranes known as the

nuclear envelope The outer membrane of the nuclear envelope is continuous with the endoplasmic reticulum (Figure 117 on page 15) The nuclear envelope has many small pores called nuclear pores These allow and control exchange between the nucleus and the cytoplasm Examples of substances leaving the nucleus through the pores are mRNA and ribosomes for protein synthesis Examples of substances entering through the nuclear pores are proteins to help make ribosomes nucleotides ATP (aderosine triphosphate) and some hormones such as thyroid hormone T3Within the nucleus the chromosomes are in a loosely coiled state known as chromatin (except during nuclear division see Chapter 5) Chromosomes contain DNA which is organised into functional units called genes Genes control the activities of the cell and inheritance thus the nucleus controls the cellrsquos activities When a cell is aboutto divide the nucleus divides first so that each new cell will have its own nucleus (Chapters 5 and 19) Also within the nucleus the nucleolus makes ribosomes using the information in its own DNAEndoplasmic reticulum and ribosomesWhen cells were first seen with the electron microscope biologists were amazed to see so much detailed structure The existence of much of this had not been suspected This was particularly true of an extensive system of membranes running through the cytoplasm which became knownas the endoplasmic reticulum (ER) (Figure 119 on page 15 ndash see also Figures 118 on page 15 and 122 on page 17) The ER is continuous with the outer membrane of the nuclear envelope (Figure 117)There are two types of ER rough ER and smooth ER Rough ER is so called because it is covered with many tinyFigure 115 A TEM in usecollided with air molecules they would scatter making it impossible to achieve a sharp picture Also water boils at room temperature in a vacuum so all specimens must be dehydrated before being placed in the microscope This means that only dead material can be examined Great efforts are therefore made to try to preserve material in a life-like state when preparing it for the microscopeSAQ 13Explain why ribosomes are not visible using a light microscopeUltrastructure of an animal cellThe lsquofinersquo or detailed structure of a cell as revealed by the electron microscope is called its ultrastructure Figure 116 shows the appearance of typical animal cells as seen with an electron microscope and Figure 117 on page 15 is a diagram based on many other such micrographsSAQ 14Compare Figure 117 on page 15 with Figure 13 on page 2 Name the structures which can be seen with the electron microscope but not with the light microscope1 Cell structure 13lysosomeGGolgi apparatusendoplasmic reticulumglycogen granulesccell surface membrane

mmitochondriachromatinnnucleolusmmicrovillusrribosomesFFigure 116 Representative animal cells as seen with a TEM The cells are liver cells from a rat (1113088 9600) The nucleus is clearly visible in one of the cells 1441 1 Cell structurenucleusnnuclear envelopetwo centrioles close to the nucleus and at right angles to each othermitochondrion lysosomerough endoplasmic reticulumnucleoluschromatin nucleusnuclear porenuclear envelope (two membranes)microvillismooth endoplasmic reticulumFigure 117 Ultrastructure of a typical animal cell as seen with an electron microscope In reality the ER is more extensive than shown and free ribosomes may be more extensive Glycogen granules are sometimes present in the cytoplasmFigure 118 TEM of the nucleus of a cell from the pancreas of a bat (1113088 7500) The circular nucleus is surrounded by a double-layered nuclear envelope containing nuclear pores The nucleolus is more Figure 119 TEM of rough ER covered with ribosomes (black dots) darkly stained Rough ER is visible in the surrounding cytoplasm (1113088 17 000) Some free ribosomes can also be seen in the cytoplasmGolgi vesicle Golgi apparatusribosomescell surface membranecytoplasm1 Cell structure 15organelles called ribosomes These are just visible as black dots in Figures 118 and 119 on page 15 At very high magnifications they can be seen to consist of two subunits a large and a small subunit Ribosomes are the sites of protein synthesis (see pages 111ndash112) They can be found free in the cytoplasm as well as on the rough ER They are very small only about 25 nm in diameter They are made of RNA (ribonucleic acid) and protein The rough ER forms an extensive system of flattened sacs spreading in sheets throughout the cell Proteins made by the ribosomes on the rough ER enter the sacs and move through them The proteins are often processed in some way on their journey Small sacs called vesicles can break off from the ER and these can join together to form the Golgi apparatus Proteins can be exported from the cell via the Golgi apparatus (see page 80)Smooth ER so called because it lacks ribosomes has a completely different function It makes lipids and steroids such as cholesterol and the reproductive hormones oestrogen and testosteroneGolgi apparatus (Golgi body or Golgi complex)The Golgi apparatus is a stack of flattened sacs (Figure 120) This stack of sacs is sometimes referred to as theFigure 120 TEM of a Golgi apparatus A central stack of saucer-shaped sacs can be seen budding off small Golgi vesicles (green) These may form secretory vesicles whose contents can be released at the cell surface by exocytosis (see page 80)Golgi body More than one may be present in a cell The stack is constantly being formed at one end from vesicles which bud off from the ER and broken down again at the other end to form Golgi vesicles The stack of sacs with the associated vesicles is referred to as the Golgi apparatus or Golgi complex

The Golgi apparatus collects processes and sorts molecules (particularly proteins from the rough ER) ready for transport in Golgi vesicles either to other parts of the cell or out of the cell (secretion) Two examples of protein processing in the Golgi apparatus are the addition of sugars to proteins to make molecules known as glycoproteins and the removal of the first amino acid methionine from newly formed proteins to make a functioning protein In plants enzymes in the Golgi apparatus convert sugars into cell wall components Golgi vesicles are also used to make lysosomesLysosomesLysosomes (Figure 121) are spherical sacs surrounded by a single membrane and having no internal structure They are commonly 01ndash 05 1113088m in diameter They contain digestive (hydrolytic) enzymes which must be kept separateFigure 121 Lysosomes (orange) in a mouse kidney cell (1113088 55 000) They contain cell structures in the process of digestion and vesicles (green) Cytoplasm is coloured blue here1661 1 Cell structurefrom the rest of the cell to prevent damage Lysosomes are responsible for the breakdown (digestion) of unwanted structures such as old organelles or even whole cells as in mammary glands after lactation (breast feeding) In white blood cells lysosomes are used to digest bacteria (see endocytosis page 80) Enzymes are sometimes released outside the cell ndash for example in the replacement of cartilage with bone during development The headsof sperm contain a special lysosome the acrosome for digesting a path to the ovum (egg)MitochondriaMitochondria (singular mitochondrion) are usually about 1 1113088m in diameter and can be various shapes often sausage- shaped as in Figure 122 They are surrounded by two membranes (an envelope) The inner of these is foldedto form finger-like cristae which project into the interior solution or matrixThe main function of mitochondria is to carry out aerobic respiration As a result of respiration they make ATP the universal energy carrier in cells (see Chapter 16) They are also involved in the synthesis of lipids (page 37)Figure 122 Mitochondrion (orange) with its double membrane (envelope) the inner membrane is folded to form cristae (1113088 20 000) Mitochondria are the sites of aerobic cell respiration Note also the rough ERIn the 1960s it was discovered that mitochondria and chloroplasts contain ribosomes which are slightly smaller than those in the cytoplasm and are the same size as those found in bacteria The size of ribosomes is measured in lsquoS unitsrsquo which are a measure of how fast they sediment in a centrifuge Cytoplasmic ribosomes are 80S while those of bacteria mitochondria and chloroplasts are 70S It was also discovered in the 1960s that mitochondria and chloroplasts contain small circular DNA molecules also like those found in bacteria Not surprisingly it was later provedthat mitochondria and chloroplasts are in effect ancient bacteria which now live inside the larger cells typical of animals and plants (see

prokaryotic and eukaryotic cells page 18) This is known as the endosymbiont theory lsquoEndorsquo means lsquoinsidersquo and a lsquosymbiontrsquo is an organism which lives in a mutually beneficial relationship with another organism The DNA and ribosomes of mitochondria and chloroplasts are still active and responsible for the coding and synthesis of certain vital proteins but mitochondria and chloroplasts can no longer live independentlyMitochondrial ribosomes are just visible as tiny dark dots in the mitochondrial matrix in Figure 122Cell surface membraneThe cell surface membrane is extremely thin (about 7 nm) However at very high magnifications at least 1113088 100 000 it can be seen to have three layers described as a trilaminar appearance This consists of two dark lines (heavily stained) either side of a narrow pale interior (Figure 123) The membrane is partially permeable and controls exchange between the cell and its environment Membrane structure is discussed further in Chapter 4Figure 123 Cell surface membrane (1113088 250 000) At this magnification the membrane appears as two dark lines at the edge of the cellMicrovilliMicrovilli (singular microvillus) are finger-like extensions of the cell surface membrane typical of certain epithelial cells (cells covering surfaces of structures) They greatly1 Cell structure 17increase the surface area of the cell surface membrane (see Figure 117 on page 15) This is useful for example for absorption in the gut and for reabsorption in the proximal convoluted tubules of the kidney (see page 307)CentriolesThe extra resolution of the electron microscope reveals that just outside the nucleus there are really two centrioles (see Figure 124) not one as it appears under the light microscope (compare with Figure 13 on page 2) They lie close together at right-angles to each other A centriole is a hollow cylinder about 04 1113088m long formed from a ring of short microtubules tiny tubes made of a protein called tubulin These microtubules are used as a starting point for growing the spindle microtubules for nuclear division (see page 92) Centrioles are not found in plant cellsUltrastructure of a plant cellAll the structures except centrioles and microvilli so far described in animal cells are also found in plant cells The appearance of a plant cell as seen with the electron microscope is shown in Figure 125 while Figure 126 is a diagram based on many such micrographs The relatively thick cell wall and the large central vacuole are obvious as are the chloroplasts two of which are shown in detail in Figure 127 These structures and their functions have been described on pages 5 and 6 The electron microscopeFigure 124 Centrioles in transverse and longitudinal section (TS and LS) (1113088 86 000) The one on the left is seen in TS and clearly shows the nine triplets of microtubules which make up the structurereveals that chloroplasts contain 70S ribosomes and small circular

DNA molecules as do mitochondria and bacteria This has already been discussed with mitochondria above (page 17)SAQ 15Compare Figure 126 with Figure 15 on page 3 Name the structures which can be seen with the electron microscope but not with the light microscopeTwo fundamentally different types of cellAt one time it was common practice to try to classify all living organisms as either animals or plants With advances in our knowledge of living things it has become obvious that the living world is not that simple Fungi and bacteria for example are very different from animals and plants and from each other Eventually it was discovered that there are two fundamentally different types of cell The most obvious difference betweenthese types is that one possesses a nucleus and the other does notOrganisms that lack nuclei are called prokaryotes (lsquoprorsquo means before lsquokaryonrsquo means nucleus) All prokaryotes are now referred to as bacteria They are on average about 1000 to 10 000 times smaller in volume than cells with nuclei and are much simpler in structure ndash for example their DNA lies free in the cytoplasmOrganisms whose cells possess nuclei are called eukaryotes (lsquoeursquo means true) Their DNA lies inside a nucleus Eukaryotes include animals plants fungi and a group containing most of the unicellular eukaryotes known as protoctists Most biologists believe that eukaryotes evolved from prokaryotes one-and-a-half thousand million years after prokaryotes first appeared on Earth We mainly study animals and plants in this book but all eukaryotic cells have certain features in commonA generalised prokaryotic cell is shown in Figure 128 A comparison of prokaryotic and eukaryotic cells is given in Table 12 on page 211881 1 Cell structure

those electrons that are transmitted (pass through the specimen) are seen This allows us to see thin sections of specimens and thus to see inside cells In the scanning electron microscope (SEM) on the other hand the electron beam is used to scan the surfaces of structures and only the reflected beam is observedAn example of a scanning electron micrograph is shown in Figure 113 The advantage of this microscope is that surface structures can be seen Also great depth of field is obtained so that much of the specimen is in focus at the same time and a three-dimensional appearance is obtained Such a picture would be impossible to obtain with a light microscope even using the same magnification and resolution because you would have to keep focusing up and down with the objective lens to see different parts of the specimen The disadvantage of the SEM is that it cannot achieve the same resolution as a TEM Resolution is between 3 nm and 20 nmViewing specimens with the electron microscopeFigure 114 shows how an electron microscope works and Figure 115 shows one in useFigure 113 False-colour SEM of the head of a cat flea (1113088 100)

electron gun and anode which E produce a beam of electronselectron beam vacuumpathway of electronscondenser electromagnetic lens which directs the electron beam onto the specimenspecimen which is placed on a gridobjective electromagnetic lens which produces an imageprojector electromagnetic lenses which focus the magnified image onto the screenscreen or photographic plate which shows the image of the specimenFigure 114 How an electron microscope worksIt is not possible to see an electron beam so to make the image visible the electron beam has to be projected onto a fluorescent screen The areas hit by electrons shine brightly giving overall a lsquoblack and whitersquo picture The stains used to improve the contrast of biological specimens for electron microscopy contain heavy metal atoms which stop the passage of electrons The resulting picture is like an X-ray photograph with the more densely stained parts of the specimen appearing blacker lsquoFalse-colourrsquo images can be created by colouring the standard black and white image using a computerTo add to the difficulties of electron microscopy the electron beam and therefore the specimen and the fluorescent screen must be in a vacuum If electrons1221 1 Cell structure

EStructures and functions of organellesCompartmentalisation and division of labour within the cell are even more obvious with an electron microscope than with a light microscopeWe will now consider the structures and functions of some of the cell components in more detailNucleusThe nucleus (Figure 118 on page 15) is the largest cell organelle (see also page 5) It is surrounded by two membranes known as the

nuclear envelope The outer membrane of the nuclear envelope is continuous with the endoplasmic reticulum (Figure 117 on page 15) The nuclear envelope has many small pores called nuclear pores These allow and control exchange between the nucleus and the cytoplasm Examples of substances leaving the nucleus through the pores are mRNA and ribosomes for protein synthesis Examples of substances entering through the nuclear pores are proteins to help make ribosomes nucleotides ATP (aderosine triphosphate) and some hormones such as thyroid hormone T3Within the nucleus the chromosomes are in a loosely coiled state known as chromatin (except during nuclear division see Chapter 5) Chromosomes contain DNA which is organised into functional units called genes Genes control the activities of the cell and inheritance thus the nucleus controls the cellrsquos activities When a cell is aboutto divide the nucleus divides first so that each new cell will have its own nucleus (Chapters 5 and 19) Also within the nucleus the nucleolus makes ribosomes using the information in its own DNAEndoplasmic reticulum and ribosomesWhen cells were first seen with the electron microscope biologists were amazed to see so much detailed structure The existence of much of this had not been suspected This was particularly true of an extensive system of membranes running through the cytoplasm which became knownas the endoplasmic reticulum (ER) (Figure 119 on page 15 ndash see also Figures 118 on page 15 and 122 on page 17) The ER is continuous with the outer membrane of the nuclear envelope (Figure 117)There are two types of ER rough ER and smooth ER Rough ER is so called because it is covered with many tinyFigure 115 A TEM in usecollided with air molecules they would scatter making it impossible to achieve a sharp picture Also water boils at room temperature in a vacuum so all specimens must be dehydrated before being placed in the microscope This means that only dead material can be examined Great efforts are therefore made to try to preserve material in a life-like state when preparing it for the microscopeSAQ 13Explain why ribosomes are not visible using a light microscopeUltrastructure of an animal cellThe lsquofinersquo or detailed structure of a cell as revealed by the electron microscope is called its ultrastructure Figure 116 shows the appearance of typical animal cells as seen with an electron microscope and Figure 117 on page 15 is a diagram based on many other such micrographsSAQ 14Compare Figure 117 on page 15 with Figure 13 on page 2 Name the structures which can be seen with the electron microscope but not with the light microscope1 Cell structure 13lysosomeGGolgi apparatusendoplasmic reticulumglycogen granulesccell surface membrane

mmitochondriachromatinnnucleolusmmicrovillusrribosomesFFigure 116 Representative animal cells as seen with a TEM The cells are liver cells from a rat (1113088 9600) The nucleus is clearly visible in one of the cells 1441 1 Cell structurenucleusnnuclear envelopetwo centrioles close to the nucleus and at right angles to each othermitochondrion lysosomerough endoplasmic reticulumnucleoluschromatin nucleusnuclear porenuclear envelope (two membranes)microvillismooth endoplasmic reticulumFigure 117 Ultrastructure of a typical animal cell as seen with an electron microscope In reality the ER is more extensive than shown and free ribosomes may be more extensive Glycogen granules are sometimes present in the cytoplasmFigure 118 TEM of the nucleus of a cell from the pancreas of a bat (1113088 7500) The circular nucleus is surrounded by a double-layered nuclear envelope containing nuclear pores The nucleolus is more Figure 119 TEM of rough ER covered with ribosomes (black dots) darkly stained Rough ER is visible in the surrounding cytoplasm (1113088 17 000) Some free ribosomes can also be seen in the cytoplasmGolgi vesicle Golgi apparatusribosomescell surface membranecytoplasm1 Cell structure 15organelles called ribosomes These are just visible as black dots in Figures 118 and 119 on page 15 At very high magnifications they can be seen to consist of two subunits a large and a small subunit Ribosomes are the sites of protein synthesis (see pages 111ndash112) They can be found free in the cytoplasm as well as on the rough ER They are very small only about 25 nm in diameter They are made of RNA (ribonucleic acid) and protein The rough ER forms an extensive system of flattened sacs spreading in sheets throughout the cell Proteins made by the ribosomes on the rough ER enter the sacs and move through them The proteins are often processed in some way on their journey Small sacs called vesicles can break off from the ER and these can join together to form the Golgi apparatus Proteins can be exported from the cell via the Golgi apparatus (see page 80)Smooth ER so called because it lacks ribosomes has a completely different function It makes lipids and steroids such as cholesterol and the reproductive hormones oestrogen and testosteroneGolgi apparatus (Golgi body or Golgi complex)The Golgi apparatus is a stack of flattened sacs (Figure 120) This stack of sacs is sometimes referred to as theFigure 120 TEM of a Golgi apparatus A central stack of saucer-shaped sacs can be seen budding off small Golgi vesicles (green) These may form secretory vesicles whose contents can be released at the cell surface by exocytosis (see page 80)Golgi body More than one may be present in a cell The stack is constantly being formed at one end from vesicles which bud off from the ER and broken down again at the other end to form Golgi vesicles The stack of sacs with the associated vesicles is referred to as the Golgi apparatus or Golgi complex

The Golgi apparatus collects processes and sorts molecules (particularly proteins from the rough ER) ready for transport in Golgi vesicles either to other parts of the cell or out of the cell (secretion) Two examples of protein processing in the Golgi apparatus are the addition of sugars to proteins to make molecules known as glycoproteins and the removal of the first amino acid methionine from newly formed proteins to make a functioning protein In plants enzymes in the Golgi apparatus convert sugars into cell wall components Golgi vesicles are also used to make lysosomesLysosomesLysosomes (Figure 121) are spherical sacs surrounded by a single membrane and having no internal structure They are commonly 01ndash 05 1113088m in diameter They contain digestive (hydrolytic) enzymes which must be kept separateFigure 121 Lysosomes (orange) in a mouse kidney cell (1113088 55 000) They contain cell structures in the process of digestion and vesicles (green) Cytoplasm is coloured blue here1661 1 Cell structurefrom the rest of the cell to prevent damage Lysosomes are responsible for the breakdown (digestion) of unwanted structures such as old organelles or even whole cells as in mammary glands after lactation (breast feeding) In white blood cells lysosomes are used to digest bacteria (see endocytosis page 80) Enzymes are sometimes released outside the cell ndash for example in the replacement of cartilage with bone during development The headsof sperm contain a special lysosome the acrosome for digesting a path to the ovum (egg)MitochondriaMitochondria (singular mitochondrion) are usually about 1 1113088m in diameter and can be various shapes often sausage- shaped as in Figure 122 They are surrounded by two membranes (an envelope) The inner of these is foldedto form finger-like cristae which project into the interior solution or matrixThe main function of mitochondria is to carry out aerobic respiration As a result of respiration they make ATP the universal energy carrier in cells (see Chapter 16) They are also involved in the synthesis of lipids (page 37)Figure 122 Mitochondrion (orange) with its double membrane (envelope) the inner membrane is folded to form cristae (1113088 20 000) Mitochondria are the sites of aerobic cell respiration Note also the rough ERIn the 1960s it was discovered that mitochondria and chloroplasts contain ribosomes which are slightly smaller than those in the cytoplasm and are the same size as those found in bacteria The size of ribosomes is measured in lsquoS unitsrsquo which are a measure of how fast they sediment in a centrifuge Cytoplasmic ribosomes are 80S while those of bacteria mitochondria and chloroplasts are 70S It was also discovered in the 1960s that mitochondria and chloroplasts contain small circular DNA molecules also like those found in bacteria Not surprisingly it was later provedthat mitochondria and chloroplasts are in effect ancient bacteria which now live inside the larger cells typical of animals and plants (see

prokaryotic and eukaryotic cells page 18) This is known as the endosymbiont theory lsquoEndorsquo means lsquoinsidersquo and a lsquosymbiontrsquo is an organism which lives in a mutually beneficial relationship with another organism The DNA and ribosomes of mitochondria and chloroplasts are still active and responsible for the coding and synthesis of certain vital proteins but mitochondria and chloroplasts can no longer live independentlyMitochondrial ribosomes are just visible as tiny dark dots in the mitochondrial matrix in Figure 122Cell surface membraneThe cell surface membrane is extremely thin (about 7 nm) However at very high magnifications at least 1113088 100 000 it can be seen to have three layers described as a trilaminar appearance This consists of two dark lines (heavily stained) either side of a narrow pale interior (Figure 123) The membrane is partially permeable and controls exchange between the cell and its environment Membrane structure is discussed further in Chapter 4Figure 123 Cell surface membrane (1113088 250 000) At this magnification the membrane appears as two dark lines at the edge of the cellMicrovilliMicrovilli (singular microvillus) are finger-like extensions of the cell surface membrane typical of certain epithelial cells (cells covering surfaces of structures) They greatly1 Cell structure 17increase the surface area of the cell surface membrane (see Figure 117 on page 15) This is useful for example for absorption in the gut and for reabsorption in the proximal convoluted tubules of the kidney (see page 307)CentriolesThe extra resolution of the electron microscope reveals that just outside the nucleus there are really two centrioles (see Figure 124) not one as it appears under the light microscope (compare with Figure 13 on page 2) They lie close together at right-angles to each other A centriole is a hollow cylinder about 04 1113088m long formed from a ring of short microtubules tiny tubes made of a protein called tubulin These microtubules are used as a starting point for growing the spindle microtubules for nuclear division (see page 92) Centrioles are not found in plant cellsUltrastructure of a plant cellAll the structures except centrioles and microvilli so far described in animal cells are also found in plant cells The appearance of a plant cell as seen with the electron microscope is shown in Figure 125 while Figure 126 is a diagram based on many such micrographs The relatively thick cell wall and the large central vacuole are obvious as are the chloroplasts two of which are shown in detail in Figure 127 These structures and their functions have been described on pages 5 and 6 The electron microscopeFigure 124 Centrioles in transverse and longitudinal section (TS and LS) (1113088 86 000) The one on the left is seen in TS and clearly shows the nine triplets of microtubules which make up the structurereveals that chloroplasts contain 70S ribosomes and small circular

DNA molecules as do mitochondria and bacteria This has already been discussed with mitochondria above (page 17)SAQ 15Compare Figure 126 with Figure 15 on page 3 Name the structures which can be seen with the electron microscope but not with the light microscopeTwo fundamentally different types of cellAt one time it was common practice to try to classify all living organisms as either animals or plants With advances in our knowledge of living things it has become obvious that the living world is not that simple Fungi and bacteria for example are very different from animals and plants and from each other Eventually it was discovered that there are two fundamentally different types of cell The most obvious difference betweenthese types is that one possesses a nucleus and the other does notOrganisms that lack nuclei are called prokaryotes (lsquoprorsquo means before lsquokaryonrsquo means nucleus) All prokaryotes are now referred to as bacteria They are on average about 1000 to 10 000 times smaller in volume than cells with nuclei and are much simpler in structure ndash for example their DNA lies free in the cytoplasmOrganisms whose cells possess nuclei are called eukaryotes (lsquoeursquo means true) Their DNA lies inside a nucleus Eukaryotes include animals plants fungi and a group containing most of the unicellular eukaryotes known as protoctists Most biologists believe that eukaryotes evolved from prokaryotes one-and-a-half thousand million years after prokaryotes first appeared on Earth We mainly study animals and plants in this book but all eukaryotic cells have certain features in commonA generalised prokaryotic cell is shown in Figure 128 A comparison of prokaryotic and eukaryotic cells is given in Table 12 on page 211881 1 Cell structure

nuclear envelope The outer membrane of the nuclear envelope is continuous with the endoplasmic reticulum (Figure 117 on page 15) The nuclear envelope has many small pores called nuclear pores These allow and control exchange between the nucleus and the cytoplasm Examples of substances leaving the nucleus through the pores are mRNA and ribosomes for protein synthesis Examples of substances entering through the nuclear pores are proteins to help make ribosomes nucleotides ATP (aderosine triphosphate) and some hormones such as thyroid hormone T3Within the nucleus the chromosomes are in a loosely coiled state known as chromatin (except during nuclear division see Chapter 5) Chromosomes contain DNA which is organised into functional units called genes Genes control the activities of the cell and inheritance thus the nucleus controls the cellrsquos activities When a cell is aboutto divide the nucleus divides first so that each new cell will have its own nucleus (Chapters 5 and 19) Also within the nucleus the nucleolus makes ribosomes using the information in its own DNAEndoplasmic reticulum and ribosomesWhen cells were first seen with the electron microscope biologists were amazed to see so much detailed structure The existence of much of this had not been suspected This was particularly true of an extensive system of membranes running through the cytoplasm which became knownas the endoplasmic reticulum (ER) (Figure 119 on page 15 ndash see also Figures 118 on page 15 and 122 on page 17) The ER is continuous with the outer membrane of the nuclear envelope (Figure 117)There are two types of ER rough ER and smooth ER Rough ER is so called because it is covered with many tinyFigure 115 A TEM in usecollided with air molecules they would scatter making it impossible to achieve a sharp picture Also water boils at room temperature in a vacuum so all specimens must be dehydrated before being placed in the microscope This means that only dead material can be examined Great efforts are therefore made to try to preserve material in a life-like state when preparing it for the microscopeSAQ 13Explain why ribosomes are not visible using a light microscopeUltrastructure of an animal cellThe lsquofinersquo or detailed structure of a cell as revealed by the electron microscope is called its ultrastructure Figure 116 shows the appearance of typical animal cells as seen with an electron microscope and Figure 117 on page 15 is a diagram based on many other such micrographsSAQ 14Compare Figure 117 on page 15 with Figure 13 on page 2 Name the structures which can be seen with the electron microscope but not with the light microscope1 Cell structure 13lysosomeGGolgi apparatusendoplasmic reticulumglycogen granulesccell surface membrane

mmitochondriachromatinnnucleolusmmicrovillusrribosomesFFigure 116 Representative animal cells as seen with a TEM The cells are liver cells from a rat (1113088 9600) The nucleus is clearly visible in one of the cells 1441 1 Cell structurenucleusnnuclear envelopetwo centrioles close to the nucleus and at right angles to each othermitochondrion lysosomerough endoplasmic reticulumnucleoluschromatin nucleusnuclear porenuclear envelope (two membranes)microvillismooth endoplasmic reticulumFigure 117 Ultrastructure of a typical animal cell as seen with an electron microscope In reality the ER is more extensive than shown and free ribosomes may be more extensive Glycogen granules are sometimes present in the cytoplasmFigure 118 TEM of the nucleus of a cell from the pancreas of a bat (1113088 7500) The circular nucleus is surrounded by a double-layered nuclear envelope containing nuclear pores The nucleolus is more Figure 119 TEM of rough ER covered with ribosomes (black dots) darkly stained Rough ER is visible in the surrounding cytoplasm (1113088 17 000) Some free ribosomes can also be seen in the cytoplasmGolgi vesicle Golgi apparatusribosomescell surface membranecytoplasm1 Cell structure 15organelles called ribosomes These are just visible as black dots in Figures 118 and 119 on page 15 At very high magnifications they can be seen to consist of two subunits a large and a small subunit Ribosomes are the sites of protein synthesis (see pages 111ndash112) They can be found free in the cytoplasm as well as on the rough ER They are very small only about 25 nm in diameter They are made of RNA (ribonucleic acid) and protein The rough ER forms an extensive system of flattened sacs spreading in sheets throughout the cell Proteins made by the ribosomes on the rough ER enter the sacs and move through them The proteins are often processed in some way on their journey Small sacs called vesicles can break off from the ER and these can join together to form the Golgi apparatus Proteins can be exported from the cell via the Golgi apparatus (see page 80)Smooth ER so called because it lacks ribosomes has a completely different function It makes lipids and steroids such as cholesterol and the reproductive hormones oestrogen and testosteroneGolgi apparatus (Golgi body or Golgi complex)The Golgi apparatus is a stack of flattened sacs (Figure 120) This stack of sacs is sometimes referred to as theFigure 120 TEM of a Golgi apparatus A central stack of saucer-shaped sacs can be seen budding off small Golgi vesicles (green) These may form secretory vesicles whose contents can be released at the cell surface by exocytosis (see page 80)Golgi body More than one may be present in a cell The stack is constantly being formed at one end from vesicles which bud off from the ER and broken down again at the other end to form Golgi vesicles The stack of sacs with the associated vesicles is referred to as the Golgi apparatus or Golgi complex

The Golgi apparatus collects processes and sorts molecules (particularly proteins from the rough ER) ready for transport in Golgi vesicles either to other parts of the cell or out of the cell (secretion) Two examples of protein processing in the Golgi apparatus are the addition of sugars to proteins to make molecules known as glycoproteins and the removal of the first amino acid methionine from newly formed proteins to make a functioning protein In plants enzymes in the Golgi apparatus convert sugars into cell wall components Golgi vesicles are also used to make lysosomesLysosomesLysosomes (Figure 121) are spherical sacs surrounded by a single membrane and having no internal structure They are commonly 01ndash 05 1113088m in diameter They contain digestive (hydrolytic) enzymes which must be kept separateFigure 121 Lysosomes (orange) in a mouse kidney cell (1113088 55 000) They contain cell structures in the process of digestion and vesicles (green) Cytoplasm is coloured blue here1661 1 Cell structurefrom the rest of the cell to prevent damage Lysosomes are responsible for the breakdown (digestion) of unwanted structures such as old organelles or even whole cells as in mammary glands after lactation (breast feeding) In white blood cells lysosomes are used to digest bacteria (see endocytosis page 80) Enzymes are sometimes released outside the cell ndash for example in the replacement of cartilage with bone during development The headsof sperm contain a special lysosome the acrosome for digesting a path to the ovum (egg)MitochondriaMitochondria (singular mitochondrion) are usually about 1 1113088m in diameter and can be various shapes often sausage- shaped as in Figure 122 They are surrounded by two membranes (an envelope) The inner of these is foldedto form finger-like cristae which project into the interior solution or matrixThe main function of mitochondria is to carry out aerobic respiration As a result of respiration they make ATP the universal energy carrier in cells (see Chapter 16) They are also involved in the synthesis of lipids (page 37)Figure 122 Mitochondrion (orange) with its double membrane (envelope) the inner membrane is folded to form cristae (1113088 20 000) Mitochondria are the sites of aerobic cell respiration Note also the rough ERIn the 1960s it was discovered that mitochondria and chloroplasts contain ribosomes which are slightly smaller than those in the cytoplasm and are the same size as those found in bacteria The size of ribosomes is measured in lsquoS unitsrsquo which are a measure of how fast they sediment in a centrifuge Cytoplasmic ribosomes are 80S while those of bacteria mitochondria and chloroplasts are 70S It was also discovered in the 1960s that mitochondria and chloroplasts contain small circular DNA molecules also like those found in bacteria Not surprisingly it was later provedthat mitochondria and chloroplasts are in effect ancient bacteria which now live inside the larger cells typical of animals and plants (see

prokaryotic and eukaryotic cells page 18) This is known as the endosymbiont theory lsquoEndorsquo means lsquoinsidersquo and a lsquosymbiontrsquo is an organism which lives in a mutually beneficial relationship with another organism The DNA and ribosomes of mitochondria and chloroplasts are still active and responsible for the coding and synthesis of certain vital proteins but mitochondria and chloroplasts can no longer live independentlyMitochondrial ribosomes are just visible as tiny dark dots in the mitochondrial matrix in Figure 122Cell surface membraneThe cell surface membrane is extremely thin (about 7 nm) However at very high magnifications at least 1113088 100 000 it can be seen to have three layers described as a trilaminar appearance This consists of two dark lines (heavily stained) either side of a narrow pale interior (Figure 123) The membrane is partially permeable and controls exchange between the cell and its environment Membrane structure is discussed further in Chapter 4Figure 123 Cell surface membrane (1113088 250 000) At this magnification the membrane appears as two dark lines at the edge of the cellMicrovilliMicrovilli (singular microvillus) are finger-like extensions of the cell surface membrane typical of certain epithelial cells (cells covering surfaces of structures) They greatly1 Cell structure 17increase the surface area of the cell surface membrane (see Figure 117 on page 15) This is useful for example for absorption in the gut and for reabsorption in the proximal convoluted tubules of the kidney (see page 307)CentriolesThe extra resolution of the electron microscope reveals that just outside the nucleus there are really two centrioles (see Figure 124) not one as it appears under the light microscope (compare with Figure 13 on page 2) They lie close together at right-angles to each other A centriole is a hollow cylinder about 04 1113088m long formed from a ring of short microtubules tiny tubes made of a protein called tubulin These microtubules are used as a starting point for growing the spindle microtubules for nuclear division (see page 92) Centrioles are not found in plant cellsUltrastructure of a plant cellAll the structures except centrioles and microvilli so far described in animal cells are also found in plant cells The appearance of a plant cell as seen with the electron microscope is shown in Figure 125 while Figure 126 is a diagram based on many such micrographs The relatively thick cell wall and the large central vacuole are obvious as are the chloroplasts two of which are shown in detail in Figure 127 These structures and their functions have been described on pages 5 and 6 The electron microscopeFigure 124 Centrioles in transverse and longitudinal section (TS and LS) (1113088 86 000) The one on the left is seen in TS and clearly shows the nine triplets of microtubules which make up the structurereveals that chloroplasts contain 70S ribosomes and small circular

DNA molecules as do mitochondria and bacteria This has already been discussed with mitochondria above (page 17)SAQ 15Compare Figure 126 with Figure 15 on page 3 Name the structures which can be seen with the electron microscope but not with the light microscopeTwo fundamentally different types of cellAt one time it was common practice to try to classify all living organisms as either animals or plants With advances in our knowledge of living things it has become obvious that the living world is not that simple Fungi and bacteria for example are very different from animals and plants and from each other Eventually it was discovered that there are two fundamentally different types of cell The most obvious difference betweenthese types is that one possesses a nucleus and the other does notOrganisms that lack nuclei are called prokaryotes (lsquoprorsquo means before lsquokaryonrsquo means nucleus) All prokaryotes are now referred to as bacteria They are on average about 1000 to 10 000 times smaller in volume than cells with nuclei and are much simpler in structure ndash for example their DNA lies free in the cytoplasmOrganisms whose cells possess nuclei are called eukaryotes (lsquoeursquo means true) Their DNA lies inside a nucleus Eukaryotes include animals plants fungi and a group containing most of the unicellular eukaryotes known as protoctists Most biologists believe that eukaryotes evolved from prokaryotes one-and-a-half thousand million years after prokaryotes first appeared on Earth We mainly study animals and plants in this book but all eukaryotic cells have certain features in commonA generalised prokaryotic cell is shown in Figure 128 A comparison of prokaryotic and eukaryotic cells is given in Table 12 on page 211881 1 Cell structure

mmitochondriachromatinnnucleolusmmicrovillusrribosomesFFigure 116 Representative animal cells as seen with a TEM The cells are liver cells from a rat (1113088 9600) The nucleus is clearly visible in one of the cells 1441 1 Cell structurenucleusnnuclear envelopetwo centrioles close to the nucleus and at right angles to each othermitochondrion lysosomerough endoplasmic reticulumnucleoluschromatin nucleusnuclear porenuclear envelope (two membranes)microvillismooth endoplasmic reticulumFigure 117 Ultrastructure of a typical animal cell as seen with an electron microscope In reality the ER is more extensive than shown and free ribosomes may be more extensive Glycogen granules are sometimes present in the cytoplasmFigure 118 TEM of the nucleus of a cell from the pancreas of a bat (1113088 7500) The circular nucleus is surrounded by a double-layered nuclear envelope containing nuclear pores The nucleolus is more Figure 119 TEM of rough ER covered with ribosomes (black dots) darkly stained Rough ER is visible in the surrounding cytoplasm (1113088 17 000) Some free ribosomes can also be seen in the cytoplasmGolgi vesicle Golgi apparatusribosomescell surface membranecytoplasm1 Cell structure 15organelles called ribosomes These are just visible as black dots in Figures 118 and 119 on page 15 At very high magnifications they can be seen to consist of two subunits a large and a small subunit Ribosomes are the sites of protein synthesis (see pages 111ndash112) They can be found free in the cytoplasm as well as on the rough ER They are very small only about 25 nm in diameter They are made of RNA (ribonucleic acid) and protein The rough ER forms an extensive system of flattened sacs spreading in sheets throughout the cell Proteins made by the ribosomes on the rough ER enter the sacs and move through them The proteins are often processed in some way on their journey Small sacs called vesicles can break off from the ER and these can join together to form the Golgi apparatus Proteins can be exported from the cell via the Golgi apparatus (see page 80)Smooth ER so called because it lacks ribosomes has a completely different function It makes lipids and steroids such as cholesterol and the reproductive hormones oestrogen and testosteroneGolgi apparatus (Golgi body or Golgi complex)The Golgi apparatus is a stack of flattened sacs (Figure 120) This stack of sacs is sometimes referred to as theFigure 120 TEM of a Golgi apparatus A central stack of saucer-shaped sacs can be seen budding off small Golgi vesicles (green) These may form secretory vesicles whose contents can be released at the cell surface by exocytosis (see page 80)Golgi body More than one may be present in a cell The stack is constantly being formed at one end from vesicles which bud off from the ER and broken down again at the other end to form Golgi vesicles The stack of sacs with the associated vesicles is referred to as the Golgi apparatus or Golgi complex

The Golgi apparatus collects processes and sorts molecules (particularly proteins from the rough ER) ready for transport in Golgi vesicles either to other parts of the cell or out of the cell (secretion) Two examples of protein processing in the Golgi apparatus are the addition of sugars to proteins to make molecules known as glycoproteins and the removal of the first amino acid methionine from newly formed proteins to make a functioning protein In plants enzymes in the Golgi apparatus convert sugars into cell wall components Golgi vesicles are also used to make lysosomesLysosomesLysosomes (Figure 121) are spherical sacs surrounded by a single membrane and having no internal structure They are commonly 01ndash 05 1113088m in diameter They contain digestive (hydrolytic) enzymes which must be kept separateFigure 121 Lysosomes (orange) in a mouse kidney cell (1113088 55 000) They contain cell structures in the process of digestion and vesicles (green) Cytoplasm is coloured blue here1661 1 Cell structurefrom the rest of the cell to prevent damage Lysosomes are responsible for the breakdown (digestion) of unwanted structures such as old organelles or even whole cells as in mammary glands after lactation (breast feeding) In white blood cells lysosomes are used to digest bacteria (see endocytosis page 80) Enzymes are sometimes released outside the cell ndash for example in the replacement of cartilage with bone during development The headsof sperm contain a special lysosome the acrosome for digesting a path to the ovum (egg)MitochondriaMitochondria (singular mitochondrion) are usually about 1 1113088m in diameter and can be various shapes often sausage- shaped as in Figure 122 They are surrounded by two membranes (an envelope) The inner of these is foldedto form finger-like cristae which project into the interior solution or matrixThe main function of mitochondria is to carry out aerobic respiration As a result of respiration they make ATP the universal energy carrier in cells (see Chapter 16) They are also involved in the synthesis of lipids (page 37)Figure 122 Mitochondrion (orange) with its double membrane (envelope) the inner membrane is folded to form cristae (1113088 20 000) Mitochondria are the sites of aerobic cell respiration Note also the rough ERIn the 1960s it was discovered that mitochondria and chloroplasts contain ribosomes which are slightly smaller than those in the cytoplasm and are the same size as those found in bacteria The size of ribosomes is measured in lsquoS unitsrsquo which are a measure of how fast they sediment in a centrifuge Cytoplasmic ribosomes are 80S while those of bacteria mitochondria and chloroplasts are 70S It was also discovered in the 1960s that mitochondria and chloroplasts contain small circular DNA molecules also like those found in bacteria Not surprisingly it was later provedthat mitochondria and chloroplasts are in effect ancient bacteria which now live inside the larger cells typical of animals and plants (see

prokaryotic and eukaryotic cells page 18) This is known as the endosymbiont theory lsquoEndorsquo means lsquoinsidersquo and a lsquosymbiontrsquo is an organism which lives in a mutually beneficial relationship with another organism The DNA and ribosomes of mitochondria and chloroplasts are still active and responsible for the coding and synthesis of certain vital proteins but mitochondria and chloroplasts can no longer live independentlyMitochondrial ribosomes are just visible as tiny dark dots in the mitochondrial matrix in Figure 122Cell surface membraneThe cell surface membrane is extremely thin (about 7 nm) However at very high magnifications at least 1113088 100 000 it can be seen to have three layers described as a trilaminar appearance This consists of two dark lines (heavily stained) either side of a narrow pale interior (Figure 123) The membrane is partially permeable and controls exchange between the cell and its environment Membrane structure is discussed further in Chapter 4Figure 123 Cell surface membrane (1113088 250 000) At this magnification the membrane appears as two dark lines at the edge of the cellMicrovilliMicrovilli (singular microvillus) are finger-like extensions of the cell surface membrane typical of certain epithelial cells (cells covering surfaces of structures) They greatly1 Cell structure 17increase the surface area of the cell surface membrane (see Figure 117 on page 15) This is useful for example for absorption in the gut and for reabsorption in the proximal convoluted tubules of the kidney (see page 307)CentriolesThe extra resolution of the electron microscope reveals that just outside the nucleus there are really two centrioles (see Figure 124) not one as it appears under the light microscope (compare with Figure 13 on page 2) They lie close together at right-angles to each other A centriole is a hollow cylinder about 04 1113088m long formed from a ring of short microtubules tiny tubes made of a protein called tubulin These microtubules are used as a starting point for growing the spindle microtubules for nuclear division (see page 92) Centrioles are not found in plant cellsUltrastructure of a plant cellAll the structures except centrioles and microvilli so far described in animal cells are also found in plant cells The appearance of a plant cell as seen with the electron microscope is shown in Figure 125 while Figure 126 is a diagram based on many such micrographs The relatively thick cell wall and the large central vacuole are obvious as are the chloroplasts two of which are shown in detail in Figure 127 These structures and their functions have been described on pages 5 and 6 The electron microscopeFigure 124 Centrioles in transverse and longitudinal section (TS and LS) (1113088 86 000) The one on the left is seen in TS and clearly shows the nine triplets of microtubules which make up the structurereveals that chloroplasts contain 70S ribosomes and small circular

DNA molecules as do mitochondria and bacteria This has already been discussed with mitochondria above (page 17)SAQ 15Compare Figure 126 with Figure 15 on page 3 Name the structures which can be seen with the electron microscope but not with the light microscopeTwo fundamentally different types of cellAt one time it was common practice to try to classify all living organisms as either animals or plants With advances in our knowledge of living things it has become obvious that the living world is not that simple Fungi and bacteria for example are very different from animals and plants and from each other Eventually it was discovered that there are two fundamentally different types of cell The most obvious difference betweenthese types is that one possesses a nucleus and the other does notOrganisms that lack nuclei are called prokaryotes (lsquoprorsquo means before lsquokaryonrsquo means nucleus) All prokaryotes are now referred to as bacteria They are on average about 1000 to 10 000 times smaller in volume than cells with nuclei and are much simpler in structure ndash for example their DNA lies free in the cytoplasmOrganisms whose cells possess nuclei are called eukaryotes (lsquoeursquo means true) Their DNA lies inside a nucleus Eukaryotes include animals plants fungi and a group containing most of the unicellular eukaryotes known as protoctists Most biologists believe that eukaryotes evolved from prokaryotes one-and-a-half thousand million years after prokaryotes first appeared on Earth We mainly study animals and plants in this book but all eukaryotic cells have certain features in commonA generalised prokaryotic cell is shown in Figure 128 A comparison of prokaryotic and eukaryotic cells is given in Table 12 on page 211881 1 Cell structure

The Golgi apparatus collects processes and sorts molecules (particularly proteins from the rough ER) ready for transport in Golgi vesicles either to other parts of the cell or out of the cell (secretion) Two examples of protein processing in the Golgi apparatus are the addition of sugars to proteins to make molecules known as glycoproteins and the removal of the first amino acid methionine from newly formed proteins to make a functioning protein In plants enzymes in the Golgi apparatus convert sugars into cell wall components Golgi vesicles are also used to make lysosomesLysosomesLysosomes (Figure 121) are spherical sacs surrounded by a single membrane and having no internal structure They are commonly 01ndash 05 1113088m in diameter They contain digestive (hydrolytic) enzymes which must be kept separateFigure 121 Lysosomes (orange) in a mouse kidney cell (1113088 55 000) They contain cell structures in the process of digestion and vesicles (green) Cytoplasm is coloured blue here1661 1 Cell structurefrom the rest of the cell to prevent damage Lysosomes are responsible for the breakdown (digestion) of unwanted structures such as old organelles or even whole cells as in mammary glands after lactation (breast feeding) In white blood cells lysosomes are used to digest bacteria (see endocytosis page 80) Enzymes are sometimes released outside the cell ndash for example in the replacement of cartilage with bone during development The headsof sperm contain a special lysosome the acrosome for digesting a path to the ovum (egg)MitochondriaMitochondria (singular mitochondrion) are usually about 1 1113088m in diameter and can be various shapes often sausage- shaped as in Figure 122 They are surrounded by two membranes (an envelope) The inner of these is foldedto form finger-like cristae which project into the interior solution or matrixThe main function of mitochondria is to carry out aerobic respiration As a result of respiration they make ATP the universal energy carrier in cells (see Chapter 16) They are also involved in the synthesis of lipids (page 37)Figure 122 Mitochondrion (orange) with its double membrane (envelope) the inner membrane is folded to form cristae (1113088 20 000) Mitochondria are the sites of aerobic cell respiration Note also the rough ERIn the 1960s it was discovered that mitochondria and chloroplasts contain ribosomes which are slightly smaller than those in the cytoplasm and are the same size as those found in bacteria The size of ribosomes is measured in lsquoS unitsrsquo which are a measure of how fast they sediment in a centrifuge Cytoplasmic ribosomes are 80S while those of bacteria mitochondria and chloroplasts are 70S It was also discovered in the 1960s that mitochondria and chloroplasts contain small circular DNA molecules also like those found in bacteria Not surprisingly it was later provedthat mitochondria and chloroplasts are in effect ancient bacteria which now live inside the larger cells typical of animals and plants (see

prokaryotic and eukaryotic cells page 18) This is known as the endosymbiont theory lsquoEndorsquo means lsquoinsidersquo and a lsquosymbiontrsquo is an organism which lives in a mutually beneficial relationship with another organism The DNA and ribosomes of mitochondria and chloroplasts are still active and responsible for the coding and synthesis of certain vital proteins but mitochondria and chloroplasts can no longer live independentlyMitochondrial ribosomes are just visible as tiny dark dots in the mitochondrial matrix in Figure 122Cell surface membraneThe cell surface membrane is extremely thin (about 7 nm) However at very high magnifications at least 1113088 100 000 it can be seen to have three layers described as a trilaminar appearance This consists of two dark lines (heavily stained) either side of a narrow pale interior (Figure 123) The membrane is partially permeable and controls exchange between the cell and its environment Membrane structure is discussed further in Chapter 4Figure 123 Cell surface membrane (1113088 250 000) At this magnification the membrane appears as two dark lines at the edge of the cellMicrovilliMicrovilli (singular microvillus) are finger-like extensions of the cell surface membrane typical of certain epithelial cells (cells covering surfaces of structures) They greatly1 Cell structure 17increase the surface area of the cell surface membrane (see Figure 117 on page 15) This is useful for example for absorption in the gut and for reabsorption in the proximal convoluted tubules of the kidney (see page 307)CentriolesThe extra resolution of the electron microscope reveals that just outside the nucleus there are really two centrioles (see Figure 124) not one as it appears under the light microscope (compare with Figure 13 on page 2) They lie close together at right-angles to each other A centriole is a hollow cylinder about 04 1113088m long formed from a ring of short microtubules tiny tubes made of a protein called tubulin These microtubules are used as a starting point for growing the spindle microtubules for nuclear division (see page 92) Centrioles are not found in plant cellsUltrastructure of a plant cellAll the structures except centrioles and microvilli so far described in animal cells are also found in plant cells The appearance of a plant cell as seen with the electron microscope is shown in Figure 125 while Figure 126 is a diagram based on many such micrographs The relatively thick cell wall and the large central vacuole are obvious as are the chloroplasts two of which are shown in detail in Figure 127 These structures and their functions have been described on pages 5 and 6 The electron microscopeFigure 124 Centrioles in transverse and longitudinal section (TS and LS) (1113088 86 000) The one on the left is seen in TS and clearly shows the nine triplets of microtubules which make up the structurereveals that chloroplasts contain 70S ribosomes and small circular

DNA molecules as do mitochondria and bacteria This has already been discussed with mitochondria above (page 17)SAQ 15Compare Figure 126 with Figure 15 on page 3 Name the structures which can be seen with the electron microscope but not with the light microscopeTwo fundamentally different types of cellAt one time it was common practice to try to classify all living organisms as either animals or plants With advances in our knowledge of living things it has become obvious that the living world is not that simple Fungi and bacteria for example are very different from animals and plants and from each other Eventually it was discovered that there are two fundamentally different types of cell The most obvious difference betweenthese types is that one possesses a nucleus and the other does notOrganisms that lack nuclei are called prokaryotes (lsquoprorsquo means before lsquokaryonrsquo means nucleus) All prokaryotes are now referred to as bacteria They are on average about 1000 to 10 000 times smaller in volume than cells with nuclei and are much simpler in structure ndash for example their DNA lies free in the cytoplasmOrganisms whose cells possess nuclei are called eukaryotes (lsquoeursquo means true) Their DNA lies inside a nucleus Eukaryotes include animals plants fungi and a group containing most of the unicellular eukaryotes known as protoctists Most biologists believe that eukaryotes evolved from prokaryotes one-and-a-half thousand million years after prokaryotes first appeared on Earth We mainly study animals and plants in this book but all eukaryotic cells have certain features in commonA generalised prokaryotic cell is shown in Figure 128 A comparison of prokaryotic and eukaryotic cells is given in Table 12 on page 211881 1 Cell structure

prokaryotic and eukaryotic cells page 18) This is known as the endosymbiont theory lsquoEndorsquo means lsquoinsidersquo and a lsquosymbiontrsquo is an organism which lives in a mutually beneficial relationship with another organism The DNA and ribosomes of mitochondria and chloroplasts are still active and responsible for the coding and synthesis of certain vital proteins but mitochondria and chloroplasts can no longer live independentlyMitochondrial ribosomes are just visible as tiny dark dots in the mitochondrial matrix in Figure 122Cell surface membraneThe cell surface membrane is extremely thin (about 7 nm) However at very high magnifications at least 1113088 100 000 it can be seen to have three layers described as a trilaminar appearance This consists of two dark lines (heavily stained) either side of a narrow pale interior (Figure 123) The membrane is partially permeable and controls exchange between the cell and its environment Membrane structure is discussed further in Chapter 4Figure 123 Cell surface membrane (1113088 250 000) At this magnification the membrane appears as two dark lines at the edge of the cellMicrovilliMicrovilli (singular microvillus) are finger-like extensions of the cell surface membrane typical of certain epithelial cells (cells covering surfaces of structures) They greatly1 Cell structure 17increase the surface area of the cell surface membrane (see Figure 117 on page 15) This is useful for example for absorption in the gut and for reabsorption in the proximal convoluted tubules of the kidney (see page 307)CentriolesThe extra resolution of the electron microscope reveals that just outside the nucleus there are really two centrioles (see Figure 124) not one as it appears under the light microscope (compare with Figure 13 on page 2) They lie close together at right-angles to each other A centriole is a hollow cylinder about 04 1113088m long formed from a ring of short microtubules tiny tubes made of a protein called tubulin These microtubules are used as a starting point for growing the spindle microtubules for nuclear division (see page 92) Centrioles are not found in plant cellsUltrastructure of a plant cellAll the structures except centrioles and microvilli so far described in animal cells are also found in plant cells The appearance of a plant cell as seen with the electron microscope is shown in Figure 125 while Figure 126 is a diagram based on many such micrographs The relatively thick cell wall and the large central vacuole are obvious as are the chloroplasts two of which are shown in detail in Figure 127 These structures and their functions have been described on pages 5 and 6 The electron microscopeFigure 124 Centrioles in transverse and longitudinal section (TS and LS) (1113088 86 000) The one on the left is seen in TS and clearly shows the nine triplets of microtubules which make up the structurereveals that chloroplasts contain 70S ribosomes and small circular

DNA molecules as do mitochondria and bacteria This has already been discussed with mitochondria above (page 17)SAQ 15Compare Figure 126 with Figure 15 on page 3 Name the structures which can be seen with the electron microscope but not with the light microscopeTwo fundamentally different types of cellAt one time it was common practice to try to classify all living organisms as either animals or plants With advances in our knowledge of living things it has become obvious that the living world is not that simple Fungi and bacteria for example are very different from animals and plants and from each other Eventually it was discovered that there are two fundamentally different types of cell The most obvious difference betweenthese types is that one possesses a nucleus and the other does notOrganisms that lack nuclei are called prokaryotes (lsquoprorsquo means before lsquokaryonrsquo means nucleus) All prokaryotes are now referred to as bacteria They are on average about 1000 to 10 000 times smaller in volume than cells with nuclei and are much simpler in structure ndash for example their DNA lies free in the cytoplasmOrganisms whose cells possess nuclei are called eukaryotes (lsquoeursquo means true) Their DNA lies inside a nucleus Eukaryotes include animals plants fungi and a group containing most of the unicellular eukaryotes known as protoctists Most biologists believe that eukaryotes evolved from prokaryotes one-and-a-half thousand million years after prokaryotes first appeared on Earth We mainly study animals and plants in this book but all eukaryotic cells have certain features in commonA generalised prokaryotic cell is shown in Figure 128 A comparison of prokaryotic and eukaryotic cells is given in Table 12 on page 211881 1 Cell structure

DNA molecules as do mitochondria and bacteria This has already been discussed with mitochondria above (page 17)SAQ 15Compare Figure 126 with Figure 15 on page 3 Name the structures which can be seen with the electron microscope but not with the light microscopeTwo fundamentally different types of cellAt one time it was common practice to try to classify all living organisms as either animals or plants With advances in our knowledge of living things it has become obvious that the living world is not that simple Fungi and bacteria for example are very different from animals and plants and from each other Eventually it was discovered that there are two fundamentally different types of cell The most obvious difference betweenthese types is that one possesses a nucleus and the other does notOrganisms that lack nuclei are called prokaryotes (lsquoprorsquo means before lsquokaryonrsquo means nucleus) All prokaryotes are now referred to as bacteria They are on average about 1000 to 10 000 times smaller in volume than cells with nuclei and are much simpler in structure ndash for example their DNA lies free in the cytoplasmOrganisms whose cells possess nuclei are called eukaryotes (lsquoeursquo means true) Their DNA lies inside a nucleus Eukaryotes include animals plants fungi and a group containing most of the unicellular eukaryotes known as protoctists Most biologists believe that eukaryotes evolved from prokaryotes one-and-a-half thousand million years after prokaryotes first appeared on Earth We mainly study animals and plants in this book but all eukaryotic cells have certain features in commonA generalised prokaryotic cell is shown in Figure 128 A comparison of prokaryotic and eukaryotic cells is given in Table 12 on page 211881 1 Cell structure