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Al Wadi International School BIOLOGY 9700-201

1

All organisms are composed of cells. Knowledge of their structure and funct-

ion underpins much of biology. The fundamental differences between eukaryotic andprokaryotic cells are explored and provide useful biological background for the section

on Infectious disease. Viruses are introduced as non-cellular structures, which gives

candidates the opportunity to consider whether cells are a fundamental property of life.

The use of light microscopes is a fundamental skill that is developed in this section and

applied throughout several other sections of the syllabus. Throughout the course,

photomicrographs and electron micrographs from transmission and scanning electron

microscopes should be studied.

1.1 The microscope in cell studies

d) explain and distinguish between resolution and magnification, with reference to

light microscopy and electron microscopy

Most cells are very small, and their structures

can only be seen by using a microscope.

Microscopy is the use of microscopes to study

the structural details of organisms and the

organelles within the cell by magnifying theimage.

Different kinds of Microscope.

1. Light Microscope. This is the oldest, simplest and most widely-used form of

microscope.

Specimens are illuminated with light, which is focused using glass lenses and viewed using

the eye or photographic film. Specimens can be living or dead, but often need to be coloured

with a coloured stain to make them visible. Many different stains are available that stainspecific parts of the cell such as DNA, lipids, cytoskeleton, etc. All light microscopes today

are compound microscopes, which mean they use several lenses to obtain high

magnification.

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Light microscope has a resolution of about 200

nm, which is good enough to see tissues and

cells, but not the details of cell organelles. There

has been a recent resurgence in the use of lightmicroscopy, partly due to technical

improvements, which have dramatically

improved the resolution far beyond the

theoretical limit. For example fluorescence

microscopy has a resolution of about 10 nm,

while interference microscopy has a resolution

of about 1 nm.

2.

Electron Microscope.This uses a beam of electrons, rather

than electromagnetic radiation, to

"illuminate" the specimen. This

may seem strange, but electrons

behave like waves and can easily

be produced (using a hot wire),

focused (using electromagnets)

and detected (using a phosphor

screen or photographic film).

A beam of electrons has an effective

wavelength of less than 1 nm, so can be

used to resolve small sub-cellular

ultrastructure. The development of the

electron microscope in the 1930s

revolutionised biology, allowing

organelles such as mitochondria, ER and

membranes to be seen in detail for the

first time.

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There are several problems with the electron microscopy:

the electron beam is scattered by air molecules, so to avoid this there is a vacuum

inside an electron microscope, so it can't be used for living organisms.

specimens must be very thin, so are embedded in plastic for support, so can't bemanipulated under the microscope.

specimens can be damaged by the electron beam, so delicate structures and

molecules can be destroyed.

specimens are usually transparent to electrons, so must be stained with an electron-

dense chemical (usually heavy metals like osmium, lead or gold).

Initially there was a problem of artefacts (i.e. observed structures that were due to

the preparation process and were not real), but improvements in technique have

eliminated most of these.

There are two kinds of electron microscope.

The transmission electron microscope (TEM) works much like a light microscope,

transmitting a beam of electrons through a thin specimen and then focusing the

electrons to form an image on a screen or on film. This is the most common form of

electron microscope and has the best resolution.

The scanning electron microscope (SEM) scans a fine beam of electron onto a

specimen and collects the electrons scattered by the surface. This has poorer

resolution, but gives excellent 3-dimensional images of surfaces.

TEM SEM

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Measurements used in Microscopy

1 centimeter (cm) = 1/100 meter = 0.4 inch = 10-2 m

1 millimeter (mm) = 1/1,000 meter = 1/10 cm = 10-3 m

1 micrometer (m) = 1/1,000,000 meter = 1/10,000 cm = 10-6 m

1 nanometer (nm) = 1/1,000,000,000 meter = 1/10,000,000 cm = 10-9 m1 angstrom (A) = 1/10,000,000,000 meter = 1/100,000,000 cm = 10-10 m

1 meter = 102 cm = 103 mm = 106 m = 109 nm = 1010 A

3 a) Convert the following. All the answers are to be written in standard form.

0.00254 micrometer into millimeter

1.0665x10-5 nanometer into centimeter

6.211 x10-5 millimeter into nanometer

2449.88 micrometer into nanometer

b) Calculate the magnification of a drawing 22 mm long of an object having an actual

length of 0.06 micrometer.

c) What is the actual length of an organelle 4mm long shown in a drawing with a

magnification of x4000.

Q. The scanning electron micrograph below shows the surface of the nuclear

envelope with numerous nuclear pores.

(i) Calculate the power of

magnification of the image.

......................................................... (1)

(ii) State the diameter of the pore

labelled

X........................................................(1)

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Comparison between the Light microscope & Electron Microscope

Light Microscope Electron Microscope

1. Source of

radiation

Uses light rays of wavelength

between 400 to 700 nm

Uses electron beams of

wavelength 0.005 nm2. Lenses Eyepiece and objective lenses

made of glass

Electromagnets

3. Stains used Dyes with suitable colours Heavy metals such as lead

4. Maximum

magnification

5.Maximumresolution

6. Depth of field

Disadvantages

It only magnifies objects to an

extent of 1500 times.

200 nm

The depth of field is restricted.

Advantages

It is able to magnify objects

over 250,000 times (for TEM)

and over 100,000 times (SEM)

0.5 nm

The depth of field possible is

greater.

7. Price and

operation

8. Portability

9. Type of

specimen

10. Magnetic fields

11.

Preparation of

specimen

12.

Image produced

Advantages

Inexpensive to purchase and

easier to operate.

Easy and small.

Living or non-living organisms.

Magnetic fields have no effect on

it.

Preparation of specimen is

simple and fast

Coloured, with the natural

colour of specimen or dye

Disadvantages

Expensive to purchase and

operation requires expertise.

Bulky and less portable.

Only non-living organisms.

Magnetic fields have an effect

on it.

Preparation of specimen is

complex and needs

considerable time and

experience.

Black and white only

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b) calculate the linear magnifications of drawings, photomicrographs and

electron micrographs

Magnification

Magnification is the size of an image of an object compared to the actual size. It iscalculated using the formula M = I/A (M is magnification, I is the size of the image and A is

the actual size of the object, using the same units for both sizes). This formula can be

rearranged to give the actual size of an object where the size of the image and magnification

are known: A = I/M.

e.g., if a cell is 10m in diameter, and a microscope produces an image of it which is 1mm

(1000m) in diameter, than the microscope has magnified the image 100 times. (x100)

Magnification Calculations

Microscope drawings and photographs (micrographs) are usually magnified. There are

two ways of doing this:

1. Using a Magnification Factor

Sometimes the image has a magnification factor on it. The formula for the magnification is

Amount of magnification depends on the resolution of the microscope (ability to

distinguish 2 objects as separate).

The smaller the objects that can be distinguished --> the higher the resolution.

wavelength: beam of electrons > light microscope

with electron microscope, we can see much more fine detail of a cell.

Units: millimetre, micrometre, nanometre

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c) use an eyepiece graticule and stage micrometer scale to measure cells and be

familiar with units (millimetre, micrometre, nanometre) used in cell studies

Measuring cells using a graticule

Eyepiece graticule is a little scale bar placed in the eyepiece of light microscope.

The graticule is marked off in 'graticule units'.

Turn the eyepiece so that the graticule scale lies over the object: the width of one

cell is 23 graticule units.

Calibration: the conversion of graticule units into real units (mm, µm).

use a special slide called a stage micrometer that is marked off in a tiny scale. The

smallest markings are often 0.01 mm (10 µg) apart.

Take the specimen off the stage or the microscope and replace it with the stagemicrometer. Use the same objective lens.

Line up the micrometer scale and the eyepiece graticule scale (by turning

the eyepiece and moving the micrometer on the stage). Make sure that 2 large

markings on each scale are lined up.

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The 50 mark (stage micrometer) is lined up with the 1.0 mark (eyepiece graticule).

Work towards the right until you see another two lines lined up.

The 68 mark (stage micrometer) is lined up with the 9.0 mark (eyepiece graticule).

So you can say that:

80 small eyepiece graticule markings = 18 stage micrometer markings

= 18 x 0.01 mm = 0.18 mm= 180 µm

so 1 small eyepiece graticule marking = 180: 80 = 2.25 µm

The plant cell was 23 eyepiece graticule units long --> its real width is: 23 x 2.25 =

51.75 µm

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e) calculate actual sizes of specimens from drawings, photomicrographs and

electron micrographs

Work out the actual size of an object knowing the magnification:

a. This drawing of a mitochondrion has been magnified 100 000 times.

• Use ruler to measure its length in mm (50 mm).

• Convert this measurement to µm by multiplying by 1 000.

50 x 1 000 = 50 000 µg

• Substitute into the equation:

b. This is the drawing of a chloroplast:

- The magnification for this drawing:

- The length of the chloroplast:

• Measure the length of the image in mm (80 mm)

and convert to µm ---> 80 000 µm.

• Calculate its real length:

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a) compare the structure of typical animal and plant cells by making temporary

preparations of live material and using photomicrographs

When preparing microscope slides for observation, it is important first to have all

necessary materials on hand. This includes slides, cover slips, droppers or pipettes andany chemicals or stains you plan to use.

The most common slide preparation is called the "wet mount" slide and utilizes a flat

slide and a cover slip.

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The common flat glass slide is rectangular and measure approximately 1 x 3 inches (25 x

75 mm). A cover slip or cover glass (18-20mm) is a very thin square piece of glass that is

placed over the water drop. Because of surface tension, the water drop alone tends to sit

in a thick dome. With a cover slip in place, the drop is flattened out allowing to focus with

high power very close to the specimen. The cover glass also confines the specimen to asingle plane and thereby reduces the amount of focusing necessary. Finally, the cover

glass protects the objective lens from immersion into the water drop.

To make a slide, place a drop of the sample in the middle of a clean slide and lower a

cover slip gently over the drop at an angle, with one edge touching the slide first (See

Figure). Allow the liquid to spread out between the two pieces of glass without applying

pressure. It takes some practice to determine just how much liquid to use. If too much is

placed on the slide, the cover slip will "float", creating a water layer that is too thick. If too

little liquid is used, the organisms may be crushed by the cover glass and evaporation willdry up the specimens quickly. A well prepared slide will last for 15 -30 minutes before it

dries up.

1.2 Cells as the basic units of living organisms

b) recognise the following cell structures and outline their functions:

• cell surface membrane

• nucleus, nuclear envelope and nucleolus • rough endoplasmic reticulum

• smooth endoplasmic reticulum

• Golgi body (Golgi apparatus or Golgi complex)

• mitochondria (including small circular DNA)

• ribosomes (80S in the cytoplasm and 70S in chloroplasts and mitochondria)

• lysosomes

• centrioles and microtubules

• chloroplasts (including small circular DNA)

• cell wall

• plasmodesmata • large permanent vacuole and tonoplast of plant cells

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The Cell

All living things are made of cells, and cells are the smallest units that can be alive. There

are thousands of different kinds of cell, but the biggest division is between the cells of the

prokaryote kingdom (the bacteria) and those of the other four kingdoms (animals, plantsfungi and protoctista), which are all eukaryotic cells. Prokaryotic cells are smaller and

simpler than eukaryotic cells, and do not have a nucleus.

Prokaryote = without a nucleus

Eukaryote = with a nucleus

These two kinds of cell are being examined in detail, based on structures seen in electron

micrographs.

Euakryotic Cells

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Cytoplasm (or Cytosol).

This is the solution within the cell membrane. It contains enzymes for glycolysis (part of

respiration) and other metabolic reactions together with sugars, salts, amino acids,

nucleotides and everything else needed for the cell to function.

Membrane Systems and Organelles:

Endoplasmic Reticulum (ER)

This is a series of membrane channels involved in synthesising and transporting materials

Rough Endoplasmic Reticulum (RER) is studded with numerous ribosomes, which give it

its rough appearance. The ribosomes synthesise proteins, which are processed in the RER

(e.g. by enzymatically modifying the polypeptide chain, or adding carbohydrates), before

being exported from the cell via the Golgi apparatus. Smooth Endoplasmic Reticulum (SER)does not have ribosomes and is used to process materials, mainly lipids, needed by the cell

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Golgi Apparatus

Another series of flattened membrane

vesicles, formed from the endoplasmic

reticulum. Its job is to transport proteinsfrom the RER to the cell membrane for

export.

Parts of the RER containing proteins fuse

with one side of the Golgi body membranes,

while at the other side small vesicles bud off

and move towards the cell membrane,

where they fuse, releasing their contents by exocytosis.

Role of Golgi apparatus

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Mitochondrion (pl. Mitochondria)

This is a sausage shaped

organelle (8μm long), and is

where aerobic respirationtakes place in all eukaryotic

cells. Mitochondria are

surrounded by a double

membrane: the outer

membrane is simple and quite

permeable, while the inner

membrane is highly folded

into cristae (C), which give it a

large surface area for enzyme

reactions. The space enclosed

by the inner membrane is called the mitochondrial matrix, and contains small circular

strands of DNA. They contain 70S Ribosomes in their matrix for the synthesis of enzymes

needed for aerobic respiration. The inner membrane is studded with stalked particles,

which are the site of ATP synthesis during respiration.

RibosomesThese are the smallest and most

numerous of the cell organelles, and

are the sites of protein synthesis.

They are composed of protein and

RNA, and are manufactured in the

nucleolus of the nucleus.

Ribosomes are either found free in

the cytoplasm, where they make

proteins for the cell's own use, or

they are found attached to the rough endoplasmic reticulum, where they make proteins for

export from the cell. All eukaryotic ribosomes are of the larger, "80S" type. However,

eukaryotic cells possess 70S ribosomes as well but inside the mitochondria and

chloroplasts (plant cells only).

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LysosomesThese are small membrane-bound vesicles formed from the RER containing a cocktail of

digestive enzymes. They are used to break down unwanted chemicals, toxins, organelles

or even whole cells, so that the materials may be recycled. They can also fuse with a feedingvacuole to digest its contents.

It is very important that the enzymes contained within lysosomes are isolated from the rest

of the cell inside the lysosomes membrane, otherwise their release would result in self-

digestion of the cell. In fact, this sometimes happens in certain tissues, such as the tadpole’s

tail when it is changing into a frog; this process is called apoptosis or programmed cell

death

Role of Lysosome in a cell

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Centrioles and MicrotubulesThis is a special pair of short cytoskeleton fibres

involved in cell division. They initiate the

formation of spindle microtubules thatorganises and separates the chromosomes

during the cell division.

Recently, experiments have shown that the

centrioles are the site of formation of the whole

cytoskeleton network made up of microtubules,

not just the spindle fibres. This has led to them

being renamed microtubule organising

centers.

Cell Surface Membrane

The cell surface membrane is the boundary between the cell and its environment. It has

little mechanical strength but plays a vital role in controlling which materials pass in and

out of the cell.

Although basically a double layer of phospholipids molecules, arranged tail to tail, cell

surface membrane is a complex structure, studded with proteins. These can be embedded

in the membrane or they can penetrate the bilayer forming pores through which molecules

can pass.

Centrioles

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Chloroplasts

Bigger and fatter than mitochondria, chloroplasts are where photosynthesis takes place, so

are only found in photosynthetic organisms (plants and algae).

Like mitochondria they are enclosed by a double membrane, but chloroplasts also have a

third membrane called the thylakoid membrane. The thylakoid membrane is folded into

thylakoid disks, which are then stacked into piles called grana. The space between the inner

membrane and the thylakoid is called the stroma. The thylakoid membrane contains

chlorophyll and chloroplasts also contain starch grains, ribosomes and circular DNA.

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Large Permanent Vacuole and Tonoplast (of

Plant Cells):

Vacuoles are membrane-bound sacs within the

cytoplasm of a cell that function in several

different ways. In mature plant cells, vacuolestend to be very large and are extremely important

in providing structural support, as well as serving

functions such as storage, waste disposal,

protection, and growth. Many plant cells have a

large, single central vacuole that typically takes

up most of the room in the cell (80 percent or

more). Vacuoles in animal cells, however, tend to

be much smaller, and are more commonly used to

temporarily store materials or to transport substances.

The central vacuole in plant cells is enclosed by a membrane termed the tonoplast , an

important and highly integrated component of the plant internal membrane network

(endomembrane) system. This large vacuole slowly develops as the cell matures by fusion

of smaller vacuoles derived from the endoplasmic reticulum and Golgi apparatus. Because

the central vacuole is highly selective in transporting materials through its membrane, the

chemical palette of the vacuole solution (termed the cell sap) differs markedly from that

of the surrounding cytoplasm.

Cell Wall:

Components

The main ingredient in cell walls are polysaccharides (or complex carbohydrates or

complex sugars) which are built from monosaccharides (or simple sugars). Eleven

different monosaccharides are common in these polysaccharides including glucose and

galactose. Carbohydrates are good building blocks because they can produce a nearly

infinite variety of structures. There are a variety of other components in the wall

including protein, and lignin.

i) Cellulose:

β1,4-glucan made of as many as 25,000 individual glucose molecules. Every other

molecule (called residues) is "upside down". Cellobiose (glucose-glucose disaccharide) is

the basic building block. Cellulose readily forms hydrogen bonds with itself (intra-

molecular H-bonds) and with other cellulose chains (inter-molecular H-bonds). A

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cellulose chain will form hydrogen bonds with about 36 other chains to yield a

microfibril . This is somewhat analogous to the formation of a thick rope from thin fibers.

Microfibrils are 5-12 nm wide and give the wall strength - they have a tensile strength

equivalent to steel. Some regions of the microfibrils are highly crystalline while others

are more "amorphous".

ii) Cross-linking glycans (Hemicellulose)

Diverse group of carbohydrates that used to be called hemicellulose. Characterized by

being soluble in strong alkali. They are linear (straight), flat, with a β-1,4 backbone and

relatively short side chains. The main feature of this group is that they don’t aggregate

with themselves - in other words, they don’t form microfibrils. However, they form

hydrogen bonds with cellulose and hence the reason they are called "cross-linking

glycans".

iii) Pectic polysaccharides

They are the easiest constituents to remove from the wall. They form gels (i.e., used in

jelly making). They are also a diverse group of polysaccharides and are particularly rich

in galacturonic acid. They are polymers of primarily β 1,4 galacturonans

(polygalacturonans). These are helical in shape. Divalent cations, like calcium, also form

cross-linkages to join adjacent polymers creating a gel.

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Functions of the cell wall:

The cell wall serves a variety of purposes including:

1.

Maintaining/determining cell shape (analogous to an external skeleton for everycell). Since protoplasts are invariably round, this is good evidence that the wall

ultimately determines the shape of plant cells.

2.

Support and mechanical strength (allows plants to get tall, hold out thin leaves to

obtain light).

3.

prevents the cell membrane from bursting in a hypotonic medium (i.e., resists

water pressure).

4. controls the rate and direction of cell growth and regulates cell volume.

5.

ultimately responsible for the plant architectural design and controlling plant

morphogenesis since the wall dictates that plants develop by cell addition (not cellmigration).

6. has a metabolic role (i.e., some of the proteins in the wall are enzymes for transport,

secretion).

7.

physical barrier to: (a) pathogens; and (b) water in suberized cells. However,

remember that the wall is very porous and allows the free passage of small

molecules, including proteins up to 60,000 MW. The pores are about 4 nm.

8. carbohydrate storage - the components of the wall can be reused in other metabolic

processes (especially in seeds). Thus, in one sense the wall serves as a storage

repository for carbohydrates.

9.

signaling - fragments of wall, called oligosaccharins, act as hormones.Oligosaccharins, which can result from normal development or pathogen attack,

serve a variety of functions including: (a) stimulate ethylene synthesis; (b) induce

phytoalexin (defense chemicals produced in response to a fungal/bacterial

infection) synthesis; (c) induce chitinase and other enzymes; (d) increase

cytoplasmic calcium levels and (d) cause an "oxidative burst". This burst produces

hydrogen peroxide, superoxide and other active oxygen species that attack the

pathogen directly or cause increased cross-links in the wall making the wall harder

to penetrate.

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Plasmodesmata:

Plasmodesmata (singular, plasmodesma) are small channels that directly connect the

cytoplasm of neighboring plant cells to each other, establishing living bridges between

cells. Similar to the gap junctions found in animal cells, the plasmodesmata, which

penetrate both the primary and secondary cell walls, allow certain molecules to passdirectly from one cell to another and are important in cellular communication.

Nucleus:

This is the largest

organelle. It is surroundedby a nuclear envelope,

which is a double

membrane with nuclear

pores–large holes

containing proteins that

control the exit of

substances such as RNA

and ribosomes from the

nucleus. The interior is

called the nucleoplasm,

which is full of chromatin –

the DNA/protein complex.

During cell division the chromatin becomes condensed into discrete observable

chromosomes.

The nucleolus is a dark region of chromatin, involved in making ribosomes.

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Euchromatin and Heterochromatin

The DNA in the nucleus exists in two forms that reflect the level of activity of the cell

Heterochromatin appears as small, darkly staining, irregular particles scattered

throughout the nucleus or accumulated adjacent to the nuclear envelope. Euchromatin is

dispersed and not readily stainable. Euchromatin is prevalent in cells that are active in the

transcription of many of their genes while heterochromatin is most abundant in cells that

are less active or not active.

Functions of Nucleus

To control all cellular activities.

To produce RNA.

To control the synthesis of proteins, including enzymes, in the cell, and so control the

cell’s activities.

To undergo nuclear division in the start of cell division, ensuring that the daughter

cells have exact copies of the cell’s genetic material in their chromosomes. To assemble ribosomes (function of the nucleolus).

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Comparison of Animal and Plant Cells

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Summary of Cell Organelles

OrganelleLocation and

occurrence in cell

Size Function

Nucleus Usually one per cell

in cytoplasm

10 - 20m Contains the hereditary

material (DNA) which codes for

synthesis of proteins in the

cytoplasm

Nucleolus One to several in

nucleus

1 - 2m Synthesizes ribosomal RNA and

manufactures ribosomes

Rough

endoplasmic

reticulum

Throughout

cytoplasm

Membranes

about 4nm

thick,

enclosing

Transport of proteins

synthesized on ribosomes

Smooth

endoplasmic

reticulum

In cytoplasm. Extent

depends on types of

cell

Cisternae of

varying

diameter

Synthesis of lipids

Ribosomes Attached to rough

endoplasmicreticulum or free in

cytoplasm

20 – 25 nm Site of protein synthesis

Golgi

apparatus

In cytoplasm Variable Synthesis of glycoproteins,

packaging of proteins.

Lysosomes In cytoplasm 100nm Digestion of unwanted

materials and worn-out

organelles

Mitochondria In cytoplasm. Several

to thousands per cell.

1 m wide

and up to

10 m in

length

Production of energy by aerobic

respiration

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Double membranous

organelles

Single membranous

organelles

Non-membranous

organelles

Nucleus Endoplasmic reticula ribosome

Chloroplast Golgi apparatus

Mitochondria Lysosome

Centrioles Pair, in cytoplasm,

usually near nucleus

0.5 m X

0.2 m

Form the spindle fibres during

cell division of animal and

fungal cells

Chloroplasts In cytoplasm of someplant cells

2 – 10 min diameter

Site of photosynthesis

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a) describe and interpret electron micrographs and drawings of typical animal and

plant cells as seen with the electron microscope

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c) state that ATP is produced in mitochondria during respiration and chloroplasts

during photosynthesis and outline the role of ATP in cells

Adenosine Triphosphate (ATP), an energy-bearing molecule found in all living cells.

Formation of nucleic acids, transmission of nerve impulses, muscle contraction, and manyother energy-consuming reactions of metabolism are made possible by the energy in ATP

molecules. The energy in ATP is obtained from the breakdown of complex organic

molecules.

The energy in ATP can be released as heat or can be used in the cell as a power source to

drive various types of chemical and mechanical activities. For example, when the terminal

phosphate group of the ATP molecule is removed by hydrolysis (a decomposition process

that occurs when a substance reacts with water), energy in the form of heat is released

and adenosine diphosphate (ADP) and inorganic phosphate (Pi) are formed.

The hydrolysis of ATP is accelerated by an enzyme called adenosine triphosphatase, or

ATP-ase.

ATP ADP + Pi + energy

The regeneration of ATP from ADP requires energy, which is obtained in the process of

oxidation. The energy released in the oxidation of carbohydrates and fats initiates a

complex series of chemical reactions that ultimately regenerate ATP molecules from ADP

molecules. The complete oxidation of a typical molecule of fat results in the formation ofabout 150 molecules of ATP.

d)

outline key structural features of typical prokaryotic cells as seen in a typical

bacterium (including: unicellular, 1-5μ m diameter, peptidoglycan cell walls,

lack of organelles surrounded by double membranes, naked circular DNA,

70S ribosomes)

Prokaryotic Cells

1.

The word prokaryotes means ‘before nucleus’. This describes the main differencebetween eukaryotic and prokaryotic cells: prokaryotes have no nucleus or nuclear

membrane. Their DNA is therefore not separated from the cytoplasm, but forms a

single circular loop, sometimes called a bacterial chromosome.

2.

Their DNA is not associated with proteins, unlike eukaryotic chromosomes. Bacteria

also have smaller loops of DNA in the cytoplasm, called plasmids.

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3. Prokaryotic cells are much smaller than eukaryotic ones, and much simpler in their

structure.

4. They lack endoplasmic reticulum and membrane-bound organelles like

mitochondria and chloroplasts and any complex structures such as Golgi bodies,

cytoskeleton or lyososmes.

Structure of Bacterium (A Prokaryotic Cell)

Prokaryotic cells are smaller than

eukaryotic cells and do not have a nucleus

or indeed any membrane-bound

organelles. All prokaryotes are bacteria.

Prokaryotic cells are much older than

eukaryotic cells and they are far moreabundant (there are ten times as many

bacteria cells in a human than there are

human cells). The main features of

prokaryotic cells are:

Cytoplasm. Contains all the enzymes

needed for all metabolic reactions, since

there are no organelles

Ribosomes. The smaller (70S) type, all

free in the cytoplasm and never attached to membranes. Used for protein synthesis.

Nuclear Zone (or Nucleoid). The region of the cytoplasm that contains DNA. It is not

surrounded by a nuclear membrane.

DNA. Always circular (i.e. a closed loop), and not associated with any proteins to form

chromatin. Sometimes confusingly referred to as the bacterial chromosome.

Plasmid. Small circles of DNA, separate from the main DNA loop. Used to exchange DNA

between bacterial cells, and also very useful for genetic engineering.

Cell membrane. Made of phospholipids and proteins, like eukaryotic membranes.

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Mesosome. A tightly-folded region of the cell membrane containing all the membrane-

bound proteins required for respiration and photosynthesis. Can also be associated with

the nucleoid.

This is now thought to be an artefact of the electron microscope and not real structure.

Cell Wall. Made of murein (not cellulose), which is a glycoprotein (i.e. a

protein/carbohydrate complex, also called peptidoglycan).

Capsule. A thick polysaccharide layer outside of the cell wall. Used for sticking cells

together, as a food reserve, as protection against desiccation and chemicals, and as

protection against phagocytosis. In some species the capsules of many cells fuse together

forming a mass of sticky cells called a biofilm. Dental plaque is an example of a biofilm.

Flagellum. A rigid rotating helical-shaped tail used for propulsion. The motor is embeddedin the cell membrane and is driven by a H+ gradient across the membrane. Anticlockwise

rotation drives the cell forwards, while clockwise rotation causes a chaotic spin.

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e) compare and contrast the structure of typical prokaryotic cells with typical

eukaryotic cells (reference to mesosomes should not be included)

Differences between eukaryotic and prokaryotic cells

Eukaryotic Cells Prokaryotic Cells

1. True nucleus surrounded by a nuclear

envelop.

No true nucleus

2. Linear DNA associated with histoneproteins forming true chromosomes.

Circular DNA not associated with proteins.Separate loops of DNA called plasmids.

3. Cell wall, if present, made of cellulose

(plants and algae) or chitin (fungi).

Cell wall containing peptidoglycan

4. Endoplasmic reticulum present. No endoplasmic reticulum or associated

organelles such as Golgi apparatus.

5. Membrane–bound organelles such as

mitochondria and chloroplast (in plants

and algae).

No membrane-bound organelles.

Mesosomes and thylakoids present in

some bacteria.

6. Large (80S) ribosomes attached toendoplasmic reticulum.

Small (70S) ribosomes scattered in thecytoplasm.

7. If present, flagella have (9+2)

arrangement of microtubules.

If present, flagella are made of a single

microtubule.

8. Cells are large, typically 10-100m in

diameter, some cells can be up to 400m.

Cells are small, typically 0.5-3m in

diameter, Volume may be as little as

1/10,000th of eukaryotic cell.

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f) outline the key features of viruses as non-cellular structures (limited to

protein coat and DNA/RNA)

Viruses are small obligate intracellular parasites, which by definition contain either a

RNA or DNA genome surrounded by a protective, virus-coded protein coat. They are verysmall and are measured in nanometers, which is one-billionth of a meter. Viruses can

range in the size between 20 to 750nm, which is 45,000 times smaller than the width of a

human hair. The majority of viruses cannot be seen with a light microscope because the

resolution of a light microscope is limited to about 200nm, so a scanning electron

microscope is required to view most viruses.

The basic structure of a virus is made up of a genetic information molecule and a protein

layer that protects that information molecule. The arrangement of the protein layer and

the genetic information comes in a variety of presentations. The core of the virus is madeup of nucleic acids, which then make up the genetic information in the form of RNA or

DNA. The protein layer that surrounds and protects the nucleic acids is called the capsid.

When a single virus is in its complete form and has reached full infectivity outside of the

cell, it is known as a virion. The main function of the virion is to deliver its DNA or RNA

genome into the host cell so that the genome can be expressed (transcribed and

translated) by the host cell. The viral genome, often with associated basic proteins, is

packaged inside a symmetric protein capsid. The nucleic acid-associated protein, called

nucleoprotein, together with the genome, forms the nucleocapsid. In enveloped viruses,

the nucleocapsid is surrounded by a lipid bilayer derived from the modified host cell

membrane and studded with an outer layer of virus envelope glycoproteins.

A virus structure can be one of the following: icosahedral, enveloped, complex or helical.

Icosahedral

These viruses appear spherical in shape, but a closer look actually

reveals they are icosahedral. The icosahedron is made up of

equilateral triangles fused together in a spherical shape. This is the

most optimal way of forming a closed shell using identical proteinsub-units. The genetic material is fully enclosed inside of the

capsid. Viruses with icosahedral structures are released into the

environment when the cell dies, breaks down and lyses, thus

releasing the virions. Examples of viruses with an icosahedral

structure are the polio virus, rhinovirus, and a denovirus.

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Envelope

This virus structure is a conventional icosahedral or helical structure

that is surrounded by a lipid bilayer membrane, meaning the virus is

encased or enveloped.

The envelope of the virus is formed when the virus is exiting the cellvia budding, and the infectivity of these viruses is mostly dependent

on the envelope. The most well known examples of enveloped viruses

are the influenza virus, Hepatitis C and HIV.

Complex

These virus structures have a combination of icosahedral

and helical shape and may have a complex outer wall or

head-tail morphology. The head-tail morphology structure

is unique to viruses that only infect bacteria and are knownas bacteriophages. The head of the virus has an icosahedral

shape with a helical shaped tail. The bacteriophage uses its

tail to attach to the bacterium, creates a hole in the cell

wall, and then inserts its DNA into the cell using the tail as a

channel. The Poxvirus is one of the largest viruses in size

and has a complex structure with a unique outer wall and

capsid. One of the most famous types of poxviruses is the

variola virus which causes smallpox.

HelicalThis virus structure has a capsid with a central cavity or

hollow tube that is made by proteins arranged in a circular fashion,

creating a disc like shape. The disc shapes are attached helically

(like a toy slinky) creating a tube with room for the nucleic acid in

the middle. All filamentous viruses are helical in shape. They are

usually 15-19nm wide and range in length from 300 to 500nm

depending on the genome size. An example of a virus with a

helical symmetry is the tobacco mosaic virus.

1. Cell Structure 2015-16

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