Week 5 Advanced

www.ck12.org Chapter 3. Cell Biology - Advanced

3.1 Cells - Advanced

Describe the importance of cells to biology.

Why is a cell so complex?

Cells have lots of things to do. Some cells make the whole organism, so that one cell must do everything thatorganism needs to do to live. Other cells perform specific functions, so they must be designed to do that specificactivity.

Introduction to Cells

The cell is the smallest unit of structure and function of all living organisms. A cell is also the smallest unit of life,with single-celled organisms present on this planet for over 3.5 billion years. Single-celled ( unicellular) organismslike bacteria are obviously composed of just one cell, whereas multicellular organisms can be composed of trillionsof cells. Multicellular organisms include protists (though single-celled protists also exist), fungi, plants and animals.Most plant and animal cells are between 1 and 100 m and therefore can only be observed under the microscope.

The one cell of a unicellular organism must be able to perform all the functions necessary for life. These functionsinclude metabolism, homeostasis and reproduction. Specifically, these single cells must transport materials, obtainand use energy, dispose of wastes, and continuously respond to their environment. The cells of a multicellularorganism also perform these functions, but they may do so in collaboration with other cells.

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Cells are essentially carbohydrates, lipids, proteins and nucleic acids in a water-based environment. It is thelipid (phospholipid) membrane that keeps the water-based environment in the cell separate from the water-basedenvironment outside the cell. But a cell, even the single cell of a unicellular organism, must be able to interact withits external environment. The cell must be able to bring molecules in from the outside, and expel unwanted wasteproducts. Knowing the components of cells and how cells work is necessary to all of the biological sciences.

Learning about the similarities and differences between cell types is particularly important to the fields of cell biologyand molecular biology. Cell biology is the field of biology that studies cells. In particular, cell biologists study a cellsphysiological properties, structure, organelles, interactions with the extracellular environment, life cycle, divisionand death. Molecular biology concerns itself with understanding the interactions between the various systems of acell, including the relationships between DNA, RNA and proteins.

Research in cell biology is closely linked to molecular biology, as well as genetics, biochemistry and developmentalbiology. The importance of the similarities and differences between cell types is a unifying theme in biology. Theyallow the principles learned from studying one cell type to be applied when learning about other cell types. Forexample, learning about how single-celled bacteria function can help us understand more about how human cellsfunction. Understanding basic cellular processes, such as cell division or metabolism in bacteria, gives informationabout similar processes in our cells.

Vocabulary

cell: The basic unit of structure and function of all living organisms.

cell biology: The field of biology that studies cells.

molecular biology: The field of biology that deals with the molecular basis of biological activity; the studyof molecules that make up living organisms.

multicellular organism: Organism made up of more than one type of cell; most have specialized cells thatare grouped together to carry out specialized functions.

organelle: A structure within the cytoplasm of a cell; may be enclosed within a membrane; performs a specificfunction.

unicellular organism: An organism that consists of only one cell; also known as a single-celled organism.

Summary

A cell is the smallest unit of structure and function of all living organisms. The understanding of cells is integral to other biological fields, including molecular biology, genetics, bio-

chemistry and developmental biology.

Explore More

Use this resource to answer the questions that follow.

Introduction to Cells at http://www.youtube.com/watch?v=gFuEo2ccTPA

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MEDIAClick image to the left or use the URL below.URL: http://www.ck12.org/flx/render/embeddedobject/139336

1. Concerning cells, what does all life have in common?2. How many cells are in the human body?3. How big is a cell?4. Each second, what is happening in your cells that keeps you alive?

Review

1. What is a cell?2. List some of the functions of a cell.3. Describe the relationship between cell biology and molecular biology.

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3.2 Discovery of Cells - Advanced

Identify the scientists that first observed cells. Describe the first cells identified.

What was needed to discover the cell?

The microscope of course. Objects that were too small to be seen with the human eye were unknown until themicroscope was developed. Once this instrument was developed, a whole new field of science was initiated.

Discovery of Cells

If you look at living organisms under a microscope you will see they are made up of cells. The word cell, derivedfrom the Latin word cellula meaning small compartment, was first used by Robert Hooke, a British biologist andearly microscopist. Hooke looked at thin slices of cork under a microscope. The structure he saw looked likea honeycomb as it was made up of many tiny units. Hookes drawing is shown in Figure 3.1. In 1665 Hookepublished his book Micrographia, in which he wrote:

... I could exceedingly plainly perceive it to be all perforated and porous, much like a Honey-comb, but that the poresof it were not regular.... these pores, or cells, ... were indeed the first microscopical pores I ever saw, and perhaps,that were ever seen, for I had not met with any Writer or Person, that had made any mention of them before this...

During the 1670s, the Dutch tradesman Antony van Leeuwenhoek, shown in Figure 3.2, used microscopes to observemany microbes and body cells. Leeuwenhoek developed an interest in microscopy and ground his own lenses tomake simple microscopes. Leeuwenhoek was so good at making lenses that his simple microscopes were ableto magnify much more clearly than the compound microscopes of his day. His microscopes increased ability to

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FIGURE 3.1This figure is a drawing of the structureof cork from Micrographia as it appearedunder the microscope to Robert Hooke.

magnify over 200x is comparable to a modern compound light microscope. Compound microscopes, which aremicroscopes that use more than one lens, had been invented around 1595 by Zacharias Jansen, a Dutch spectacle-maker. Several people, including Robert Hooke, had built compound microscopes and were making importantdiscoveries with them during Leeuwenhoeks time.

Fortunately, Leeuwenhoek took great care in writing detailed reports of what he saw under his microscope. He wasthe first person to report observations of many microscopic organisms. Some of his discoveries included tiny animalssuch as ciliates, foraminifera, roundworms, and rotifers, shown in Figure 3.3. He discovered blood cells and wasthe first person to see living sperm cells. In 1683, Leeuwenhoek wrote to the Royal Society of London about hisobservations on the plaque between his own teeth, "a little white matter, which is as thick as if twere batter." Hecalled the creatures he saw in the plaque animacules, or tiny animals. This report was among the first observationson living bacteria ever recorded.

Vocabulary


compound microscope: An optical microscopes that has a series of lenses; has uses in many fields of science,particularly biology and geology.

microscope: An instrument used to view objects that are too small to be seen by the naked eye.

microscopist: A scientist who specializes in research with the use of microscopes.

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FIGURE 3.2Antony van Leeuwenhoek (1632-1723).His carefully crafted microscopes and in-sightful observations of microbes led tothe title the "Father of Microscopy."

FIGURE 3.3A rotifer, the microscopic organismLeeuwenhoek saw under his micro-scope.

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Summary

Before the development of microscopes, the existence of cellular life was unknown. By examining a piece of cork, Robert Hooke first saw and named cells. Antony van Leeuwenhoek was the first person to see living cells.

Explore More


Discovering Cells at http://www.youtube.com/watch?v=FUqORLDDwVM


1. How did Hooke first observe cells?2. What did Leeuwenhoek look at through his microscope?

Review

1. Describe the contributions of Hooke and Leeuwenhoek to cell biology.2. What enabled Leeuwenhoek to observe things that nobody else had seen before?

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3.3. Microscopes in Biology - Advanced www.ck12.org

3.3 Microscopes in Biology - Advanced

Outline the importance of microscopes in the discovery of cells. Describe contemporary microscopes that are used for biological research.

How do you see something that is too small to be seen by a light microscope?

Use an electron microscope. This instrument has a resolution many times greater than a light microscope, and canbe used to see the details on the outside of a cell. Some electron microscopes can also be used to see the detailsinside a cell.

Microscopes

Hookes and Leeuwenhoeks studies and observations filled people with wonder because their studies were of lifeforms that were everywhere, but too small to see with the naked eye. Just think how amazed you would be ifyou were to read about the first accounts of a newly discovered microorganism from the moon or Mars. Your firstthought might be "Things can live there?!" which was probably the first thought of the people who read Hookes andLeeuwenhoeks accounts. The microscope literally opened up an amazing new dimension in the natural sciences,and became a critical tool in the progress of biology.

Magnifying glasses had been in use since the 1300s, but the use of lenses to see very tiny objects was a slowly-developing technology. The magnification power of early microscopes was very limited by the glass quality usedin the lenses and the amount of light reflected off the object. These early light microscopes had poor resolutionand a magnification power of about 10 times. Compare this to the over 200 times magnification that Leeuwenhoek

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was able to achieve by carefully grinding his own lenses. However, in time the quality of microscopes was muchimproved with better lighting and resolution. It was through the use of light microscopes that the first discoveriesabout the cell and the cell theory (1839) were developed. The Figure 3.4 is an example of an early light microscopeused by Robert Hooke (1665), the microscopist who was the first to discover cells.

However, by the end of the 19th century, light microscopes had begun to hit resolution limits, and microscopybecame a significant tool in the biological sciences. Resolution is a measure of the clarity of an image; it is theminimum distance that two points can be separated by and still be distinguished as two separate points. Becauselight beams have a physical size, it is difficult to see an object that is about the same size as the wavelength of light.Objects smaller than about 0.2 micrometers appear fuzzy, and objects below that size just cannot be seen. Lightmicroscopes were still useful, but most organelles and tiny cell structures were invisible to the light microscope.

FIGURE 3.4(A) English Scientist and MicroscopistRobert Hookes light microscope. (B)Modern electron microscope.

Electron Microscopes

In the 1930s, a new system was developed that could use a beam of electrons to resolve very tiny dimensions at themolecular level. Electron microscopes, one of which is shown in Figure 3.4, have been used to produce imagesof molecules and atoms. They have been used to visualize the tiny sub-cellular structures that were invisible tolight microscopes. Many of the discoveries made about the cell since the 1950s have been made with electronmicroscopes. The first electron microscope was the transmission electron microscope (TEM). The TEM workson the same principle as an optical microscope, but uses electrons instead of light and electromagnets in the placeof glass lenses. Electrons have a much lower wavelength, which makes it possible to get a resolution a thousandtimes better than with a light microscope. The TEM allows scientists to study the topographical, morphological,compositional and crystalline details in the cell or different materials near the atomic levels, as seen in the Figure3.5.

Development of the TEM was quickly followed in 1935 by the development of the scanning electron microscope(SEM). The SEM forms an image of a sample by scanning it with a beam of electrons. The electrons interact withthe atoms that make up the sample producing signals that contain information about the samples surface topography,morphology and composition. Although it is not as powerful as its TEM counterpart, the interactions that take placeon the surface of the specimen provide a greater depth of view, higher resolution and more detailed surface picture.

The 1980s saw the development of the first scanning probe microscopes. The first was the scanning tunnelingmicroscope in 1981. Scanning Probe Microscopy forms images of surfaces using a physical probe that scans thespecimen. The most recent developments in light microscope largely center on the rise of fluorescence microscopy.A fluorescence microscope is an optical microscope that uses fluorescence and phosphorescence instead of (or

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FIGURE 3.5Electron Microscope Image ofOrganelles. An electron microscopeproduced this image of a cell.

sometimes, in addition to) reflection and absorption to study properties of organic or inorganic samples.

How to use a microscope can be viewed at http://www.youtube.com/watch?v=FuDcge0Zuak (1:52).


KQED: The Worlds Most Powerful Microscope

Lawrence Berkeley National labs uses a $27 million electron microscope to make images to a resolution of halfthe width of a hydrogen atom. This makes it the worlds most powerful microscope. See http://www.kqed.org/quest/television/the-worlds-most-powerful-microscope and http://www.kqed.org/quest/slideshow/web-extra-images-from-the-worlds-most-powerful-microscope for more information.


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KQED: Confocal Microscopy

Confocal microscopy is an optical imaging technique used to increase optical resolution and contrast of a mi-crograph by using point illumination and a spatial pinhole to eliminate out-of-focus light in specimens that arethicker than the focal plane. These important features also allow confocal microscopes to collect double andtriple labels(most commonly used for detecting fluorescent labels), since precise colocalizations can be performed.Examples of specific uses of the confocal microscope are:

1. to resolve the structure of various complex three-dimensional objects, such as networks of cytoskeletal fibersin the cytoplasm of cells

2. the arrangements of chromosomes and genes in a cells nucleus during different points in the cell divisioncycle

3. observing pollen grains whose complex cell wall can only be seen clearly using confocal microscopy

Cutting-edge confocal microscopes, at the University of California, San Francisco are helping scientists create three-dimensional images of cells, and may help lead to new medical breakthroughs, including a treatment for Type 1diabetes. For a description of this work, see http://www.kqed.org/quest/television/super-microscope


Vocabulary

confocal microscopy: An optical imaging technique used to increase optical resolution and contrast of amicrograph by using point illumination and a spatial pinhole to eliminate out-of-focus light in specimens thatare thicker than the focal plane.

electron microscope: A microscope that uses electrons instead of light; allows a researcher to see things atvery high magnification, far higher than an optical microscope can magnify.

magnification: Enlarging an image of an object so that it appears much bigger than its actual size; also refersto the number of times an object is magnified.

microscopist: A scientist who specializes in research with the use of microscopes.

microscopy: The scientific field of using microscopes to view samples and objects that cannot be seen withthe unaided eye.

optical microscope: A microscope that uses visible light and lenses to magnify objects.


resolution: A measure of the clarity of an image; the minimum distance that two points can be separated byand still be distinguished as two separate points.

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scanning electron microscope (SEM): Electron microscope that scans an electron beam over the surface ofan object; measures how many electrons are scattered back.

transmission electron microscope (TEM): Electron microscope that shoots electrons through the sample;measures how the electron beam changes because it is scattered in the sample.

Summary

The development of light microscopes and later electron microscopes helped scientists learn about the cell. Early light microscopes had poor resolution and a magnification power of only about 10 times. Electron microscopes have much higher resolution and magnification; they have been used to produce images

of molecules and atoms. Many of the discoveries about cell structure since the 1950s have been made with the use of electron micro-

scopes.

Explore More

Use the three videos within this resource to answer the questions that follow.

Types of Microscopy at http://www.wellcome.ac.uk/Education-resources/Teaching-and-education/Big-Picture/All-issues/The-Cell/Videos-Types-of-microscopy/index.htm

1. Differentiate between light and electron microscopy.2. What is the role of the objective lenses of a light microscope?3. What is the upper magnification of an electron microscope?

Review

1. Relate resolution to magnification.2. Compare the different types of electron microscopy.

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3.4 The Cell Theory - Advanced

Summarize the principle points of the Cell Theory.

Where do cells come from?

All cells come from other cells. It was the advent of the microscope that allowed this discovery to be made. And it isone of the three basic points of the Cell Theory. This picture represents cell division, the process of one cell dividinginto two cells.

The Cell Theory

Over the next two centuries after the discoveries of Hooke and Leeuwenhoek, biologists found cells everywhere.Biologists in the early part of the 19th century suggested that all living things were made of cells, but the role of cellsas the primary building block of life was not discovered until 1839 when two German scientists, Theodor Schwann,a zoologist, and Matthias Jakob Schleiden, a botanist, suggested that cells were the basic unit of structure andfunction of all living things. Later, in 1858, the German doctor Rudolf Virchow observed that cells divide to producemore cells. He proposed that all cells arise only from other cells. The collective observations of all three scientistsform the Cell Theory, which states that:

all organisms are made up of one or more cells, all the life functions of an organism occur within cells, all cells come from preexisting cells.

Though no one point of the Cell Theory is more important than another, the theory clearly states that the functionsnecessary for life occur in the cell. Findings since the time of the original Cell Theory have enabled scientists to"modernize" the theory, including points related to biochemistry and molecular biology. The modern version ofthe Cell Theory includes:

all known living things are made up of one or more cells, all living cells arise from pre-existing cells by division,

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the cell is the fundamental unit of structure and function in all living organisms, the activity of an organism depends on the total activity of independent cells, energy flow ( metabolism and biochemistry) occurs within cells, cells contain hereditary information ( DNA) which is passed from cell to cell during cell division, all cells are basically the same in chemical composition in organisms of similar species.

The Cell Theory is one of the main principles of biology. The points of the theory have been found to be true for alllife. As with any scientific theory, the Cell Theory is based on observations that over many years upheld the basicconclusions of Schwanns 1839 paper. However, one of Schwanns original conclusions stated that cells formed ina similar way to crystals. This observation, which refers to spontaneous generation of life, was discounted whenVirchow proposed that all cells arise only from other cells. The Cell Theory has withstood intense examinationof cells by modern powerful microscopes and other instruments. Scientists continue to use new techniques andequipment to look into cells to discover additional explanations for how they work.

Vocabulary

biochemistry: The study of the structure, composition, and chemical reactions of substances in living systems.

botanist: A person engaged in botany, the science of plant life.


cell division: The process of cell formation from the division of older cells.

cell theory: One of the foundations of biology; refers to the idea that cells are the basic unit of structure andfunction of all life.

deoxyribonucleic acid (DNA): Double-stranded nucleic acid that composes genes and chromosomes; thehereditary material.

metabolism: The sum of all the chemical reactions in a cell and/or organism.

molecular biology: The field of biology that deals with the molecular basis of biological activity; the studyof molecules that make up living organisms.

spontaneous generation: An obsolete principle regarding the origin of life from inanimate matter.

zoologist: A person engaged in zoology, the branch of biology that focuses on the animal kingdom; studies thestructure, embryology, evolution, classification, habits, and distribution of all animals, both living and extinct.

Summary

The Cell Theory states that all living things are made of one or more cells, that cells are the basic unit of life,and that cells come only from other cells.

The Cell Theory has been updated to include findings based on more recent findings.

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Explore More


The wacky history of cell theory at http://ed.ted.com/lessons/the-wacky-history-of-cell-theory

1. What are the three parts of the Cell Theory?2. Who was Zacharias Jensen?3. How was bacteria discovered?4. Who were Matthias Schleiden and Theodore Schwann? Discuss their agreements and disagreements.5. Who was Rudolf Virchow? What was his role in the formation of the cell theory?

Review

1. What three things does the original Cell Theory propose?2. Compare the modern Cell Theory to the original Cell Theory.3. How has the theory developed?

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3.5 Cell Size and Shape - Advanced

Identify the limitations on cell size. Describe the relationship between volume and surface area. Discuss cell shape and its relationship to cell function.

What determines a cells function?

The cells structure has a lot to do with it. Notice in the representation of skin that there are different layers. Theselayers have different functions. Also notice the difference in cell shape within the different layers. The structure-function relationship is a central theme running throughout biology.

Diversity of Cells

Different cells within a single organism can come in a variety of sizes and shapes. They may not be very big, buttheir shapes can be very different from each other. However, these cells all have common abilities, such as obtainingand using food energy, responding to the external environment, and reproducing. In part, a cells shape determinesits function.

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Cell Size

If cells are the main structural and functional unit of an organism, why are they so small? And why are there noorganisms with huge cells? The answers to these questions lie in a cells need for fast, easy food. The need to beable to pass nutrients and gases into and out of the cell sets a limit on how big cells can be. The larger a cell gets,the more difficult it is for nutrients and gases to move in and out of the cell.

As a cell grows, its volume increases more quickly than its surface area. If a cell was to get very large, the smallsurface area would not allow enough nutrients to enter the cell quickly enough for the cells needs. This idea isexplained in Figure 3.6. However, large cells have a way of dealing with some size challenges. Big cells, suchas some white blood cells, often grow more nuclei so that they can supply enough proteins and RNA for the cellsrequirements. Large, metabolically active cells often have lots of cell protrusions, resulting in many folds throughoutthe membrane. These folds increase the surface area available for transport of materials into or out of the cell. Suchcell types are found lining your small intestine, where they absorb nutrients from your food through protrusionscalled microvilli.

Scale of Measurements

1 centimeter (cm) = 10 millimeters (mm) = 102 meters (m) 1 mm = 1000 micrometers (m) = 103 m 1 m = 1000 nanometers (nm) = 106 m 1 nm = 103 m

FIGURE 3.6A small cell (left), has a larger surface-area to volume ratio than a bigger cell(center). The greater the surface-area tovolume ratio of a cell, the easier it is forthe cell to get rid of wastes and take inessential materials such as oxygen andnutrients. In this example, the large cellhas the same area as 27 small cells, butmuch less surface area.

Imagine cells as little cube blocks. If a small cube cell like the one in the Figure 3.6 is one unit (u) in length, thenthe total surface area of this cell is calculated by the equation:

height width number of sides number of boxes 1u 1u 6 1 = 6u2

The volume of the cell is calculated by the equation:

height width length number of boxes 1u 1u 1u 1 = 1u3

The surface-area to volume ratio is calculated by the equation:

area volume

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6 1 = 6

A larger cell that is 3 units in length would have a total surface area of

3u 3u 6 1 = 54u2

and a volume of:

3u 3u 3u 1 = 27u3

The surface-area to volume ratio of the large cell is:

54 27 = 2

Now, replace the three unit cell with enough one unit cells to equal the volume of the single three unit cell. This canbe done with 27 one unit cells. Find the total surface area of the 27 cells:

1u 1u 6 27 = 162u2

The total volume of the block of 27 cells is:

1 1 1 27 = 27u3

The surface-area to volume ratio of the 27 cells is:

162 27 = 6

An increased surface area to volume ratio means increased exposure to the environment. This means that nutrientsand gases can move in and out of a small cell more easily than in and out of a larger cell.

FIGURE 3.7Ostrich eggs (A) can weigh as much as1.5 kg and be 13 cm in diameter, whereaseach of the mouse cells (B) shown at rightare each about 10 m in diameter, muchsmaller than the period at the end of thissentence.

The cells you have learned about so far are much smaller than the period at the end of this sentence, so they arenormally measured on a very small scale. The smallest prokaryotic cell currently known has a diameter of only 400nm. Eukaryotic cells normally range between 1100m in diameter. The mouse cells in Figure 3.7 are about 10m in diameter. One exception, however, is eggs. Eggs contain the largest known single cell, and the ostrich egg isthe largest of them all. The ostrich egg in Figure 3.7 is over 10,000 times larger than the mouse cell.

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Cell Shape

The variety of cell shapes seen in prokaryotes and eukaryotes reflects the functions that each cell has, confirmingthe structure-function relationship seen throughout biology. Each cell type has evolved a shape that is best relatedto its function. For example, the neuron in Figure 3.8 has long, thin extensions ( axons and dendritres) that reachout to other nerve cells. The extensions help the neuron pass chemical and electrical messages quickly through thebody. The shape of the red blood cells ( erythrocytes) enable these cells to easily move through capillaries. Thespikes on the pollen grain help it stick to a pollinating insect or animal so that it can be transferred to and pollinateanother flower. The long whip-like flagella (tails) of the algae Chlamydomonas help it swim in water.

FIGURE 3.8Cells come in very different shapes. Leftto right, top row: Long, thin nerve cells;biconcave red blood cells; curved-rodshaped bacteria. Left to right, bottomrow: oval, flagellated algae and round,spiky pollen grains are just a sample ofthe many shapes.

Vocabulary

axon: A long, slender projection of a neuron that conducts electrical impulses away from the neurons cellbody.

capillary: The smallest of a bodys blood vessels.

dendrite: Branched projections of a neuron; conducts the electrochemical stimulation received from othercells to the cell body.

egg (Latin, ovum): Cell in which an embryo first begins to develop.

erythrocyte: Flattened, doubly concave cells that carry oxygen; also known as red blood cells.

eukaryotic cell: Typical of multi-celled organisms; have membrane bound organelles; usually larger thanprokaryotic cells.

flagella (singular, flagellum): A "tail-like" appendage that protrudes from the cell body of certain prokaryoticand eukaryotic cells; used for locomotion.

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microvilli: Cellular membrane protrusions that increase the surface area of cells.

neuron: An electrically excitable cell that processes and transmits information by electrical and chemicalsignaling; a nerve cell.

prokaryotic cell: Typical of simple, single-celled organisms, such as bacteria; lack a nucleus and othermembrane bound organelles.

structure-function relationship: Principle that states the function of a biological item (molecule, protein,cell) is determined by its structure.

Summary

Cell size is limited by a cells surface area to volume ratio. A smaller cell is more effective and transportingmaterials, including waste products, than a larger cell.

Cells come in many different shapes. A cells function is determined, in part, by its shape.

Explore More


Cell Shape and Size at http://www.youtube.com/watch?v=wgnG64ieUkE


1. Describe the relationship between the cell surface area and cell membrane.2. Why is a smaller volume of the cell better?3. What are ways to "get around" the SA:V ratio?

Review

1. What limits the size of a cell? Why?2. A cell has a volume of 64 units, and total surface area of 96 units. What is the cells surface area to volume

ratio?3. What is the largest single cell?4. Describe the relationship between cell shape and function? Give an example of cell shape influencing cell

function.

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3.6 Common Parts of Cells - Advanced

Identify the parts common to all cells.

What do your cells share with a bacterium?

This bacterial cell has the features all cells have in common-the ribosomes and DNA can be seen floating around inthe cytoplasm, which is surrounded by the plasma membrane.

Parts of a Cell

There are many different types of cells, but all cells fall into two general categories: prokaryotic and eukaryotic.These cells can be vastly different, but still similar in some ways. All cells, whether from a simple bacterium or acell from a large whale, have a few things in common. These are:

a cell membrane (also known as the plasma membrane) cytoplasm ribosomes DNA (genetic information)

The cell membrane (also called the plasma membrane) is the physical boundary between the intracellular space(the inside of the cell) and the extracellular environment. It acts almost like the "skin" of the cell, controlling themovement of substances in and out of cells. The cell membrane is semi-permeable, allowing only select ions and

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organic molecules to enter and/or leave the cell. The cell membrane consists of two layers of phospholipids (a lipidbilayer) with embedded proteins which have numerous functions. More about the cell membrane will be discussedin the The Plasma Membrane (Advanced) concepts.

Cytoplasm is the general term for all of the material inside the cell, excluding the nucleus of eukaryotic cells. Allthe contents of a prokaryotic cell are contained within the cytoplasm. Cytoplasm is made up of cytosol, a wateryfluid that contains cytoskeletal fragments, dissolved particles and organelles. Organelles are structures that carryout specific functions inside the cell. It is within the cytoplasm that most cellular activities occur, such as manymetabolic pathways and processes such as cell division. More about the cytoplasm will be discussed in the CellStructures: The Cytoplasm and Cytoskeleton (Advanced) concept.

Ribosomes are the organelles on which proteins are made during protein synthesis. Ribosomes are found through-out the cytosol of the cell and attached to the endoplasmic reticulum organelle. Ribosomes order amino acids usingmessenger RNA (mRNA) as a template in a process called translation. Ribosomes are made from complexes ofribosomal RNAs (rRNA) and proteins called ribonucleoproteins. Each ribosome is divided into two subunits.The smaller subunit binds to the mRNA pattern, while the larger subunit binds to the transfer RNA (tRNA) and thegrowing polypeptide chain. More about the ribosome will be discussed in the Cell Structures: Ribosomes (Advanced)concept.

All cells also have DNA. DNA contains the genetic information needed for building structures such as proteins andRNA molecules in the cell.

An introduction to the cell, discussing various parts of the cell, is available at http://www.youtube.com/user/khanacademy#p/c/7A9646BC5110CF64/33/Hmwvj9X4GNY (21:03).


Vocabulary

cell membrane: Thin coat of lipids (phospholipids) that surrounds and encloses a cell; physical boundarybetween the intracellular space and the extracellular environment; also called the plasma membrane.

cytoplasm: The gel-like material inside the plasma membrane of a cell; holds the cells organelles (excludingthe nucleus).

cytosol: A watery cytoplasmic fluid that contains cytoskeletal fragments, dissolved particles and organelles.

deoxyribonucleic acid (DNA): Double-stranded nucleic acid that composes genes and chromosomes; thehereditary material.

eukaryotic: From an organism that has cells containing a nucleus and other membrane-bound organelles;eukaryote.

messenger RNA (mRNA): Type of RNA that copies genetic instructions from DNA in the nucleus and carriesthem to the cytoplasm.

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plasma membrane: Thin coat of lipids (phospholipids) that surrounds and encloses a cell; physical boundarybetween the intracellular space and the extracellular environment; also called the cell membrane.

prokaryotic: From a single-celled organism that lacks a nucleus; prokaryote.

protein synthesis: The process in which cells make proteins; includes transcription of DNA and translationof mRNA.

ribonucleoprotein: A nucleoprotein that contains RNA; includes the ribosome, vault ribonucleoproteins, andsmall nuclear RNPs (snRNPs).

ribosomal RNA: Type of RNA that helps form ribosomes and assemble proteins.

ribosome: A non-membrane bound organelle inside all cells; site of protein synthesis (translation).

semi-permeable: The feature of a cell membrane that allows only select molecules (ions and organic molecules)to enter and/or leave the cell.

transfer RNA (tRNA): Type of RNA that brings amino acids to ribosomes where they are joined together toform proteins.

translation: The process of synthesizing a polypeptide/protein from the information in a mRNA sequence;occurs on ribosomes.

Summary

Parts common to all cells are the plasma membrane, the cytoplasm, ribosomes, and genetic material.

Review

1. What are the common parts of all cells?2. What is one general feature of the plasma membrane?3. What occurs on the ribosomes? What are the three types of RNAs?4. What is the relationship between cytosol and cytoplasm?

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3.7 Two Types of Cells - Advanced

Define prokaryotic and eukaryotic. Describe common features of prokaryotic cells. Compare prokaryotic and eukaryotic cells.

How many different types of cells are there?

There are many different types of cells. For example, in you there are blood cells and skin cells and bone cells andeven bacteria. Here we have drawings of bacteria and animal cells. Can you tell which depicts various types ofbacteria? However, all cells - whether from bacteria, human, or any other organism - will be one of two generaltypes: prokaryotic or eukaryotic. In fact, all cells other than bacteria will be one type, and bacterial cells will be theother. And it all depends on how the cell stores its DNA.

Two Types of Cells

There are two cell types: prokaryotes and eukaryotes. Prokaryotic cells are usually single-celled and smaller thaneukaryotic cells. Eukaryotic cells are usually found in multicellular organisms, but there are some single-celledeukaryotes.

Prokaryotic Cells

The bacterium in Figure 3.9 is a prokaryote. Prokaryotes are microscopic organisms that have neither a membrane-bound nucleus nor membrane-bound organelles. Some cell biologists consider the term "organelle" to describemembrane-bound structures only, whereas other cell biologists define organelles as discrete structures that have aspecialized function. Prokaryotes have ribosomes, which are not surrounded by a membrane but do have a special-ized function, and could therefore be considered organelles. All metabolic functions carried out by a prokaryote takeplace in the plasma membrane or the cytosol.

Prokaryotes are the smallest types of cells, averaging 2-5m in diameter. Despite their small size, inside each cellthere is chemical and biochemical machinery necessary for growth, reproduction, and the acquisition and utilizationof energy. The common features of prokaryotic cells are:

cell wall plasma membrane ribosomes genetic material

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FIGURE 3.9Diagram of a typical prokaryotic cell.Among other things, prokaryotic cellshave a plasma membrane, cytoplasm, ri-bosomes, and DNA. Prokaryotes do nothave membrane-bound organelles or acell nucleus.

capsule (most, but not all) flagella (most, but not all) pili (most, but not all) lack of compartmentalization plasmid (most, but not all) binary fission

All prokaryotes have a cell wall that adds structural support, acts as a barrier against outside forces and anchors thewhip-like flagella. Some prokaryotes have an extra layer outside their cell wall called a capsule, which protects thecell when it is engulfed by other organisms, assists in retaining moisture, and helps the cell adhere to surfaces andnutrients. Pili are hair-like structures on the surface of the cell that attach to other bacterial cells or surfaces.

Within the plasma membrane, the cytoplasm is not subdivided by membranes into organelles, a lack of compart-mentalization that is most evident in the organization of the genetic material. Prokaryotic cells contain only a singlecircular piece of chromosomal DNA stored in an area called the nucleoid. Some prokaryotes also carry smallercircles of DNA called plasmids. Plasmids are physically separate from, and can replicate independently of, thechromosomal DNA. The genetic information on the plasmids is transferable between cells, allowing prokaryotes toshare abilities, such as antibiotic resistance.

Scientists have discovered that plasmids serve as important tools in genetics and biotechnology labs, most commonlyfor their ability to amplify (make many copies of) or to express particular genes. For example, the pGLO plasmidis a genetically engineered plasmid used in biotechnology as a vector for creating genetically modified organisms.Below is a video that will demonstrate the pGLO transformation.

pGLO Transformation at http://www.youtube.com/watch?v=yI9IXHw0j1U


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Reproduction in prokaryotic cells is by binary fission; a process of growth, enlargement and division. This will bediscussed in the Cell Division: Prokaryotic (Advanced) concept.

Prokaryotes have a large array of characteristics, that enable them to withstand different conditions, environmentsand resources. Some live in the absence of oxygen, some in extreme cold or hot temperatures, and some in thebottom of the ocean where their only resource is hot hydrogen sulfide, bubbling up from the core of the Earth. Theyare spectacularly resourceful organisms.

Eukaryotic Cells

FIGURE 3.10A eukaryotic cell, represented here by amodel animal cell is much more complexthan a prokaryotic cell. Eukaryotic cellscontain many organelles that do specificjobs. No single eukaryotic cell has all theorganelles shown here, and this modelshows all eukaryotic organelles.

A eukaryote is an organism whose cells are organized into complex structures by internal membranes and acytoskeleton, as shown in Figure 3.10. The most characteristic membrane-bound structure of eukaryotes is thenucleus. This feature gives them their name, which comes from Greek and means "true nucleus." The nucleus is themembrane-enclosed organelle that contains DNA. Eukaryotic DNA is organized in one or more linear molecules,called chromosomes. Some eukaryotes are single-celled, but many are multicellular. Eukaryotes include all protists,fungi, plants and animals.

In addition to having a plasma membrane, cytoplasm, a nucleus and ribosomes, eukaryotic cells also containmembrane-bound organelles. Each organelle in a eukaryote has a distinct function. Because of their complex levelof organization, eukaryotic cells can carry out many more functions than prokaryotic cells. The main differencesbetween prokaryotic and eukaryotic cells are shown in Figure 3.11 and listed in Table 3.1. Keep in mind that someeukaryotic cells may have characteristics or features that other eukaryotic cells lack, such as the cell wall.

A Comparison

Eukaryotic cells are about 10 times the size of a typical prokaryote; they range between 10 and 100 m in diameterwhile prokaryotes range between 1 and 10 m in diameter, as shown in Figure 3.12. Scientists believe thateukaryotes developed about 1.6 2.1 billion years ago. The earliest fossils of multicellular organisms that havebeen found are 1.2 billion years old.

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FIGURE 3.11The main differences between prokaryoticand eukaryotic cells. Eukaryotic cellshave membrane bound organelles, shownhere as a mitochondria, while prokaryoticcells do not. The nucleoid is the areawithin the cytoplasm of a prokaryotic cellthat contains the genetic material.

FIGURE 3.12The relative scale of prokaryotic and eu-karyotic cells. See how eukaryotic cellsare generally 10 to 100 times larger thanprokaryotic cells.

TABLE 3.1: Structural Differences Between Prokaryotic Cells and Eukaryotic Cells

Presence of Prokaryote EukaryotePlasma membrane yes yesGenetic material (DNA) yes yesCytoplasm yes yesRibosomes yes yesNucleus no yesNucleolus no yesMitochondria no yesOther membrane-bound organelles no yesCell wall yes some (plant cells)Capsule yes noFlagellum yes yesPili yes noAverage diameter 0.4 to 10 m 1 to 100 m

Vocabulary

binary fission: Asexual reproduction in prokaryotic organisms; produces two identical cells.

chromosome: The coiled structure of DNA and histone proteins; allows for the precise separation of replicatedDNA; forms during prophase of mitosis and meiosis.

eukaryote: An organism whose cells are organized into complex structures by internal membranes and acytoskeleton; all organisms other than bacteria.

eukaryotic cells: Typical of multi-celled organisms; have membrane bound organelles; usually larger thanprokaryotic cells.

nucleoid: The area within the cytoplasm of a prokaryotic cell where the DNA is concentrated.

nucleus (plural, nuclei): The membrane-enclosed organelle found in most eukaryotic cells that contains thegenetic material (DNA); control center of the cell.


pili: Hair-like structures on the surface of a prokaryotic cell that attach to other cells or surfaces; another namefor a fimbria.

plasmid: A small circular piece of DNA that is physically separate from, and can replicate independently of,chromosomal DNA within a cell.

prokaryote: An organism that does not have a cell nucleus nor any organelles that are surrounded by amembrane; bacteria.

prokaryotic cells: Typical of simple, single-celled organisms, such as bacteria; lack a nucleus and othermembrane bound organelles.

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Summary

There are only two main types of cells: prokaryotic and eukaryotic. Prokaryotic cells lack a nucleus and other membrane-bound organelles. Eukaryotic cells have a nucleus and other membrane-bound organelles. This allows these cells to have

complex functions.

Review

1. What is a prokaryotic cell? What is an eukaryotic cell? What is the major difference between the two celltypes?

2. What are the common features of prokaryotic cells?3. Give examples of prokaryotic and eukaryotic cells.4. Identify three differences between prokaryotic and eukaryotic cells.

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3.8 Viruses - Advanced

Explain why viruses are not considered living organisms.

What is a virus? Is it even a living organism?

This alien-looking thing is a representation of a virus. But is it prokaryotic or eukaryotic? Or neither? Or both? Avirus is essentially genetic material surrounded by protein. Thats it. So, is a virus prokaryotic or eukaryotic? Orneither? Or both?

Are Viruses Prokaryotic or Eukaryotic?

A virus is a sub-microscopic particle that can infect living cells. Viruses are much smaller than prokaryoticorganisms. In essence, a virus is simply a nucleic acid surrounded by a protein coat, as seen in the Figure 8.3.This outer coat is called a capsid. Viruses will be discussed in more detail in the Viruses concepts.

Are viruses prokaryotic or eukaryotic? Neither. Viruses are not made up of cells, so they do not have a cell membraneor any cytoplasm, ribosomes, or other organelles, nor do they have their own metabolism. The viral DNA (orsometimes RNA) encodes for proteins that make the capsid. However, viruses cannot make their own proteins; theyuse the ribosomes of a host cell to make proteins. A host cell is a cell infected by a virus.

Viruses do not reproduce by themselves, instead, they use their host cell to make additional copies of themselves.The host cell both replicates the viral genome (DNA) and produces the viral capsid. The viral genome is thenpackaged into the capsid, resulting in new viruses. So most virologists consider viruses non-living. But, they doevolve, which is a characteristic of living organisms.

Viruses do have significant use in research and medicine, including gene therapy. Understanding the structure ofviruses and understanding their interaction with host organisms (including how they infect and exploit host cells toreproduce) and understanding their physiology and immunity is beneficial to human health.

An overview of viruses can be seen at http://www.youtube.com/watch?v=0h5Jd7sgQWY (23:17).

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FIGURE 3.13Structural overview of a virus, the T2phage. A 2-dimensional representationis on the left, and a 3-dimensional rep-resentation is on the right. The virus isessentially nucleic acid surrounded by aprotein coat.


Viruses were first described by Dmitri Ivanovsky in 1892. He described a "non-bacterial pathogen" infecting tobaccoplants. This was soon followed by the identification of the tobacco mosaic virus by Martinus Beijerinck in 1898.Since then, about 5,000 viruses have been described in detail, although it is believed that there are millions ofdifferent types. Viruses are found in almost every ecosystem on Earth, and are the most abundant type of biologicalentity. Viruses can be classified with a taxonomic structure from order to species. No kingdom classification exists.Viruses, not being made of cells, do not fall into any of the six prokaryotic or eukaryotic kingdoms.

Vocabulary

capsid: The protective protein coat that surrounds the DNA or RNA of a virus particle.

gene therapy: Process to potentially cure genetic disorders; involves inserting normal genes into cells withmutant genes.

genome: The complete set of an organisms hereditary information; may be either DNA or, for many types ofvirus, RNA; includes both the genes and the non-coding sequences of the DNA/RNA.

virologist: A scientist who studies viruses and virus-like agents.

virus: A sub-microscopic particle that can infect living cells; contains DNA (or RNA) and can evolve, butlacks other characteristics of living organisms.

Summary

Viruses are neither prokaryotic or eukaryotic.

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Viruses are not made of cells. Viruses cannot replicate on their own. Most scientists do not consider viruses to be living.

Explore More


http://www.hippocampus.org/Biology Non-Majors Biology Search: Viruses

1. Describe a virus.2. What are the two types of replication a viral genome can undergo? Explain.

Review

1. What is a virus?2. Are viruses considered living? Explain your answer.

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Week 5 Advanced

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Transcript of Week 5 Advanced