Week 5.1 Advanced

3.9. Cell Structures - Advanced www.ck12.org

3.9 Cell Structures - Advanced

Describe the cellular structure-function relationship.

What are cell structures?

The contents of the cell, or the structures of the cell, allow the cell to be "specialized." Together with the cellsproteins, they allow the cell to do specific things. They allow a cell to act like a neuron or a bone cell or a skin cell.

Introduction to Cellular Structures

The invention of the microscope opened up a previously unknown world. Before the invention of the microscope,very little was known about what made up living things and non-living things, or where living things came from.During the discovery of cells, spontaneous generation the belief that living organisms grow directly fromdecaying organic substances was the accepted explanation for the appearance of small organisms. For example,people accepted that mice spontaneously appeared in stored grain, and maggots formed in meat with no apparentexternal influence. Once cells were discovered, the search for answers to such questions as "What are cells madeof?" and "What is the function of cells?" became the focus of study.

Cell Function

Cells share the same needs: the need to get energy from their environment, the need to respond to their environment,and the need to reproduce. Cells must also be able to separate their relatively stable interior from the ever-changing

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FIGURE 3.14The structure and contents of a typicalanimal cell. Every animal cell has a cellmembrane, cytoplasm, ribosomes, and anucleus, but not all cells have every struc-ture shown here. For example, some cellssuch as red blood cells do not have anymitochondria, yet others such as musclecells may have thousands of mitochon-dria.

external environment. They do this by coordinating many processes that are carried out within organelles, or othercellular structures. Structures that are common to many different cells indicate the common history shared by cell-based life. Examples of these common structures include the components of both the cell (or plasma) membraneand the cytoskeleton, and other structures shown in Figure 3.14.

Is there a relationship between the cell structure and its function? Of course there is. The structure-functionrelationship describes a pattern evident throughout biological systems. This relationship is evident in proteins(protein structure determines its function), nucleic acids (nucleic acid structure results in a genetic code), anatomy(longer necked giraffes are more functional than short neck giraffes), as well as cells. Using the human body as anexample, specialized cells perform many diverse functions, from digestion and excretion to message transmissionand oxygen distribution. The structure of each type of human cell depends on what function it will perform. Thisstructure-function relationship can be extended to all other organisms, from the largest whale to the smallest bacteria.

The variability between cell function is related to the proteins expressed in a particular type of cell. For example,though they do have many proteins in common, a neuron is going to use select different proteins than muscle cell.A direct relationship exists between the proteins expressed, the size and shape of every cell and the tasks it needs toaccomplish. Examples can easily be seen in red blood cells, neurons muscle cells and sperm cells.

Red blood cells are flat, round, and very small. Their small size allows easy maneuverability through thecapillaries, the narrowest blood vessels, where oxygen is transferred into body cells.

Neurons have a long, thin cellular extension, allowing for very quick and accurate communication andresponses. The long length allows a neuron to send electrical messages extremely quickly.

Skeletal muscle cells have an arrangement of linear protein fibers. The elongated shape allows for musclecontraction.

Sperm cells are the only human cell with flagella. This is because of their need to "swim" long distances toreach an egg for fertilization.

Vocabulary

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

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

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

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

Summary

A cells function is usually directly related to its structure; this is known as the structure-function relationship. The structure-function relationship is evident throughout biology.

Explore More

Use this resource to answer the questions that follow.

The Theme of Structure and Function in Cells at http://www.shmoop.com/biology-cells/structure-function.html .

1. What is meant by structure dictates function?2. Describe how the structure-function relationships relates to the following:

a. mitochondria.b. chloroplasts.c. ribosomes.

Review

1. Describe the structure-function relationship of cells. Give two examples.2. Discuss the role of proteins in the structure-function relationship of cells.

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3.10 The Plasma Membrane - Advanced

Outline the structure of the plasma membrane.

All cells have a plasma membrane. This membrane surrounds the cell. So what is its role?

Can molecules enter and leave the cell? Yes. Can anything or everything enter or leave? No. So, what determineswhat can go in or out? Is it the nucleus? The DNA? Or the plasma membrane?

Plasma Membrane

The plasma membrane (also called the cell membrane) is a lipid bilayer that is common to all living cells. Itsfunction is to keep the cell as a distinct entity in a water-based environment. A phospholipid bilayer is a doublelayer of closely-packed phospholipid molecules. It is this orientation of the phospholipids into the bilayer thatbiochemically gives the membrane its specific functional characteristics.

Organelle membranes are also composed of phospholipids. For example, mitochondria are bounded by a doublemembrane. Each membrane has a phospholipid bilayer with embedded proteins. This division from the rest of thecell makes the mitochondria only partially dependent on the cell. More on the structure of the phospholipid bilayerwill be presented in the The Plasma Membrane: The Phospholipid Bilayer (Advanced) concept.

Along the phospholipid bilayer, numerous proteins are embedded within the membrane. This structure is called theFluid Mosaic Model which will be discussed in the The Plasma Membrane: The Fluid Mosaic Model (Advanced)concept. These proteins have a variety of important roles; hormone binding sites, electron carriers, pumps for activetransport, channels for passive transport, enzymes, cell signaling and cell adhesion.

The plasma membranes allows only certain molecules, such as ions and small organic molecules, into and out ofthe cell. The ability to allow only certain molecules in or out of the cell is referred to as selective permeability orsemipermeability. This characteristic helps the cell to regulate its interactions between the internal machinery andthe external surroundings, helping to maintain homeostasis.

The plasma membrane also acts as the attachment point for both the intracellular cytoskeleton and, if present, thecell wall.

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The plasma membrane is discussed at http://www.youtube.com/watch?v=4Ug5Vf_lxtI . The video is below.

MEDIAClick image to the left or use the URL below.URL: http://www.ck12.org/flx/render/embeddedobject/139345

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.

Fluid Mosaic Model: Model of the plasma membrane; proposes that the membrane behaves like a fluid withan embedded mosaic of proteins.

phospholipid bilayer: A bilayer (2 layers) of phospholipids that surrounds and encloses a cell; physicalboundary between the intracellular space and the extracellular environment.

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.

selective permeability: The ability to allow only certain molecules in or out of the cell; characteristic of thecell membrane; also called semipermeability.

semipermeability: The ability to allow only certain molecules in or out of the cell; characteristic of the cellmembrane; also called selective permeability.

Summary

The plasma membrane forms a barrier between the cytoplasm and the environment outside the cell. A main characteristic of the plasma membrane is selective permeability.

Explore More

Use these resources to answer the questions that follow.

Cell Membranes at http://johnkyrk.com/cellmembrane.html .

1. Are all cells surrounded by a membrane?2. Why are phospholipids considered an amphipathic molecule?3. What is a glycolipid?4. Describe the role of cholesterol in the cell membrane.

http://www.hippocampus.org/Biology Non-Majors Biology Search: Plasma Membrane Structure

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5. What are the roles of the plasma membrane?

Construction of the Cell Membrane at http://www.wisc-online.com/Objects/ViewObject.aspx?ID=AP1101.

6. What are the two main components of the cell membrane?

Review

1. Describe the role of the plasma membrane.2. What is meant by semipermeable?

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3.11. The Phospholipid Bilayer - Advanced www.ck12.org

3.11 The Phospholipid Bilayer - Advanced

Describe the structure and function of the plasma membrane.

Why a bilayer?

Whats on the inside of the cell and on the outside? Mostly water. As you can see here, the water-based interior ofthe cell has lots of components. These need to be kept inside the cell. And it is the nature of the phospholipid bilayerto keep the inside of the cell separate from the outside.

Phospholipids

The cell membrane (or plasma membrane) is composed mainly of phospholipids with embedded proteins. Themembrane is a lipid bilayer, with the phospholipids oriented in a distinct manner to provide qualities necessary tomaintain a cell in a water-based environment.

A phospholipid is made up of a polar, phosphorus-containing head, and two long fatty acid (hydrocarbon), non-polar"tails." That is, the head of the molecule is hydrophilic (water-loving), and the tail is hydrophobic (water-fearing).Cytosol and extracellular fluid - the insides and outsides of the cell - are made up of mostly water. In this wateryenvironment, the water loving heads point out towards the water, and the water fearing tails point inwards, and pushthe water out. The resulting double layer is called a phospholipid bilayer. A phospholipid bilayer is made up oftwo layers of phospholipids, in which hydrophobic fatty acids are in the middle of the plasma membrane, and thehydrophilic heads are on the outside. An example of a simple phospholipid bilayer is illustrated in Figure 3.15.

The cell membrane also decides what may enter or leave a cell. The membrane is said to be semipermeableor selectively permeable, allowing only certain ions and organic molecules to cross the membrane. The plasmamembrane contain many proteins, as well as other lipids called sterols. The proteins have various functions, such aschannels (channel proteins) that allow certain molecules into the cell, and receptors (receptor proteins) that bind to

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FIGURE 3.15Phospholipid Bilayer. The phospholipidbilayer consists of two layers of phos-pholipids, with a hydrophobic, or water-hating, interior and a hydrophilic, or water-loving, exterior. The hydrophilic (polar)head group and hydrophobic tails (fattyacid chains) are depicted in the singlephospholipid molecule. The polar headgroup and fatty acid chains are attachedby a 3-carbon glycerol unit. The hy-drophobic fatty acids point towards themiddle of the plasma membrane, and thehydrophilic heads point outwards. Themembrane is stabilized by cholesterolmolecules (green). This self-organizationof phospholipids results in a semiperme-able membrane which allows only certainmolecules in or out of the cell.

signal molecules. In Figure 3.15, the smaller (green) molecules shown between the phospholipids are cholesterolmolecules. Cholesterol helps keep the plasma membrane firm and stable over a wide range of temperatures. At leastten different types of lipids are commonly found in plasma membranes. Each type of cell or organelle will have adifferent percentage of each lipid, protein and carbohydrate.

Vocabulary

cholesterol: A steroid alcohol that is present in animal cells and body fluids, regulates membrane fluidity, andfunctions as a precursor molecule in various metabolic pathways.

hydrophilic: Characteristic of the phospholipid head group; water-loving.

hydrophobic: Characteristic of the phospholipid tails; water-hating.

phospholipid: A major component of the cell membrane; consists of two hydrophobic tails and a hydrophilicphosphate head group.



Summary

A phospholipid is a lipid molecule with a polar head group ( a phosphate group) and two non-polar hydrocar-bon tails.

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3.11. The Phospholipid Bilayer - Advanced www.ck12.org

The plasma membrane is a selectively permeable lipid bilayer that contains mostly lipids and proteins. Theselipids and proteins are involved in many cellular processes.

Explore More


What Is the Phospholipid Bilayer? at http://www.wisegeek.com/what-is-the-phospholipid-bilayer.htm .

1. What is the phospholipid bilayer?2. Describe the structure of a phospholipid.3. What are the phospholipid bilayers problems?

Review

1. Why can hydrophobic (water-hating) molecules easily cross the plasma membrane, while hydrophilic (water-loving) molecules cannot?

2. Describe the composition of the plasma membrane.3. Describe the orientation of the phospholipids in the cell membrane.4. What is the role of cholesterol in the plasma membrane?

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3.12 Membrane Proteins - Advanced

Describe the structures and roles of proteins associated with the cell membrane.

Membrane Proteins

The second main component of plasma membranes are the variety of proteins. A membrane protein is a proteinmolecule that is attached to, or associated with the membrane of a cell or an organelle. Membrane proteins can beput into two groups based on how the protein is associated with the membrane: (1) integral membrane proteins and(2) peripheral membrane proteins.

Integral membrane proteins, also called intrinsic proteins, are permanently embedded within the plasma mem-brane. Structurally, the integral proteins contain residues with hydrophobic side chains that penetrate the fatty acylregions of the phospholipid bilayer, thus anchoring the protein to the membrane. The only way to remove the integralproteins from the membrane are with synthetic detergents, nonpolar solvents and denaturing agents that disrupt thehydrophobic interactions of the bilayer.

Integral membrane proteins can be classified according to their relationship with the bilayer:

Transmembrane proteins span the entire plasma membrane. Their function is mainly to regulate the transport

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of specific molecules across the membrane. There are two basic types of transmembrane proteins, alpha-helical and beta-barrels, which are discussed in Organic Compounds: Proteins (Advanced).

Integral monotopic proteins are permanently attached to the membrane from only one side.

Examples of integral membrane proteins and their functions are:

1. glycoprotein (cell-to-cell interactions)2. Na+/K+ ATPase (responsible for establishing and maintaining the electrochemical gradients of Sodium and

Potassium ions across the plasma membrane)3. glucose permease (the reversible transporter protein of glucose)4. ion channels gates (the flow of ions across the cell membrane)5. gap junction proteins (a direct connection between the cytoplasm of two cells, which allows various molecules

and ions to pass freely between cells)6. Bacterial rhodopsins (a protein in archaeans that uses the energy of light to pump protons across the mem-

brane)

Peripheral membrane proteins, also called extrinsic proteins, are only temporarily associated with the membrane.Most peripheral membrane proteins are hydrophilic so usually they are either attached to integral membrane proteins,or they can directly bound to a polar head group of the bilayer. This way they can be easily removed, which allowsthem to be involved in cell signaling. Peripheral membrane proteins are often associated with ion channels andtransmembrane receptors.

Examples of peripheral membrane proteins and their functions are:

1. spectrin (links the plasma membrane to the actin cytoskeleton for determination of cell shape, arrangement oftransmembrane proteins, and organization of organelles)

2. Kinase C (enzyme that helps mediate signal transduction cascades by hydrolyzing lipids)3. phospholipases (hydrolyze various bonds in the the polar head group of phospholipids which are vital to the

degredation of damaged or aged cell membranes)4. hormone receptors (binds a hormone outside the cell membrane and activates a protein kinase inside the cell)

Glycoproteins and glycolipids, in particular, have a carbohydrate chain that acts as a label to identify the celltype. Specifically, A, B, O blood groups result from having different carbohydrate chains on the cell surface ofred blood cells and other types of cells. Everyone has glycolipids and glycoproteins with the particular type ofcarbohydrate chain that signals type O. However, people with type A also have an additional carbohydrate calledN-Acetylgalactosamine and those with type B have an added galactose. Those with type AB have some glycolipidsand glycoproteins with N-Acetylgalactosamine added and others with galactose added.

Shown in Figure 3.16 are two different types of membrane proteins and associated molecules.

Vocabulary

integral membrane proteins: Proteins that are permanently embedded within the plasma membrane of a cellor organelle.

membrane protein: A protein molecule that is attached to, or associated with, the membrane of a cell ororganelle.

peripheral membrane proteins: Proteins that are only temporarily associated with the cell membrane; canbe easily removed.


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FIGURE 3.16Some of the membrane proteins make up a major transport system that moves molecules and ions through thepolar phospholipid bilayer.

Summary

The plasma membrane has many proteins that assist other substances in crossing the membrane. Membrane proteins may be permanently attached/embedded (integral membrane proteins) to the membrane,

or just temporarily associated with the membrane (peripheral membrane proteins).

Explore More


Cell Membranes at http://www.sparknotes.com/biology/cellstructure/cellmembranes/section2.rhtml

1. Distinguish between integral and peripheral proteins.2. List three facts about integral proteins.3. What is the glycocalyx?

Review

1. What are the main differences between the types of proteins associated with the plasma membrane?2. Name three membrane protein functions.

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3.13. The Fluid Mosaic Model - Advanced www.ck12.org

3.13 The Fluid Mosaic Model - Advanced

Describe the Fluid Mosaic Model of the plasma membrane.

Fluid and mosaic?

In this artistic impression of a plasma membrane of a human cell, the plasma membrane is shown as a bilayercomposed of phospholipids with transmembrane and surface proteins. The phospholipids create a fluid environmentfull of a mosaic of proteins.

The Fluid Mosaic Model

In 1972, S. J. Singer and G. L. Nicolson proposed the now widely accepted Fluid Mosaic Model of the structureof cell membranes (Science, 175: 720-731). Remember, it is the cell membrane that keeps the cells internalenvironment separate from its surroundings, but it is also this membrane that constantly and consistently allowsthe cell to interact and exchange materials with its environment.

The Fluid Mosaic Model proposes that integral membrane proteins are embedded in the phospholipid bilayer, asseen in the opening image. The bilayer results from the chemical nature of the phospholipids in a polar environment.The phospholipids create a double layer - or bilayer - when placed in a polar environment like water. Some of theseproteins associated with the membrane extend all the way through the bilayer, and some only partially across it. Inthis model, the integral membrane proteins have their polar groups protruding from the membrane into the aqueousenvironment, while the non polar regions of the protein are buried within the hydrophobic interior of the membrane.

This model also proposed that the membrane behaves like a fluid. Scanning electron microscope images demon-strated that the embedded molecules can move sideways throughout the membrane, meaning the membrane is notsolid, but more like a fluid. The membrane proteins and lipids of the membrane can move laterally around themembrane, much like buoys in water, or sideways throughout the membrane. Such movement causes a constantchange in the "mosaic pattern" of the plasma membrane. The mosaic pattern results from the many different com-ponents of the bilayer. These components include the phospholipids, integral and peripheral proteins, glycoproteins

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and glycolipids, which aid in their location and identification of food, water, waste, and other membrane traffic.Each cell has a particular glycoprotein structure extruding from the cell membrane, based on its need to attract orrepel membrane traffic. The cell is constantly interacting with its environment, bring certain molecules such as ions,hormones and food into the cell, and exporting materials, such as wastes, out of the cell.

A further description of the fluid mosaic model can be viewed in Fluid Mosaic Model of the Cell Membrane at http://www.youtube.com/watch?v=LKN5sq5dtW4 (1:27).


Vocabulary

Fluid Mosaic Model: Model of the plasma membrane; proposes that the membrane behaves like a fluid withan embedded mosaic of proteins.

integral membrane proteins: Proteins that are permanently embedded within the plasma membrane of a cellor organelle.


Summary

The Fluid Mosaic Model depicts the biological nature of the plasma membrane, with a fluid phospholipidbilayer and a mosaic of proteins.

Review

1. Describe the Fluid Mosaic Model.

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3.14. The Cytoplasm and Cytoskeleton - Advanced www.ck12.org

3.14 The Cytoplasm and Cytoskeleton - Ad-vanced

Distinguish cytoplasm from cytosol. Name and describe three types of protein fibers that make up the cytoskeleton.

Does a cell have, or even need, a "skeleton"?

What do you get if you take some tubing, and make the tubes smaller and smaller and smaller? You get verysmall tubes, or microtubes. Very small tubes, or microtubules, together with microfilaments, form the basis of the"skeleton" inside the cell.

Cytoplasm

Cytoplasm is one component of cells that is common to all cells. Cytoplasm is the gel-like material between the cellmembrane and the nucleus. The cytoplasm plays an important role in a cell, serving as a "jelly" in which organellesare suspended and held together by the cell membrane. Though prokaryotic cells do not have organelles (thoughthey do have ribosomes), they still have cytoplasm. It is within the cytoplasm that most cellular activities occur,including the many metabolic pathways that occur within organelles, such as photosynthesis and aerobic respiration.

The cytosol, which is the watery substance that does not contain organelles, is made up of 80% to 90% water. Thecytosol plays a mechanical role by exerting pressure against the cells plasma membrane. This helps keep the shapeof the cell. Cytosol also acts as the site of biochemical reactions such as anaerobic glycolysis and protein synthesis.In prokaryotes all chemical reactions take place in the cytosol.

Cytoskeleton

The cytoskeleton is a cellular "scaffolding" or "skeleton" that crisscrosses the cytoplasm. All eukaryotic cellshave a cytoskeleton, and recent research has shown that prokaryotic cells also have a cytoskeleton. The eukaryoticcytoskeleton is made up of a network of long, thin protein fibers and has many functions. It helps to maintain cell

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shape, holds organelles in place, and for some cells, it enables cell movement. The cytoskeleton plays important rolesin both the intracellular movement of substances and in cell division. Certain proteins act like a path that vesicles andorganelles move along within the cell. The threadlike proteins that make up the cytoskeleton continually rebuild toadapt to the cells constantly changing needs. Three main kinds of cytoskeleton fibers are microtubules, intermediatefilaments, and microfilaments.

Microtubules, shown as (a) in Figure 3.17, are hollow cylinders and are the thickest of the cytoskeletonstructures. They are most commonly made of filaments which are polymers of alpha and beta tubulin, andradiate outwards from an area near the nucleus called the centrosome. Tubulin is the protein that formsmicrotubules. Two forms of tubulin, alpha and beta, form dimers (pairs) which come together to form thehollow cylinders. The cylinders are twisted around each other to form the microtubules. Microtubules helpthe cell keep its shape. They hold organelles in place and allow them to move around the cell, and they formthe mitotic spindle during cell division. Microtubules also make up parts of cilia and flagella, the organellesthat help a cell move.

Microfilaments, shown as (b) in Figure 3.17, are made of two thin actin chains that are twisted aroundone another. Microfilaments are mostly concentrated just beneath the cell membrane, where they support thecell and help the cell keep its shape. Microfilaments form cytoplasmatic extentions, such as pseudopodiaand microvilli, which allow certain cells to move. The actin of the microfilaments interacts with the proteinmyosin to cause contraction in muscle cells. Microfilaments are found in almost every cell, and are numerousin muscle cells and in cells that move by changing shape, such as phagocytes (white blood cells that searchthe body for bacteria and other invaders).

Intermediate filaments differ in make-up from one cell type to another. Intermediate filaments organize theinside structure of the cell by holding organelles and providing strength. They are also structural componentsof the nuclear envelope. Intermediate filaments made of the protein keratin are found in skin, hair, and nailscells.

FIGURE 3.17(a) The eukaryotic cytoskeleton. Micro-filaments are shown in red, microtubulesin green, and the nuclei are in blue. Bylinking regions of the cell together, thecytoskeleton helps support the shape ofthe cell. (b) Microscopy of microfilaments(actin filaments), shown in green, insidecells. The nucleus is shown in blue.

TABLE 3.2: Cytoskeleton Structure

Microtubules Intermediate Filaments MicrofilamentsFiber Diameter About 25 nm 8 to 11 nm Around 7 nmProtein Composition Tubulin, with two sub-

units, alpha and beta tubu-lin

One of different types ofproteins such as lamin, vi-mentin, and keratin

Actin

Shape Hollow cylinders made oftwo protein chains twistedaround each other

Protein fiber coils twistedinto each other

Two actin chains twistedaround one another

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TABLE 3.2: (continued)

Microtubules Intermediate Filaments MicrofilamentsMain Functions Organelle and vesicle

movement; form mitoticspindles during cellreproduction; cellmotility (in cilia andflagella)

Organize cell shape; po-sitions organelles in cy-toplasm structural supportof the nuclear envelopeand sarcomeres; involvedin cell-to-cell and cell-to-matrix junctions

Keep cellular shape; al-lows movement of certaincells by forming cytoplas-matic extensions or con-traction of actin fibers; in-volved in some cell-to-cell or cell-to-matrix junc-tions

Representation

The cytoskeleton is discussed in the following video: http://www.youtube.com/watch?v=5rqbmLiSkpk (4:50).


Vocabulary

actin: A thin, threadlike protein filament found in muscle; the protein component of microfilaments.

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

cytoskeleton: The structure of filaments and tubules in the cytoplasm; provides a cell with an internalframework.

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

intermediate filaments: Intermediate component of the cytoskeleton; made of protein fiber coils twisted intoeach other.

microfilaments: Smallest component of the cytoskeleton; made of two actin chains twisted around oneanother.

microtubules: Largest component of the cytoskeleton; hollow protein cylinders made of alpha and betatubulin; also found in flagella.

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

tubulin: Protein component of microtubules; alpha-tubulin and beta-tubulin combine to form components ofmicrotubules.

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Summary

The cytoplasm consists of everything between the plasma membrane of the cell and the nucleus (of aneukaryotic cell).

The cytoskeleton is a cellular "skeleton" that crisscrosses the cytoplasm. Three main cytoskeleton fibers aremicrotubules, intermediate filaments, and microfilaments.

Microtubules are the thickest of the cytoskeleton structures and are most commonly made of filaments whichare polymers of alpha and beta tubulin.

Microfilament are the thinnest of the cytoskeleton structures and are made of two thin actin chains that aretwisted around one another.

Review

1. What is the difference between cytoplasm and cytosol?2. Name the three main parts of the cytoskeleton.3. List two functions of the eukaryotic cytoskeleton.

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3.15 External Structures of Cells - Advanced

Distinguish between cilia and flagella.

What propels a bacteria along?

Bacteria, being single-celled organisms, cannot just get up and walk from place to place. So they have to "swim." Todo this, they must have some sort of structure that propels them through their environment. Such a tail-like structureis a flagellum or set of flagella. These protein containing structures spin around a biological motor, allowing thebacteria to move.

External Structures of the Cell

Flagella ( flagellum, singular) are long, thin structures that protrude from the cell membrane. Both eukaryotic andprokaryotic cells can have flagella. Flagella help single-celled organisms move or swim towards food. The flagellaof eukaryotic cells are normally used for movement too, such as in the movement of sperm cells, which have onlya single flagellum. The flagella of either group are very different from each other. Prokaryotic flagella, shown inFigure 3.18, are spiral-shaped and stiff. They spin around in a fixed base much like a screw does, which moves thecell in a tumbling fashion. Eukaryotic flagella are made of microtubules that bend and flex like a whip.

Cilia ( cilium, singular) are made up of microtubule containing extensions of the cell membrane. Although bothcilia and flagella are used for movement, cilia are much shorter than flagella. Cilia cover the surface of some single-celled organisms, such as paramecium. Their cilia beat together to move the little animal-like protists through thewater. In multicellular animals, including humans, cilia are usually found in large numbers on a single surface ofcells. Multicellular animals cilia usually move materials inside the body. For example, the mucociliary escalator ofthe respiratory system is made up of mucus-secreting ciliated cells that line the trachea and bronchi. These ciliatedcells, shown in Figure 3.19, move mucus away from the lungs. This mucus catches spores, bacteria, and debris andmoves to the esophagus, where it is swallowed.

A video showing flagella and cilia can be viewed at http://www.youtube.com/watch?v=QGAm6hMysTA (3:12).

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FIGURE 3.18Bacterial flagella spin about in place,which causes the bacterial cell to "tum-ble."


FIGURE 3.19Left: Scanning electron micrograph(SEM), of the cilia protruding from humanlung cells. Right: Electron micrographof cross-section of two cilia, showing thepositions of the microtubules inside. Notehow there are nine groups of two mi-crotubules (called dimers) in each cilium.Each dimer is made up of an alpha anda beta tubulin protein that are connectedtogether.

Vocabulary

cilia (singular, cilium): Short, hairlike projection, similar to flagella, that allows some cells to move.

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

microtubules: Largest component of the cytoskeleton; hollow protein cylinders made of alpha and betatubulin; also found in flagella.

Summary

Cilia and flagella are extensions of the cell membrane that contain microtubules, and are usually used formovement.

Cilia cover the surface of some single-celled animals, such as paramecium, but cover only one side of cells insome multicellular organisms.

Explore More


Structure and Function of Bacterial Cells at http://textbookofbacteriology.net/structure_2.html .

1. What is the role of the flagellum motor?2. What powers the flagulla motor?3. Describe the process needed to propel the bacterium.4. Describe the structure and function of the basal body and hook of the flagella.

Review

1. Compare and contrast cilia and flagella.

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Week 5.1 Advanced

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