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A fundamental statement of life is that all organisms are composed of one ormore cells. The cell is the structural and functional unit of life. A cell is thesimplest collection of matter that has the properties of life. All living organismsare composed of one or more cells. For single-celled organisms, all requirementsfor life: growth, development, regulation, reproduction, etc., are met within asingle cell. Multicellular plants, fungi and animals are comprised of collections ofcells and tissues, each with specialized structures and functions that contribute tothe whole. These two statements (Living organisms are composed of cells andcells are the structural and functional unit of life) along with a third: All cells comefrom pre-existing cells form the cell theory.

Each cell is unique, composed of molecules organized into an orderly structural andfunctional unit. Just as we saw that macromolecules are remarkable in theirstructure to function relationship, we shall, in this chapter, see how the structureof cells and, in particular, the structure of cell components, facilitates thefunctioning of cells.

Much of what we will be discussing about cells was learned using methods wecannot see with our un-assisted eyes. Cells, for the most part, are microscopic.Moreover, when we turn to the functioning of cells, and their component parts, theorganelles, we must often affect the very cell that we are hoping to learn aboutwith our manipulations.

Until the first light microscopes of 300 years ago, no one had really seen cells.However, even the best light microscope can not clearly resolve images magnifiedmore than 1000x times (for detail about 0.2 υm.) There are a variety of methodsused to enhance the quality of images viewed with a light microscope, which, incombination, can help researchers.

Until the use of transmission electron microscopes and scanning electronmicroscopes during the past fifty or so years, many cell components that wediscuss today were not known or seen. (Electron microscopes can resolve imagesof about 2 nm. The scanning-tunneling microscope can generate computer surfaceimages of single atoms magnified about 2 million times.) BCC is fortunate to havehad a scanning electron microscope donated for student (and faculty) use by alocal resident.

As mentioned, cell biologists frequently study cell components outside of the cellusing techniques such as cell fractionation, in which differential centrifugation isused to separate and isolate different organelles based on their density. Some ofyou will have done a cell fractionation lab in Biology 101 or in other courses.

At any rate, in this chapter we are going to look at structure and organization oftypical cells, based on the knowledge we have of cells today.

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First, every organism is composed of one of two fundamental types of cells:prokaryotic or eukaryotic .

Prokaryotic cells do not have their genetic material enclosed within a membrane-bounded structure (no nucleus). Their DNA is concentrated in a region of the cellcalled the nucleoid. Nor do prokaryotic cells have a set of membrane-boundedorganelles within their cells.

The DNA of eukaryotic cells is contained within a membrane-bounded nucleus. Thenucleus is surrounded by the cytoplasm of the cell, much of which is the semi-fluid matrix, the cytosol, in which organelles are suspended. The boundary ofall cells is the plasma, or cell, membrane.

As briefly mentioned in our introduction, the world of life is currently organizedinto three domains and six Kingdoms. Two of the domains, Archaea and Bacteriaare prokaryotic. The Domain, Eukarya, is comprised of four Kingdoms: Protista,Fungi, Plantae and Animalia, whose members are composed of Eukaryotic cells.

Brief Review of Domains and Kingdoms – See Table 32.2 p. 662Prokaryotic Organisms

Domain and Kingdom Archaea• A group of prokaryotes that diverged early in evolution, based on

ribosomal RNA studies• Cell walls lack peptidoglycan,• Archaebacteria membrane lipids and rRNA are unique• Possess common DNA sequences (signature DNA) found in no other

groups of organisms• Biochemical versatility in obtaining nutrients (Methanogens)• Often restricted to harsh environments (Extremophiles: Halophiles,

Thermophiles, as well as pH and pressure extremes)

Domain and Kingdom Bacteria (Eubacteria)• Cell walls contain peptidoglycan• Some have photosynthesis, most are heterotrophs (obtain their nutrients

by processing some organic molecules)• Include Bacteria and Cyanobacteria• Probably the original organisms on earth from which both Archaeabacteria

and the Eukarya diverged

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Eukaryotic Organisms (Domain Eukarya)Pro t is ta• Organisms which lack "true" tissue development• Have a variety of means of nutrition

Plantae• Photosynthetic Autotrophs

Obtain inorganic materials from the external environment and processthem into the organic compounds needed for life.

• Cells secrete a cell wall exterior to the plasma membrane

Fungi• Non-photosynthetic Heterotrophic

Obtain organic materials from the external environment and assimilatethem for their needs

• Cells secrete a cell wall exterior to the plasma membrane

Animalia• Heterotrophic• Cells lack a cell wall

Evolutionary Relationships among the Six Kingdoms

We shall be focusing on the structure and function of the eukaryotic cell for mostof this chapter (and Biology 201). The diversity of organisms is covered in Biology202 and 203, and microbiology focuses on the prokaryotic kingdoms. We will dosome work with Bacteria in lab during our unit on Genetics and Biotechnology.

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Cell Organization and Cell DimensionsWhile the benefits of a cellular organization seem fairly clear, we must look moreclosely at how a cell functions to understand why most cells are very small, andwhy multicellular organisms are comprised of many, many microscopic cells, ratherthan just a few enormous ones.

Each cell needs to perform a number of metabolic functions while maintaining apretty constant internal environment. Cells must exchange materials with theexternal environment, and undergo any number of chemical reactions, each withspecific chemical requirements, in order to stay alive and do their jobs. Cells thathave an internal organization can perform these functions more efficiently.

Chemical communication and movement of materials within and between cells iscritical to cell functioning. Anything that enters or leaves a cell must passthrough the plasma or cell membrane. The more things needed in a cell, the moreexchanges have to occur through the membrane and within the cell's environment.If the volume of a cell becomes too large, there is not enough membrane surfacearea to accomplish all exchanges, and the "travel time" for movement of moleculeswithin the cell increases.

The overall limit to cell size seems to be the surface area/volume ratio. As thevolume of a cell increases, the cell has proportionally less surface to exchangenutrients, gases and wastes with its environment to sustain the increasing volume.Within the cytoplasm, materials move by diffusion, a physical process that canwork only for short distances. A large volume would inhibit the rate of movementtoo much for cells to function.

Cells with minimal metabolic needs can have larger volumes. Some highlyspecialized cells, such as red blood cells, can be also be very tiny (in part to dotheir job). But most cells average between 5 and 20 nms in diameter. Thesmallest cells are the mycoplasmas, which are about 1 micrometer in diameter.Most bacteria range from 1 - 10 micrometers, and eukaryotic cells are about 10 -100 micrometers. There are some notable exceptions:

• The yolk of a bird is a single cell.• Some nerve cells run from the spine to the toes of mammals (although the

diameter is small and they are microscopic, maintaining a good surface area tovolume ratio.)

• Some green algae, such as Caulerpa and Acetabularia (Kingdom, Protista) havehuge cells, and often are multinucleate.

Let us now turn to the features of the prokaryotic and eukaryotic cell.

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Features of Prokaryotic Cells• Generally very small and relatively simple• External Features

• Boundary is the plasma membraneMay have infoldings called mesosomesPhotosynthetic cyanobacteria have significant infoldings of themembrane in which the photosynthetic pigments are embedded.

• Rigid wall composed of a unique substance, found only in the walls ofprokaryotes called Peptidoglycan (and absent in the Archaebacteria). Thecell wall layers of bacteria are used in classification (Gram+ or Gram-)

May secrete a slime sheath or capsule to protect• May have motile structures called flagella, but they are different from the

flagella of eukaryotic cells. Prokaryotic flagella propel by rotating. Themotor is embedded in the plasma membrane and wall.

• Some bacteria have tiny projections called pili, which help them attach tosurfaces.

• Interior of Prokaryotic Cell• Concentrated DNA molecule (circular), called a nucleoid, not surrounded by

protein. May have more than one copy of the DNA molecule.• May have plasmids, independent DNA fragments that carry a specific piece

of genetic information. Plasmids can be transmitted from one bacterium toanother, or from the environment to a bacterium. Plasmids are important inrecombinant DNA research.

• Cytoplasm• Ribosomes, composed of RNA and protein, of 70s density.• No internal membrane-bounded structures (organelles)

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Features of the Eukaryotic Cell• Eukaryotic cells have internal membrane-bounded structures: organelles• Nucleus bounded by the nuclear envelope (Literal meaning = true nucleus)• Cytoplasm of cytosol in which specialized organelles are suspended

• Organelles physically separate different types of cell activities in thecytoplasm space for greater efficiency

• Organelles allow for separation of cell activities in time, to provide forsequential cell activities

• May or may not (animals) secrete an external cell wall

Eukaryotic Cell Components (pp. 84-85 of your text show an overview of cell components)

Nucleus• Nuclear Envelope• Chromatin - Chromosomes• Nucleolus

Cytoplasm• Cytosol (fluid matrix)• Organelles

• Endomembrane System (Internal Membranes)• Rough Endoplasmic Reticulum

• Ribosomes• Smooth Endoplasmic Reticulum• Golgi Complex

• Vesicles• Lysosomes• Other "–somes"

• Mitochondria• Plastids

• Chloroplast• Amyloplast• Chromoplast

• Central Plant Vacuole• Cytoskeleton (Internal Skeleton)

• Microfilaments• Intermediate Filaments• Microtubules

• CentriolesCell Movements

• Intracellular Movement• Motor Molecules• Sol-Gel State

• Cilia and FlagellaExternal Structures

• Cell Wall• Extracellular Matrix

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NucleusThe nucleus is generally the largest or most "conspicuous" (except for whenstudents are trying to find one) structure within the eukaryotic animal cell. Themature plant central vacuole, which you usually cannot see, takes much more ofthe volume of the plant cell. The nucleus is spherical and quite dense. Althoughmost cells have a single nucleus, some fungi and some protists have multiple nucleiper "cell", as do some muscle cells of vertebrates.

Nucleus Functions• Contains and stores the genetic information, DNA, that determines how the cell

will function, as well as the basic structure of that cell. (A few organelles,mitochondria and chloroplasts, do have some DNA, but the vast majority of acell's DNA is contained within the nucleus.)

• Manufactures all RNA, including ribosomal, transfer and messenger RNA• Duplicates the DNA of the cell prior to cell division

Nucleus Structure

Nuclear EnvelopeThe nucleus is bounded by the nuclear envelope• A double membrane structure. The outer membrane is continuous with the

membranes of the Endomembrane system, or endoplasmic reticulum.• Perforated with pores comprised of RNA and protein, with a channel for

exchanging substances with the cytoplasm of the cell. The two membranelayers are sealed to each other at the nuclear pores. The specific proteinlinings of the pores determine the passage of substances in and out of thenucleus, generally protein messengers and RNA and RNA-protein complexes forexport to the cytoplasm. The pores in the surface of the nuclear envelope arequite conspicuous in scanning electron micrographs.

• The inner membrane is lined with a protein network, the nuclear lamina, whichmaintains the shape of the nucleus. The nuclear lamina may also affecttranscription of genes, where portions of DNA are attached to it, and/or to theinternal nucleus protein framework.

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DNA - Chromatin• The DNA of the nucleus appears as a granular mass when observed with a

microscope. The DNA of eukaryotes is organized into chromosomes, longmolecules of DNA, surrounded by histone proteins (which have large amounts ofpositively charged lysine and arginine that bind to the negatively charged DNA).The thread-like chromosomes are not visible as distinct structures except whencells are dividing. The collection of chromosomes is called chromatin. Whencells prepare to divide, the DNA of the chromosomes coils around the histonesforming bead-like aggregates called nucleosomes. The nucleosomes condenseinto a compact mass.

• Each type of organism has a set number of chromosomes; it is a species

Nucleolus• Small concentrated masses DNA, RNA and protein that are sites of ribosome

subunit synthesis found within the nucleus.• Multiple copies of the DNA needed for the manufacture of ribosomal RNA

cluster in the nucleolus, where they direct ribosomal RNA synthesis. Ribosomalsubunits are assembled in the nucleolus from rRNA and protein

• Nucleoli are so dense that they can be seen with a compound light microscope.

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The Cell’s Endomembrane SystemThe membranes of cells are, as will be discussed in much detail soon, composed ofphospholipid bilayers, embedded with a variety of proteins and other associatedmolecules. Not only do membranes form the boundary of the cell, the plasmamembrane, but within the cell we find a membrane system composed of a numberof components, each of which may connect to the plasma membrane at some timeor another, and to the nuclear envelope as well. In addition, small membranefragments may be pinched off forming vesicles, used for transport. Membranesvary in composition, size and function.

Endomembrane Components• Endoplasmic Reticulum

• Rough Endoplasmic Reticulum and associated Ribosomes• Smooth Endoplasmic Reticulum

• Golgi Complex• Assorted Vesicles

Endoplasmic Reticulum• Series of interconnected membrane flattened tubes or channels that

compartmentalize the cytoplasm, and run throughout the cytosol. Themembranes of the ER separates the compartment within the ER, called thecisternal spaces, from the cytosol. The ER is about half of the totalmembrane volume of a typical cell. Projections of endoplasmic reticulumconnect the nuclear envelope with the endoplasmic reticulum and otherprojections connect to the plasma membrane.

• Endoplasmic reticulum synthesizes, transports and isolates intracellularcontents

• There are two forms of endoplasmic reticulum: smooth and rough.

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Smooth Endoplasmic Reticulum• Many enzymes are associated with the surfaces of smooth ER, including

enzymes for the manufacture of lipids and for carbohydrate metabolism• The enzymes needed to synthesize the phospholipids for membranes are on the

smooth ER• Enzymes needed to synthesize many steroids are also associated with smooth

ER• Smooth ER in liver cells of animals contains enzymes for many of the liver's

regulatory metabolic functions, including detoxifying alcohol and many othertoxins. The enzymes that do this generally add hydroxyl functional groups tothe toxin, which makes it more soluble for removal from the body. Cells thatperform such functions generally produce more smooth ER. (This can benegative, because the more smooth ER with detoxing enzymes, the moretolerant one becomes of the drugs in some cases.)

• The sarcoplasmic reticulum of muscle tissue is a form of smooth ER. Thiscontains the calcium reservoirs needed to trigger muscle contraction.

Rough Endoplasmic Reticulum• Endoplasmic reticulum that has ribosomes attached to its surface is called

rough endoplasmic reticulum. This is most abundant in cells that secrete alot of proteins.

• Rough endoplasmic reticulum may synthesize itself, and also can be used tomaintain and replace nuclear or plasma membrane as needed. Both membraneprotein making enzymes and phospholipid making enzymes are embedded in therough ER membranes.

• Ribosomes• Ribosomes are the site for the assembly of proteins.• Ribosomes consist of two subunits, and are composed of RNA and protein,

In eukaryotes, the ribosomes are 80s, whereas the ribosomes ofprokaryotes are 70s, one of the reasons some antibiotics are effectiveagainst bacteria, and don't harm us.

• Ribosomal subunits, as stated, are made in the nucleolus and move intothe cytoplasm. Some ribosomes are located freely in the cytosol, whileothers are attached to ER for protein synthesis. Ribosomes that attachto ER give the ER a rough appearance, hence the term, rough endoplasmicreticulum.

• Ribosomes attach to ER with the aid of signal molecules that move theribosome with its growing polypeptide to the ER.

• Proteins synthesized at free ribosomes are generally enzymes thatfunction in cytoplasmic metabolic reactions. Those proteins synthesizedon ribosomes associated with rough endoplasmic reticulum are generallyproteins bound into membranes, or packaged into organelles, such as thelysosome, or packaged for export from the cell in transport vesicles.

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Golgi Complex (Apparatus)

The Golgi complex consists of stacks of flattened disk-like membrane sacs. Itfunctions more or less as a processing center for materials to be packaged up anddistributed in. The Golgi complex gets materials from one place in the cell,packages them in vesicles pinched off of the tips of the Golgi membranes anddistributes them to organelles or exports (secretes) from the cell. Transportrelies on transport vesicles.• The two poles (front and back) of a Golgi body differ in polarity and thickness.

One end (the cis end) is associated with the ER and receives substances; theother (the trans end) pinches off product vesicles and ships them out.

• Golgi modifications are done in stages within the sequential cisternae of theGolgi, moving from one to another from the cis to trans poles of the Golgi. Oneof the stages is attaching the specific recognition markers that target theproduct for its specific destination.

• Among the functions of the Golgi are modification of glycoproteins andglycolipids for their specific oligosaccharide portions and the manufacture ofpolysaccharides.

Of note is the use of synthetic vesicles called liposomes, whose membranecomposition mimics the natural membrane sufficiently to be used as a deliverymeans for certain substances, such as medications, that might be useful and nototherwise capable of entering target cells and tissues.

LysosomesLysosomes contain hydrolytic enzymes, which can breakdown carbohydrates,proteins, nucleic acids, and many lipids. Since these hydrolytic enzymes functionbest in an acid pH, the lysosome maintains a pH of about 5 by actively moving H+

through the lysosome membrane. This pH difference usually protects the cellfrom damage if a lysosome membrane is broken. The neutral pH of the cytosoldoes not favor the functioning of the enzymes.

Lysosomes are manufactured from enzymes and membranes of the rough ER andpackaged in the Golgi complex.

The Lysosome is responsible for disassembly or breakdown of cell componentswhen no longer needed or when damaged or in need of recycling. The monomers

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formed during hydrolysis are freed into the cytosol for new assembly. It is anormal part of cell maintenance and renewal.

Lysosomes can also destroy or degrade bacteria and foreign substances.Macrophages for example, contain large numbers of lysosomes as do organisms,such as the Amoeba, that feed by phagocytosis. Amoeba surround their prey itemwith a portion of their plasma membrane, and engulf the item by fusing themembrane around it and moving the now "food vacuole" into the cytosol. The foodvacuole formed when an Amoeba engulfs its prey merges with lysosomes fordigestion.

During development, lysosomes are important in digestion of parts. Reabsorptionof tadpole tails and formation of fingers and toes are two examples of this.

There are some genetic disorders that are associated with lysosomes. If alysosome lacks a needed enzyme, it can not do its job and the lysosome becomesfilled with the substance needing degrading. Ultimately these lysosomes interferewith the cell's functioning. In Tay-Sachs, lipid-digesting enzymes are missing, andbrain cells become impaired. Tay-Sachs is fatal at a relatively young age.

Other "–somes"Cells contain a number of membrane bounded bodies, often called microbodies, thatstore specific sets of enzymes needed for specialized functions. Peroxisomesand Glyoxysomes are two such microbodies. Peroxisomes and glyoxysomes aresingle membrane-bounded organelles and are not derived from ER. Theirmembranes are formed from lipids and proteins in the cytosol.

• Peroxisomes contain enzymes that transfer hydrogen in biochemicalreactions to oxygen, forming hydrogen peroxide as a by-product. Theseinclude the breakdown of fatty acids for fuel purposes and the detoxificationof alcohol in the liver. Since H2O2 is toxic, peroxisomes also contain anenzyme, catalase, which breaks down the H2O.

• Plant cells, especially in seeds, contain glyoxysomes. These cells store oilsso that the germinating seed has a fuel supply. During germination, thefatty acids are converted to sugar molecules for the rapid cell respirationneeded for successful germination and seedling establishment.

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Mitochondria

Function of Mitochondria:• Mitochondria contain the enzymes needed to obtain energy stored in

carbohydrate (and other nutrient) molecules and use that energy to form ATP,the molecule needed to do cell work.

• The processes that occur in the mitochondrion are a part of aerobic cellrespiration, specifically known as the Krebs Cycle and Electron Transport,and require oxygen. We will devote some time to the discussion of these vitalmetabolic processes of cell respiration in our next unit!

Structure of Mitochondria• Most are 1 - 5 micrometers in length and oval to elongate in shape.• The mitochondrion has a double membrane system; the outer membrane is

smooth; the inner membrane is deeply folded and convoluted, forming cristae.• The membrane's proteins are formed on ribosomes in the free cytosol and on

ribosomes found within the mitochondrion itself, rather than being derived fromER. This is also true of the chloroplast, an organelle to be discussed in a bit.Mitochondria and chloroplasts also contain their own DNA for the proteinssynthesized at their internal ribosomes and can be self-replicating (more aboutthis later, too).

• The double membrane of the mitochondrion forms two compartments filled withfluid: The intercompartment space is between the outer membrane and thecristae, and the central mitochondrial matrix is formed by the inner cristaemembrane. This arrangement facilitates the functions of the mitochondria.Some respiration processes occur within the matrix and others on the innermembranes. The cristae folds ensure that there is much surface area for theenzymes needed for cell respiration.

• Cells may have few to many mitochondria, depending on the energyrequirements of cell.

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PlastidsPlastids are found in the cells of plants. Animal cells do not contain plastids.There are two common plastids in addition to the chloroplast. In general, a plastidis a membrane bounded organelle that stores something (yes, same as a vacuole).

ChloroplastsChloroplasts contain the pigments, including chlorophyll, and the enzymesnecessary for photosynthesis, the process by which light energy is converted tochemical energy, which is used to manufacture carbohydrate (fuel) molecules.Chloroplasts are found in plants and in some protists. In fact some protistchloroplasts are very elaborate structures. Chloroplasts are not found inheterotrophic organisms. Some bacteria have chlorophyll and canphotosynthesize, but lack the membrane-bounded chloroplasts. Some bacteriaalso have photosynthetic pigments other than chlorophyll.

Typical Chloroplast Structure:The plant chloroplast is a double layered membrane bounded organelle, with aninner compartment that contains an additional set of internal membranes. Theouter and inner membranes are smooth, and oval shaped in higher plants. Mostchloroplasts are about 2 - 5 micrometers in diameter.• The internal membrane system has a series of folds or layers within a fluid

matrix. The fluid is called the stroma• The internal membranes are disc like in structure and called thylakoids. These

flattened discs stack up to form grana. The photosynthetic pigments arearranged on the grana and are connected to each other within the stroma bysimple lamellae (thylakoid membranes that are not stacked to form grana andconnect one granum to another).

This distinctive structure is important for the multiple processes which occurduring photosynthesis, a process which we will discuss in great detail later...

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Other PlastidsChromoplasts• Store the plant pigments (notably the yellow, orange and red carotenoids) which

are not water soluble, and not involved in photosynthesis• Chromoplasts are abundant in orange, golden and scarlet pigmented regions of

plants.

Amyloplasts• Amyloplasts store starch, which is unpigmented. (There is a general term,

leucoplast, which means unpigmented plastid, but is not as descriptive asamyloplast, which identifies what is stored in the plastid). Amyloplasts are alsocalled starch grains, but not by biology students who know the correct term.

• Amyloplasts vary in size depending on how much starch is being deposited.They are also species specific in overall design; a specialist can identify thesource of starch grains.

• Starch is usually deposited in concentric layers within an amyloplast, which canbe seen sometimes with the microscope.

• Amyloplasts are abundant in the storage cells of most plants.

Amyloplasts

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Central Plant VacuoleAll living, mature plant cells have a large membrane bounded organelle, filled withfluid, called the Central Plant Vacuole. The central vacuole occupies as muchas 90 - 95% of the volume of the mature cell. The membrane of the vacuole iscalled the tonoplast. The tonoplast is poorly permeable to water and watersoluble materials.

Functions of the Central Plant Vacuole• Stores metabolic products including:

• proteins found in seeds• ions, such as potassium and chorine• the plant's water soluble pigments (the anthocyanins)• toxic substances• secondary metabolites, some of which serve to defend the plant against

unwanted munching by predators• Stored substances in the vacuole attract water that increases fluid pressure

within the vacuole. This pressure is known as turgor pressure and isimportant in increasing cell size and surface area during cell growth. Thispressure also forces the cytoplasm against the plasma membrane and cell wall,helping to keep the cell rigid, maintaining a condition of turgor. Turgorprovides support and strength for herbaceous plants and other plant partslacking secondary cell walls. When plant cells lose turgor, they wilt, a conditionknown biologically as plasmolysis. "Permanent wilt" is a botanical euphemismfor death.

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CytoskeletonThe cytoskeleton is the internal, fibrous framework of cells, composed of protein.Many organelles and some enzymes are organized along this framework.• The cytoskeleton maintains the shape of animal cells by its architectural design

and anchors organelles.• The cytoskeleton is responsible for motility within cells, most notably, muscle

contraction. Vesicles move throughout the cytosol along cytoskeletal fibrils.During cyclosis, organelles are transported along cytoskeletal tracks within thecytosol. Movement involves interaction of the cytoskeletal molecules withspecial proteins, called motor molecules, which activate the contraction ofthe cytoskeletal molecules. The cytoskeleton is also responsible for motility ofcells and external movement such as the amoeboid movement of white bloodcells and the migration of cells during development. The cytoskeleton also has arole in cell division.

• The cytoskeleton may also be involved with mechanical signals for cell activities.Mechanical pressure on the cell may promote certain activities, particularly cellrearrangements and cell elongation.

Components of the cytoskeleton• Microfilaments (Actin Filaments)• Microtubules• Intermediate Filaments

Microfilaments (Actin Filaments)• Tiny solid fibers of coiled globular protein, actin. Two actin filaments coil to

form each microfilament.• Functions

• Microfilaments make up the core of microvilli, projections of some cellsthat increase surface area for transport of materials. The cells of thesmall intestine have microvilli.

• Muscle contraction (actin filaments alternate with thicker fibers ofmyosin in muscle tissue)

• Cyclosis (the movement of cytoplasm contents within the cell).• "Amoeboid" movement and phagocytosis.• Responsible for the cleavage furrow in animal cytokinesis

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Microtubules• Hollow cylindrical tubules composed of tubulin, a dumbbell shaped protein. Each

microtubule is made of 13 columns of tubulin. Tubulin is a quaternary protein;each tubulin molecule is comprised of two polypeptides (a dimer of α and βtubulin)

• Provides an internal structural framework for animal cells• Can generate movement as microtubule aggregates slide past one another.• Microtubules that have motor molecules associated with them form the

tracks for the movement of vesicles from the Golgi complex to the plasmamembrane.

• Microtubules can be assembled and disassembled on demand in most cells froma microtubule organizing center (MTOC), called the centrosome, locatednear the nucleus, and in other locations. Microtubules grow at their ends byadding more tubulin molecules. Microtubules can generate new ones by pinchingoff portions of an existing microtubule. GTP bonded to the tip stabilizesmicrotubules.

CentriolesCentrioles, composed of microtubules, are located in the microtubule organizingcenter. Centrioles, which always occur in pairs found at right angles to each other,have a precise arrangement of microtubules, consisting of 9 groups of 3microtubules (9 X 3 arrangement). Centrioles are self-replicating and contain DNA.In animal cells, centrioles are important in microtubule assembly. Plants and fungido not have centrioles

Intermediate Filaments• Made of fibrous protein forming a solid, intertwined rope-like structure• Intermediate filaments are composed of keratins and vimentins. There are

several different keratins.• Intermediate filaments tend to be stable and fixed in position within the cell,

rather than being more "mobile" or transitory as microfilaments andmicrotubules are.

• Functions• Anchor for other cell components, particularly the nucleus• Reinforce cells under tension, maintaining shape. Nerve cell axons maintain

their shape with intermediate filaments called neurofilaments• Form the nuclear lamina (layer beneath the nuclear envelope)

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Cell MovementsIntracellular MovementWe have already mentioned that both microfilaments and microtubules are involvedin movement within the cytoplasm for muscle contraction as well as for cyclosisand vesicle transport. Microtubules form the spindle complex that moveschromosomes during cell division. Microfilaments in animal cells are responsible forthe cleavage furrow that separates the original cell into two new cells to completecell division. To successfully move organelles or vesicles within cells, motormolecules, proteins of kinesin are needed to provide the energy-driven force toliterally pull the vesicles along microtubules as well as connector molecules,used to connect a vesicle to the microtubule. ATP provides the energy source.Motor molecules move materials both from the interior of the cell towards theperiphery and from the periphery to the interior. The direction depends on themotor molecule.

Cell Locomotion via PseudopodiaAt any given time, the cytoplasm of a cell exists in a mixture of fluid and morerigid regions. The more solid cytoplasm is often called the "gel" in contrast to themore fluid "sol" state. Some cells use this changing sol-gel state to generatemovement. Macrophages and some protists, such as Amoeba, move this way. Aweak area of gel perimeter gets pushed out by the sol interior forming apseudopod, so that a portion of cytoplasm oozes outward. It then solidifies withmicrofilaments in the new position.

Cell Locomotion via Cilia and FlagellaCilia and flagella, which extend from the plasma membrane, are composed ofmicrotubules, coated with plasma membrane material. Eukaryotic cilia and flagellahave an arrangement of microtubules, known as the 9 + 2 arrangement (9 pairs ofmicrotubules (doublets) around the circumference plus 2 central microtubules)."Spokes" radiate from the microtubules towards the central microtubules to helpmaintain the structure of the cilium or flagellum.• Each of the microtubule doublets has motor molecule "arms", the dynein arms,

which can grip and pull an adjacent microtubule to generate the sliding motion.(The protein of this motor molecule is dynein.)

• Cilia are generally small in length, and a ciliated cell will have many. Flagella arerelatively long, and cells will have one or very few.

• Cilia and flagella are embedded in the plasma membrane of cells and extendoutward into the environment. They are organized from the basal body, withinthe membrane. Basal bodies are identical in structure to centrioles. There is atransition zone where the two microtubules of the cilia join a third microtubuleforming the basal body ring. Basal bodies are in fact, centrioles that areembedded in the plasma membrane. The basal body of a sperm cell migratesinto the cytoplasm of an egg to form the first centriole.

• (Recall that the flagella of prokaryotic cells have a structure and mode ofgenerating motion very different from the eukaryotic flagella.)

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The Cell Surface – External StructuresCell Walls

Although the boundary of any cell is its cell or plasma membrane, the cells of manytypes of organisms, such as plants, fungi, bacteria and many protists, have one ormore rigid surface layers exterior to the plasma membrane. These surfacelayers are called the cell wall, or for some protists, the pellicle. Cell walls aresecreted by the cell they surround, and are composed of a number of differentkinds of materials, depending on the Kingdom or Domain. Let's look a bit at the wallstructure of plants.

Cell walls provide strength, shape and mechanical protection for plants (and otherorganisms that have cell walls). The collective cell walls form the supportstructure of plants. In addition, cell wall strength is critical to maintaining turgorin plant cells, which, along with providing strength to the cell, also prevents cellsfrom taking in too much water (We will discuss this later with osmosis.)

Plant Cell Wall LayersPrimary Wall• All plant cells secrete a primary cell wall exterior to the plasma membrane.• The primary wall is composed extensively of cellulose, a polysaccharide of

glucose discussed previously. Walls may also contain other cellulose-relatedmaterials as well as gums, pectins and other materials. Cellulose is the mainconstituent of wood, pulp and paper, and cotton and linen as well as other"fibers". It is also the bulk of the "fiber" of our diets.

Secondary Walls• Secondary walls consist of layers secreted interior to the primary wall and

provide additional strength to those cells.• Secondary walls will have lignin as well as cellulose materials and are important

in woody tissues, and in the support and conducting tissues of plants• Certain cell types in plants secrete suberin, an impermeable substance that

prevents the movement of substances through the walls. The Casparian stripof the endodermis cells in roots, cork tissue cells, and the abscission zone cellsin leaves contain suberin.

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Middle LamellaWhen plant cells divide, vesicles secrete pectins (Calcium pectate) and other"gummy" substances between the adjacent cell membranes of the divided cell. Thismaterial forms the middle lamella which "glues" plant cells together (Unlikemembranes, walls are not sticky.)

PlasmodesmataPlasmodesmata are membrane connections (little channels) between adjacent plantcells that pass through the wall layers. Plasmodesmata provide for intercellularcytoplasm communication.

Extracellular MatrixThe surfaces of animal cell membranes have a variety of substances that areattached to the plasma membrane. These materials form the extracellularmatrix. Many of the molecules of the matrix are glycoproteins that form aproteoglycan network. Collagen fibers are typically embedded into a network inthe matrix, and provide strength. One glycoprotein, fibronectin, connects theothers to the plasma membrane. Another, integrin, connects the plasmamembrane to microfilaments of the cytoskeleton.

The communication role of the extracellular matrix appears to be important indevelopmental processes as well as in normal cell behavior by affecting geneticcontrols in the nucleus through both mechanical and chemical signaling.

In addition to the extracellular matrix of animal cells, the plasma membranes ofcells have receptor proteins, molecules that chemically detect signals from othercells or chemicals in the environment. For example, hormones often "work" bybinding to receptors, or passing through protein channels and binding to chemicalswithin the cell to direct specific functions. Such chemical communications are vitalto the functioning of all cells and organisms. We will discuss the membraneproteins in more detail next.