45

Chapter Concepts

Cell Structure andFunction

3.1 The Cellular Level of Organization• All organisms are composed of cells, which arise

from preexisting cells. 46• A microscope is usually needed to see a cell

because most cells are quite small. 48

3.2 Eukaryotic Cells• All cells have a plasma membrane that regulates

the entrance and exit of molecules into and outof the cell. Some cells also have a protective cellwall. 49

• Eukaryotic cells have a number of membranousorganelles that carry out specific functions. 49

• The nucleus controls the metabolic functions andthe structural characteristics of the cell. 52

• A system of membranous canals and vacuoleswork together to produce, modify, transport,store, secrete, and/or digest macromolecules. 53

• Mitochondria and chloroplasts transform oneform of energy into another. Mitochondriaproduce ATP and chloroplasts producecarbohydrates. 57

• The cell has a cytoskeleton composed ofmicrotubules and filaments. The cytoskeletongives the cell shape and allows it and itsorganelles to move. 58

• Centrioles are related to cilia and flagella, whichenable the cell to move. 60

3.3 Prokaryotic Cells• In contrast to the eukaryotic cell, the prokaryotic

cell lacks a well-defined nucleus. 62

3.4 Evolution of the Eukaryotic Cell• The endosymbiotic hypothesis suggests that

certain organelles of eukaryotic cells were onceprokaryotic cells. 63

Artist’s representation of a cell’s interior illustrates its complexity.Vacuoles and the Golgi apparatus glide past a sectioned endoplas-mic reticulum outside the nucleus.

Three types of microscopes are most commonly used: the com-pound light microscope, transmission electron microscope, andscanning electron microscope (Fig. 3A). In a compound lightmicroscope, light rays passing through a specimen are broughtto a focus by a set of glass lenses, and the resulting image is thenviewed by the human eye. In the transmission electron micro-scope, electrons passing through a specimen are brought to afocus by a set of magnetic lenses, and the resulting image is pro-jected onto a fluorescent screen or photographic film.

46

3.1 The Cellular Level ofOrganizationAntonie van Leeuwenhoek of Holland, who lived in the sev-enteenth century, is now famous for making his own micro-scopes and observing all sorts of tiny things that no one hadseen before. The Englishman Robert Hooke was the first touse the term cell for walled-off chambers he observed incork, a material used to make stoppers today. A hundredyears later—in the 1830s—the German microscopistMatthias Schleiden said that plants are composed of cells;his counterpart Theodor Schwann said that animals are alsomade up of cells. This was quite a feat, because aside fromtheir own exhausting work, they had to be aware of thestudies of many other microscopists. Rudolf Virchow, anoth-er German microscopist, later came to the conclusion thatcells don’t suddenly appear; rather, they come from preex-isting cells. Today, the cell theory, which states that allorganisms are made up of cells and that cells come onlyfrom preexisting cells, is a unifying concept of biology.

The cell marks the boundary between the nonliving andthe living. The molecules that serve as food for a cell and theorganic molecules that make up a cell are not alive, and yet thecell is alive. The answer to what life is will have to be foundwithin the cell, because the smallest living organisms areunicellular, while larger organisms are multicellular—theyare composed of many cells. The diversity of cells is exem-plified by the many types in the human body. But despitevariety of form and function, cells contain the same compo-nents. The basic elements that are common to all cells regard-less of their specializations are the subject of this chapter.

All organisms are composed of self-reproducingunits called cells. Microscopy revealed thepresence of cells and even today is making knowntheir detailed structure.

If you've ever tried to stuff a week's worth of clothing, toi-letries, and other necessities into a piece of carry-on lug-gage, you would be amazed at what cells can pack into a

space smaller than the period at the end of this sentence.Like the human body, which is composed of many trillions ofcells working in harmony, cells have an internal skeleton thatgives them shape and controls their movement. They harbormicroscopic assembly lines that manufacture a wide array ofproteins. Every cell even has its own power stations that pro-duce energy. And nestled among such structures, often in aspecialized area walled off from the rest of the cell, are theall-important chromosomes, which store the instructionmanual for life. It took the development of powerful micro-scopes before biologists could visualize the innards of cells.Let us now take you on a guided tour.

ye

ocular lens

objective lensspecimencondenser

light source

Compound light microscope

bloodvessel

red bloodcells

e

25 µm

Figure 3A Blood vessels and red blood cells viewedwith three different types of microscopes.

A scanning electron microscope provides a three-dimensionalview of the surface of an object. A narrow beam of electrons isscanned over the surface of the specimen, which has been coat-ed with a thin layer of metal. The metal gives off secondary elec-trons, which are collected to produce a television-type picture ofthe specimen’s surface on a screen.

A picture obtained using a light microscope sometimes iscalled a photomicrograph, and a picture resulting from the useof an electron microscope is called a transmission electronmicrograph (TEM) or a scanning electron micrograph (SEM),depending on the type of microscope used.

The magnification produced by an electron microscope is muchhigher than that of a light microscope (50,000� compared to1,000�). Also, the ability of the electron microscope to make outdetail is much greater. The distance needed to distinguish twopoints as separate is much less for an electron microscope than alight microscope (10 nm compared to 200 nm1). The greaterresolving power of the electron microscope is due to the fact thatelectrons travel at a much shorter wavelength than do light rays.However, because electrons only travel in a vacuum, the objectis always dried out, whereas even living objects can be observedwith a light microscope.

1See the metric system in Appendix C.

Microscopy of Today

blood

red bloodcells

electron beam

objective lens

specimen

condenser

projector lens

observationorphotograph

ransmission electron microscope

vessel

electron beam

condensers

scanning coil

objectivelens

secondaryelectrons

specimen

detector

TVviewing screen

Scanning electron microscope

blood vessel

cells

electron

red blood

T

10 µm

14 µm

Cell SizeFigure 3.1 outlines the visual range of the eye, light micro-scope, and electron microscope. Cells are quite small. Afrog’s egg, at about one millimeter (mm) in diameter, is largeenough to be seen by the human eye. Most cells are farsmaller than one millimeter; some are even as small as onemicrometer (µm)—one thousandth of a millimeter. Cellinclusions and macromolecules are even smaller than amicrometer and are measured in terms of nanometers (nm).

An explanation for why cells are so small and why weare multicellular is explained by considering thesurface/volume ratio of cells. Nutrients enter a cell andwastes exit a cell at its surface; therefore, the amount of sur-face represents the ability to get material in and out of thecell. A large cell requires more nutrients and produces morewastes than a small cell. In other words, the volume repre-sents the needs of the cell. Yet as cells get larger in volume,the proportionate amount of surface area actually decreases,as you can see by comparing these two cells:

A small cube that is 1 mm tall has a surface area of 6 mm2

and a volume of 1 mm3. This is a ratio of surface area to vol-ume of 6:1. But a cube that is 2 mm tall has a surface area of24 mm2 and a volume of 8 mm3. This is a ratio of only 3:1.Therefore a small cell has more surface area per volume thandoes a large cell.2

Small cells, not large cells, are likely to have an adequatesurface area for exchange of wastes for nutrients. We wouldexpect, then, a size limitation for an actively metabolizingcell. A chicken’s egg is several centimeters in diameter, butthe egg is not actively metabolizing. Once the egg is incu-bated and metabolic activity begins, the egg divides repeat-edly without growth. Cell division restores the amount ofsurface area needed for adequate exchange of materials.Further, cells that specialize in absorption have modifica-tions that greatly increase the surface area per volume of thecell. The columnar cells along the surface of the intestinalwall have surface foldings called microvilli (sing., micro-villus), which increase their surface area.

A cell needs a surface area that can adequatelyexchange materials with the environment. Surface-area-to-volume considerations require that cellsstay small.

48 Part 1 Cell Biology 3-4

mouse

frogegg

humanegg

plant andanimalcells

most bacteriaviruses

proteins

amino acids

atoms

antelectron microscope

light microscope

human eye

1km100m10m1m0.1m1cm1mm100µm10µm1µm100nm10nm1nm0.1nm

human

blue whale

chloroplast

Figure 3.1 The sizes of living things and their components. It takes a microscope to see most cells and lower levels of biological organization. Cells are visible with the light microscope, but not in muchdetail. It takes an electron microscope to see organelles in detail and to make out viruses and molecules. Notice that in this illustration eachhigher unit is 10 times greater than the lower unit. (In the metric system, 1 meter � 102 cm � 103 mm � 106 mm � 109 nm—see Appendix C.)

large cell—less surface areaper volume

small cell—more surface areaper volume

surface area = 6 (2 � 2) = 24 mm2

volume = 2 � 2 � 2 = 8 mm2

surface area/volume = 3 : 1

2surface area = 6 (1 � 1) = 6 mm2

volume = 1 � 1 � 1 = 1 mm2

surface area/volume = 6 : 1

3.2 Eukaryotic CellsEukaryotic cells have a nucleus, a large structure that con-trols the workings of the cell because it contains the genes.

Outer Boundaries of Animal and Plant CellsAll cells, including plant and animal cells, are surroundedby a plasma membrane, a phospholipid bilayer in whichprotein molecules are embedded.

The plasma membrane is a living boundary that separatesthe contents of the cell from the surrounding environment.Inside the cell, the nucleus is surrounded by the cytoplasm,a semifluid medium that contains organelles. The plasmamembrane regulates the entrance and exit of molecules intoand out of the cytoplasm.

Plant cells (but not animal cells) have a permeable but pro-tective cell wall in addition to a plasma membrane. Manyplant cells have both a primary and secondary cell wall. Amain constituent of a primary cell wall is cellulose molecules.Cellulose molecules form fibrils that lie at right angles to oneanother for added strength. Acell wall sometimes forms insidethe primary cell wall. Secondary cell walls contain lignin, a sub-stance that makes them even stronger than primary cell walls.

Organelles of Animal and Plant CellsAnimal and plant cells contain organelles, small bodies thathave a specific structure and function. Originally the termorganelle referred to only membranous structures, but wewill use it to include any well-defined internal subcellularstructure (Table 3.1). Still, membrane compartmentalizes thecell so that the various functions of the cell are kept separatefrom one another. Just as all the assembly lines of a factoryare in operation at the same time, so all the organelles of acell function simultaneously. Raw materials enter a factoryand then are turned into various products by differentdepartments. In the same way, chemicals are taken up by thecell and then processed by the organelles. The cell is a bee-hive of activity the entire twenty-four hours of every day.

Both animal (Fig. 3.2) and plant (Fig. 3.3) cells containmitochondria, while only plant cells have chloroplasts. Onlyanimal cells have centrioles. The color chosen to representeach structure in the plant and animal cell is used for thatstructure throughout the chapters of this part.

proteinmolecules

phospholipidlayer

Chapter 3 Cell Structure and Function 493-5

Table 3.1 Eukaryotic Structures in Animal Cellsand Plant Cells

Name Composition Function

Cell wall* Contains cellulose Support and protectionfibrils

Plasma Phospholipid bilayer Define cell boundary;membrane with embedded regulation of molecule

proteins passage into and out ofcells

Nucleus Nuclear envelope Storage of geneticsurrounding information; synthesis ofnucleoplasm, DNA and RNAchromatin, and nucleoli

Nucleolus Concentrated area Ribosomal subunitof chromatin, RNA, formationand proteins

Ribosome Protein and RNA in Protein synthesistwo subunits

Endoplasmic Membranous Synthesis and/orreticulum flattened channels modification of proteins(ER) and tubular canals and other substances,

and transport byvesicle formation

Rough ER Studded with Protein synthesisribosomes

Smooth ER Having no ribosomes Various; lipid synthesisin some cells

Golgi Stack of membranous Processing, packaging,apparatus saccules and distribution of

proteins and lipids

Vacuole and Membranous sacs Storage of substancesvesicle

Lysosome Membranous vesicle Intracellular digestioncontaining digestiveenzymes

Peroxisome Membranous vesicle Various metabolic taskscontaining specificenzymes

Mitochondrion Membranous cristae Cellular respirationbounded by anouter membrane

Chloroplast* Membranous grana Photosynthesisbounded by twomembranes

Cytoskeleton Microtubules, Shape of cell andintermediate movement of itsfilaments, actin partsfilaments

Cilia and 9 � 2 pattern of Movement of cellflagella microtubules

Centriole** 9 � 0 pattern of Formation of basalmicrotubules bodies

*Plant cells only**Animal cells only

50 Part 1 Cell Biology 3-6

Figure 3.2 Animal cell anatomy.a. Generalized drawing. b. Transmission electron micrograph. See Table 3.1 for a description of these structures, along with a listing of theirfunctions.

chromatin

nucleolus

lysosome

microtubule

centriole

ribosome

mitochondrion

nuclear pore

rough ER

smooth ER

vacuole

cytoplasm

Golgi apparatus

plasma membrane

vesicle

nuclear envelope

nucleus

polyribosome

plasma membrane

nuclear envelope

chromatin

nucleolus

endoplasmic reticulum

actin filament

peroxisome

a.

b.

Chapter 3 Cell Structure and Function 513-7

1 µm

peroxisome

mitochondrion

central vacuole

chloroplast cell wall

plasma membrane

nucleus

ribosomes

chromatin

nucleolus

nuclear pore

nuclear envelope

nucleus

rough ER

smooth ER

central vacuole

cytosol

Golgi apparatus

plasma membrane

microtubule

ribosome

mitochondrion

actin filament

cell wall

intracellular space

chloroplast

cell wall of adjacent cell

Figure 3.3 Plant cell anatomy.a. Generalized drawing. b. Transmission electron micrograph of young leaf cell. See Table 3.1 for a description of these structures, along with alisting of their functions.

a.

b.

The NucleusThe nucleus, which has a diameter of about 5 µm, is a promi-nent structure in the eukaryotic cell. The nucleus is of primaryimportance because it stores genetic information that deter-mines the characteristics of the body’s cells and their metabol-ic functioning. Every cell contains a complex copy of geneticinformation, but each cell type has certain genes, or segmentsof DNA, turned on, and others turned off. Activated DNA,with RNA acting as an intermediary, specifies the sequence ofamino acids during protein synthesis. The proteins of a celldetermine its structure and the functions it can perform.

When you look at the nucleus, even in an electron micro-graph, you cannot see DNA molecules but you can see chro-matin (Fig. 3.4). Chromatin looks grainy, but actually it is athreadlike material that undergoes coiling into rodlike struc-tures called chromosomes, just before the cell divides. Chemi-cal analysis shows that chromatin, and therefore chromosomes,contains DNA and much protein, and some RNA. Chromatinis immersed in a semifluid medium called the nucleoplasm.A difference in pH between the nucleoplasm and cytoplasmsuggests that the nucleoplasm has a different composition.

Most likely, too, when you look at an electron micro-graph of a nucleus, you will see one or more regions that lookdarker than the rest of the chromatin. These are nucleoli(sing., nucleolus) where another type of RNA, called riboso-mal RNA (rRNA), is produced and where rRNA joins withproteins to form the subunits of ribosomes. (Ribosomes aresmall bodies in the cytoplasm that contain rRNAand proteins.)

The nucleus is separated from the cytoplasm by a dou-ble membrane known as the nuclear envelope which is con-tinuous with the endoplasmic reticulum discussed on thenext page. The nuclear envelope has nuclear pores of suffi-cient size (100 nm) to permit the passage of proteins into thenucleus and ribosomal subunits out of the nucleus.

The structural features of the nucleus includethe following.

Chromatin: DNA and proteinsNucleolus: chromatin and ribosomal

subunits

Nuclear envelope: double membrane with pores

52 Part 1 Cell Biology 3-8

nuclear pores

nucleoluschromatin

Electron micrographs ofnuclear envelope showingpores.

nuclear envelope

outer membrane

inner membrane

Figure 3.4 The nucleus and the nuclear envelope.The nucleoplasm contains chromatin. Chromatin has a special region called the nucleolus, which is where rRNA is produced and ribosomalsubunits are assembled. The nuclear envelope, consisting of two membranes separated by a narrow space, contains pores. The electronmicrographs show that the pores cover the surface of the envelope.

RibosomesRibosomes are composed of two subunits, one large andone small. Each subunit has its own mix of proteins andrRNA. Protein synthesis occurs on the ribosomes. Ribo-somes occur free within the cytoplasm either singly or ingroups called polyribosomes. Ribosomes are often attachedto the endoplasmic reticulum, a membranous system of sac-cules and channels discussed in the next section. Proteinssynthesized by cytoplasmic ribosomes are used in the cell,such as in the mitochondria and chloroplasts. Those pro-duced by ribosomes attached to endoplasmic reticulum mayeventually be secreted from the cell.

Ribosomes are small organelles where proteinsynthesis occurs. Ribosomes occur in thecytoplasm, both singly and in groups (i.e.,polyribosomes). Numerous ribosomes areattached to the endoplasmic reticulum.

The Endomembrane SystemThe endomembrane system consists of the nuclear envelope,the endoplasmic reticulum, the Golgi apparatus, and severalvesicles (tiny membranous sacs). This system compartmental-izes the cell so that particular enzymatic reactions are restrictedto specific regions. Membranes that make up the endomem-brane system are connected by direct physical contactand/or by the transfer of vesicles from one part to the other.

The Endoplasmic ReticulumThe endoplasmic reticulum (ER), a complicated system ofmembranous channels and saccules (flattened vesicles), isphysically continuous with the outer membrane of thenuclear envelope. Rough ER is studded with ribosomes onthe side of the membrane that faces the cytoplasm (Fig. 3.5).Here proteins are synthesized and enter the ER interiorwhere processing and modification begin. Smooth ER,which is continuous with rough ER, does not have attachedribosomes. Smooth ER synthesizes the phospholipids thatoccur in membranes and has various other functionsdepending on the particular cell. In the testes, it producestestosterone, and in the liver it helps detoxify drugs. Regard-less of any specialized function, smooth ER also forms vesi-cles in which large molecules are transported to other partsof the cell. Often these vesicles are on their way to the plas-ma membrane or the Golgi apparatus.

ER is involved in protein synthesis (rough ER) andvarious other processes such as lipid synthesis(smooth ER). Molecules that are produced ormodified in the ER are eventually enclosed invesicles that often transport them to the Golgiapparatus.

Chapter 3 Cell Structure and Function 533-9

Figure 3.5 The endoplasmic reticulum (ER).a. Rough ER has attached ribosomes but smooth ER does not.b. Rough ER appears to be flattened saccules, while smooth ER is anetwork of interconnected tubules. c. A protein made on a ribosomemoves into the lumen of the system and eventually is packaged in atransport vesicle for distribution inside the cell.

a.

c.

b.

smoothER

ribosome

ribosomevesicle formation transport vesicle

proteinERlumen

400 nm

The Golgi ApparatusThe Golgi apparatus is named for Camillo Golgi, who dis-covered its presence in cells in 1898. The Golgi apparatusconsists of a stack of three to twenty slightly curved sacculeswhose appearance can be compared to a stack of pancakes(Fig. 3.6). In animal cells, one side of the stack (the innerface) is directed toward the ER, and the other side of thestack (the outer face) is directed toward the plasma mem-brane. Vesicles can frequently be seen at the edges of thesaccules.

The Golgi apparatus receives protein and/or lipid-filled vesicles that bud from the ER. Some biologistsbelieve that these fuse to form a saccule at the inner faceand that this saccule remains as a part of the Golgi appara-tus until the molecules are repackaged in new vesicles atthe outer face. Others believe that the vesicles from the ERproceed directly to the outer face of the Golgi apparatus,where processing and packaging occurs within its saccules.

The Golgi apparatus contains enzymes that modifyproteins and lipids. For example, it can add a chain of sug-ars to proteins, thereby making them glycoproteins andglycolipids, which are molecules found in the plasmamembrane.

The vesicles that leave the Golgi apparatus move todifferent locations in the cell. Some vesicles proceed to theplasma membrane, where they discharge their contents.Because this is secretion, it is often said that the Golgiapparatus is involved in processing, packaging, andsecretion. Other vesicles that leave the Golgi apparatusare lysosomes.

The Golgi apparatus processes, packages, anddistributes molecules about or from the cell. It isalso said to be involved in secretion.

54 Part 1 Cell Biology 3-10

Golgi apparatus

lysosome

Lysosomes

lysosome combineswith new vesicle, and substance is digested

transport vesiclesmove from thesmooth ER tothe Golgi apparatus

Golgi apparatus

substance is takeninto cell by vesicleformation

secretory vesicledischarges aproduct at theplasma membrane

Figure 3.6 The Golgi apparatus.The Golgi apparatus receives transport vesicles containing proteinsfrom smooth ER. After modifying the proteins, it repackages them ineither secretory vesicles or in lysosomes. When lysosomes combinewith newly formed vesicles, their contents are digested. Lysosomesalso break down cellular components.

LysosomesLysosomes are membrane-bounded vesicles produced bythe Golgi apparatus in animal cells and plant cells. Lyso-somes contain hydrolytic digestive enzymes.

Sometimes macromolecules are brought into a cell byvesicle formation at the plasma membrane (Fig. 3.6). Whena lysosome fuses with such a vesicle, its contents are digest-ed by lysosomal enzymes into simpler subunits that thenenter the cytoplasm. Some white blood cells defend thebody by engulfing bacteria that are then enclosed withinvesicles. When lysosomes fuse with these vesicles, the bac-teria are digested. It should come as no surprise, then, thateven parts of a cell are digested by its own lysosomes(called autodigestion). Normal cell rejuvenation most likelytakes place in this manner, but programmed cell destruc-tion occurs during development. For example, when a tad-pole becomes a frog, lysosomes digest away the cells of thetail. The fingers of a human embryo are at first webbed, butthey are freed from one another as a result of lysosomalaction.

Occasionally, a child is born with a metabolic disorderinvolving a missing or inactive lysosomal enzyme. In thesecases, the lysosomes fill to capacity with macromoleculesthat cannot be broken down. The cells become so full ofthese lysosomes that the child dies. Someday soon it maybe possible to provide the missing enzyme for thesechildren.

Lysosomes are produced by a Golgi apparatus,and their hydrolytic enzymes digestmacromolecules from various sources.

PeroxisomesPeroxisomes, similar to lysosomes, are membrane-boundedvesicles that enclose enzymes (Fig. 3.7). These enzymes weresynthesized by free ribosomes and imported directly into aperoxisome. Peroxisomes contain enzymes for oxidizingcertain organic substances with the formation of hydrogenperoxide (H2O2):

RH2 + O2 £ R + H2O2

Hydrogen peroxide, a toxic molecule, is immediately brokendown to water and oxygen by another peroxisomal enzymecalled catalase. Peroxisomes are abundant in cells thatmetabolize lipids and in liver cells that metabolize alcohol.They help detoxify alcohol.

Peroxisomes play additional roles in plants. In germi-nating seeds, they oxidize fatty acids into molecules that canbe converted to sugars needed by the growing plant. Inleaves, peroxisomes can carry out a reaction that is oppositeto photosynthesis—the reaction uses up oxygen and releasescarbon dioxide.

Chapter 3 Cell Structure and Function 553-11

100 nm

Figure 3.7 Peroxisome in a tobacco leaf.Peroxisomes are vesicles that oxidize organic substances with aresulting build-up of hydrogen peroxide. The crystalline, squarelikecore of a peroxisome contains the enzyme catalase, which breaksdown hydrogen peroxide (H2O2) to water and oxygen.

VacuolesA vacuole is a large membranous sac. A vesicle is smallerthan a vacuole. Animal cells have vacuoles, but they aremuch more prominent in plant cells. Typically, plant cellshave a large central vacuole so filled with a watery fluid thatit gives added support to the cell (see Fig. 3.3).

Vacuoles store substances. Plant vacuoles contain notonly water, sugars, and salts but also pigments and toxicmolecules. The pigments are responsible for many of thered, blue, or purple colors of flowers and some leaves. Thetoxic substances help protect a plant from herbivorous ani-mals. The vacuoles present in unicellular protozoans arequite specialized, and they include contractile vacuoles forridding the cell of excess water and digestive vacuoles forbreaking down nutrients.

The organelles of the endomembrane systemare as follows:

Endoplasmic reticulum (ER): synthesis andmodification and transport of proteins andother substancesRough ER: protein synthesisSmooth ER: lipid synthesis in particular

Golgi apparatus: processing, packaging, and distribution of protein molecules

Lysosomes: intracellular digestionPeroxisomes: various metabolic tasksVacuoles: storage areas

Energy-Related OrganellesLife is possible only because of a constant input of energyused for maintenance and growth. Chloroplasts and mito-chondria are the two eukaryotic membranous organellesthat specialize in converting energy to a form that can beused by the cell. Chloroplasts use solar energy to synthesizecarbohydrates, and carbohydrate-derived products are bro-ken down in mitochondria (sing., mitochondrion) to pro-duce ATP molecules.

56 Part 1 Cell Biology 3-12

ATP

carbohydrate(high chemical energy)

CO2 + H2O(low chemical energy)

solarenergy

chloroplast mitochondrion

usableenergyfor cells

stroma

doublemembrane

grana

b.a.

outer membrane

inner membrane

500 nm

Photosynthesis, which occurs in chloroplasts, is theprocess by which solar energy is converted to chemical ener-gy within carbohydrates. Photosynthesis can be representedby this equation:

Here the word energy stands for solar energy, the ultimatesource of energy for cellular organization. Only plants,algae, and cyanobacteria are capable of carrying on photo-synthesis in this manner.

Cellular respiration is the process by which the chemicalenergy of carbohydrates is converted to that of ATP (adeno-sine triphosphate), the common carrier of chemical energyin cells. Aerobic cellular respiration can be represented bythis equation:

Here the word energy stands for ATP molecules. When acell needs energy, ATP supplies it. The energy of ATP isused for synthetic reactions, active transport, and allenergy-requiring processes in cells. All organisms carry oncellular respiration, and all organisms except bacteriacomplete the process of aerobic cellular respiration inmitochondria.

carbon dioxide + water + energycarbohydrate + oxygen

light energy + carbon dioxide + water carbohydrate + oxygen

Figure 3.8 Chloroplast structure.a. Electron micrograph. b. Generalized drawing in which the outer and inner membranes have been cut away to reveal the grana.

cristaedoublemembrane

outer membrane

inner membranematrix

a.200 nm

b.

ChloroplastsPlant cells contain chloroplasts, the organelles that allowthem to produce their own organic food. Chloroplasts areabout 4–6 µm in diameter and 1–5 µm in length; they belongto a group of organelles known as plastids. Among the plas-tids are also the amyloplasts, common in roots, which storestarch; and the chromoplasts, common in leaves, which con-tain red and orange pigments. A chloroplast is green, ofcourse, because it contains the green pigment chlorophyll.

A chloroplast is bounded by two membranes thatenclose a fluid-filled space called the stroma. A membranesystem within the stroma is organized into interconnectedflattened sacs called thylakoids. In certain regions, the thy-lakoids are stacked up in structures called grana (sing.,granum). There can be hundreds of grana within a singlechloroplast (Fig. 3.8). Chlorophyll, which is located withinthe thylakoid membranes of grana, captures the solar ener-gy that is needed to allow chloroplasts to produce carbohy-drates. The stroma contains DNA, ribosomes, and enzymesthat synthesize carbohydrates from carbon dioxide andwater.

MitochondriaAll eukaryotic cells, including plant cells, contain mitochon-dria. This means that plant cells contain both chloroplastsand mitochondria. Most mitochondria are usually 0.5–1.0 µm in diameter and 2–5 µm in length.

Mitochondria, like chloroplasts, are bounded by a dou-ble membrane (Fig. 3.9). In mitochondria the inner fluid-filled space is called the matrix. The matrix contains DNA,ribosomes, and enzymes which break down carbohydrateproducts, releasing energy that is used for ATP production.

The inner membrane of a mitochondrion invaginates toform cristae. Cristae provide a much greater surface area toaccommodate the protein complexes and other participantsthat produce ATP.

Mitochondria and chloroplasts are able to make someproteins, but others are imported from the cytoplasm.

Chloroplasts and mitochondria are membranousorganelles whose structure lends itself to theprocesses that occur within them.

Chapter 3 Cell Structure and Function 573-13

Figure 3.9 Mitochondrion structure.a. Electron micrograph. b. Generalized drawing in which the outer membrane and portions of the inner membrane have been cut away to revealthe cristae.

The CytoskeletonThe cytoskeleton is a network of interconnected filamentsand tubules that extends from the nucleus to the plasmamembrane in eukaryotic cells. Prior to the 1970s, it wasbelieved that the cytoplasm was an unorganized mixture oforganic molecules. Then, high-voltage electron micro-scopes, which can penetrate thicker specimens, showedthat the cytoplasm was instead highly organized. The tech-nique of immunofluorescence microscopy identified themakeup of specific protein fibers within the cytoskeletalnetwork (Fig. 3.10).

The name cytoskeleton is convenient in that it allows usto compare the cytoskeleton to the bones and muscles of ananimal. Bones and muscles give an animal structure andproduce movement. Similarly, we will see that the ele-ments of the cytoskeleton maintain cell shape and causethe cell and its organelles to move. The cytoskeleton isdynamic; elements undergo rapid assembly and disassem-bly by monomers continuously entering or leaving thepolymer. These changes occur at rates that are measured inseconds and minutes. The entire cytoskeletal network caneven disappear and reappear at various times in the life ofa cell. Before a cell divides, for instance, the elements dis-assemble and then reassemble into a structure called aspindle that distributes chromosomes in an orderly man-ner. At the end of cell division, the spindle disassemblesand the elements reassemble once again into their formerarray.

The cytoskeleton contains three types of elements: actinfilaments, intermediate filaments, and microtubules, whichare responsible for cell shape and movement.

Actin FilamentsActin filaments (formerly called microfilaments) are long,extremely thin fibers (about 7 nm in diameter) that occur inbundles or meshlike networks. The actin filament containstwo chains of globular actin monomers twisted about oneanother in a helical manner.

Actin filaments play a structural role when they form adense complex web just under the plasma membrane, towhich they are anchored by special proteins. They are alsoseen in the microvilli that project from intestinal cells, andtheir presence most likely accounts for the ability ofmicrovilli to alternately shorten and extend into the intes-tine. In plant cells, they apparently form the tracks alongwhich chloroplasts circulate or stream in a particular direc-tion. Also, the presence of a network of actin filaments lyingbeneath the plasma membrane accounts for the formation ofpseudopods, extensions that allow certain cells to move inan amoeboid fashion.

How are actin filaments involved in the movement ofthe cell and its organelles? They interact with motor mole-cules, which are proteins that move along either actin

filaments or microtubules. These motor molecules accom-plish this by attaching, detaching, and reattaching fartheralong the actin filament or microtubule. In the presence ofATP, the motor molecule myosin attaches, detaches, andreattaches to actin filaments. Myosin has both a head anda tail. In muscle cells, the tails of several muscle myosinmolecules are joined to form a thick filament. In nonmus-cle cells, cytoplasmic myosin tails are bound to mem-branes but the heads still interact with actin.

During animal cell division the two new cells form whenactin, in conjunction with myosin, pinches off the cells fromone another.

MicrotubulesMicrotubules are small hollow cylinders about 25 nm indiameter and from 0.2–25 µm in length.

Microtubules are made of a globular protein calledtubulin. When microtubules assemble, tubulin moleculescome together as dimers and the dimers arrange themselvesin rows. Microtubules have 13 rows of tubulin dimers sur-rounding what appears in electron micrographs to be anempty central core.

In many cells the regulation of microtubule assemblyis under the control of a microtubule organizing center,called the centrosome, which lies near the nucleus. Micro-tubules help maintain the shape of the cell and act astracks along which organelles can move. Whereas themotor molecule myosin is associated with actin filaments,the motor molecules kinesin and dynein move alongmicrotubules. One type of kinesin is responsible for mov-ing vesicles along microtubules, including those that arisefrom the ER.

vesicle moves, not microtubule

ATP

vesicle

kinesin

kinesinreceptor

actin filament

myosinmolecules

anchor head membrane

ATP ADP + P

58 Part 1 Cell Biology 3-14

Chapter 3 Cell Structure and Function 593-15

actin filament

intermediate filament

microtubule

plasmamembrane

b. Actin filament

7 nm

10 µm

d. Intermediate filament

10 nm

10 µm

c. Microtubule

25 nm

10 µm

a.

Figure 3.10 The cytoskeleton.a. Diagram comparing the size relationship of actin filaments,intermediate filaments, and microtubules. b. Actin filaments as theyappear in a cell and in diagram. c. Microtubules as they appear in thecell and in diagram. The filaments and tubules are visible followingimmunofluorescence, a technique that binds fluorescent antibodiesto specific proteins in cells. d. Intermediate filaments as they appearin the cell and in diagram.

There are different types of kinesin proteins, each special-ized to move one kind of vesicle or cellular organelle. Asecond type of cytoplasmic motor molecule is called cyto-plasmic dynein because it is closely related to the moleculedynein found in flagella.

Intermediate Filaments Intermediate filaments (8–11 nm in diameter) are intermedi-ate in size between actin filaments and microtubules. Theyare a ropelike assembly of fibrous polypeptides that supportthe nuclear envelope and the plasma membrane. In the skin,intermediate filaments made of the protein keratin givegreat mechanical strength to skin cells. Recent work hasshown intermediate filaments to be highly dynamic. Theyalso are able to assemble and disassemble in the same man-ner as actin filaments and microtubules.

The cytoskeleton contains actin filaments,intermediate filaments, and microtubules. Thesemaintain cell shape and allow organelles to movewithin the cytoplasm. Sometimes they are alsoinvolved in movement of the cell itself.

CentriolesCentrioles are short cylinders with a 9 � 0 pattern of micro-tubule triplets—that is, a ring having nine sets of tripletswith none in the middle (Fig. 3.11). In animal cells and mostprotists, a centrosome contains two centrioles lying at rightangles to each other. The centrosome is the major micro-tubule organizing center for the cell. Therefore, it is possiblethat centrioles are also involved in the process by whichmicrotubules assemble and disassemble.

Before an animal cell divides, the centrioles replicateand the members of each pair are at right angles to oneanother (Fig. 3.11). Then each pair becomes part of a sepa-rate centrosome. During cell division the centrosomes moveapart and may function to organize the mitotic spindle. Inany case, each new cell has its own centrosome. Plant cellshave the equivalent of a centrosome but it does not containcentrioles, suggesting that centrioles are not necessary to theassembly of cytoplasmic microtubules.

In cells with cilia and flagella, centrioles are believed togive rise to basal bodies that direct the organization ofmicrotubules within these structures. In other words, a basalbody may do for a cilium (or flagellum) what the centro-some does for the cell.

Centrioles, which are short cylinders with a 9 + 0pattern of microtubule triplets, may be involved inmicrotubule organization and in the formation ofcilia and flagella.

Cilia and FlagellaCilia and flagella are hairlike projections that can moveeither in an undulating fashion, like a whip, or stiffly, like anoar. Cells that have these organelles are capable of move-ment. For example, unicellular paramecia move by means ofcilia, whereas sperm cells move by means of flagella. Thecells that line our upper respiratory tract have cilia thatsweep debris trapped within mucus back up into the throat,where it can be swallowed. This action helps keep the lungsclean.

In eukaryotic cells, cilia are much shorter than flagella,but they have a similar construction. Both are membrane-bounded cylinders enclosing a matrix area. In the matrix arenine microtubule doublets arranged in a circle around twocentral microtubules. Cilia and flagella move when themicrotubule doublets slide past one another (Fig. 3.12).

As mentioned, each cilium and flagellum has a basalbody lying in the cytoplasm at its base. Basal bodies have thesame circular arrangement of microtubule triplets as centri-oles and are believed to be derived from them. It is possiblethat basal bodies organize the microtubules within cilia andflagella, but this is not supported by the observation that cil-ia and flagella grow by the addition of tubulin dimers totheir tips.

Cilia and flagella, which have a 9 � 2 pattern ofmicrotubules, are involved in the movement ofcells.

60 Part 1 Cell Biology 3-16

Figure 3.11 Centrioles.In a nondividing cell there is a pair of centrioles in a centrosome outside the nucleus. Just before a cell divides, the centrosome divides so thatthere are two pairs of centrioles. During cell division, the centrosomes separate so that each new cell has one pair of centrioles.

one pair of centrioles

two pairs of centrioles

one microtubuletriplet

200 nm

Chapter 3 Cell Structure and Function 613-17

Figure 3.12 Structure of a flagellum or cilium.A basal body derived from a centriole is at the base of a flagellum or cilium. The shaft of a flagellum (or cilium) contains microtubule doubletswhose side arms are motor molecules that cause the flagellum (such as those of sperm) to move. Without the ability of sperm to move to theegg, human reproduction would not be possible.

shaft

Flagellum

Dynein side arms

ATP

The shaft of the flagellum has a ring of nine microtubule doublets anchored to a central pair of microtubules. The side arms of each

doublet are composedof dynein, a motormolecule.

In the presence of ATP,the dynein side armsreach out to theirneighbours and bendingoccurs.

The basal body of a flagellumhas a ring of nine microtubule triplets with nocentral microtubules.

Flagellum cross section

Sperm

25 nmflagellum

plasmamembrane

Basal bodycross section

Basal body

triplets

100 nm

outermicrotubuledoublet

dyneinside armscentralmicrotubules

radialspoke

3.3 Prokaryotic CellsBacteria are prokaryotic cells in the kingdom Monera. Mostbacteria are 1–10 µm in size; therefore, they are just visiblewith the light microscope.

Figure 3.13 illustrates the main features of bacterialanatomy. The cell wall contains peptidoglycan, a complexmolecule with chains of a unique amino disaccharide joinedby peptide chains. In some bacteria, the cell wall is furthersurrounded by a capsule and/or gelatinous sheath called aslime layer. Motile bacteria usually have long, very thinappendages called flagella (sing., flagellum) that are com-posed of subunits of the protein called flagellin. The flagella,which rotate like propellers, rapidly move the bacterium ina fluid medium. Some bacteria also have fimbriae, which areshort appendages that help them attach to an appropriatesurface.

A membrane called the plasma membrane regulates themovement of molecules into and out of the cytoplasm, theinterior of the cell. Cytoplasm in a prokaryotic cell consists ofa semifluid medium, and thousands of granular inclusionscalled ribosomes that coordinate the synthesis of proteins. Inprokaryotes, most genes are found within a single chromo-some (loop of DNA, or deoxyribonucleic acid) located withinthe nucleoid, but they may also have small accessory rings of

DNA called plasmids. In addition, the photosyntheticcyanobacteria have light-sensitive pigments, usually withinthe membranes of flattened disks called thylakoids.

Although bacteria seem fairly simple, they are actuallymetabolically diverse. Bacteria are adapted to living inalmost any kind of environment and are diversified to theextent that almost any type of organic matter can be used asa nutrient for some particular bacterium. Given an energysource, most bacteria are able to synthesize any kind of mol-ecule they may need. Therefore, the cytoplasm is the site ofthousands of chemical reactions and bacteria are more meta-bolically competent than are human beings. Indeed, themetabolic capability of bacteria is exploited by humans whouse them to produce a wide variety of chemicals and prod-ucts for human use.

Bacteria are prokaryotic cells with theseconstant features.

Outer boundary: cell wallplasma membrane

Cytoplasm: ribosomesthylakoids (cyanobacteria)innumerable enzymes

Nucleoid: chromosome (DNA only)

62 Part 1 Cell Biology 3-18

25 µm250 nm

plasma membrane

plasma membrane

cell wall

cell wall

capsule

cytoplasm

flagellum

ribosome

slimelayer ribosome

nucleoid region

nucleoid region

cytoplasm

thylakoid

Figure 3.13 Prokaryotic cells.a. Nonphotosynthetic bacterium. b. Cyanobacterium, a photosynthetic bacterium, formerly called a blue-green alga.

a. b.

3.4 Evolution of the Eukaryotic CellInvagination of the plasma membrane might explain theorigin of the nuclear envelope and organelles such as theendoplasmic reticulum and the Golgi apparatus. Somebelieve that the other organelles could also have arisen inthis manner. But another hypothesis has been put forth. Ithas been observed that in the laboratory an amoeba in-fected with bacteria can become dependent upon them.Some investigators believe that mitochondria and chloro-plasts are derived from prokaryotes that were taken up bya much larger cell (Fig. 3.14). Perhaps mitochondria wereoriginally aerobic heterotrophic bacteria and chloroplastswere originally cyanobacteria. The host cell would havebenefited from an ability to utilize oxygen or synthesizeorganic food when by chance the prokaryote was notdestroyed. Therefore, after these prokaryotes entered byendocytosis, a symbiotic relationship was established. Someof the evidence for the endosymbiotic hypothesis is asfollows:

1. Mitochondria and chloroplasts are similar to bacteria insize and in structure.

2. Both organelles are bounded by a double membrane—the outer membrane may be derived from theengulfing vesicle, and the inner one may be derivedfrom the plasma membrane of the original prokaryote.

3. Mitochondria and chloroplasts contain a limitedamount of genetic material and divide by splitting.Their DNA (deoxyribonucleic acid) is a circular looplike that of bacteria.

4. Although most of the proteins within mitochondriaand chloroplasts are now produced by the eukaryotichost, they do have their own ribosomes and they doproduce some proteins. Their ribosomes resemble thoseof bacteria.

5. The RNA (ribonucleic acid) base sequence of theirribosomes suggests a eubacterial origin for chloroplastsand mitochondria.

It’s just possible also that the flagella of eukaryotes arederived from an elongated bacterium that became attachedto a host cell (Fig. 3.14). However, it is important to remem-ber that the flagella of eukaryotes are constructed different-ly. In any case, the acquisition of basal bodies, which couldhave become centrioles, may have led to the ability to forma spindle during cell division.

According to the endosymbiotic hypothesis,heterotrophic bacteria became mitochondria andcyanobacteria became chloroplasts after beingtaken up by precursors to modern-day eukaryoticcells.

Chapter 3 Cell Structure and Function 633-19

plasma membrane

endoplasmicreticulumnucleus

aerobicbacterium

spirochete

cell has a flagellum

cyanobacterium

cell has chloroplastscell has mitochondriacell has a nucleusand other organelles

Figure 3.14 Evolution of the eukaryotic cell.Invagination of the plasma membrane could account for the formation of the nucleus and certain other organelles. The endosymbiotic hypothesissuggests that mitochondria, chloroplasts, and flagella are derived from prokaryotes that were taken up by a much larger eukaryotic cell.

Summarizing the Concepts

3.1 The Cellular Level of OrganizationAll organisms are composed of cells, the smallest units of livingmatter. Cells are capable of self-reproduction, and existing cellscome only from preexisting cells. Cells are very small and are mea-sured in micrometers. The plasma membrane regulates exchangeof materials between the cell and the external environment. Cellsmust remain small in order to have an adequate amount of surfacearea per cell volume.

3.2 Eukaryotic Cells The nucleus of eukaryotic cells, represented by animal and plantcells, is bounded by a nuclear envelope containing pores. Thesepores serve as passageways between the cytoplasm and the nucle-oplasm. Within the nucleus, the chromatin undergoes coiling intochromosomes at the time of cell division. The nucleolus is a specialregion of the chromatin where rRNA is produced and where pro-teins from the cytoplasm gather to form ribosomal subunits. Thesesubunits are joined in the cytoplasm.

Ribosomes are organelles that function in protein synthesis.They can be bound to ER or exist within the cytoplasm singly or ingroups called polyribosomes.

The endomembrane system includes the ER (both rough andsmooth), the Golgi apparatus, the lysosomes, and other types ofvesicles and vacuoles. The endomembrane system serves to com-partmentalize the cell and keep the various biochemical reactionsseparate from one another. Newly produced proteins enter the ERlumen, where they may be modified before proceeding to the inte-rior of the smooth ER. The smooth ER has various metabolic func-tions depending on the cell type, but it also forms vesicles thatcarry proteins and lipids to different locations, particularly to theGolgi apparatus. The Golgi apparatus processes proteins andrepackages them into lysosomes, which carry out intracellulardigestion, or into vesicles that fuse with the plasma membrane.

Following fusion, secretion occurs. The endomembrane systemalso includes peroxisomes that have special enzymatic functions.Peroxisomes contain enzymes that oxidize molecules by producinghydrogen peroxide that is subsequently broken down. The largesingle plant cell vacuole not only stores substances but lends sup-port to the plant cell.

Cells require a constant input of energy to maintain theirstructure. Chloroplasts capture the energy of the sun and carry onphotosynthesis, which produces carbohydrate. Carbohydrate-derived products are broken down in mitochondria as ATP is pro-duced. This is an oxygen-requiring process called aerobicrespiration.

The cytoskeleton contains actin filaments, intermediate fila-ments, and microtubules. These maintain cell shape and allow itand the organelles to move. Actin filaments, the thinnest filaments,interact with the motor molecule myosin in muscle cells to bringabout contraction; in other cells, they pinch off daughter cells andhave other dynamic functions. Intermediate filaments support thenuclear envelope and the plasma membrane and probably partici-pate in cell-to-cell junctions. Microtubules radiate out from the cen-trosome and are present in centrioles, cilia, and flagella. They serveas tracks along which vesicles and other organelles move due tothe action of specific motor molecules.

3.3 Prokaryotic Cells There are two major groups of cells: prokaryotic and eukaryotic.Both types have a plasma membrane and cytoplasm. Eukaryoticcells also have a nucleus and various organelles. Prokaryotic cellshave a nucleoid that is not bounded by a nuclear envelope. Theyalso lack most of the other organelles that compartmentalizeeukaryotic cells.

3.4 Evolution of the Eukaryotic CellThe nuclear envelope most likely evolved through invagination ofthe plasma membrane, but mitochondria and chloroplasts mayhave arisen through endosymbiotic events.

64 Part 1 Cell Biology 3-20

As the cell theory tells us, cells can’t bemanufactured. Therefore, unlike medi-

cations such as certain hormones and vac-cines, which are now biotechnologyproducts, the only way to get human cellsis from previous human cells. No wonder,then, there is a market in blood cells whichare easily accessible. Viacord is in the busi-ness of storing umbilical blood for possi-ble future use by expectant parents. It onlycosts an initial outlay of $2,500 and a year-ly fee of $95.

The blood in a baby’s umbilical cord isrich in stem cells, which give rise to all theother types of blood cells within an adult’sred bone marrow. Should the parent’s orthe child’s red bone marrow be destroyedby disease or cancer treatment, these stem

cells will bring the red bone marrow backto life. Viacord says that stem-cell bankingis a kind of insurance, but there is no doubtit capitalizes on our fear of future illness.

The banking of umbilical cord blood isa new idea. Suppose you are an elderly can-cer patient, could you buy stem cells fromsomeone else? The U.S. National OrganTransplant Act (NOTA) bans interstate com-merce in the sale of transplant organs butexcludes tissues such as blood or sperm,which are replenishable. The answer, then,is most likely yes, especially if the parentsnow find themselves in need of cash.

Where would you draw the line in thetraffic of human parts? In 1984, John Mooresued his doctor for using, without his con-sent, tissue from his cancerous spleen to

create a commercial cell line now valued atabout $3 billion in profits. The CaliforniaSupreme Court ruled against him becauseit might deter the work of research scien-tists. Was that right and proper?

Questions1. Should there be any restrictions in the

buying and selling of transplant organs,blood, or blood products? Why or whynot?

2. Who should have the rights to tissues thatare used to start up commercial cell lines,the person who donated the cells or theinvestigator who established the line, orboth?

3. Who should get to use stored umbilicalblood? Only family members, or also apaying customer? After all, some parentshave several children.

Studying the Concepts

1. What are the two basic tenets of the cell theory? 462. Why is it advantageous for cells to be small? 483. Distinguish between the nucleolus, rRNA, and ribosomes.

52–534. Describe the structure and the function of the nuclear enve-

lope and the nuclear pores. 525. Trace the path of a protein from rough ER to the plasma

membrane. 536. Give the overall equations for photosynthesis and cellular

respiration, contrast the two, and tell how they are related.56

7. What are the three components of the cytoskeleton? What aretheir structures and functions? 58–59

8. What similar features do prokaryotic cells and eukaryoticcells have? What is their major difference? 62

9. Contrast the structure of chloroplasts and mitochondria. 6310. Describe the endosymbiotic hypothesis and the evidence to

support it. 63

Testing Yourself

Choose the best answer for each question.1. The small size of cells is best correlated with

a. the fact they are self-reproducing.b. their prokaryotic versus eukaryotic nature.c. an adequate surface area for exchange of materials.d. their vast versatility.e. All of these are correct.

2. Which of these is not found in the nucleus?a. functioning ribosomesb. chromatin that condenses to chromosomesc. nucleolus that produces rRNAd. nucleoplasm instead of cytoplasme. DNA making up the genes.

3. Vesicles from the smooth ER most likely are on their way tothea. rough ER.b. lysosomes.c. Golgi apparatus.d. plant cell vacuole only.e. cell wall of adjoining cells.

4. Lysosomes function in a. protein synthesis.b. processing and packaging.c. intracellular digestion.d. lipid synthesis.e. All of these are correct.

5. Mitochondriaa. are involved in cellular respiration.b. break down ATP to release energy for cells.c. contain grana and cristae.d. have a convoluted outer membrane. e. All of these are correct.

6. Which organelle releases oxygen?a. ribosomeb. Golgi apparatusc. mitochondriond. peroxisome

7. Which of these is not true?a. Actin filaments are found in muscle cells.b. Microtubules radiate out from the ER.c. Intermediate filaments sometimes contain keratin.d. Motor molecules use microtubules as tracks.e. Cilia and flagella are constructed similarly.

8. Cilia and flagellaa. bend when microtubules try to slide past one another.b. contain myosin that pulls on actin filaments.c. are organized by basal bodies derived from centrioles.d. are of the same length.e. Both a and c are correct.

9. Which organelle would not have originated by endosymbio-sis?a. mitochondriab. flagellac. nucleusd. chloroplastse. All of these are correct.

10. Which structure would be found in a prokaryotic cell?a. cell wall, ribosomes, thylakoids, chromosomeb. cell wall, plasma membrane, nucleus, flagellumc. nucleoid region, ribosomes, chloroplasts, capsuled. plasmid, ribosomes, enzymes, DNA, mitochondriae. chlorophyll, enzymes, Golgi apparatus, plasmids

11. Label these parts of the cell that are involved in protein syn-thesis and modification. Give a function for each structure.

Chapter 3 Cell Structure and Function 653-21

e.

d.b.a.

c.

66 Part 1 Cell Biology 3-22

Understanding the Terms

bacteria 62capsule 62cell 46cell theory 46cell wall 49centriole 60centrosome 58chloroplast 56chromatin 52chromosome 52cilium (pl., cilia) 60cristae 57cytoplasm 49cytoskeleton 58endoplasmic reticulum 53endosymbiotic hypothesis

63eukaryotic cell 49flagellum (pl., flagella) 60Golgi apparatus 54granum 57lysosome 55matrix 57

microtubule 58mitochondrion 56motor molecule 58nuclear envelope 52nuclear pore 52nucleoid 62nucleolus (pl., nucleoli) 52nucleoplasm 52nucleus 52organelle 49peroxisome 55plasma membrane 49plasmid 62polyribosome 53prokaryotic cell 62ribosome 53secretion 54slime layer 62stroma 57thylakoid 57vacuole 55vesicle 53

Match the terms to these definitions:a. Area in prokaryotic cell where DNA is found. b. Dark-staining, spherical body in the nucleus that

produces ribosomal subunits.c. Internal framework of the cell, consisting of

microtubules, actin filaments, and intermediate filaments.d. Organelle consisting of saccules and vesicles that

processes, packages, and distributes molecules about or fromthe cell.

e. System of membranous saccules and channels inthe cytoplasm, often with attached ribosomes.

Thinking Scientifically

1. To study movement of molecules in the cell:a. It is possible to bathe a cell in radioactively tagged amino

acids and then later to detect photographically the locationof radiation, and therefore the amino acids in the cell. Whywould you suggest using radioactive sulfur (i.e., the aminoacids cysteine and methionine contain sulfur) rather thanradioactive carbon?

b. An investigator uses the procedure outlined in question ato support the belief that proteins move from the cyto-plasm into the nucleus. If so, where will radiation firstappear, and where will it subsequently appear in the cell?(page 52)

c. An investigator uses this same procedure to support thebelief that proteins move from the rough ER to secretoryvesicles. Where will the radiation first appear, and wherewill it subsequently appear in the cell? (page 53)

2. A microtubular spindle apparatus appears in plant and ani-mal cells at the time of cell division.a. Why does this suggest that centrioles are not necessary to

the formation of the spindle apparatus?b. What evidence is there to suggest centrioles are necessary

to microtubule organization in animal cells?c. In animal cells, each newly formed cell receives a pair of

centrioles. Why might centrioles be necessary to animalcells but not to most plant cells? (page 60)

Using Technology

Your study of cell structure and function is supported by theseavailable technologies:

Essential Study Partner CD-ROMCells ££ Cell Structures

Visit the Mader web site for related ESP activities.

Exploring the InternetThe Mader Home Page provides resources and tools asyou study this chapter.

http://www.mhhe.com/biosci/genbio/mader

Dynamic Human 2.0 CD-ROMHuman Body ££ Clinical Concepts Cell Components

Life Science Animations 3D Video3 Cellular Secretion