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Cell Structure andFunction

C h a p t e r C o n c e p t s

3.1 the cellular level of Organization■ What does the cell theory state? 46

■ What instruments would a scientist use to study and view small cells? 46–48

3.2 Eukaryotic Cells■ What boundary is found in all cells? What additional boundary isfound in plant cells? 49

■ What do you call the small structures in eukaryotic cells that carry outspecific functions? 49

■ What is the function of the nucleus? 52

■ What membranous system within eukaryotic cells is involved in theproduction, modification, transportation, storage, secretion, and/ordigestion of macromolecules? 53–55

■ What energy transformation structures are present in plant and animalcells? What does each structure produce? 56–57

■ What is the composition of the cytoskeleton, and what is its function? 58–59

■ What structures are responsible for movement of the cell? 60–61

3.3 Prokaryotic Cells■ What is the major difference between prokaryotic and eukaryotic cells? 62

3.4 Evolution of the Eukaryotic Cell■ What hypothesis suggests how eukaryotic cells arose? 63

chapter3

A CHICKEN’s egg is largeenough to hold in yourhand while most cells take a microscope to

see them. A chicken’s egg is big because much of

it is stored food for growth of an embryo.

Cells are incredibly variable. Bacterial cell

looks simple compared to those within our

nervous system. However, the bacterial cell is

the entire organism; that senses the

environment, obtains food, gets rid of waste,

and reproduces. In contrast, a human being is

composed of approximately 10 trillion cells.

Each type of human cell is specialized to

perform specific functions, but it still has to do

many of the same things a bacterial cell is

required to do.

Cells have still other differences. Some

bacteria, such as thermophilic (heat-loving)

ones, live in boiling sulfur springs, while others

exist as parasites within the human body. Most

bacterial and fungal cells, acquire energy by

decomposing the dead remains of organisms,

but plant cells are able to get their energy from

the sun. Regardless of their differences, there are

certain structures that every cell must have and

certain functions it must perform. This chapter

discusses the contents of a generalized cell.

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3.1 The Cellular Level of Organization

The cell marks the boundary between the nonliving and theliving. The molecules that serve as food for a cell and themacromolecules that make up a cell are not alive, and yetthe cell is alive. Thus, the answer to what life is must liewithin the cell, because the smallest living organisms areunicellular, while larger organisms are multicellular—thatis, composed of many cells. The diversity of cells is exem-plified by the many types in the human body, such asmuscle cells and nerve cells. But despite variety of form andfunction, cells contain the same components. The basiccomponents that are common to all cells regardless of theirspecializations are the subject of this chapter. The ScienceFocus on these two pages introduces you to the micro-scopes most used today to study cells. Electron microscopyand biochemical analysis have revealed that a cell actuallycontains tiny specialized structures called organelles thatperform specific cellular functions.

Today, we are accustomed to thinking of living things asbeing constructed of cells. But the word cell didn’t enter biol-ogy until the seventeenth century. Antonie van Leeuwenhoekof Holland is now famous for making his own microscopes andobserving all sorts of tiny things that no one had seen before.Robert Hooke, an Englishman, confirmed Leeuwenhoek’sobservations and was the first to use the term cell. The tinychambers he observed in the honeycomb structure of corkreminded him of the rooms, or cells, in a monastery.

A hundred years later—in the 1830s—the Germanmicroscopist Matthias Schleiden said that plants are com-posed of cells; his counterpart, Theodor Schwann, saidthat animals are also made up of living units called cells.This was quite a feat, because aside from their own exhaust-ing work, both had to take into consideration the studiesof many other microscopists. Rudolf Virchow, anotherGerman microscopist, later came to the conclusion thatcells don’t suddenly appear; rather, they come from pre-existing cells.

Today, the cell theory, which states that all organ-isms are made up of basic living units called cells and thatcells come only from preexisting cells, is a basic theory ofbiology.

The cell theory states the following:■ All organisms are composed of one or more cells.■ Cells are the basic living unit of structure and

function in organisms.■ All cells come only from other cells.

46 Part I cell biology

hree types of microscopes are most commonly used to-day: the compound light microscope, transmission elec-tron microscope, and scanning electron microscope.

Figure 3A depicts these microscopes, along with a micrograph ofred blood cells viewed with each one.

In a compound light microscope, light rays passing through aspecimen are brought to a focus by a set of glass lenses, and theresulting image is then viewed by the human eye. In the transmis-sion electron microscope, electrons passing through a specimen arebrought to a focus by a set of magnetic lenses, and the resulting

T

science focusMicroscopy Today

FIGURE 3A Blood vessels and red bloodcells viewed with three different types ofmicroscopes.

t

eye

ocular lens

objective lensspecimencondenser

light source

Compound light microscope

Tissue was stained.

bloodvessel

red bloodcells

25 µm

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image is projected onto a fluorescent screen orphotographic film.

The magnification produced by an elec-tron microscope is much higher than that ofa light microscope (50,000X compared to1,000X). Also, the ability of the electron micro-scope to make out detail is much greater.The distance needed to distinguish twopoints as separate is much less for an elec-tron microscope than for a light microscope(10 nm compared to 200 nm1). The greater

resolving power of the electron microscopeis due to the fact that electrons travel at amuch shorter wavelength than do light rays.However, because electrons travel only in avacuum, the object is always dried out be-fore viewing, whereas even living objectscan be observed with a light microscope.

A scanning electron microscope providesa three-dimensional view of the surface of anobject. A narrow beam of electrons is scannedover the surface of the specimen, which has

been coated with a thin layer of metal. Themetal gives off secondary electrons, whichare collected to produce a television-type pic-ture of the specimen’s surface on a screen.

A picture obtained using a light micro-scope is sometimes called a photomicro-graph, and a picture resulting from the useof an electron microscope is called a trans-mission electron micrograph (TEM) or ascanning electron micrograph (SEM), depend-ing on the type of microscope used.

1nm � nanometer. See Appendix C, Metric System.

Chapter 3 cell structure and function 47

Micrograph was colored.

blood vessel

cellsred blood

10 µm

blood

red bloodcellselectron beam

objective lens

specimen

condenser

projector lens

observationorphotograph

ransmission electron microscope

vessel

T

14 µmMicrograph was colored.

electron beam

condensers

scanning coil

objectivelens

secondaryelectrons

specimen

detector

TVviewing screen

Scanning electron microscope

electron

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Cell SizeFigure 3.1 outlines the visual ranges of the eye, light micro-scope, and electron microscope. Cells are usually quite small.A frog’s egg, at about one millimeter (mm) in diameter, islarge enough to be seen by the human eye. But most cells arefar smaller than one millimeter; some are even as small asone micrometer (�m)—one-thousandth of a millimeter. Cellinclusions and macromolecules are even smaller than amicrometer and are measured in terms of nanometers (nm).

To understand why cells are so small and why we aremulticellular, consider the surface/volume ratio of cells.Nutrients enter a cell and wastes exit a cell at its surface;therefore, the amount of surface affects the ability to get ma-terial in and out of the cell. A large cell requires morenutrients and produces more wastes than a small cell. Inother words, the volume represents the needs of the cell. Yet,as cells get larger in volume, the proportionate amount ofsurface area actually decreases, as you can see by comparingthese two cells:

A small cube, 1 mm tall, has a volume of 1 mm3 (height �width � depth is 1 mm3). The surface area is 6 mm2. (Eachside has a surface area of 1 mm2, and 6 � 1 mm2 is 6 mm2).Therefore, the ratio of surface area to volume is 6:1 becausethe surface area is 6 mm2 and the volume is 1 mm3.

Contrast this with a larger cube that is 2 mm tall. Thesurface area is 24 mm2. (Each side has a surface area of 4 mm2,and 6 � 4 is 24 mm2). The volume of this larger cube is8 mm3; therefore, the ratio of surface area to volume of thelarger cube is 3:1. We can conclude then that a small cell hasa greater surface area to volume ratio than does a larger cell.

Therefore, small cells, not large cells, are likely to havean adequate surface area for exchanging wastes for nutrients.We would expect, then, a size limitation for an activelymetabolizing cell. You can hold a chicken’s egg in your hand,but the egg is not actively metabolizing. Once the egg is incu-bated and metabolic activity begins, the egg dividesrepeatedly without growth. Cell division restores the amountof surface area needed for adequate exchange of materials.Further, cells that specialize in absorption have modificationsthat greatly increase the surface area per volume of the cell.For example, the columnar cells along the surface of theintestinal wall have surface foldings called microvilli (sing.,microvillus), which increase their surface area.

A cell needs a surface area that can adequately ex-change materials with the environment. Surface-area-to-volume considerations require that cells stay small.


1 km100 m10 m1 m0.1 m1 cm1 mm100 µm10 µm1 µm100 nm10 nm1 nm0.1 nm

mouse

frogegg

humanegg

plant andanimalcells

most bacteria

virus

protein

amino acid

atom

antelectron microscope

light microscope

human eye

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 much detail.An electron microscope is needed to see organelles in detail and to make out viruses and molecules. Notice that in this illustration each measurementis 10 times greater than the lower one. (In the metric system, 1 meter � 102 cm � 103 mm � 106 �m � 109 nm—see Appendix C.)

small cell—more surface areaper volume

large cell—less surface areaper volume

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3.2 Eukaryotic Cells

Eukaryotic cells, one of the two major types of cells, have anucleus. A nucleus is a large structure that controls theworkings of the cell because it contains the genes. Both ani-mals and plants have eukaryotic cells.

Outer Boundaries of Animal and Plant CellsAnimal and plant cells are surrounded by a plasmamembrane, that consists of a phospholipid bilayer in whichprotein molecules are embedded:


proteinmolecules

phospholipidbilayer

The plasma membrane is a living boundary that separatesthe living contents of the cell from the nonliving surround-ing environment. Inside the cell, the nucleus is surroundedby the cytoplasm, a semifluid medium that containsorganelles. The plasma membrane regulates the entranceand exit of molecules into and out of the cytoplasm.

Plant cells (but not animal cells) have a permeable butprotective cell wall, in addition to a plasma membrane.Many plant cells have both a primary and secondary cellwall. A main constituent of a primary cell wall is cellulosemolecules. Cellulose molecules form fibrils that lie at rightangles to one another for added strength. The secondary cellwall, if present, forms inside the primary cell wall. Such sec-ondary cell walls contain lignin, a substance that makesthem even stronger than primary cell walls.

Organelles of Animal and Plant CellsOriginally the term organelle referred to only membranousstructures, but we will use it to include any well-definedsubcellular structure (Table 3.1). Just as all the assemblylines of a factory are in operation at the same time, so all theorganelles of a cell function simultaneously. Raw materialsenter a factory and then are turned into various products bydifferent departments. In the same way, chemicals are takenup by the cell and then processed by the organelles. The cellis a beehive of activity the entire 24 hours of every day.

Both animal cells (Fig. 3.2) and plant cells (Fig. 3.3) con-tain mitochondria, while only plant cells have chloroplasts.Only animal cells have centrioles. All the organelles have anassigned color that is used to represent them in the illustra-tions throughout the text.

TABLE 3.1 EUKARYOTIC STRUCTURES IN ANIMAL

CELLS AND PLANT CELLS

Name Composition Function

Cell wall* Contains cellulose Support and protectionfibrils

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

proteins passage into and out ofcells

Nucleus Nuclear envelope, Storage of geneticnucleoplasm, information; synthesis ofchromatin, and DNA and RNAnucleoli

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 distribution 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

Lysosome Membranous vesicle Intracellular digestioncontaining digestiveenzymes

Vacuole and Membranous sacs Storage of substancesvesicle

Peroxisome Membranous vesicle Various metabolic taskscontaining specificenzymes

Mitochondrion Inner membrane Cellular respiration(cristae) bounded byan outer 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 nm

endoplasmic reticulum

nucleolus

chromatin

nuclear envelope

plasma membrane

ribosomes(in cytoplasm)

chromatin

nucleolus

lysosome

microtubule(within cytoskeleton)

centriole

mitochondrion

nuclear pore

rough ER

smooth ER

vacuole

peroxisome

cytoplasm

Golgi apparatus

plasma membrane

vesicle

nuclear envelope

nucleus

polyribosome

actin filament(withincytoskeleton)

ribosome(attached to rough ER)

a.

b.

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 their functions.



chromatin

nucleolus

nuclear pore

nuclear envelope

nucleus

rough ER

smooth ER

central vacuole

cytoplasm

Golgi apparatus

plasma membrane

microtubule(within cytoskeleton)

mitochondrionactin filament(within cytoskeleton)

cell wall

ribosome(attached torough ER)

chloroplast

ribosomes(in cytoplasm)

cell wall of adjacent cell

1 µm

mitochondrion

central vacuole

chloroplast cell wall

plasma membrane

nucleus

ribosomes

peroxisome

FIGURE 3.3 Plant cell anatomy.a. Generalized drawing. b. Transmission electron micrograph of a 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 �m, is aprominent structure in the eukaryotic cell. The nucleus is ofprimary importance because it stores the genetic materialDNA which governs the characteristics of the cell and itsmetabolic functioning. Every cell in the same individualcontains the same DNA, but, in each cell type, certain genesare turned on and certain others are turned off. ActivatedDNA, with RNA acting as an intermediary, specifies the se-quence of amino acids when a protein is synthesized. Theproteins of a cell determine its structure and the functions itcan perform.

When you look at the nucleus, even in an electron micro-graph, you cannot see a DNA molecule. You can see chro-matin, which consists of DNA and associated proteins (Fig.3.4). Chromatin looks grainy, but actually it is a threadlike ma-terial that undergoes coiling to form rodlike structures, calledchromosomes, just before the cell divides. Chromatin is im-mersed in a semifluid medium called the nucleoplasm. A dif-ference in pH between the nucleoplasm and the 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 ribosomalRNA (rRNA), is produced and where rRNA joins with pro-teins to form the subunits of ribosomes. (Ribosomes are smallbodies in the cytoplasm that contain rRNA and proteins.)

The nucleus is separated from the cytoplasm by a dou-ble membrane known as the nuclear envelope, which iscontinuous with the endoplasmic reticulum discussed onthe next page. The nuclear envelope has nuclear pores ofsufficient size (100 nm) to permit the passage of proteins intothe nucleus and ribosomal subunits out of the nucleus.

The structural features of the nucleus include thefollowing.Chromatin: DNA and proteinsNucleolus: Chromatin and ribosomal

subunitsNuclear envelope: Double membrane with pores


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, where rRNA is produced and ribosomal subunits areassembled. The nuclear envelope, consisting of two membranes separated by a narrow space, contains pores. The electron micrographs show that thepores 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 at the ribosomes. Ribosomescan be found within the cytoplasm, either singly or ingroups called polyribosomes. Ribosomes can also be foundattached to the endoplasmic reticulum, a membranous sys-tem of saccules and channels discussed in the next section.Proteins synthesized at ribosomes attached to the endoplas-mic reticulum have a different fate. They are eventually se-creted from the cell or become a part of its external surface.

Ribosomes are small organelles where proteinsynthesis occurs. Ribosomes occur in the cytoplasm,both singly and in groups (i.e., polyribosomes).Numerous ribosomes are also attached to theendoplasmic reticulum.

The Endomembrane SystemThe endomembrane system consists of the nuclear enve-lope, the endoplasmic reticulum, the Golgi apparatus, andseveral vesicles (tiny membranous sacs). This system com-partmentalizes the cell so that particular enzymatic reac-tions are restricted to specific regions. Organelles that makeup the endomembrane system are connected either directlyor by transport vesicles.

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 interior,where processing and modification begin. Most proteins aremodified by the addition of a sugar chain, which makesthem a glycoprotein.

Smooth ER, which is continuous with rough ER, doesnot have attached ribosomes. Smooth ER synthesizes thephospholipids that occur in membranes and has variousother functions depending on the particular cell. In thetestes, it produces testosterone, and in the liver, it helpsdetoxify drugs. Regardless of any specialized function,smooth ER also forms vesicles in which proteins are trans-ported to the Golgi apparatus.

ER is involved in protein synthesis (rough ER) andvarious other processes, such as lipid synthesis(smooth ER). Vesicles transport proteins from theER to the Golgi apparatus.


a.

c.

b.

smoothER

ribosome

ribosome

polypeptidecarbohydrate chain

400 nm

rough ER

glycoprotein

transportvesicle

vesicleformation

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 at a ribosomemoves into the lumen of the system, is modified, and is eventuallypackaged in a transport vesicle for distribution to the Golgi apparatus.


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 pan-cakes (Fig. 3.6). In animal cells, one side of the stack (theinner face) is directed toward the ER, and the other side ofthe stack (the outer face) is directed toward the plasmamembrane. Vesicles can frequently be seen at the edges ofthe saccules.

The Golgi apparatus receives protein and also lipid-filled vesicles that bud from the smooth ER. These moleculesthen move through the Golgi from the inner face to the outerface. How this occurs is still being debated. According to thematuration saccule model, the vesicles fuse to form an innerface saccule, which matures as it gradually becomes a sac-cule at the outer face. According to the stationary sacculemodel, the molecules move through stable saccules from the

inner face to the outer face by shuttle vesicles. It is likely thatboth models apply, depending on the organism and the typeof cell.

During their passage through the Golgi apparatus,glycoproteins have their sugar chains modified before theyare repackaged in secretory vesicles. Secretory vesicles pro-ceed to the plasma membrane, where they discharge theircontents. Because this is secretion, the Golgi apparatus issaid to be involved in processing, packaging, and secretion.

The Golgi apparatus is also involved in the formationof lysosomes, vesicles that contain proteins and remainwithin the cell. How does the Golgi apparatus direct traffic—in other words, what makes it direct the flow of proteins todifferent destinations? It now seems that proteins made atthe rough ER have specific molecular tags that serve as “zipcodes” to tell the Golgi apparatus whether they belong in alysosome or in a secretary vesicle. The final sugar chainserves as a tag that directs proteins to their final destination.


incoming vesicle brings substances into the cell.

secretory vesiclesfuse with the plasma membrane as secretion occurs.

Golgi apparatusmodifies lipids and proteins; sorts and packages themin vesicles.

transport vesiclesfrom rough ER.

rough ERsynthesizes proteins and packages them in vesicles.

lysosomedigest molecules or oldcell parts.

transport vesicles from smooth ER.

smooth ERsynthesizes lipids and performs other functions.

FIGURE 3.6 Endomembrane system.The organelles in the endomembrane system work together to carry out the functions noted.


LysosomesLysosomes are membrane-bounded vesicles produced bythe Golgi apparatus. Lysosomes contain hydrolytic diges-tive enzymes.

Sometimes macromolecules are brought into a cell byvesicle formation at the plasma membrane (Fig. 3.6). When alysosome fuses with such a vesicle, its contents are digestedby lysosomal enzymes into simpler subunits that then enterthe cytoplasm. Some white blood cells defend the body byengulfing pathogens by vesicle formation. When lysosomesfuse with these vesicles, the bacteria are digested. Evenparts of a cell are digested by its own lysosomes (calledautodigestion). Normal cell rejuvenation takes place inthis manner.

Lysosomes contain many enzymes for digesting allsorts of molecules. The absence or malfunction of one ofthese results in a so-called lysosomal storage disease. Occa-sionally, a child inherits the inability to make a lysosomal en-zyme, and therefore has a lysosomal storage disease. Insteadof being degraded, the molecule accumulates inside lyso-somes, and illness develops when they swell and crowd theother organelles. In Tay Sachs disease, the cells that surroundnerve cells cannot break down a particular lipid, and thenervous system is affected. At about six months, the infantcan no longer see and, then, gradually loses hearing andeven the ability to move. Death follows at about three yearsof age.

The endomembrane system consists of theendoplasmic reticulum, Golgi apparatus,lysosomes, and transport vesicles.

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.

Vacuoles are larger than vesicles. Plants are wellknown for having a large central vacuole area forstorage of various molecules.

PeroxisomesPeroxisomes, similar to lysosomes, are membrane-boundedvesicles that enclose enzymes (Fig. 3.7). However, theenzymes in peroxisomes are synthesized by cytoplasmic ri-bosomes and transported into a peroxisome by carrier pro-teins. Typically, peroxisomes contain enzymes whose actionresults in hydrogen peroxide (H2O2):

RH2 � O2 → R � H2O2

R � remainder of moleculeHydrogen peroxide, a toxic molecule, is immediately brokendown to water and oxygen by another peroxisomal enzymecalled catalase.

The enzymes in a peroxisome depend on the functionof the cell. Peroxisomes are especially prevalent in cells thatare synthesizing and breaking down fats. In the liver, someperoxisomes break down fats and others produce bile saltsfrom cholesterol. In the movie Lorenzo’s Oil, the cells lackeda carrier protein to transport an enzyme into peroxisomes. Asa result, long chain fatty acids accumulate in his brain andLorenzo suffers from neurological damage.

Plant cells also have peroxisomes. In germinating seeds,they oxidize fatty acids into molecules that can be converted tosugars needed by the growing plant. In leaves, peroxisomescan carry out a reaction that is opposite to photosynthesis—the reaction uses up oxygen and releases carbon dioxide.

Typically, the enzymes in peroxisomes break downmolecules and as a result produce hydrogenperoxide molecules.


0.5 µm

FIGURE 3.7 Peroxisomes.Peroxisomes are vesicles that oxidize organic substances with aresulting buildup of hydrogen peroxide. Peroxisomes contain theenzyme catalase, which breaks down hydrogen peroxide (H2O2) towater and oxygen.


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, as shown in the following diagram:

Only plants, algae, and cyanobacteria are capable ofcarrying on photosynthesis in this manner:


ATP

carbohydrate(high chemical energy)

CO2 + H2O(low chemical energy)

solarenergy

chloroplast mitochondrion

usableenergyfor cells

solar energy + carbon dioxide + water carbohydrate + oxygen

Plants and algae have chloroplasts while cyanobacteriacarry on photosynthesis within independent thylakoids.Solar energy is the ultimate source of energy for cells becausenearly all organisms, either directly or indirectly, use thecarbohydrates produced by photosynthesizers as an energysource.

All organisms carry on cellular respiration, the processby which the chemical energy of carbohydrates is convertedto that of ATP (adenosine triphosphate). ATP is the commoncarrier of chemical energy in cells. All organisms, except bac-teria, complete the process of cellular respiration in mito-chondria. Cellular respiration can be represented by thisequation:

carbon dioxide + water + energycarbohydrate + oxygen

Here energy is in the form of ATP molecules. When a cellneeds energy, ATP supplies it. The energy of ATP is used forsynthetic reactions, active transport, and all energy-requiringprocesses in cells.

independentthylakoids

overlappingthylakoids

stroma

doublemembrane

granum

outer membraneinner membrane

500 nm

a. b.

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.


ChloroplastsPlant and algal cells contain chloroplasts, the organelles thatallow them to produce their own organic food. Chloroplastsare about 4–6 �m in diameter and 1–5 �m in length; theybelong to a group of organelles known as plastids. Amongthe plastids are also the amyloplasts, common in roots, whichstore starch, and the chromoplasts, common in leaves, whichcontain 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 (Fig. 3.8). Amembrane system within the stroma is organized into in-terconnected flattened sacs called thylakoids. In certain re-gions, the thylakoids are stacked up in structures calledgrana (sing., granum). There can be hundreds of granawithin a single chloroplast (Fig. 3.8). Chlorophyll, which islocated within the thylakoid membranes of grana, capturesthe solar energy needed to enable chloroplasts to producecarbohydrates. The stroma also contains DNA, ribosomes,and enzymes that synthesize carbohydrates from carbondioxide and water.

MitochondriaAll eukaryotic cells, including those of plants and algae,contain mitochondria. This means that plant cells containboth chloroplasts and mitochondria. Most mitochondria areusually 0.5–1.0 �m in diameter and 2–5 �m 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 that break down carbohydrateproducts, releasing energy to be used for ATP production.

The inner membrane of a mitochondrion invaginatesto form cristae. Cristae provide a much greater surface areato accommodate the protein complexes and other partici-pants that produce ATP.

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

Chloroplasts and mitochondria are membranousorganelles whose structures lend themselves to theenergy transfers that occur within them.


cristaedoublemembrane

outer membrane

inner membranematrix

200 nm

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 reveal the cristae.

a.

b.


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 microscopes,which can penetrate thicker specimens, showed that thecytoplasm is instead highly organized. It contains actin fila-ments, microtubules, and intermediate filaments. Thetechnique of immunofluorescence microscopy identifiedthe makeup of these protein fibers within the cytoskeletalnetwork (Fig. 3.10).

The name cytoskeleton is convenient in that it comparesthe cytoskeleton to the bones and muscles of an animal.Bones and muscles give an animal structure and producemovement. Similarly, the fibers of the cytoskeleton maintaincell shape and cause the cell and its organelles to move. Thecytoskeleton is dynamic; assembly occurs when monomersjoin a fiber and disassembly occurs when monomers leave afiber. Assembly and disassembly occur at rates that are meas-ured in seconds and minutes. The entire cytoskeletal net-work can even disappear and reappear at various times inthe life of a cell.

MicrotubulesMicrotubules are small, hollow cylinders about 25 nm indiameter and from 0.2 to 25 �m in length.

Microtubules are made of a globular protein calledtubulin. When microtubules assemble, tubulin moleculescome together as dimers, and the dimers arrange them-selves in rows. Microtubules have 13 rows of tubulindimers surrounding what appears in electron micrographsto be an empty central core. In many cells, microtubuleassembly is under the control of a microtubule organizingcenter, MTCO, called the centrosome. The centrosome liesnear the nucleus. Before a cell divides, the microtubulesassemble into a structure called a spindle that distributeschromosomes in an orderly manner. At the end of cell di-vision, the spindle disassembles, and the microtubules re-assemble once again into their former array.

When the cell is not dividing, microtubules helpmaintain the shape of the cell and act as tracks alongwhich organelles can move. Motor molecules are proteinsthat derive energy from ATP to propell themselves along aprotein filament or microtubule. Whereas, the motor mole-cule myosin is associated with actin filaments, the motor


intermediate filament

actin filament

plasmamembranemicrotubule

centrosome

FIGURE 3.10 The cytoskeleton.Diagram comparing the size relationship of microtubules, intermediatefilaments, and intermediate filaments. Microtubule construction iscontrolled by the centrosome.

ATP

vesicle

kinesin

kinesinreceptor

a. microtubules in cell

b. vesicles move along microtubules

FIGURE 3.11 Microtubules.a. Microtubules are visible in this cell due to a technique calledimmunofluorescence. b. Microtubules act as tracks along whichorganelles move. The motor molecule kinesin, bound to a vesicle, breaksdown ATP and uses the energy to move along the microtubule.


molecules kinesin and dynein move along microtubules.One type of kinesin is responsible for moving vesicles alongmicrotubules, including microtubules, including the transportvesicles of the endomembrane system. The vesicle is bondedto the kinesin, and then kinensin “walks” along the micro-tubule by attaching and reattaching itself further along themicrotubule. There are different types of kinesin proteins, eachspecialized to move one kind of vesicle or cellular organelle.One type of dynein molecule, called cytoplasmic dynein, isclosely related to the dynein found in flagella (Fig. 3.11).

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 by forming adense complex web just under the plasma membrane, towhich they are anchored by special proteins. Also, the as-sembly and disassembly of a network of actin filaments ly-ing beneath the plasma membrane accounts for the forma-tion of pseudopods, extensions that allow certain cells tomove in an amoeboid fashion.

Actin filaments are seen in the microvilli that projectfrom intestinal cells, and their presence most likely accountsfor the ability of microvilli to alternately shorten and extendinto the intestine. In plant cells, actin filaments apparentlyform the tracks along which chloroplasts circulate or streamin a particular direction.

How are actin filaments involved in the movement ofthe cell and its organelles? They interact with motor mole-cule called myosin. Myosin has both a head and a tail. In thepresence of ATP, the myosin head attaches, and then reat-taches to an actin filament at a more distant location (Fig. 3.12).In muscle cells, the tails of several muscle myosin moleculesare joined to form a thick filament. In nonmuscle cells, cyto-plasmic myosin tails are bound to membranes, but the headsstill interact with actin. During animal cell division, the twonew cells form when actin, in conjunction with myosin,pinches off the cells from one another.

Intermediate FilamentsIntermediate filaments (8–11 nm in diameter) are intermedi-ate in size between actin filaments and microtubules. Theyare ropelike assemblies of fibrous polypeptides (Fig. 3.13)that support the nuclear envelope and the plasma mem-brane. In the skin, intermediate filaments made of the proteinkeratin give great mechanical strength to skin cells. Recentwork has shown intermediate filaments to be highly dy-namic. They also are able to assemble and disassemble in thesame manner as actin filaments and microtubules.

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


actin filament

myosinmolecules

anchor head membrane

ATP ADP + P

a. Actin filaments in cell

b. Actin and myosin interact

FIGURE 3.12 Actin filaments.a. Actin filaments are visible in this cell due to a technique calledimmunofluorescence. b. In the presence of ATP, myosin, a motormolecule, attaches to an actin filament, pulls it, and then reattaches at adifferent location. This is the mechanism that allows muscle to contract.

fibroussubunits

a. Intermediate filaments (IF)in cell

b. IF anchor organelles

FIGURE 3.13 Intermediate filaments.a. Intermediates are visible in this cell due to a technique calledimmunofluorescence. b. Fibrous subunits account for the ropelikestructure of intermediate filaments.

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CentriolesCentrioles are short cylinders with a 9 � 0 pattern ofmicrotubule triplets—that is, a ring having nine sets oftriplets with none in the middle (Fig. 3.14). In animal cells, acentrosome contains two centrioles lying at right angles toeach other. The centrosome is the major microtubule organ-izing center for the cell, and centrioles may be involved inthe process of microtubule assembly and disassembly.

Before an animal cell divides, the centrioles replicate,and the members of each pair are again at right angles to oneanother (Fig. 3.14). Then, each pair becomes part of a sepa-rate centrosome. During cell division, the centrosomes moveapart and may function to organize the mitotic spindle.Plant cells have the equivalent of a centrosome, but it doesnot contain centrioles, suggesting that centrioles are notnecessary to the assembly of cytoplasmic microtubules.

Centrioles are believed to give rise to basal bodies thatdirect the organization of microtubules within cilia andflagella. In other words, a basal body does for a cilium (orflagellum) what the centrosome does for the cell.

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

Cilia and FlagellaCilia and flagella are hairlike projections that can move eitherin an undulating fashion, like a whip, or stiffly, like an oar.Cells that have these organelles are capable of movement. Forexample, unicellular paramecia move by means of cilia,whereas sperm cells move by means of flagella. The cells thatline our upper respiratory tract have cilia that sweep debristrapped within mucus back up into the throat, where it can beswallowed. This action helps keep the lungs clean.

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. Therefore, they have a 9 � 2 pattern ofmicrotubules. Cilia and flagella move when the microtubuledoublets slide past one another (Fig. 3.15).

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. The basalbody initiates polymerization of the nine outer doublets of acilium or flagellum

Cilia and flagella, which have a 9 � 2 pattern ofmicrotubules, enable some cells to move.


one pair of centrioles

two pairs of centrioles

one microtubuletriplet

200 nm

FIGURE 3.14 Centrioles.Left and top right. A nondividing cell contains a pair of centrioles in a centrosome outside the nucleus. Bottom, right. Just before a cell divides, thecentrosome divides so that there are two pairs of centrioles. During cell division, the centrosomes separate so that each new cell has one pair ofcentrioles.


61

visual focus-

plasmamembrane

Basal bodycross section

Basal body

triplets

100 nm

outermicrotubuledoublet

dyneinside armscentralmicrotubules

radialspokes

shaft

Flagellum

dynein side arm

ATP

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

The side arms of eachdoublet are composedof dynein, a motormolecule.

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

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

Flagellum cross section

Sperm

25 nmflagellum

FIGURE 3.15 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 microtubuledoublets whose side arms are motor molecules that cause the flagellum (such as those of sperm) to move. Without the ability of sperm tomove to the egg, human reproduction would not be possible.


3.3 Prokaryotic Cells

Prokaryotic cells, the other major type of cell, does nothave a nucleus as eukaryotic cells do. Archaea and bacteriaare both prokaryotes, cells so small they are just visiblewith the light microscope.

Figure 3.16 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.

The cytoplasm of prokaryotic cells like that of eukary-otic cells is bounded by a plasma membrane. Prokaryoteshave a single chromosome (loop of DNA) located within a re-gion called the nucleoid but it is not bounded by membrane.Many prokaryotes also have small accessory rings of DNAcalled plasmids. The cytoplasm has thousands of ribosomesfor the synthesis of proteins. In addition, the photosynthetic

cyanobacteria have light-sensitive pigments, usually withinthe membranes of flattened disks called thylakoids.

Although prokaryotes are structurally simple, they areactually metabolically complex and contain many differentkinds of enzymes. Prokaryotes are adapted to living inalmost any kind of environment and are diversified to theextent that almost any kind of organic matter can be used asa nutrient for some particular type. The cytoplasm is the siteof thousands of chemical reactions, and prokaryotes aremore metabolically competent than are human beings.Given adequate nutrients, most prokaryotes are able to syn-thesize any kind of molecule they may need. 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 these constantfeatures.Outer boundaries: Cell wall

Plasma membraneCytoplasm: Ribosomes

Thylakoids (cyanobacteria)Innumerable enzymes

Nucleoid: Chromosome (DNA only)


25 µm250 nm

plasma membrane

plasma membrane

cell wall

cell wall

capsule

cytoplasmflagellum

ribosomeslime layer

ribosome

nucleoid

nucleoid

cytoplasm

thylakoid

FIGURE 3.16 Bacterial cells.a. Nonphotosynthetic bacterium. b. Cyanobacterium, a photosynthetic bacterium, formerly called a blue-green alga.

a. b.


3.4 Evolution of the Eukaryotic Cell

How did the eukaryotic cell arise? Invagination of theplasma membrane might explain the origin of the nuclearenvelope and organelles, such as the endoplasmic reticu-lum and the Golgi apparatus. Some believe that the otherorganelles could also have arisen in this manner.

Another, more interesting, hypothesis has been putforth. It has been observed that, in the laboratory, an amoebainfected with bacteria can become dependent upon them.Some investigators believe that mitochondria and chloro-plasts are derived from prokaryotes that were taken up by amuch larger cell (Fig. 3.17). Perhaps mitochondria were orig-inally aerobic heterotrophic bacteria, and chloroplasts wereoriginally cyanobacteria. The host eukaryotic cell wouldhave benefited from an ability to utilize oxygen or synthesizeorganic food when, by chance, the prokaryote was taken upand not destroyed. In other words, after these prokaryotesentered by endocytosis, a symbiotic relationship would havebeen established. Some of the evidence for this endosymbiotichypothesis is as follows:

1. Mitochondria and chloroplasts are similar to bacteriain size and in structure.

2. Both organelles are bounded by a double membrane—the outer membrane may be derived from the engulfing

vesicle, and the inner one may be derived from theplasma 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 prokaryotes.

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 resemblethose of prokaryotes.

5. The RNA (ribonucleic acid) base sequence of theribosomes in chloroplasts and mitochondria alsosuggests a prokaryotic origin of these organelles.

It is also just possible that the flagella of eukaryotes arederived from an elongated bacterium that became attachedto a host cell (Fig. 3.17). However, it is important to remem-ber that the flagella of eukaryotes are constructed differ-ently. In any case, the acquisition of basal bodies, whichcould have become centrioles, may have led to the ability toform a spindle during cell division.

According to the endosymbiotic hypothesis,heterotrophic bacteria became mitochondria, andcyanobacteria became chloroplasts after being takenup by precursors to modern-day eukaryotic cells.


Cell has mitochondria.

plasma membrane

endoplasmicreticulum

nucleus

nuclearenvelope

aerobicbacterium

bacterium

Animal cell has a flagellum and mitochondria.

Plant cell has chloroplasts andmitochondria.

Cell has a nucleusand other organelles.

cyanobacterium

FIGURE 3.17 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.


64

bioethical focusUse of Stem Cells

u

tem cells are immature cells thatdevelop into mature, differentiatedcells that make up the adult body. For

example, the red bone marrow contains stemcells for all the many different types of bloodcells in the bloodstream. Embryonic cells arean even more suitable source of stem cells.The early embryo is simply a ball of cells, andeach of these cells has the potential to becomeany type of cell in the body—a muscle cell, anerve cell, or a pancreatic cell, for example.

The use of stem cells from aborted em-bryos or frozen embryos left over from fertil-ity procedures is controversial. Even thoughquadriplegics, like Christopher Reeve, andothers with serious illnesses may benefitfrom this research, so far the governmentwill not fund such research. One senatorsaid it reminds him of the rationalizationused by Nazis when they experimented on

S death camp inmates—“after all, they are go-ing to be killed anyway.”

Parkinson disease and Alzheimer diseaseare debilitating neurological disorders thatpeople fear. It is possible that one day thesedisorders could be cured by supplying the pa-tient with new nerve cells in a critical area ofthe brain. Suppose you had one of these disor-ders. Would you want to be denied a cure be-cause the government doesn’t support experi-mentation on human embryonic stem cells?

There are other possible sources of stemcells. It turns out that the adult body notonly has blood stem cells, it also has neuralstem cells in the brain. It has even beenpossible to coax blood stem cells and neu-ral stem cells to become some other types ofmature cells found in the body. A possiblesource of blood stem cells is a baby’s umbilicalcord, and it is now possible to store umbilical

blood for future use. Once researchers havethe know-how, it may eventually be possibleto use any type of stem cell to cure many of thedisorders afflicting human beings.

Decide Your Opinion1. Should researchers have access to embry-

onic stem cells? Any source or just certainsources? Which sources and why?

2. Should an individual have access tostem cells from just his own body?Also from a relative’s body? Also froma child’s umbilical cord? From embry-onic cells?

3. Should differentiated cells fromwhatever source eventually be availablefor sale to patients who need them?After all, you are now able to buyartificial parts, why not living parts?

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 new cells comeonly from preexisting cells. Cells are so small they are measured inmicrometers. Cells must remain small in order to have an adequateamount of surface area per cell volume for exchange of moleculeswith the environment.

3.2 Eukaryotic CellsThe nucleus of eukaryotic cells, which include animal and plant cells,is bounded by a nuclear envelope containing pores. These poresserve as passageways between the cytoplasm and the nucleoplasm.Within the nucleus, the chromatin is a complex of DNA and protein.In dividing cells, the DNA is found in discrete structures called chro-mosomes. The nucleolus is a special region of the chromatin whererRNA is produced and where proteins from the cytoplasm gather toform ribosomal subunits. These subunits are joined in the cytoplasm.

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

The endomembrane system includes the ER (both rough andsmooth), the Golgi apparatus, the lysosomes, and other types of vesi-cles and vacuoles. The endomembrane system serves to compartmen-talize the cell and keep the various biochemical reactions separatefrom one another. Newly produced proteins enter the ER lumen,where they may be modified before proceeding to the interior of thesmooth ER. The smooth ER has various metabolic functions depend-ing on the cell type, but it also forms vesicles that carry proteins andlipids to the Golgi apparatus. The Golgi apparatus processes proteinsand repackages them into lysosomes, which carry out intracellulardigestion, or into vesicles that fuse with the plasma membrane. Fol-lowing fusion, secretion occurs. Vacuoles are large storage sacs, and

vesicles are smaller ones. The large single plant cell vacuole not onlystores substances but also lends support to the plant cell.

Peroxisomes contain enzymes that were produced by cyto-plasmic ribosomes. These enzymes oxidize molecules by produc-ing hydrogen peroxide that is subsequently broken down.

Cells require a constant input of energy to maintain their struc-ture. Chloroplasts capture the energy of the sun and carry onphotosynthesis, which produces carbohydrates. Carbohydrate-derived products are broken down in mitochondria at the sametime as ATP is produced. This is an oxygen-requiring processcalled cellular respiration.

The cytoskeleton contains actin filaments, intermediate fila-ments, and microtubules. These maintain cell shape and allow thecell and its organelles to move. Microtubules radiate out from thecentrosome and are present in centrioles, cilia, and flagella. Theyserve as tracks along which vesicles and other organelles move dueto the action of specific motor molecules. Actin filaments, thethinnest filaments, interact with the motor molecule myosin inmuscle cells to bring about contraction; in other cells, they pinch offdaughter cells and have other dynamic functions. Intermediate fil-aments support the nuclear envelope and the plasma membraneand probably participate in cell-to-cell junctions.

3.3 Prokaryotic CellsProkaryotic cells do not have a nucleus. They do have a plasmamembrane and cytoplasm. Prokaryotic cells have a nucleoid that isnot bounded by a nuclear envelope. They also lack most of the otherorganelles that compartmentalize eukaryotic 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.

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Testing Yourself

Choose the best answer for each quest ion.1. Proteins produced at cytoplasmic ribosomes are

a. used in the cell.b. used by other cells.c. never used at all.d. None of the above is correct.

2. Which is associated with DNA?a. chromatinb. chromosomec. nucleusd. All of the above are correct.

3. Secondary cell walls are found in ____________ and contain________________.a. animals, ligandb. animals, cellulosec. plants, ligandd. plants, cellulose

4. Which is characteristic of prokaryotic cells?a. nucleusb. mitochondriac. nucleoidd. All of the above are correct.

5. Eukaryotic cells are associated with ____________, andprokaryotic cells are associated with ____________.a. chloroplasts, nucleusb. chloroplasts, chloroplastsc. thylakoids, chloroplastsd. chloroplasts, thylakoids

For questions 6–9, match the statements to the terms in the key.Key:a. chloroplastsb. amyloplastsc. vacuolesd. nucleus

6. All eukaryotic cells7. Photosynthetic site8. Stores starch9. May contain toxins

10. The Golgi apparatus can be found in ____________ cells.a. animalb. plantc. bacteriad. Both a and b are correct.

11. ___________ are (is) produced by the Golgi and contains____________.a. Lysosomes, DNAb. Mitochondria, DNAc. Lysosomes, enzymesd. Nucleus, DNA

12. Mitochondria and chloroplasts contain ____________ and areable to synthesize ___________.a. RNA , fatty acidsb. DNA, proteinsc. DNA, fatty acidsd. cholesterol, fatty acids

13. Which of these are involved in the movement of structuresinside a cell?a. actinb. microtubulesc. centriolesd. All of the above are correct.

14. Peroxisomes import enzymes from the ____________ and arenumerous in cells breaking down ______________.a. Golgi, glucoseb. Golgi, fatsc. cytoplasm, glucosed. cytoplasm, fats

15. Which of these could you see with a light microscope?a. atomb. amino acidc. proteinsd. None of the above is correct.

For questions 16–20, match the structure to the function in the key.Key:a. movement of cellb. processing of proteinsc. photosynthesisd. ribosome formatione. synthesizes phospholipids

16. Chloroplasts17. Flagella18. Golgi19. Smooth ER20. Nucleolus21. Which of these depicts a hypothesized evolutionary scenario?

a. cyanobacteria→mitochondriab. Golgi→mitochondriac. mitochondria→cyanobacteriad. cyanobacteria→chloroplast

22. Vacuoles are more common ina. animal cells.b. plant cells.c. Both a and b are correct.d. Neither a nor b is correct.

23. A “9 � 2” formation refers toa. cilia.b. flagella.c. centrioles.d. Both a and b are correct.



24. The “waste” products of photosynthesis area. carbohydrates and oxygen.b. carbon dioxide and water.c. oxygen and water.d. carbohydrates and water.

25. Label the structures in this prokaryotic cell.

31. Which of the following structures would be found in bothplant and animal cells?a. centriolesb. chloroplastsc. cell walld. mitochondriae. All of these are found in both types of cells.

Understanding the Terms


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

(ER) 53endosymbiotic

hypothesis 63eukaryotic cell 49flagellum (pl., flagella)

60, 62glycoprotein 53Golgi apparatus 54granum (pl., grana) 57lysosome 55

matrix 57microtubule 58mitochondrion 56motor molecule 58nuclear envelope 52nuclear pore 52nucleoid 62nucleolus (pl., nucleoli) 52nucleoplasm 52nucleus 52organelle 46peroxisome 55plasma membrane 49, 62plasmid 62polyribosome 53prokaryotic cell 62ribosome 53, 62secretion 54slime layer 62stroma 57thylakoid 57, 62vacuole 55vesicle 53

Match the terms to these def init ions:a.____________________ Unstructured semifluid substance

that fills the space between cells in connective tissuesor inside organelles.

b.____________________ Dark-staining, spherical body in thenucleus that produces ribosomal subunits.

c.____________________ Internal framework of the cell,consisting of microtubules, actin filaments, andintermediate filaments.

d.____________________ Organelle consisting of saccules andvesicles that processes, packages, and distributesmolecules about or from the cell.

e.____________________ System of membranous saccules andchannels in the cytoplasm, often with attachedribosomes.

d.c.

b.

e.

a.

26. The cell theory states:a. Cells form as organelles and molecules become groupedtogether in an organized manner.b. The normal functioning of an organism does not dependon its individual cells.c. The cell is the basic unit of life.d. Only eukaryotic organisms are made of cells.

27. The small size of cells is best correlated witha. the fact that 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 the above are correct.

28. 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 the above are correct.

29. Which of these is broken down during cellular respiration?a. carbon dioxideb. waterc. carbohydrated. oxygene. Both c and d are correct.

30. Which of the following is not one of the three components ofthe cytoskeleton?a. flagellab. actin filamentsc. microtubulesd. intermediate filaments


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