Chapter 7: A View of the Cell - Polson...

174 A VIEW OF THE CELL

A View of the Cell

What You’ll Learn■ You will distinguish eukary-

otic and prokaryotic cells.■ You will learn the structure

and function of the plasmamembrane.

■ You will relate the structureof cell parts to their functions.

Why It’s ImportantA knowledge of cell structureand function is essential to abasic understanding of life.

Go through the chapter,and note the figures thatdepict different types of cellstructures. For each figure,note the various compo-nents of the cell. As youread the text, write thecharacteristics of each celltype by its name.

To find out more about cells,visit the Glencoe Science Web site.science.glencoe.com

READING BIOLOGYREADING BIOLOGY

7

Cells are amazinglydiverse. Yet for all theirdiversity, cells such asthis nerve cell and theprotist Euglena sharemany common features.

ChapterChapter

Magnification: 17 270�

Magnification: 9310�

BIOLOGY

http://science.glencoe.com

Section

The History of the Cell Theory

Before microscopes were invented,people believed that diseases werecaused by curses and supernaturalspirits. They had no idea that organ-isms such as bacteria existed. As sci-entists began using microscopes, theyquickly realized they were entering anew world—one of microorganisms(my kroh OR guh nihz umz).Microscopes enabled scientists toview and study cells, the basic unitsof living organisms.

Development of light microscopesThe microscope van Leeuwenhoek

(LAY vun hook) used is considered asimple light microscope because it

contained one lens and used naturallight to view objects. Over the next200 years, scientists greatly improvedmicroscopes by grinding higher qual-ity lenses and creating the compoundlight microscope. Compound lightmicroscopes use a series of lenses tomagnify objects in steps. Thesemicroscopes can magnify objects up to1500 times. As the observations ofplants and animals viewed under amicroscope expanded, scientists beganto draw conclusions about the orga-nization of living matter. With themicroscope established as a valid sci-entific tool, scientists had to learn thesize relationship of magnified objectsto their true size. Look at Focus onMicroscopes to see what specimenslook like at different magnifications.

7.1 THE DISCOVERY OF CELLS 175

Have you ever used a magnifyingglass to examine somethingyou’d found? Hundreds of years

ago, scientists, too, were fascinated and motivated to study their environment.The first microscopes were not much different from hand-held magnifyingglasses, consisting of only one lens.Through a single-lens microscope, the Dutch scientist Anton vanLeeuwenhoek in the mid-1600s wasthe first person to record looking atwater under a microscope. He wasamazed to find it full of living things.

SECTION PREVIEW

ObjectivesRelate advances inmicroscope technologyto discoveries aboutcells and cell structure.Compare the operationof a compound lightmicroscope with that ofan electron microscope.Identify the main ideasof the cell theory.

Vocabularycellcompound light

microscopecell theoryelectron microscopeprokaryoteeukaryoteorganellenucleus

7.1 The Discovery of Cells

Magnificationreveals minutedetails.

OriginWORDWORD

microscopeFrom the Greekwords mikros,meaning “small”,and skopein, mean-ing to “look.” Amicroscope is usedto examine smallobjects.

The cell theoryRobert Hooke was an

English scientist who lived atthe same time as van

Leeuwenhoek. Hooke used acompound light microscope to

study cork, the dead cells of oakbark. In cork, Hooke observed smallgeometric shapes, like those shown inFigure 7.1. Hooke gave these box-shaped structures the name cellsbecause they reminded him of thesmall rooms monks lived in at amonastery. Cells are the basic build-ing blocks of all living things. Hookepublished his drawings and descrip-tions, which encouraged other scien-tists to search for cells in the materi-als they were studying.

Several scientists extended Hooke’sobservations and drew some impor-tant conclusions. In the 1830s, theGerman scientist Matthias Schleidenobserved a variety of plants and con-cluded that all plants are composedof cells. Another German scientist,Theodore Schwann, Figure 7.1,made similar observations on animals.The observations and conclusions ofthese scientists are summarized as thecell theory, one of the fundamentalideas of modern biology.

The cell theory is made up ofthree main ideas:1. All organisms are composed of one or

more cells. An organism may be asingle cell, such as the organismsvan Leeuwenhoek saw in water.Others, like most of the plants andanimals with which you are mostfamiliar, are multicellular, or madeup of many cells.

2. The cell is the basic unit of organiza-tion of organisms. Although organ-isms can become very large andcomplex, such as humans, dogs,and trees, the cell remains the sim-plest, most basic component of anyorganism.

3. All cells come from preexisting cells.Before the cell theory, no oneknew how cells were formed,where they came from, or whatdetermined the type of cell theybecame. The cell theory states thata cell divides to form two identicalcells.

Development of electron microscopes

The microscopes we have dis-cussed so far use a light source andcan magnify an object up to about1500 times its actual size. Althoughlight microscopes continue to bevaluable tools, scientists knewanother world existed within a cellthat they could not yet see. In the1940s a new type of microscope, theelectron microscope, was invented.This microscope uses a beam of elec-trons instead of natural light to mag-nify structures up to 500 000 timestheir actual size, allowing scientists tosee structures within a cell.

There are two basic types of elec-tron microscopes. Scientists com-monly use the scanning electronmicroscope (SEM) to scan the sur-faces of cells to learn their three-dimensional shape. The transmission



Figure 7.1Cork cells (top) wereobserved by RobertHooke using a crudecompound lightmicroscope that mag-nified structures only30 times. TheodoreSchwann (inset)made similar obser-vations in animals.

Measuring Objects Under aMicroscope Knowing the diameter of the circle of lightyou see when lookingthrough a microscope allowsyou to measure the size of objects that are beingviewed. For most micro-scopes, the diameter of thecircle of light is 1.5 mm, or1500 µm (micrometers), underlow power and 0.375 mm, or375 µm, under high power.

Refer to Practicing Scientific Methods in the Skill Handbook if you need help with SI units.

Procedure! Look at diagram A that shows an object viewed under

low power. Knowing the circle diameter to be 1500 µm,the estimated length of object (a) is 400 µm. What is theestimated length of object (b)?

@ Look at diagram B that shows an object viewed underhigh power. Knowing the circle diameter to be 375 µm,the estimated length of object (c) is 100 µm. What is theestimated length of object (d)?

# Prepare a wet mount of a strand of your hair. Yourteacher can help with this procedure. CAUTION: Use caution when handling microscopes and glass slides.Measure the width of your hair strand while viewing itunder low and then high power.

Analysis1. An object can be magnified 100, 200, or 1000 times when

viewed under a microscope. Does the object’s actual sizechange with each magnification? Explain.

2. Do your observations of the size of your hair strand underlow and high power support the answer to question 1? Ifnot, offer a possible explanation why.

MiniLab 7-1MiniLab 7-1electron microscope (TEM) allowsscientists to study the structures con-tained within a cell.

New types of microscopes and newtechniques are continually beingdesigned. The scanning tunnelingmicroscope (STM) uses the flow ofelectrons to investigate atoms on thesurface of a molecule. New tech-niques using the light microscopehave increased the information scien-tists can learn with this basic tool.Most of these new techniques seek toadd contrast to structures within thecells, such as adding dyes that stainsome parts of a cell, but not others.Try the Minilab on this page to prac-tice measuring objects under amicroscope.

Two Basic Cell TypesWith the invention of light micro-

scopes, scientists noticed that cellscould be divided into two broadgroups: those with internal, mem-brane-bound structures and thosewithout. Cells lacking internal mem-brane-bound structures are calledprokaryotic (proh KER ee oh tik) cells.The cells of most unicellular organ-isms such as bacteria do not havemembrane-bound structures and aretherefore called prokaryotes.

Cells of the basic second type,those containing membrane-boundstructures, are called eukaryotic (yewKER ee oh tik) cells. Most of the mul-ticellular plants and animals we knowhave cells containing membrane-bound structures and are thereforecalled eukaryotes. It is important tonote, however, that some eukaryotes,such as some algae and yeast, are unicellular organisms.

The membrane-bound structureswithin eukaryotic cells are calledorganelles. Each organelle has a spe-cific function for cell survival.

7.1 THE DISCOVERY OF CELLS 177


Human hair

Measuring in SI

AA

1500 µm 375 µm

a c

d

b

BB

The invention and developmentof the light microscope some300 years ago allowed scien-tists to see cells for the firsttime. Improvements havevastly increased the range of visibility of microscopes.Today researchers can usethese powerful tools to studycells at the molecular level.

MicroscopesTHIS HISTORIC MICROSCOPE—held by a modern researcher—was designed by Anton vanLeeuwenhoek (above). By 1700,

Dutch scientist Leeuwenhoekhad greatly improved the accuracy of microscopes.Grinding the lenses himself,Leeuwenhoek built some 240single-lens versions. He dis-covered—and described forthe first time—red blood cellsand bacteria, taken fromscrapings from his

teeth. By 1900, problems with lenses that

had once limited image qualityhad been overcome, and the com-pound microscope had evolvedessentially into its present form.

THIS EARLY COMPOUND MICROSCOPE, housed in a gold-embossedleather case, was designed by English scientist Robert Hookeabout 1665. Using it, he observed and made drawings of corkcells. Although the microscope has three lenses, they are ofpoor quality and Hooke could see little detail.

ANTON VAN LEEUWENHOEK

▼

FOCUSON

178

▼

HOOKE’S MICROSCOPE

SCANNING ELECTRON MICROSCOPE IMAGE OF A MOSQUITO

RESEARCHER USING A TEM

RED BLOOD CELLSUNDER A COMPOUNDLIGHT MICROSCOPE

RED BLOOD CELLS UNDER A TRANSMISSIONELECTRON MICROSCOPEMagnification: 40 000x

SCANNING ELECTRON MICROSCOPEAn SEM sweeps a beam of electronsover the surface of a specimen, such as red blood cells (above),causing electrons to be emittedfrom the specimen. SEMs producea realistic, three-dimensional picture—but only the surface of an object can be observed. AnSEM can magnify about 60 000times without losing clarity.

EXPANDING Your View

HOW IT WORKS The magnifyingpower of a microscope is deter-mined by multiplying the magnifi-cation of the eyepiece and theobjective lens.

A COMPOUND LIGHT MICROSCOPE(above) uses two or more glasslenses to magnify objects. Lightmicroscopes are used to look at living cells, such as red blood cells(top), small organisms, and pre-served cells. Compound lightmicroscopes can magnify up toabout 1500 times.

RED BLOOD CELLS UNDER ASCANNING ELECTRON MICROSCOPE

TRANSMISSION ELECTRON MICROSCOPE A TEM aims a beamof electrons through a specimen. Denser portions of anobject allow fewer electrons to pass through. Thesedenser areas appear darker in the image. Two-dimension-al TEM images are used to study details of cells such asthese red blood cells (above). A TEM can magnify hundreds of thousands of times.

Magnification: 800x

Magnification: 10 000x

Magnification: 50x

SCANNING TUNNELINGMICROSCOPE IMAGEOF A DNA FRAGMENTMagnification: 2 000 000x

SCANNING TUNNELINGMICROSCOPE TheSTM revolution-

ized microscopy inthe mid-1980s by

allowing scientists tosee atoms on an object’s surface. Avery fine metal probe is brought neara specimen. Electrons flow betweenthe tip of the probe and atoms onthe specimen’s surface. As the probefollows surface contours, such asthose on this DNA molecule (above),a computer creates a three-dimen-sional image. An STM can magnifyup to one hundred million times.

▼

1 THINKING CRITICALLY Can live specimens be examined with an electron microscope? Explain. Consider how the specimen must be prepared for viewing.

2 COMPARING AND CONTRASTING Compare the images seen with an SEM and with a TEM.

Robert Brown, a Scottish scientist,first observed a prominent structurein cells that Rudolf Virchow laterconcluded was the structure respon-sible for cell division. We now knowthis structure as the nucleus, thecentral membrane-bound organellethat manages cellular functions.

Compare the prokaryotic andeukaryotic cells in Figure 7.2. Sepa-ration of organelles into distinct com-partments benefits the eukaryotic cell.One benefit is that chemical reactionsthat would normally not occur in the same area of the cell can now be carried out at the same time.


Figure 7.2Bacteria and archaebacteriaare prokaryotes. All otherorganisms are eukaryotes.

OriginWORDWORD

organelleFrom the Greekwords organon,meaning “tool” or “implement” andella, meaning “small.”Organelles are small,membrane-boundstructures in cells.

Section AssessmentSection Assessment

Understanding Main Ideas1. Why was the development of microscopes

necessary for the study of cells?2. How does the cell theory describe the organiza-

tion of living organisms?3. Compare the light sources of light microscopes

and electron microscopes.4. How are prokaryotic and eukaryotic cells

different?

Thinking Critically5. Suppose you discovered a new type of fern.

Applying the cell theory, what can you say forcertain about this organism?

6. Care and Use of a Microscope Most com-pound light microscopes have four objectivelenses with magnifications of 4�, 10�, 40�, and 100�. What magnifications are available if the eyepiece magnifies 15 times? For morehelp, refer to Practicing Scientific Methods in the Skill Handbook.

SKILL REVIEWSKILL REVIEW

A Prokaryotic cell doesnot have internal organellessurrounded by a membrane.Most of a prokaryote’smetabolic functions takeplace in the cytoplasm.

AA

This eukaryotic cellfrom an animal has distinct membrane-boundorganelles that allow different parts of the cellto perform different functions.

BB

DECODE

Cell wall

Flagellum

Plasma membranePiliDNARibosomes

Capsule

Nucleus

Plasma membrane

Chromosomes

Organelles

Nucleolus

Section

7.2 THE PLASMA MEMBRANE 181

Maintaining a BalanceYou are comfortable in your house

largely because the thermostat main-tains the temperature within a lim-ited range regardless of what’s hap-pening outside. Similarly, all livingcells must maintain a balance regard-less of internal and external condi-tions. Survival depends on the cell’sability to maintain the proper condi-tions within itself.

Why cells must control materialsYour cells need nutrients such as

glucose, amino acids, and lipids tofunction. It is the job of the plasmamembrane, the boundary betweenthe cell and its environment, to allowa steady supply of these nutrients tocome into the cell no matter what the

external conditions are. However, toomuch of any of these nutrients orother substances, especially ions, canbe harmful to the cell. If levelsbecome too high, the plasma mem-brane removes the excess. Theplasma membrane also allows wasteand other products to leave the cell.This process of maintaining the cell’senvironment is called homeostasis.

How does the plasma membranemaintain homeostasis? One mecha-nism is selective permeability, a process in which the plasma membrane of a cell allows some mol-ecules into the cell while keeping oth-ers out. Thinking back to your home,a screen in a window can performselective permeability in a similar way.When you open the window, thescreen lets fresh air inside and keeps

Think of trudging home fromschool on a cold wintry day.When you finally arrive, you

enter a room where you are shelteredfrom the wind and surrounded by awarm, comfortable environment. Yourhouse is a controlled environment justlike the cells in your body. Similar tothe way the walls of your home act asa barrier against the elements, theplasma membrane provides a barrierbetween the internal components of acell and its external environment.

SECTION PREVIEW

ObjectivesExplain how a cell’splasma membrane functions.Relate the function ofthe plasma membraneto the fluid mosaicmodel.

Vocabularyplasma membranehomeostasisselective permeabilityphospholipidfluid mosaic modeltransport protein

7.2 The Plasma Membrane

Cells, like your entire body, require a constant environment.

OriginWORDWORD

permeableFrom the Latinwords per, meaning“through,” andmeare, meaning “toglide.” Materialsmove easily (glide)through permeablemembranes.

most insects out. Some molecules,such as water, freely enter the cellthrough the plasma membrane, asshown in Figure 7.3. Other particles,such as sodium and calcium ions,must be allowed into the cell only atcertain times, in certain amounts, andthrough certain channels. The plasmamembrane must be selective in allow-ing these ions to enter. Use theProblem-Solving Lab here to evaluatethe plasma membrane of a yeast cell.

Structure of the Plasma Membrane

Now that you understand the basicfunction of the plasma membrane,you can study its structure. Recallthat lipids are insoluble moleculesthat are the primary components ofcellular membranes. The plasmamembrane is composed of a phos-pholipid bilayer, which is two layersof phospholipid back-to-back.Phospholipids are lipids with aphosphate group attached to them.The lipids in a plasma membranehave a glycerol backbone, two fattyacid chains, and a phosphate group.


Is the cell membrane a selective barrier? Yeast cells are living and contain a plasma membrane. How is it possible toshow that living yeast plasma membranes are capable oflimiting what enters the cell? Conduct an experiment to findan answer.

AnalysisDiagram A shows the appearance of yeast cells in a solu-

tion of blue stain. Note their color as well as the color of thesurrounding stain.

Diagram B also shows yeast cells in a solution of bluestain. These cells, however, were boiled for 10 minutes beforebeing placed in the stain. Again, note the color of the yeastcells as well as the color of the surrounding stain.

Thinking Critically1. Explain how boiling affected the yeast cells.2. Hypothesize why the color of the cells differs under

different conditions. Be sure that your hypothesis takesthe role of the plasma membrane into consideration.

3. Are plasma membranes selective barriers? Explain.

Problem-Solving Lab 7-1Problem-Solving Lab 7-1 Recognizing Causeand Effect

Oxygen

Amino acids

Water

Wastes

Wastes

Sugars

Plasmamembrane

Carbon dioxide

Figure 7.3The selectively permeable plasmamembrane controlssubstances enteringand leaving a cell.

A cell must take in andkeep needed materials,such as glucose. A cellmust also remove wastesand keep harmful sub-stances from entering.

BB

Screens in your win-dows are selectivelypermeable becausethey allow air insideand keep bugs out.

AA

AA BB

Makeup of the phospholipid bilayer

The addition of the phosphategroup does more than change thename of the lipid. The phosphategroup is critical for the formation andfunction of the plasma membrane.Figure 7.4 illustrates phospholipidsand their place within the structureof the plasma membrane. The twofatty acid tails of the phospholipidsare nonpolar, whereas the head of thephosphate molecule is polar.

Water is a key component of livingorganisms, both inside and outsidethe cell. The polar phosphate groupallows the cell membrane to interact

7.2 THE PLASMA MEMBRANE 183

with its watery environment because,as you recall, water is also polar. Thefatty acid tails, on the other hand,avoid water. The two layers of phos-pholipid molecules make a sandwichwith the fatty acid tails forming theinterior of the membrane and thephospholipid heads facing the wateryenvironment outside the cell. Whenmany phospholipid molecules cometogether in this manner, a barrier iscreated that is water-soluble at itsouter surfaces and water-insoluble inthe middle. Water-soluble moleculeswill not easily move through themembrane because they are stoppedby this water-insoluble layer.

Phosphategroup

Phospholipidmolecule

Fattyacids

Glycerol backbone

Polarhead

Cholesterol

Membraneprotein

Filaments of cytoskeletonCytoplasm

Carbohydratechains

Membraneprotein

Figure 7.4The structure of a plasmamembrane is a bilayer of phos-pholipids with proteins on itssurface or embedded in themembrane. The phospholipidand protein molecules are freeto move sideways within themembrane.

This model of the plasma mem-brane is called the fluid mosaicmodel. It is fluid because the mem-brane is flexible. The phospholipidsmove within the membrane just aswater molecules move with the cur-rents in a lake. At the same time, pro-teins embedded in the membranealso move among the phospholipidslike boats with their decks abovewater and hulls below water. Theseproteins create a “mosaic,” or pat-tern, on the membrane surface.

Other components of the plasma membrane

Cholesterol, shown in Figure 7.5,is also found in the plasma mem-brane where it helps stabilize the

phospholipids. Cholesterol is a com-mon topic in health issues todaybecause high levels are associatedwith reduced blood flow in bloodvessels. Yet, for all the emphasis oncholesterol-free foods, it is importantto recognize that cholesterol plays a critical role in the stability of the plasma membrane. Cholesterolprevents the fatty acid chains of the phospholipids from stickingtogether.

You’ve learned that proteins arefound within the lipid membrane.Some proteins span the entire mem-brane, creating the selectively per-meable membrane that regulates which molecules enter and whichmolecules leave a cell. These proteinsare called transport proteins.Transport proteins allow neededsubstances or waste materials tomove through the plasma membrane.Other proteins and carbohydratesthat stick out from the cell surfacehelp cells identify each other. As youwill discover later, these characteris-tics are important in protecting yourcells from infection. Proteins at theinner surface of a plasma membraneplay an important role in attachingthe plasma membrane to the cell’sinternal support structure, giving thecell its flexibility.


Section AssessmentSection AssessmentUnderstanding Main Ideas1. How is the plasma membrane a bilayer

structure?2. Explain how selective permeability maintains

homeostasis within the cell.3. What are the components of the phospholipid

bilayer, and how are they organized to form theplasma membrane?

4. Why is the plasma membrane referred to as afluid mosaic?

Thinking Critically5. Suggest what might happen if cells grow

and reproduce in an environment where no cholesterol is available.

6. Recognizing Cause and Effect Consider thatplasma membranes allow materials to passthrough them. Explain how this property con-tributes to homeostasis. For more help, refer to Thinking Critically in the Skill Handbook.


OHOH

Cholesterolmolecule

Phospholipid molecules

Figure 7.5 Eukaryotic plasmamembranes can con-tain large amounts of cholesterol—up to one molecule forevery phospholipidmolecule.

Section

7.3 EUKARYOTIC CELL STRUCTURE 185

Cellular BoundariesWhen a group works together,

someone on the team decides whatresources are necessary for the pro-ject and provides these resources. Inthe cell, the plasma membrane,shown in Figure 7.6, performs thistask by acting as a selectively perme-

able membrane. The fluid mosaicmodel describes the plasma mem-brane as a flexible boundary of a cell.However, plant cells, fungi, mostbacteria, and some protists have anadditional boundary. The cell wall isa fairly rigid structure located outsidethe plasma membrane that providesadditional support and protection.

When you work on a groupproject, each person has hisor her own skills and tal-

ents that add a particular value tothe group’s work. In the same way,each component of a eukaryotic cellhas a specific job, and all of the partsof the cell work together to help the cell survive.

SECTION PREVIEW

ObjectivesUnderstand the structure and functionof the parts of a typicaleukaryotic cell.Explain the advantagesof highly folded membranes in cells.Compare and contrastthe structures of plantand animal cells.

Vocabularycell wallchromatinnucleolusribosomecytoplasmendoplasmic reticulumGolgi apparatusvacuolelysosomechloroplastplastidchlorophyllmitochondriacytoskeletonmicrotubulemicrofilamentciliaflagella

7.3 Eukaryotic Cell Structure

Cell structures, like this team of students, worktogether for a common purpose.


Figure 7.6 The plasma membrane ismade up of two layers,which you can distinguishin this photomicrograph.

Plasmamembrane

What organelle directs cell activity? Acetabularia, a typeof marine alga, grows as single, large cells 2 to 5 cm in height.The nuclei of these cells are in the “feet.” Different species ofthese algae have different kinds of caps, some petal-like andothers that look like umbrellas. If a cap is removed, it quicklygrows back. If both cap and foot are removed from the cell ofone species and a foot from another species is attached, anew cap will grow. This new cap will have a structure withcharacteristics of both species. If this new cap is removed, thecap that grows back will be like the cell that donated thenucleus.

The scientist who discovered these properties wasJoachim Hämmerling. He wondered why the first cap thatgrew had characteristics of both species, yet the second capwas clearly like that of the cell that donated the nucleus.

AnalysisLook at the diagram below and interpret the data to

explain the results.

Thinking CriticallyWhy is the final cap like that of the cell from which the

nucleus was taken? (HINT: Recall the function of the nucleus.)

Problem-Solving Lab 7-2Problem-Solving Lab 7-2 Interpret the DataThe cell wall

The cell wall forms an inflexiblebarrier that protects the cell andgives it support. Figure 7.7 shows aplant cell wall that is made up of acarbohydrate called cellulose. Thefibers of cellulose form a thick meshof fibers. This fibrous cell wall is veryporous and allows molecules to passthrough, but unlike the plasma mem-brane, it does not select which mole-cules can enter into the cell.

Nucleus and cell controlJust as every team needs a leader to

direct activity, so the cell needs aleader to give directions. The nucleusis the leader of the eukaryotic cellbecause it contains the directions tomake proteins. Every part of the celldepends on proteins to do its job, soby containing the blueprint to makeproteins, the nucleus controls theactivity of the organelles. Read theProblem-Solving Lab on this page andconsider how the Acetabularia nucleuscontrols the cell.

The master set of directions formaking proteins is contained inchromatin, which are strands of thegenetic material, DNA. When the

Nucleus Nucleus



Cell wall

Plasmamembrane

Figure 7.7 The cell wall is a firm structure thatprotects the cell and gives the cell itsshape. Plant cell walls are mademainly of cellulose.

cell divides, the chromatin condensesto form chromosomes. Within thenucleus is another organelle calledthe nucleolus that makes ribosomes.Ribosomes are the sites where thecell assembles enzymes and otherproteins according to the directionsof DNA. Unlike other organelles,ribosomes are not bound by a mem-brane within the cell. Look at somecells as described in the MiniLabshown here and try to identify thenucleus in cells of an onion.

For proteins to be made, ribosomesmust move out of the nucleus and intothe cytoplasm, and the blueprints con-tained in DNA must be copied andsent to the cytoplasm. Cytoplasm isthe clear, gelatinous fluid inside acell. As the ribosomes and the copiedDNA are transported to the cyto-plasm, they pass through the nuclearenvelope—a structure that separatesthe nucleus from the cytoplasm asshown in Figure 7.8. The nuclearenvelope is a double membrane madeup of two phospholipid bilayers con-taining small nuclear pores for sub-stances to pass through. Ribosomesand the DNA copy pass into the cyto-plasm through the nuclear envelope.

Cell Organelles Adding stains to cellular material helps you distinquish cell organelles.

ProcedureCAUTION: Be sure to wash hands before and after this experiment.! Prepare a water wet mount of onion skin. Do this by

using your finger nail to peel off the inside of a layer ofonion bulb. The layer must be almost transparent. Use the following diagram as a guide.

@ Make sure that the onion layer is lying flat on the glassslide and not folded.

# Observe the onion cells under low- and high-power mag-nification. Identify as many organelles as possible.

$ Repeat steps 1 through 3, only this time use an iodinestain instead of water.

Analysis1. What organelles were easily seen in the unstained onion

cells? Cells stained with iodine?2. How are stains useful for viewing cells?

MiniLab 7-2MiniLab 7-2

Figure 7.8The transmission electron photomicrograph shows the nucleus of a eukaryotic cell. Thelarge holes in the nuclear envelope are pores.


Experimenting

Nuclear pores

Cytoplasm

Chromatin

Nucleolus

Nuclear envelopeof two membranes

Nucleus


Assembly, Transport,and Storage

You have begun to follow the trailof protein production as directed bythe cell manager—the nucleus. Butwhat happens to the blueprints forproteins once they pass into the cytoplasm?

Structures for assembly andtransport of proteins

The cytoplasm suspends the cell’sorganelles. One particular organellein a eukaryotic cell, the endoplasmicreticulum (ER), is the site of cellularchemical reactions. Figure 7.9 showshow the ER is a series of highlyfolded membranes suspended in thecytoplasm. The ER is basically alarge workspace within the cell. Itsfolds are similar to the folds of anaccordion in that if you spread thefolds out it would take up tremen-dous space. But by pleating and fold-ing it up, the accordion fits its surfacearea into a compact unit. So by fold-ing the membrane over and overagain, a large amount of membrane isavailable to do work.

Ribosomes in the cytoplasm attachto areas on the endoplasmic reticulum,

called rough endoplasmic reticulum,where they carry out the function ofprotein synthesis. The ribosome’sonly job is to make proteins. Eachprotein made in the rough ER has aparticular function; it may becomethe protein that forms a part of theplasma membrane, the proteinreleased from the cell, or the proteintransported to other organelles.Ribosomes can also be found floatingfreely in the cytoplasm where theymake proteins that perform taskswithin the cytoplasm itself.

Areas of the ER that are not stud-ded with ribosomes are known assmooth endoplasmic reticulum. Thesmooth ER is involved in numerousbiochemical activities, including theproduction and storage of lipids.

After proteins are produced, theyare transferred to another organellecalled the Golgi apparatus (GAWL

jee). The Golgi apparatus as shownin Figure 7.10 is a flattened systemof tubular membranes that modifiesthe proteins. The Golgi apparatusand membrane-bound structurescalled vesicles sort the proteins intopackages to be sent to the appropri-ate destination, like mail being sortedat the post office.


Ribosomes

Endoplasmicreticulum

Figure 7.9The endoplasmic reticulum is a complexsystem of membranes in the cytoplasmof eukaryotic cells. The ER forms atransport system between the nucleus and the cytoplasm.


Vacuoles and storageNow let’s look at some of the

other members of the cell teamimportant to the cell’s functioning.Cells have membrane-bound spaces,called vacuoles, for temporary stor-age of materials. A vacuole, like thosein Figure 7.11, is a sac surroundedby a membrane. Vacuoles often storefood, enzymes, and other materialsneeded by a cell, and some vacuolesstore waste products. Notice the dif-ference between vacuoles in plantand animal cells.

Lysosomes and recyclingDid anyone ever ask you to take

out the trash? You probably didn’tconsider that action as part of a teameffort, but in a cell, it is. Lysosomesare organelles that contain digestiveenzymes. They digest excess or wornout organelles, food particles, andengulfed viruses or bacteria. Themembrane surrounding a lysosomeprevents the digestive enzymes insidefrom destroying the cell. Lysosomescan fuse with vacuoles and dispensetheir enzymes into the vacuole,

Figure 7.11Plant cells usually have one large vacuole (a); animalcells contain manysmaller vacuoles (b).


Magnification: 1810� Magnification: 21 840�

a b

VesiclesGolgiapparatus

Figure 7.10The Golgi apparatus, as viewed with a TEM, looks like a side view of a stack of pancakes. Also visibleare vesicles that are involved in protein packaging.

digesting its contents. For example,when an amoeba engulfs a foodmorsel and encloses it in a vacuole, alysosome fuses to the vacuole andreleases its enzymes, which helpsdigest the food. Sometimes, lyso-somes digest the cells that containthem. For example, when a tadpoledevelops into a frog, lysosomeswithin the cells of the tadpole’s tailcause its digestion. The moleculesthus released are used to build differ-ent cells, perhaps in the newlyformed legs of the adult frog.

Energy TransformersNow that you know about a num-

ber of the cell parts and have learnedwhat they do, it’s not difficult toimagine that each of these cell teammembers requires a lot of energy.Protein production, modification,transportation, digestion—all ofthese require energy. Two otherorganelles, chloroplasts and mito-chondria, provide that energy.

Chloroplasts and energyWhen you walk through a field or

pick a vegetable from the garden, youmay not think of the plants as energy

generators. In fact, that is exactly whatyou see. Located in the cells of greenplants and some protists, chloroplastsare the heart of the generator.Chloroplasts are cell organelles thatcapture light energy and producefood to store for a later time.

A chloroplast, like a nucleus, has adouble membrane. A diagram and aTEM photomicrograph of a chloro-plast with an outer membrane and afolded inner membrane system areshown in Figure 7.12. It is withinthese thylakoid membranes that theenergy from sunlight is trapped.These inner membranes are arrangedin stacks of membranous sacs calledgrana, which resemble stacks ofcoins. The fluid that surrounds thegrana membranes is called stroma.

The chloroplast belongs to agroup of plant organelles called plastids, which are used for storage.Some plastids store starches or lipids,whereas others contain pigments,molecules that give color. Plastids arenamed according to their color or thepigment they contain. Chloroplastscontain the green pigment chloro-phyll. Chlorophyll traps light energyand gives leaves and stems theirgreen color.


Figure 7.12Chloroplasts are usually disc shaped but have the ability to change shape and position in the cell as light intensity changes. The pigment chlorophyll is embedded in the inner series of thylakoid membranes.

OriginWORDWORD

chloroplastFrom the Greekwords chloros, meaning “green,”and platos, meaning“formed object.”Chloroplasts cap-ture light energyand produce foodfor plant cells.


Stroma

Thylakoid

Granum

Two membranes

Chloroplast


Mitochondria and energyThe food energy generated by

choloroplasts is stored until it is broken down and released by mito-chondria, shown in Figure 7.13.Mitochondria are membrane-boundorganelles in plant and animal cellsthat transform energy for the cell.This energy is then stored in othermolecules that allow the cellorganelles to use the energy easilyand quickly when it is needed.

A mitochondrion has an outermembrane and a highly folded innermembrane. As with chloroplasts, thefolds of the inner membrane providea large surface area that fits in a smallspace. Energy-storing molecules areproduced on the inner folds.Mitochondria occur in varying num-bers depending on the function ofthe cell. For example, liver cells mayhave up to 2500 mitochondria.

Although the process by whichenergy is produced and used in thecells is a technical concept to learn,the Literature Connection at the end ofthis chapter explains how cellularprocesses can also be inspiring. Lookat the Inside Story on the next page to compare plant and animal cells.Notice how similar they are.

Structures for Supportand Locomotion

Scientists once thought that cellorganelles just floated in a sea ofcytoplasm. More recently, cell biolo-gists have discovered that cells have asupport structure called thecytoskeleton within the cytoplasm.The cytoskeleton is composed of avariety of tiny rods and filaments thatform a framework for the cell, likethe skeleton that forms the frame-work for your body. However, unlikeyour bones, the cytoskeleton is a con-stantly changing structure.

Cellular supportThe cytoskeleton is a network of

thin, fibrous elements that acts as asort of scaffold to provide support fororganelles. It maintains cell shapesimilar to the way that poles maintainthe shape of a tent. The cytoskeletonis composed of microtubules andmicrofilaments that are associatedwith cell shape and assist organellesin moving from place to place withinthe cell. Microtubules are thin, hol-low cylinders made of protein.Microfilaments are thin, solid pro-tein fibers.

Figure 7.13Mitochondria are granular and rod shaped, with an inner membrane that forms long, narrow folds. This TEM shows a cross section of a mitochondrion.


OriginWORDWORD

cytoskeletonFrom the Latinword cyte, meaning“cell.” Thecytoskeleton pro-vides support andstructure for thecell.

Inner membrane

Outer membrane

Mitochondrion


Comparing Animal and Plant Cells

You can easily recognize that a person does not look like a flower and an ant does not resemble

a tree. But at the cellular level under a microscope, the cells that make up all of the different animals and plants of the world are very much alike.

Critical Thinking Why are animal and plant cells similar?

INSIDESSTORTORYY

INSIDE

Plant Cells Plantcells, in general, arelarger than animal cellsand are characterizedby a cell wall andchloroplasts. Plant cellsusually have one largevacuole.

22

Golgi apparatus

Chloroplast

Plasma membrane

Cell wall

Free ribosomes

Ribosomes

Vacuolewith cellsap

Cytoskeleton

Nucleolus

Nucleus

PlantCell

Mitochondrion

Cytoplasm


Lysosome

Animal Cells Thecentriole is the onlyorganelle unique toanimal cells. Animalcells typically havemany small vacuoles.

11

Moose eating water plants

Nucleus

Golgi apparatus

Mitochondrion

Lysosome

Plasmamembrane Centriole

Nucleolus

Cytoplasm


Freeribosomes

Ribosomes

Cilium

Vacuole

AnimalCell Cytoskeleton

Cilia and flagellaSome cell surfaces have cilia and

flagella, which are structures that aidin locomotion or feeding. Cilia andflagella are composed of pairs ofmicrotubules, with a central pairsurrounded by nine additional pairs,as shown in Figure 7.14. The entirestructure is enclosed by the plasmamembrane. The outer microtubuleshave a protein that allows a pair of microtubules to slide along an

adjacent pair. This causes the ciliumor flagellum to bend.

Cilia and flagella can be distin-guished by their structure and bythe nature of their action. Cilia areshort, numerous, hairlike projec-tions that move in a wavelikemotion. Flagella are longer projec-tions that move with a whiplikemotion. In unicellular organisms,cilia and flagella are the majormeans of locomotion.


Figure 7.14Many cells of animalsand protists are cov-ered with cilia or flagella.

Section AssessmentSection AssessmentUnderstanding Main Ideas1. What is the advantage of highly folded

membranes in a cell? Name an organelle thatuses this strategy.

2. What organelles would be especially numerousin a cell that produces large amounts of a protein product?

3. Why are digestive enzymes in a cell enclosed in a membrane-bound organelle?

4. Why might a cell need a cell wall in addition to a plasma membrane?

Thinking Critically5. How do your cells and the cells of other

organisms that are not green plants obtain foodenergy from the chloroplasts of green plants?

6. Observing and Inferring Some cells have largenumbers of mitochondria with many internalfolds. Other cells have few mitochondria and,therefore, fewer internal folds. What can youconclude about the functions of these two typesof cells? For more help, refer to Observing andInferring in the Skill Handbook.


In eukaryotic cells, both ciliaand flagella are composed ofmicrotubules arranged in a ring.

AA Cilia in the windpipe beat andpropel particles of dirt andmucus toward the mouth andnose where they are expelled.

BB The flagella of thisTrichonympha move the parasitic flagellate forwardwith their whiplike action.

CC

Magnification: 97 500� Magnification: 4610� Magnification: 340�

ProblemAre all cells alike in appearance

and size?

ObjectivesIn this BioLab, you will: ■ Observe, diagram, and measure

cells and their organelles.■ Hypothesize which cells are from

prokaryotes, eukaryotes, unicellularorganisms, and multicellularorganisms.

■ List the traits of plant and animalcells.

Materialsmicroscope dropperglass slide coverslipwater forcepsprepared slides of Bacillus subtilus,

frog blood, and Elodea

Safety PrecautionsAlways wear goggles in the lab.

Skill HandbookUse the Skill Handbook if you need

additional help with this lab.

PREPARATIONPREPARATION

INVESTIGATEINVESTIGATE


Observing and ComparingDifferent Cell Types

A re all cells alike in appearance, shape, and size? Do allcells have the same organelles present within their cell

boundaries? One way to answer these questions is to observe avariety of cells using a light microscope.

1. Copy the data table.2. Examine a prepared slide of

Bacillus subtilus using both low-

and high-power magnification.(NOTE: this slide has beenstained. Its natural color is clear.)

PROCEDUREPROCEDURE

Organelles observed

Prokaryote or eukaryote

From a multicellular or unicellular organism

Diagram (with size in micrometers, µm)

Bacillus subtilus Elodea Frog blood

Data Table

CAUTION: Use care whenhandling slides. Dispose of anybroken glass in a container provided by your teacher.

3. Look for and record the names ofany observed organelles. Hypoth-esize if these cells are prokaryotesor eukaryotes. Hypothesize ifthese cells are from a unicellularor multicellular organism. Recordyour findings in the table.

4. Diagram one cell as seen underhigh-power magnification.

5. While using high power, deter-mine the length and width in micrometers of this cell. Referto Practicing Scientific Methods inthe Skill Handbook for helpwith determining magnification.Record your measurements onthe diagram.

6. Prepare a wet mount of a singleleaf from Elodea using the diagramas a guide.

7. Observe cells under low and highpower magnification.

8. Repeat steps 3 through 5 forElodea.

9. Examine a prepared slide of frogblood. (NOTE: This slide hasbeen stained. Its naturalcolor is pink.)

10. Observe cellsunder low- andhigh-powermagnification.

11. Repeat steps 3through 5 forfrog bloodcells.

1. Observing and Inferring Whichcells were prokaryotes? Howwere you able to tell?

2. Observing and Inferring Whichcells were eukaryotes? How wereyou able to tell?

3. Predicting Which cell was froma plant, from an animal? Explainyour answer.

4. Measuring Are prokaryote oreukaryote cells larger? Give specific measurements to supportyour answer.

5. Defining OperationallyDescribe how plant and

animal cells are alike and howthey differ.

ANALYZE AND CONCLUDEANALYZE AND CONCLUDE

Going FurtherGoing Further

Application Prepare a wet mount of verythin slices of bamboo (saxophone reed).Observe under low and high power. Explainwhat structures you are looking at. Explainthe absence of all other organelles from this material.

To find out more about microscopy and cell

types, visit the Glencoe Science Web site.science.glencoe.com


Leopardfrog

BIOLOGY


You may think of yourself as a body made up ofparts. Arms, legs, skin, stomach, eyes, brain,

heart, lungs. Your mind controls the whole, andyou probably believe that you own all the parts

that make up your body. In actual fact, you are acommunity of living structures that work

together for growth and survival.

Your body is made up of eukaryoticcells with organelles that work

together for each cell’s survival. Organellesmay work closely together, such as a ribo-some and the endoplasmic reticulum, or theymay perform a unique function within the cell,such as the mitochondrion.

An organism is similar to a cell in that severalparts work together. Groups of cells worktogether as tissues. Several tissues form an organand many organs form an organ system. Forexample, in an organ system such as the digestivesystem, cells and tissues form an organ such as the stomach, but several organs such as theintestines, the pancreas, and the liver, are needed to completely digest and absorb the food you eat.

In the same way, you might also considerhow all the organisms in a community are inter-connected and how the whole planet Earth is a collection of interdependent ecosystems. Lewis Thomas pondered this thought.

“I have been trying to think of the earth as a kind oforganism, but it is no go. I cannot think of it thisway. It is too big, too complex, with too many work-ing parts lacking visible connections.… I wonderedabout this. If not like an organism, what is it like,what is it most like? Then, satisfactorily for thatmoment, it came to me: it is most like a single cell.”

Words are like organelles Just as a cell is agroup of organelles working together, so is aparagraph composed of words that together

After you have studied this chapter, write aparagraph using Dr. Thomas’s style to describehow the organelles of a cell work together forcell survival.

To find out more about the works of Dr. Lewis Thomas,

visit the Glencoe Science Web site.science.glencoe.com


ConnectionLiteratureLiterature

Connection

CONNECTION TO BIOLOGYCONNECTION TO BIOLOGY

by Lewis Thomas

convey thoughts and ideas. Despite all his technical knowledge, Dr. Thomas, a physicianand medical researcher, writes simply and engag-ingly about everything from the tiny universeinside a single cell to the possibility of visitorsfrom a distant planet.

Medicine, a young science Dr. Thomas grewup with the practice of medicine. As a boy, heaccompanied his father, a family physician, onhouse calls to patients. Years later, Dr. Thomasdescribed those days in his autobiography, TheYoungest Science. The title reflects his belief thatthe practice of medicine is “still very early on”and that some basic problems of disease are justnow yielding to exploration.

Earth “is most like a single cell.”


BIOLOGY


SUMMARYSUMMARY

Section 7.1

Section 7.2

Section 7.3

Main Ideas■ Microscopes enabled biologists to see cells and

develop the cell theory.■ The cell theory states that the cell is the basic

unit of organization, all organisms are made upof one or more cells, and all cells come frompreexisting cells.

■ Using electron microscopes, scientists can studycell structure in detail.

■ Cells are classified as prokaryotic or eukaryoticbased on whether or not they have membrane-bound organelles.

Vocabularycell (p. 175)cell theory (p. 176)compound light

microscope (p. 175)electron microscope

(p. 176)eukaryote (p. 177)nucleus (p. 180)organelle (p. 177)prokaryote (p. 177)

The Discoveryof Cells

Main Ideas■ Through selective permeability, the plasma

membrane controls what enters and leaves a cell.■ The fluid mosaic model

describes the plasma membrane as a phospholipidbilayer with embedded proteins.

Vocabularyfluid mosaic model

(p. 184)homeostasis (p. 181)phospholipid (p. 182)plasma membrane

(p. 181)selective permeability

(p. 181)transport proteins

(p. 184)

The PlasmaMembrane

Main Ideas■ Eukaryotic cells have a nucleus and organelles,

are enclosed by a plasma membrane, and some have a cell wall that provides support and protection.

■ Cells make proteins on ribosomesthat are often attached to thehighly folded endoplasmic reticu-lum. Cells store materials in theGolgi apparatus and vacuoles.

■ Mitochondria break down foodmolecules to release energy.Chloroplasts convert light energyinto chemical energy.

■ The cytoskeleton helps maintaincell shape, is involved in the move-ment of organelles and cells, andresists stress placed on cells.

Vocabularycell wall (p. 185)chlorophyll (p. 190)chloroplast (p. 190)chromatin (p. 186)cilia (p. 193)cytoplasm (p. 187)cytoskeleton (p. 191)endoplasmic reticulum

(p. 188)flagella (p. 193)Golgi apparatus (p. 188)lysosome (p. 189)microfilament (p. 191)microtubule (p. 191)mitochondria (p. 191)nucleolus (p. 187)plastid (p. 190)ribosome (p. 187)vacuole (p. 189)

Eukaryotic CellStructure

CHAPTER 7 ASSESSMENT 197

Chapter 7 AssessmentChapter 7 Assessment

Chapter 7 AssessmentChapter 7 Assessment

1. What type of cell would you examine to finda chloroplast?a. prokaryote c. plantb. animal d. fungus

2. Which of the following structures utilizes thesun’s energy to make carbohydrates?a. c.

b. d.

3. Which of the following pairs of terms isNOT related?a. nucleus—DNAb. chloroplasts—chlorophyllc. flagella—chromatind. cell wall—cellulose

4. Magnifications greater than 10 000 times canbe obtained when using ________.a. light microscopesb. metric rulersc. hand lensesd. electron microscopes

5. A bacterium is classified as a prokaryotebecause it ________.a. has ciliab. has no membrane-bound nucleusc. is a single celld. has no DNA

UNDERSTANDING MAIN IDEASUNDERSTANDING MAIN IDEAS6. Which of the following structures is NOT

found in both plant and animal cells?a. chloroplast c. ribosomesb. cytoskeleton d. mitochondria

7. Which component is NOT stored in plastids?a. lipids c. amino acidsb. pigments d. starches

8. Which is a main idea of the cell theory?a. All cells have a plasma membrane.b. All cells come from preexisting cells.c. All cells are microscopic.d. All cells are made of atoms.

9. Electron microscopes can view only deadcells because ________.a. only dead cells are dense enough to be seenb. a magnetic field is needed to focus the

electronsc. the fluorescent screen in the microscope

kills the cellsd. the specimen must be in a vacuum

10. Ribosomes ________.a. do not have a cell wallb. are not surrounded by a membranec. do not contain cytoplasmd. all of the above

11. ________ are membrane-bound spaces thatserve as temporary storage areas.

12. The small bumps shown in this photo-micrograph are the site of ________.

13. The photomicrograph in question 12 was probably taken using a ________ microscope.

14. The ________ maintains a chemical balancewithin a cell by regulating the materials thatenter and leave the cell.

15. Plants are able to grow tall because their cellshave rigid ________ that contain a strongnetwork of ________.

16. Smooth ER is different from rough ER inthat smooth ER has no ________.

198 CHAPTER 7 ASSESSMENT

TEST–TAKING TIPTEST–TAKING TIP

Maximize Your ScoreAsk how your test will be scored. In order to doyour best, you need to know if there is a penaltyfor guessing, and if so, how much of a penalty. If there is no random-guessing penalty at all, youshould always fill in an answer.

Chapter 00 AssessmentChapter 00 AssessmentChapter 7 AssessmentChapter 7 Assessment

CHAPTER 7 ASSESSMENT 199

17. Although prokaryotes lack ________, theystill contain DNA.

18. Microtubules and microfilaments, whichmake up the cell cytoskeleton, are composedof ________.

19. A plant cell has a green color due to the pres-ence of ________, a pigment that is embeddedin the ________ membranes of the ________.

20. Cilia and flagella are an arrangement of________ and allow the cell to ________.

21. Explain why packets of proteins collected bythe Golgi apparatus merge with lysosomes.

22. How does the structure of the plasma mem-brane allow materials to move across it inboth directions?

23. Making Predictions Predict whether youwould expect muscle or fat cells to containmore mitochondria and explain why.

24. Concept Mapping Complete the conceptmap using the following vocabulary terms:cytoplasm, mitochondria, Golgi apparatus,ribosomes, plasma membrane, nucleus.

THINKING CRITICALLYTHINKING CRITICALLY

APPLYING MAIN IDEASAPPLYING MAIN IDEAS

ASSESSING KNOWLEDGE & SKILLSASSESSING KNOWLEDGE & SKILLS

The diagram below shows the parts of a cell.

Interpreting Scientific Illustrations Usethe diagram to answer the following ques-tions.

1. The structure labeled C represents the________.a. plasma membrane b. nuclear membranec. endoplasmic reticulumd. nucleolus

2. The function of the circular structureson membrane C is to ________.a. synthesize celluloseb. transform energyc. synthesize proteinsd. capture the sun’s energy

3. The structure labeled B represents the________.a. lysosome c. nucleus b. Golgi apparatus d. vacuole

4. The type of cell shown is a ________cell.a. plant c. animalb. fungal d. prokaryotic

5. Sequencing Structures A, B, C, and Dare involved in making a product to bereleased to the outside of the cell. Whatis the sequence of the production of thisproduct?

DD

AB

C

AB

C

surrounds the

1.

which contains

2.

3.

4. 5.

6.

For additional review, use the assessmentoptions for this chapter found on the Biology: TheDynamics of Life Interactive CD-ROM and on theGlencoe Science Web site.science.glencoe.com

CD-ROM


Chapter 7: A View of the Cell - Polson...

Documents

Transcript of Chapter 7: A View of the Cell - Polson...