section 1 looking at cells -...

Table 1 Metric Units of Length and EquivalentsUnit Prefix Metric equivalent Real-life equivalent

Kilometer (km) Kilo- 1,000 m About two-thirds of a mile

Meter (m) 1 m (SI base unit) A little more than a yard

Centimeter (cm) Centi- 0.01 m About half the diameter of a Lincoln penny

Millimeter (mm) Milli- 0.001 m About the width of a pencil tip

Micrometer (µm) Micro- 0.000001 m About the length of an average bacterial cell

Nanometer (nm) Nano- 0.000000001 m About the length of a water molecule

Section 1 Looking at Cells

50 CHAPTER 3 Cell Structure

Cells Under the MicroscopeMost cells are too small to see with the naked eye; a typical humanbody cell is many times smaller than a grain of sand. Scientistsbecame aware of cells only after microscopes were invented, in the1600s. When the English scientist Robert Hooke used a crudemicroscope to observe a thin slice of cork in 1665, he saw “a lot oflittle boxes.” The boxes reminded him of the small rooms in whichmonks lived, so he called them cells. Hooke later observed cells inthe stems and roots of plants. Ten years later, the Dutch scientistAnton van Leeuwenhoek used a microscope to view water from apond, and he discovered many living creatures. He named them“animalcules,” or tiny animals. Today we know that they were notanimals but single-celled organisms.

Measuring Cell StructuresMeasurements taken by scientists are expressed in metric units.Scientists throughout the world use the metric system. The officialname of the metric system is the International System ofMeasurements, abbreviated as SI. SI is a decimal system, so all rela-tionships between SI units are based on powers of 10. For example,scientists measure the sizes of objects viewed under a microscopeusing the SI base unit for length, which is the meter. A meter, whichis about 3.28 ft (a little more than a yard), equals 100 centimeters(cm), or 1,000 millimeters (mm). A meter also equals 0.001 kilome-ter (km). Most SI units have a prefix that indicates the relationshipof that unit to a base unit. For example, the symbol “µ” stands forthe metric prefix micro. A micrometer (µm) is a unit of linear meas-urement equal to one-millionth of a meter, or one-thousandth of amillimeter. Table 1 summarizes the SI units used to measure length.

Objectives● Describe how scientists

measure the length ofobjects.

● Relate magnification andresolution in the use ofmicroscopes.

● Analyze how lightmicroscopes function.

● Compare light microscopes with electronmicroscopes.

● Describe the scanningtunneling microscope.

Key Terms

light microscopeelectron microscopemagnificationresolutionscanning tunnelingmicroscope

2A

Copyright © by Holt, Rinehart and Winston. All rights reserved.

Characteristics of MicroscopesSince Robert Hooke first observed cork cells, micro-scopes have unveiled the details of cell structure. Thesepowerful instruments provide biologists with insight intohow cells work—and ultimately how organisms functionBiologists use different microscopes depending on theorganisms they wish to study and the questions they wantto answer. Two common kinds of microscopes are lightmicroscopes and electron microscopes. In a

, light passes through one or more lenses toproduce an enlarged image of a specimen. An

forms an image of a specimen using a beamof electrons rather than light.

An image produced by a microscope, such as the oneshown in Figure 1, is called a micrograph. Many micrographs arelabeled with the kind of microscope that produced the image—suchas a light micrograph (LM), a transmission electron micrograph(TEM), or a scanning electron micrograph (SEM). Micrographsoften are labeled with the magnification value of the image.

is the quality of making an image appear larger thanits actual size. For example, a magnification value of 200� indicatesthat the object in the image appears 200 times larger than theobject’s actual size. is a measure of the clarity of animage. Both high magnification and good resolution are needed toview the details of extremely small objects clearly. As shown inFigure 2, electron microscopes have much higher magnifying andresolving powers than light microscopes.

Resolution

Magnification

microscopeelectron

microscopelight

SECTION 1 Looking at Cells 51

A red blood cell is about 5 timeslonger than a bacterial cell.

A Lincoln penny is about 2,000times longer than a red blood cell.

A human is about 100 timeslonger than a Lincoln penny.

20 cm2 µm 2 cm 2 m

Bacterium Blood cell Penny Hand Human

0.1 nm 1 nm 10 nm 100 nm 1 µm 10 µm 100 µm 1 mm 1 cm 10 cm 1 m 10 m

Electron microscopes

Sizes of objects

Light microscopes

Unaided eye

Bacter

ium

Blood c

ell

Penn

yHan

dHum

an

Sizes of Objects and Magnifying Power of Microscopes

10 µm

Magnification: 270�

Figure 1 Micrograph. Thislight micrograph (LM) showsan amoeba.

Figure 2 Magnifyingpower of microscopes. Thescale shows the size range ofobjects that can be viewedwith electron microscopes andlight microscopes.


www.scilinks.orgTopic: MicroscopesKeyword: HX4122

Types of MicroscopesDifferent types of microscopes have different qualities and uses.Microscopes vary in magnification and resolution capabilities,which affect the overall quality of the images they produce.Microscopes also have different limitations. For example, electronmicroscopes have high magnifying power, but they cannot be usedto view living cells. Light microscopes have lower magnifyingpower, but they can be used to view living cells.

Compound Light Microscope Light microscopes that use two lenses are called compound lightmicroscopes. In a typical compound light microscope, such as theone shown in Figure 3, a light bulb in the base shines light up throughthe specimen, which is mounted on a glass slide. The objective lens,closest to the specimen, collects the light, which then travels to theocular (AHK yoo luhr) lens, closest to the viewer’s eye. Both lensesmagnify the image. Thus, a microscope with a 40� objective lens anda 10� ocular lens produces a total magnification of 400�.

Why not add a third lens and magnify even more? This approachdoes not work because you cannot distinguish between two objects,or “resolve” them, when they are closer together than a few hun-dred nm. When the objects are this close, the light beams from thetwo objects start to overlap!


Magnification: 1,500�Ocular lens

Specimen

Stage

Focusknob

Light source

Objectivelens

In a compound light microscope, a specimen is mounted on a glass slide and is illuminated with a beam of light from below.

Figure 3 Compound light microscope

LM of sperm


Real LifeThe most powerful compound light microscopes have a totalmagnification of up to 2,000�, which is sufficient for viewingobjects as small as 0.5 µm in diameter. For you to see smallerobjects, the wavelength of the light beam must be shorter than thewavelength of visible light. Electron beams have a much shorterwavelength than that of visible light, so electron microscopes aremuch more powerful than light microscopes.

Electron MicroscopesElectron microscopes can magnify an image up to 200,000�, andthey can be used to study very small structures inside cells or on cellsurfaces. In electron microscopes, both the electron beam and thespecimen must be placed in a vacuum chamber so that the electronsin the beam will not bounce off gas molecules in the air. Because liv-ing cells cannot survive in a vacuum, they cannot be viewed usingelectron microscopes.

Transmission electron microscope In a transmission electron micro-scope, shown in Figure 4, the electron beam is directed at a very thinslice of a specimen stained with metal ions. Some structures in thespecimen become more heavily stained than others. The heavilystained parts of the specimen absorb electrons, while those that arelightly stained allow electrons to pass through. The electrons thatpass through the specimen strike a fluorescent screen, forming animage on the screen. A transmission electron micrograph (TEM),such as the one of sperm cells shown in Figure 4, can reveal a cell’sinternal structure in fine detail. TEM images are always in blackand white. However, with the help of computers, scientists often addartificial colors to make certain structures more visible.

SECTION 1 Looking at Cells 53

In a transmission electron microscope, electrons pass through a specimen,forming an image of the specimen on a fluorescent screen.

Figure 4 Transmission electron microscope

TEM of sperm

Magnification: 7,730�

Thirty movies could bestored on a disk the sizeof a penny. Using the scanning inter-ferometric aperturelessmicroscope (SIAM)researchers have viewedfeatures that are aboutfour atoms (1 nm) in diam-eter. The technology couldalso be used to codeinformation on storagedisks.Applying Information Would the SIAM likely be more useful in under-standing the overallstructure of the cellor the structureof biologicalcompounds in the cell? 2A


Scanning electron microscope In a scanning electron microscope,shown in Figure 5, the electron beam is focused on a specimencoated with a very thin layer of metal. The electrons that bounce offthe specimen form an image on a fluorescent screen. A scanningelectron micrograph (SEM) shows three-dimensional images of cellsurfaces, such as the image of sperm cells shown in Figure 5. Aswith the transmission electron microscope, images produced by thescanning electron microscope are black and white, but often theyare artificially colored.

Scanning Tunneling MicroscopeNew video and computer techniques are increasing the resolution andmagnification of microscopes. The uses a needle-like probe to measure differences in voltage caused by elec-trons that leak, or tunnel, from the surface of the object being viewed. Acomputer tracks the movement of the probe across the object, enablingobjects as small as individual atoms to be viewed. The computergenerates a three-dimensional image of the specimen’s surface. Thescanning tunneling microscope can be used to study living organisms.

scanning tunneling microscope


In a scanning electron micro-scope, electrons bounce off aspecimen, forming a three-dimensional image of thespecimen on a fluorescentscreen.

Figure 5 Scanningelectron microscope

SEM of sperm

Describe the relationship between a meter, amillimeter, and a micrometer.

Describe how magnification and resolution affect the appearance of objects viewed under a microscope.

Compare the magnifying power of a light micro-scope with the magnifying power of an electronmicroscope.

Critical Thinking Recognizing DifferencesExplain why electron microscopes cannot beused to view the structure of living cells.

Critical Thinking Comparing FunctionsAssume that for the purposes of your investiga-tion, you need detailed images of the internalstructure of a bacterium. What type of micro-scope would you select for that that task? Explain your answer.

The English scientist RobertHooke used a crude microscope to examine A individual atoms C single-celled organismsB electrons D cork cells

TAKS Test PrepTAKS Test Prep

Section 1 Review

2A

2A

2A

2A

2A

3F


The Cell TheoryIt took scientists more than 150 years to fully appreciate the discov-eries of Hooke and Leeuwenhoek. In 1838, the German botanistMattias Schleiden concluded that cells make up not only the stemsand roots but every part of a plant. A year later, the German zoolo-gist Theodor Schwann claimed that animals are also made of cells.In 1858, Rudolph Virchow, a German physician, determined thatcells come only from other cells. The observations of Schleiden,Schwann, and Virchow form the , which has three parts:

1. All living things are made of one or more cells.

2. Cells are the basic units of structure and function in organisms.

3. All cells arise from existing cells.

Cell SizeSmall cells function more efficiently than large cells. There areabout 100 trillion cells in the human body, most ranging from 5 µmto 20 µm in diameter. What is the advantage of having so many tinycells instead of fewer large ones? All substances that enter or leavea cell must cross that cell’s surface. If the cell’s surface area–to-volume ratio is too low, substances cannot enter and leave the cell innumbers large enough to meet the cell’s needs. Small cells canexchange substances more readily than large cells because smallobjects have a higher surface area–to-volume ratio than largerobjects, as shown in Table 2. As a result, substances do not need totravel as far to reach the center of a smaller cell.

cell theory

Cell Features

SECTION 2 Cell Features 55

Section 2

Objectives● List the three parts of the

cell theory.

● Determine why cells mustbe relatively small.

● Compare the structure ofprokaryotic cells with thatof eukaryotic cells.

● Describe the structure ofcell membranes.

Key Terms

cell theorycell membranecytoplasmcytoskeletonribosomeprokaryotecell wallflagellumeukaryotenucleusorganelleciliumphospholipidlipid bilayer

Side length Surface area VolumeSurface area/volume ratio

1 mm 6 mm2 1 mm3 6:1

2 mm 24 mm2 8 mm3 3:1

4 mm 96 mm2 64 mm3 3:2

1 mm

2 mm

4 mm

Table 2 Relationship Between Surface Area and Volume

4A 4B

4B

4B

4B


www.scilinks.orgTopic: Cell FeaturesKeyword: HX4034

Common Features of CellsCells share common structural features, including an outer bound-ary called the . The cell membrane encloses the celland separates the cell interior, called the (SITE oh plazuhm), from its surroundings. The cell membrane also regulateswhat enters and leaves a cell—including gases, nutrients, andwastes. Within the cytoplasm are many structures, often suspendedin a system of microscopic fibers called the . All cellshave ribosomes. (RIE buh sohmz) are the cellular struc-tures on which proteins are made. All cells also have DNA, whichprovides instructions for making proteins, regulates cellular activi-ties, and enables cells to reproduce. Some specialized cells such asred blood cells, however, later lose their DNA.

Ribosomescytoskeleton

cytoplasmcell membrane


Calculating Surface Area and Volume Background

You can improve your understanding of the relationshipbetween a cell’s surface area and its volume by practicingwith the large cube in Table 2.

<x + 6x - 7 - 02

8

493 0

52

Paramecium (SEM)

Magnification: 230�

1. Find the total surface area of the cube.

• side length �l� � 4 mm• surface area of one side � l � l � l2

• surface area of one side �l2� � 4 mm � 4 mm � 16 mm2

• total surface area � 6 � l2� 6 � 16 mm2

� 96 mm2

2. Calculate the volume of the cube.

• height (h) � l � 4 mm• volume � l2

� h � 16 mm2� 4 mm � 64 mm3

3. Determine the surface area–to-volume ratio. A ratio compares two num-bers by dividing one number by the other. A ratio can be expressed in three ways:

in words as a fraction with a colonx to y x_

y x:y

For the surface area–to-volume ratio, divide total surface area by volume.

�

Divide both numbers by their greatest common factor:

�32

�96 � 32��64 � 32�

9664

total surface areavolume

Analysis

1. Calculate the surface area–to-volume ratio of the cube with a side length of 2 mm in Table 2.

2. Calculate the surface area–to-volume ratio of the cubewith a side length of 1 mm in Table 2.

3. Critical ThinkingRelating Concepts Howdoes the flatness of thesingle-celled Parameciumshown above affect the cell’ssurface area–to-volume ratio?

2C


Prokaryotes The smallest and simplest cells are prokaryotes. A (prohKAIR ee oht) is a single-celled organism that lacks a nucleus and otherinternal compartments. Without separate compartments to isolatematerials, prokaryotic cells cannot carry out many specialized func-tions. Early prokaryotes lived at least 3.5 billion years ago. For nearly2 billion years, prokaryotes were the only organisms on Earth. Theywere very simple and small (1–2 µm in diameter). Like their ancestors,modern prokaryotes are also very small (1–15 µm). The familiarprokaryotes that cause infection and cause food to spoil belong toa subset of all prokaryotes that is commonly called bacteria.

Characteristics of ProkaryotesProkaryotes can exist in a broad range of environmental conditions.Many prokaryotes, including some bacteria that cause infection inhumans, grow and divide very rapidly. Some prokaryotes do not needoxygen to survive. Other prokaryotes cannot survive in the presenceof oxygen. Some prokaryotes can even make their own food.

The cytoplasm of a prokaryotic cell includes everything inside thecell membrane. As Figure 6 shows, a prokaryote’s enzymes and ribo-somes are free to move around in the cytoplasm because there are no internal structures that divide the cell into compartments. Inprokaryotes, the genetic material is a single,circular molecule of DNA. This loop of pro-karyotic DNA is often located near the centerof the cell, suspended within the cytoplasm.

Prokaryotic cells have a sur-rounding the cell membrane that providesstructure and support. The cells of fungi andplants also have cell walls; only animal cellsand some protists lack cell walls. Prokaryoteslack an internal supporting skeleton, so theydepend on a strong cell wall to give the cellshape. A prokaryotic cell wall is made ofstrands of polysaccharides connected byshort chains of amino acids. Some prokary-otic cell walls are surrounded by a structurecalled a capsule, which is also composed of polysaccharides. The capsule enablesprokaryotes to cling to almost anything,including teeth, skin, and food.

Many prokaryotes have (fluh JELuh), which are long, threadlike structuresthat protrude from the cell’s surface andenable movement. Prokaryotic flagellarotate, propelling the organism through itsenvironment at speeds of up to 20 celllengths per second. Figure 6 shows aprokaryote with several flagella.

flagella

cell wall

prokaryote


Magnification: 61,850�

Figure 6 Prokaryotes.Prokaryotic cells have littleinternal structure. Many alsohave a capsule and flagella.

Reading EffectivelyFor many words ending in -um, the plural is formed bychanging the -um to -a. Forexample, the plural of bac-terium is bacteria, and theplural of flagellum is flagella.



Mitochondrion

Microfilaments

Lysosome

Ribosomes

Golgiapparatus

Smooth ER

Rough ER

Cell membrane

Microtubules

Nuclear poreNuclear envelopeNucleusNucleolus

Figure 7 Animal cell. Likeall eukaryotic cells, animal cellscontain a cell membrane, anucleus, and other organelles.

Eukaryotic CellsThe first cells with internal compartments were primitive eukaryoticcells, which evolved about 1.5 billion years ago. A (yooKAIR ee oht) is an organism whose cells have a nucleus. The

(NOO klee uhs) is an internal compartment that houses thecell’s DNA. Other internal compartments, or organelles, enableeukaryotic cells to function in ways different from prokaryotes. An

is a structure that carries out specific activities in the cell. The major organelles in an animal cell are shown in Figure 7. The

cytoplasm includes everything inside the cell membrane but outsidethe nucleus. A complex system of internal membranes connects theorganelles within the cytoplasm. These membranes provide chan-nels that guide the distribution of substances within the cell. Themembranes also form envelopes called vesicles that move proteinsand other molecules from one organelle to another.

Many single-celled eukaryotes use flagella for movement. Shorthairlike structures called (SIL ee uh) protrude from the surfaceof some eukaryotic cells. Flagella or cilia propel some cells throughtheir environment. In other cells, cilia and flagella move substancesacross the cell’s surface. For example, cilia on cells of the human res-piratory system, shown in Figure 8, sweep mucus and other debrisout of the lungs.

A web of protein fibers, shown in Figure 9, makes up thecytoskeleton. The cytoskeleton holds the cell together and keeps thecell’s membranes from collapsing. The fluid surrounding the cyto-plasm’s organelles, internal membranes, and cytoskeleton fibers iscalled the cytosol.

cilia

organelle

nucleus

eukaryote

Figure 8 Cilia. Cilia on cells lining the respiratory system remove debris from air passages.


The CytoskeletonThe cytoskeleton provides the inte-rior framework of an animal cell,much as your skeleton provides theinterior framework of your body. Thecytoskeleton is composed of an intri-cate network of protein fibersanchored to the inside of the plasmamembrane. By linking one region toanother, they support the shape ofthe cell, much as steel beams anchorthe sides of a building to one another.Other fibers attach the nucleus andother organelles to fixed locations inthe cell. Because protein fibers aretoo small for a light microscope toreveal, biologists visualize thecytoskeleton by attaching fluorescent dyes to antibodies. An anti-body is an immune system protein specialized to bind to oneparticular kind of molecule—in this case—cytoskeleton proteins.When the cell is examined under fluorescent light, the fibers glowbecause of the fluorescent antibody attached to them.

There are three different kinds of cytoskeleton fibers: (1) long,slender microfilaments made of the protein actin, (2) hollow tubescalled microtubules made of the protein tubulin, and (3) thick ropesof protein called intermediate fibers.

Actin Fibers The actin fibers of the cytoskeleton form a networkjust beneath the cell surface that is anchored to membrane proteinsembedded within the cell membrane. By contracting or expanding,the actin fibers play a major role in determining the shape of ani-mal cells by pulling the plasma membrane in some places andpushing it out in others. If you examine the surface of a protist suchas the one shown in Figure 10, you will find it alive with motion.Tiny projections extend out from the surface like fingers. Each is atemporary projection of the plasma membrane that shoots out andthen retracts.

Microtubules Microtubules within the cytoskeleton act as a highwaysystem for the transportation of information from the nucleus todifferent parts of the cell. RNA molecules are transported alongmicrotubular “rails” that extend through the interior of the cell liketrain tracks. The RNA molecules, in complexes with proteins, areattached to so-called motor proteins that chug along microtubuleslike locomotives on tracks. The motor proteins drag the RNA-proteincomplexes along with them like freight cars.

Intermediate Fibers The intermediate fibers of the cytoskeletonprovide a frame on which ribosomes and enzymes can be confinedto particular regions of the cell. The cell can organize complexmetabolic activities efficiently by anchoring particular enzymesnear one another.


Microtubules

Nucleus

Endoplasmicreticulum

Mitochondrion

Ribosomes

Figure 9 The cytoskeleton.The cytoskeleton’s network ofprotein fibers anchors cellsorganelles and other compo-nents of the cytoplasm.

Figure 10 Cytoskeletalprojections. The multiplespikes on the surface of thismarine amoeba are projectionsof the cytoskeleton stretchingthe cell membrane outward.


The Cell MembraneThe cytoplasm of a cell is contained by its membrane. Cell mem-branes are not rigid like an eggshell. Rather, they are fluid like asoap bubble. The fluidity of cell membranes is caused by lipids,which form the foundation of membranes. The lipids form a barrierthat separates the inside of the cell from the outside of the cell. Thisbarrier allows only certain substances in the cell’s environment topass through. This selective permeability of the cell membranedetermines which substances enter and leave the cell.

The Cell Membrane as a BarrierThe selective permeability of the cell membrane is caused mainly bythe way phospholipids interact with water. A is a lipidmade of a phosphate group and two fatty acids. As shown in Figure 11, a phospholipid has both a polar “head” and two nonpolar“tails.” You may recall that the polar ends of water molecules willform weak bonds with other polar substances. The head of a phos-pholipid, which contains a phosphate group, is polar and isattracted to water. In contrast, the two fatty acids, or tails, are non-polar and therefore are repelled by water.

In a cell membrane, the phospholipids are arranged in a doublelayer called a , as shown in Figure 11. The nonpolar tailsof the phospholipids make up the interior of the lipid bilayer.Because water both inside and outside the cell repels the nonpolartails, they are forced to the inside of the lipid bilayer. Ions and mostpolar molecules, including sugars and some proteins, are repelledby the nonpolar interior of the lipid bilayer. The lipid bilayer allowslipids and substances that dissolve in lipids to pass through.

lipid bilayer

phospholipid


Real LifeDonated blood is frozenin a special processcalled cryopreservation. Similar methods are usedto preserve human eggs,embryos, and blood fromthe umbilical cord, a richsource of immune-systemcells. Finding Information Research how cryo-preservationmethods enablecells to with-stand freezing.

Polar

Nonpolar

Polar

Lipid bilayer

Polarhead

Non-polartails

The lipid bilayer is the foundation of the cell membrane.

The arrangement of phospholipids in the lipid bilayer makes the cell membrane selectively permeable.

A phospholipid’s “head” is polar, and its two fatty acid “tails” are nonpolar.

Cell membranes are made of a double layer of phospholipids, called a lipid bilayer.

Figure 11 Lipid bilayer


Carbohydrateportion

1. Cell-surface marker:Identifies cell type

Phospholipid heads

Proteinportion

Phospholipid tails

2. Receptor protein:Recognizes andbinds to substancesoutside the cell

3. Enzyme:Assists chemicalreactions insidethe cell

4. Transport protein:Helps substancesmove across cell membrane

Lipidbilayer

Outside of cell

Inside of cell

Membrane Proteins Various proteins are located in the lipid bilayer of a cell membrane.What keeps these proteins within the lipid bilayer? You may recallthat proteins are made of amino acids and that some amino acidsare polar, while others are nonpolar. The nonpolar part of a mem-brane protein is attracted to the interior of the lipid bilayer but isrepelled by the water on either side of the lipid bilayer. In contrast,the polar parts of the protein are attracted to the water on eitherside of the lipid bilayer. This dual attraction holds the protein in thelipid bilayer. However, the motion and fluidity of phospholipidsenable membrane proteins to move around within the lipid bilayer.

As shown in Figure 12, cell membranes contain different types ofproteins. Marker proteins attached to a carbohydrate on the cell’s sur-face advertise cell type—such as a liver cell or a heart cell. Receptorproteins bind specific substances, such as signal molecules, outsidethe cell. Enzymes embedded in the cell membrane are involved inimportant biochemical reactions in the cell. Transport proteins aid themovement of substances into and out of the cell.


The cell membrane contains various proteins with specialized functions.

Figure 12 Membrane proteins

Section 2 Review

Describe the importance of the surface-area-to-volume ratio of a cell. 4B

Compare the structure of a eukaryotic cell withthat of a prokaryotic cell. 4A

Critical Thinking Comparing FunctionsDescribe the functions of two types of cell-membrane proteins. 4A 9A

Analyze the three parts of the cell theory anddescribe two observations of early scientists thatsupport it. 3A 3F

A bacterium that lost itsflagella would be unable to 4B

A divide C maintain its shapeB move D make proteins


www.scilinks.orgTopic: Cell Biology

Research in TexasKeyword: HXX4006


Nuclearpores

Nuclear envelope

Nucleolus

Section 3 Cell Organelles


The Nucleus Most functions of a eukaryotic cell are controlled by the cell’snucleus. As shown in Figure 13, the nucleus is surrounded by adouble membrane called the nuclear envelope, also called thenuclear membrane. The nuclear envelope is made of two lipidbilayers that separate the nucleus from the cytoplasm.

Scattered over the surface of the nuclear envelope are manysmall channels through the envelope called nuclear pores.Substances that are made in the nucleus, including ribosomalproteins and RNA, move into the cytoplasm by passing throughthe nuclear pores. Ribosomes are partially assembled in a regionof the nucleus called the nucleolus, which is also shown in Figure13. Recall from Section 2 that ribosomes are the structures onwhich proteins are made.

The hereditary information of a eukaryotic cell is coded in thecell’s DNA, which is stored in the nucleus. Eukaryotic DNA iswound tightly around proteins. Most of the time, DNA exists aselongated and thin strands, which appear as a dark mass undermagnification. When a cell is about to divide, however, the DNAstrands wind up into a more compact form and appear as dense,rod-shaped structures called chromosomes. The number of chro-mosomes in a eukaryotic cell differs between species. Humanbody cells have 46 chromosomes, while the cells of garden peas have 14 chromosomes. You will learn more about DNA andchromosomes later in this book.

Objectives● Describe the role of

the nucleus in cell activities.

● Analyze the role of internalmembranes in proteinproduction.

● Summarize the importanceof mitochondria in eukaryoticcells.

● Identify three structures inplant cells that are absentfrom animal cells.

Key Terms

endoplasmic reticulumvesicleGolgi apparatuslysosomemitochondrionchloroplastcentral vacuole

The nucleus is surrounded by a double membrane called the nuclear envelope.

Figure 13 Nucleus

4A 4B

4A 4B

4A

4A 4B


Ribosomes and the Endoplasmic ReticulumUnlike prokaryotic cells, eukaryotic cells have a system of internalmembranes that play an essential role in the processing of proteins.Cells make proteins on ribosomes. Each ribosome is made of dozensof different proteins as well as RNA. Some of the ribosomes in aeukaryotic cell are suspended in the cytosol, as they are in prokary-otic cells. These “free” ribosomes make proteins that remain insidethe cell, such as proteins used to build new organelles.

Production of ProteinsProteins that are exported from the cell, such as some signalmolecules, are made on the ribosomes that lie on the surface of the endoplasmic reticulum, shown in Figure 14. The

(ehn doh PLAZ mihk rih TIHK yuh luhm), orER, is an extensive system of internal membranes that move pro-teins and other substances through the cell. Like the cell mem-brane, the membranes of the ER are made of a lipid bilayer withembedded proteins.

The part of the ER with attached ribosomes is called rough ERbecause it has a rough appearance when viewed in the electronmicroscope. The rough ER helps transport the proteins that aremade by its attached ribosomes. As each protein is made, it crossesthe ER membrane and enters the ER. The portion of the ER thatcontains the completed protein then pinches off to form a vesicle. A

is a small, membrane-bound sac that transports substancesin cells. Because certain proteins are enclosed inside vesicles, theseproteins are kept separate from proteins that are produced by freeribosomes in the cytoplasm.

The rest of the ER is called smooth ER because it lacks ribosomesand thus appears smooth when viewed in the electron microscope.The smooth ER performs various functions, such as making lipidsand breaking down toxic substances.

vesicle

endoplasmic reticulum

SECTION 3 Cell Organelles 63

www.scilinks.orgTopic: ProteinsKeyword: HX4151

Smooth ER

Ribosomes

Rough ER

The ER moves proteins and other substances within eukaryotic cells.

Figure 14 Endoplasmic reticulum


Packaging and Distribution of ProteinsVesicles that contain newly made proteins move through the cytoplasmfrom the ER to an organelle called the Golgi apparatus. The (GOHL jee) is a set of flattened, membrane-bound sacs thatserves as the packaging and distribution center of the cell. Enzymesinside the Golgi apparatus modify the proteins that are received in vesicles from the ER. The modified proteins are then enclosed in newvesicles that bud from the surface of the Golgi apparatus. Some ofthese vesicles include (LIE seh sohms), which are small,spherical organelles that contain the cell’s digestive enzymes. The ER,the Golgi apparatus, and lysosomes work together in the production,packaging, and distribution of proteins, as summarized in Figure 15.

Step Ribosomes make proteins on the rough ER. The proteinsare packaged into vesicles.

Step The vesicles transport the newly made proteins from therough ER to the Golgi apparatus.

Step In the Golgi apparatus, proteins are processed and thenpackaged into new vesicles.

Step Many of these vesicles move to the cell membrane andrelease their contents outside the cell.

Step Other vesicles, including lysosomes, remain within thecytoplasm. Lysosomes digest and recycle the cell’s usedcomponents by breaking down proteins, nucleic acids,lipids, and carbohydrates.

lysosomes

apparatusGolgi


BIOgraphic

Proteins are made by ribosomes on the rough ER.

Processing of Proteins

1

Vesicles carry proteins from the rough ER to the Golgi apparatus.

2Proteins are modified in the Golgi apparatus and enter new vesicles.

3

Some vesicles release their proteins outsidethe cell.

4

Other vesicles remain in the cell and become lysosomes.

5Nucleus

Proteins are processed by an internal system of membranes.

Figure 15

Interpreting GraphicsAs you read, use Steps 1–5in the text, shown in red, tohelp you follow the samenumbered steps shown inFigure 15.


MitochondriaNearly all eukaryotic cells contain many (miet uhKAHN dree uh), like the one shown in Figure 16. A mitochondrion isan organelle that harvests energy from organic compounds to makeATP, the main energy currency of cells. Although some ATP is madein the cytosol, most of a cell’s ATP is made inside mitochondria.Cells that have a high energy requirement, such as a muscle cell,may contain hundreds or thousands of mitochondria. Figure 16shows that a mitochondrion has two membranes. The outer mem-brane is smooth. The inner membrane is greatly folded, however,and its surface area is large. The two membranes form two com-partments, one inside and one outside the mitochondrion’s innermembrane. It is here that the chemical reactions that produce ATPduring cell metabolism take place.

Mitochondrial DNAThe nucleus is not the only organelle in the cell that contains nucleicacids. Mitochondria also have DNA and ribosomes, and mitochon-dria make some of their own proteins. However, most mitochondrialproteins are made by free ribosomes in the cytosol. MitochondrialDNA is independent of nuclear DNA and similar to the circular DNAof prokaryotic cells. This fact supports the widely accepted theorythat primitive prokaryotes are the ancestors of mitochondria. Youwill learn more about the origin of mitochondrial DNA later in this book.

mitochondria

SECTION 3 Cell Organelles 65

Inner membrane

Outer membrane

In a eukaryotic cell, mitochondria make most of the ATP.

Figure 16 Mitochondrion

The word mitochondrion isfrom the Greek wordsmitos, meaning “a thread”and chrondros, meaning“grain.” Knowing this makesit easier to remember that amitonchondrion is a small,elongated cell organelle.


Structures of Plant Cells The organelles described in this section are found in both animalcells and plant cells. However, plant cells have three additionalstructures that are not found in animal cells, shown in Figure 17.

Unique Features of Plant CellsCell wall The cell membrane of a plant cell is surrounded by a thickcell wall, composed of proteins and carbohydrates, including thepolysaccharide cellulose. The cell wall helps support and maintain theshape of the cell, protects the cell from damage, and connects it withadjacent cells.

Chloroplasts Plant cells contain one ormore . Chloroplasts areorganelles that use light energy to makecarbohydrates from carbon dioxide andwater. Chloroplasts are found not only inplants but also in a wide variety of eukary-otic algae, such as seaweed. Chloroplasts,along with mitochondria, supply much ofthe energy needed to power the activitiesof plant cells. Like mitochondria, chloro-plasts are surrounded by two membranes,contain their own DNA, and are thoughtto be the descendents of ancient prokary-otic cells.

Central vacuole As shown in Figure 17,much of a plant cell’s volume is taken upby a large, membrane-bound space calledthe (VAK yoo ohl). Thecentral vacuole stores water and maycontain many substances, including ions,nutrients, and wastes. When the centralvacuole is full, it makes the cell rigid. Thisrigidity enables a plant to stand upright.

central vacuole

chloroplasts


Describe the role of the nucleus in cellactivities. 4B

Sequence the course of newly made proteinsfrom the rough ER to the outside of the cell. 4B

Describe the role of mitochondria in the metab-olism of eukaryotic cells. 4B

Explain how a plant cell’s central vacuole andcell wall help make the cell rigid. 4A

Critical Thinking Inferring RelationshipsWhat is the importance of a cell enclosing itsdigestive enzymes inside lysosomes? 4A

Which organelle serves as the packaging and distribution center of aeukaryotic cell? 4A 4B

A nucleus C mitochondrionB lysosome D Golgi apparatus


Section 3 Review

Chloroplast

Central vacuole

Cell wall

Figure 17 Plant cell. Plant cells have a cellwall, chloroplasts, and a large central vacuole(shown in blue).


CHAPTER 3 Highlights 67

Key Concepts

Study CHAPTER HIGHLIGHTS

ZONEKey Terms

Section 1light microscope (51)electron microscope (51)magnification (51)resolution (51)scanning tunneling

microscope (54)

Section 2cell theory (55)cell membrane (56)cytoplasm (56)cytoskeleton (56)ribosome (56)prokaryote (57)cell wall (57)flagellum (57)eukaryote (58)nucleus (58)organelle (58)cilium (58)phospholipid (60)lipid bilayer (60)

Section 3endoplasmic reticulum (63)vesicle (63)Golgi apparatus (64)lysosome (64)mitochondrion (65)chloroplast (66)central vacuole (66)

BIOLOGYBIOLOGYUnit 1—Use this unit to review the keyconcepts and terms in this chapter.

Looking at Cells

● Microscopes enable biologists to examine the details of cellstructure and to understand how organisms function.

● Scientists use the metric system to measure the size of objects.● Light microscopes have a low magnification and can be used

to examine living cells.● Electron microscopes have a high magnification but cannot

be used to examine living cells.● The scanning tunneling microscope uses a computer to

generate a three-dimensional image of an object.

Cell Features

● The cell theory has three parts.● Small cells function more efficiently than large cells because

small cells have a higher surface-area-to-volume ratio thanlarge cells.

● All cells have a cell membrane, cytoplasm, ribosomes, and DNA.● Prokaryotic cells lack internal compartments.● Eukaryotic cells have a nucleus and other organelles, as well

as a cytoskeleton of microscopic protein fibers.● The lipid bilayer of a cell membrane is made of a double

layer of phospholipid molecules.● Proteins in cell membranes include enzymes, receptor

proteins, transport proteins, and cell-surface markers.

Cell Organelles

● The nucleus of a eukaryotic cell directs the cell’s activities and stores DNA.

● In eukaryotic cells, an internal membrane system produces,packages, and distributes proteins.

● Mitochondria harvest energy from organic compounds to ATP.● Lysosomes digest and recycle a cell’s used components.● Plant cells have three structures that animal cells lack: a cell

wall, chloroplasts, and a central vacuole.

3

2

1


Using Key Terms1. Cell-membrane proteins do not include

a. enzymes. c. markers.b. transporters. d. lipids

2. The _____________ houses a eukaryoticcell’s DNA. a. mitochondrion c. cytoskeletonb. nucleus d. ER

3. Proteins are produced by a. vesicles. c. lysosomes.b. ribosomes. d. smooth ER.

4. Most of a cell’s ATP is produced by a. chloroplasts.b. flagella.c. the cytoskeleton.d. mitochondria.

5. For each pair of terms, explain thedifferences in their meanings. a. light microscope, electron microscope b. flagellum, cilium c. cytoplasm, cytoskeleton d. chloroplast, mitochondria

Understanding Key Ideas6. The main advantage of the transmission

electron microscope is that it showsa. three-dimensional images of cell surfaces.b. the organelles of living cells.c. a cell’s internal structure in fine detail.d. the actual colors of a cell’s components.

7. The maximum size of a cell is determinedby the ratio between the cell’sa. surface area and volume.b. volume and organelles.c. organelles and cytoplasm.d. cytoplasm and nucleus.

8. Eukaryotic cells differ from prokaryoticcells in that eukaryotic cells a. lack organelles.b. have DNA but not ribosomes.c. are smaller than prokaryotic cells.d. have a nucleus.

9. In the cell membrane, the fatty acids ofphospholipid molecules a. face the cytoplasm.b. face the outside of the cell.c. are on both sides of the membrane.d. are in the interior of the membrane.

10. One function of the Golgi apparatus is to a. store DNA.b. make carbohydrates.c. modify proteins.d. digest and recycle the cell’s wastes.

11. Structures present in plant cells but notpresent in animal cells include a. chloroplasts and the central vacuole.b. mitochondria and the cell wall.c. ribosomes and ER.d. lysosomes and the Golgi apparatus.

12. What kind of microscope produced theimage of cilia shown below?

13. Explain how the cell membrane con-tributes to a cell’s ability to maintainhomeostasis.

14. Transport proteins in the membrane of alysosome move hydrogen ions into thelysosome. Use this information to predictwhether digestive enzymes in a lysosomework best in a neutral, a basic, or an acidic environment. (Hint: See Chapter 2,Section 2.)

15. Concept Mapping Make a conceptmap that compares plant cells with animalcells. Include the following terms in yourconcept map: cell membrane, cell wall,central vacuole, chloroplasts, andmitochondria.

PerformanceZONE

68 CHAPTER 3 Review

CHAPTER REVIEW

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section 1 looking at cells -...

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