CHAPTER 3 Cell - Welcome to Mr. Walker's Class Website · Chapter 3 • Cell Structure 49 ......

Chapter 3 • Cell Structure 49

Opening ActivityCell Structure Using micrographs,show students various cells from amulticellular organism, such as neu-rons, erythrocytes, and sperm cells.First have students attempt to iden-tify the cells. After the cells havebeen identified, have students sug-gest the function of each cell andhow the structure of the cell mightbe involved in that function.

GENERAL

• Vocabulary Worksheets

• Concept Mapping

Chapter Resource File

Answers

1. Molecules with an unequal dis-tribution of electrical chargesare polar molecules. Moleculeswith an equal distribution ofelectrical charges are nonpolarmolecules.

2. Carbohydrates, lipids, pro-teins, and nucleic acids areorganic compounds containingcarbon atoms covalentlybonded to hydrogen, oxygen,and other carbon atoms.However, they each differ intheir proportions of theseatoms, the manner in whichtheir molecules are puttogether, and whether othertypes of atoms are present.

3. ATP carries energy in cells.

Quick Review

Answers

Students may know that cells are the smallestliving unit of life and are likely to know thatcells contain hereditary material in thenucleus. Some may also know that cells con-tain organelles that carry out cell functions.However, students are not likely to be able toname these organelles and describe their func-tions, or distinguish between the features ofprokaryotes and eukaryotes.

Reading Activity

Looking AheadQuick ReviewAnswer the following without referring to

earlier sections of your book.

1. Distinguish between polar and nonpolar

molecules. (Chapter 2, Section 1)

2. Compare the structures of carbohydrates,

lipids, proteins, and nucleic acids. (Chapter 2,

Section 3)

3. Describe the function of ATP. (Chapter 2,

Section 3)

Did you have difficulty? For help, review the

sections indicated.

Section 1

Looking at CellsCells Under the Microscope

Types of Microscopes

Section 2

Cell FeaturesThe Cell Theory

Prokaryotes

Eukaryotic Cells

The Cell Membrane

Section 3

Cell OrganellesThe Nucleus

Ribosomes and the Endoplasmic Reticulum

Mitochondria

Structures of Plant Cells

www.scilinks.orgNational Science Teachers Association sciLINKS Internet

resources are located throughout this chapter.

Reading ActivityWrite down the title of this chapter and the titles

of its three sections on a piece of paper or in

your notebook. Leave a few blank lines after

each section title. Then write down what you

think you will learn in each section. Save your

list, and after you finish reading this chapter,

check off everything that you learned that was

on your list.

Most cells, including this one-celled organism

Paramecium, have all the equipment necessary to

perform the essential functions of life.

CellStructure

CHAPTER

3

49

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

OverviewBefore beginning this sectionreview with your students theobjectives listed in the StudentEdition. This section begins byrecounting the development of themicroscope as a tool for visualizingand studying cells. After a shortdescription of the metric systemand the units used in measuringcells, this section describes thecharacteristics of microscopes andthe various types that are usedtoday.

Tell students to examine the SI units in Table 1. Then have students answer the following questions:

1.How many meters are there in a kilometer? (1,000 m)

2.How many centimeters are in a meter? (100 cm)

3.What is 20 micrometers innanometers? (20,000 nm)

ActivityMagnification Tell students thatmicroscopes magnify objects, makingthem appear larger than their actualsize. Pair students, and ask each pairto calculate the diameter of a Lincolnpenny if it were magnified 270 times.Have students report in meters, infeet (multiply meters by 3.28 ft/m),and in comparison to another object.(A Lincoln penny is 2 cm wide. Mag-nified 270 times, it would appear 5.4 m wide (17.7 ft), about as wideas a large room.) LogicalLS

GENERAL

MotivateMotivate

Bellringer

FocusFocus

Section 1

50 Chapter 3 • Cell Structure

• Directed Reading

• Active Reading GENERAL


• Reading Organizers

• Reading Strategies

• Basic Skills WorksheetMicroscope Magnification

• Problem Solving WorksheetOperations with Small and LargeNumbers GENERAL

Planner CD-ROM

Table 1 Metric Units of Length and Equivalents

Unit 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

Cells Under the MicroscopeMost cells are too small to see with the naked eye; a typical human

body cell is many times smaller than a grain of sand. Scientists

became aware of cells only after microscopes were invented, in the

1600s. When the English scientist Robert Hooke used a crude

microscope to observe a thin slice of cork in 1665, he saw “a lot of

little boxes.” The boxes reminded him of the small rooms in which

monks lived, so he called them cells. Hooke later observed cells in

the stems and roots of plants. Ten years later, the Dutch scientist

Anton van Leeuwenhoek used a microscope to view water from a

pond, and he discovered many living creatures. He named them

“animalcules,” or tiny animals. Today we know that they were not

animals but single-celled organisms.

Measuring Cell StructuresMeasurements taken by scientists are expressed in metric units.

Scientists throughout the world use the metric system. The official

name of the metric system is the International System of

Measurements, 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 microscope

using the SI base unit for length, which is the meter. A meter, which

is 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 relationship

of that unit to a base unit. For example, the symbol “µ” stands for

the metric prefix micro. A micrometer (µm) is a unit of linear meas-

urement equal to one-millionth of a meter, or one-thousandth of a

millimeter. Table 1 summarizes the SI units used to measure length.

Objectives

● Describe how scientists

measure the length of

objects.

● Relate magnification and

resolution in the use of

microscopes.

● Analyze how light

microscopes function.

● Compare light

microscopes with electron

microscopes.

● Describe the scanning

tunneling microscope.

Key Terms

light microscope

electron microscope

magnification

resolution

scanning tunneling

microscope

50


Interactive Reading AssignChapter 3 of the Holt BiologyGuided Audio CD Program to help students achieve greater success in reading the chapter.

Math Skills Using yard/metersticks, show students that the inchis the closest English equivalent tothe centimeter, and the yard is theclosest English equivalent to themeter. Ask students to measure 5 objects in centimeters and con-vert that measurement to meters,then to measure 5 objects in inchesand convert that measurement toyards. Which system is easier touse? Why? (Students likely had lesstrouble using the metric system andwill likely say that the metric systemis easier to use since it is based onequal increments and on multipliersof 10.) You may use this opportu-nity to have students research anddiscuss why the U.S. has not totallyswitched to the metric system.(Tradition is often given as a reason.)

KinestheticLS

GENERALBUILDERSKILL

SKILL

BUILDER

READINGREADING

TeachTeach


HISTORYHISTORYCONNECTIONCONNECTION

When Hooke observed cork using a micro-scope, he observed tiny compartments. Hegave them the Latin name cellulae, meaning“small rooms.” That is the origin of the term cell.

did you know?

Standard Measures The International Bureauof Weights and Measures in France providesthe standards of the International System of Measurements. The Bureau houses thesestandard measures, such as a one-kilogrampiece of metal and a meter-long metal bar. The current SI system was established in 1960.

Transparencies

TR Bellringer

TR A10 Metric Units of Length and Equivalents

TR A4 Object Size and Magnifying Power of Microscopes

TR A5 Compound Light Microscope

Characteristics of MicroscopesSince Robert Hooke first observed cork cells, micro-

scopes have unveiled the details of cell structure. These

powerful instruments provide biologists with insight into

how cells work—and ultimately how organisms function.

Biologists use different microscopes depending on the

organisms they wish to study and the questions they want

to answer. Two common kinds of microscopes are light

microscopes and electron microscopes. In a

, light passes through one or more lenses to

produce an enlarged image of a specimen. An

forms an image of a specimen using a beam

of electrons rather than light.

An image produced by a microscope, such as the one

shown in Figure 1, is called a micrograph. Many micrographs are

labeled with the kind of microscope that produced the image—such

as a light micrograph (LM), a transmission electron micrograph

(TEM), or a scanning electron micrograph (SEM). Micrographs

often are labeled with the magnification value of the image.

is the quality of making an image appear larger than

its actual size. For example, a magnification value of 200� indicates

that the object in the image appears 200 times larger than the

object’s actual size. is a measure of the clarity of an

image. Both high magnification and good resolution are needed to

view the details of extremely small objects clearly. As shown in

Figure 2, electron microscopes have much higher magnifying and

resolving powers than light microscopes.

Resolution

Magnification

microscope

electron

microscope

light

A red blood cell is about 5 times

longer than a bacterial cell.

A Lincoln penny is about 2,000

times longer than a red blood cell.

A human is about 100 times

longer 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

Bacterium

Blood cell

Penny

Hand

Human

Sizes of Objects and Magnifying Power of Microscopes

10 µm

Magnification: 270�

Figure 1 Micrograph. This

light micrograph (LM) shows

an amoeba.

Figure 2 Magnifying

power of microscopes. The

scale shows the size range of

objects that can be viewed

with electron microscopes and

light microscopes.

51


Teaching TipElectron Microscopy Contact a local college or university andarrange for a field trip that willintroduce your students to electronmicroscopy. Specifically requestthat the parts of the electron micro-scope be shown and explained tostudents and that the students alsobe shown how a specimen is pre-pared for viewing and how it isviewed. Also check to see if theinstitution has any other micro-scopes normally not available tohigh schools so that these too canbe shown to the students.

Visual

Using the FigureThe microscopes in your schoolmay look a bit different from theone shown in Figure 3. Give amicroscope to each pair of stu-dents. Demonstrating from thefront of the class, point out theparts of your microscopes that correspond to the microscope inthe figure. Have students find these parts on their microscopes.Encourage ELL students to make adiagram of the school microscopesthat has all of the parts labeled.

LS

GENERAL

Teach, continuedTeach, continued


English Language Learners

StrategiesStrategiesINCLUSIONINCLUSION

Have the student use reference materials (orthe Internet) to compare and contrast thelight microscope and the electron micro-scope. The student’s work should includediagrams of each microscope with partslabeled, the types of materials that can bestudied, and different applications for eachtype of microscope. Additionally, the stu-dent could interview a professional that usesan electron microscope on the job.

• Gifted and Talented

Remind students that light waves from the specimenare bent as they pass through the ocular and objec-tive lenses of the light microscope. Refraction—thebending of light waves as they pass from one mediumto another—results in a “magnified” image of thespecimen. To assess the students’ understanding ofrefraction, ask them to identify the media throughwhich the light waves are passing when they use alight microscope. Possible answers include water, air,and glass.

Integrating Physics and Chemistry

www.scilinks.org

Topic: Microscopes

Keyword: 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, electron

microscopes have high magnifying power, but they cannot be used

to view living cells. Light microscopes have lower magnifying

power, but they can be used to view living cells.

Compound Light Microscope Light microscopes that use two lenses are called compound light

microscopes. In a typical compound light microscope, such as the

one shown in Figure 3, a light bulb in the base shines light up through

the specimen, which is mounted on a glass slide. The objective lens,

closest to the specimen, collects the light, which then travels to the

ocular (AHK yoo luhr) lens, closest to the viewer’s eye. Both lenses

magnify the image. Thus, a microscope with a 403 objective lens and

a 103 ocular lens produces a total magnification of 4003.

Why not add a third lens and magnify even more? This approach

does 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 the

two objects start to overlap!

Magnification: 1,5003

Ocular lens

Specimen

Stage

Focus

knob

Light source

Objective

lens

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

52


Activity Making a Wet Mount Have students practice preparing wetmounts. CAUTION: Slides andcover slips break easily and havesharp edges. Provide students withglass slides, cover slips, smallbeakers filled with water, and eye-droppers. Students can begin byviewing cutout letters from news-papers. Challenge students to makea bubble-free slide. Then have stu-dents prepare wet mounts usingdrops of pond water. Emphasizethat it will be much easier to findand watch organisms if studentsmake their slides bubble-free.

Kinesthetic

Writing Skills Electron micro-scopes are used in areas of researchother than biology. For example,they are important tools in Earthscience. In this field, electronmicroscopes have been used todetermine the effects of weatheringon the microstructural arrangementof the mineral components ofrocks, and the effects of chemicalson rock formation and stability.Have students prepare a report thatdescribes uses of electron micro-scopes in fields other than biology.

VerbalLS

BUILDERSKILL

LS

GENERAL



Hans Janssen, Zacharias Janssen, and HansLippershey built the first microscope in theNetherlands between 1590 and 1608. It couldmagnify objects from 3X to 10X. Anton van Leeuwenhoek made significant improve-ments to the microscope, developing lenseswith magnifications of up to 300X. VanLeeuwenhoek produced about 400 micro-scopes and used them to study many things,including yeast, muscles, plants, and insects.

Answer

The SIAM makes molecules visible. Therefore it is useful instudying cells on the molecularlevel, such as the structure of certain biological compounds likeproteins. Other types of micro-scopes, such as the electron microscope, visualize cells andtheir organelles. These micro-scopes are useful in understandingthe overall structure of the cell.

Real Life

CulturalAwarenessCulturalAwareness

German-born Ernst Ruska and RheinholdRuedenberg, working indepedently of eachother, invented the transmission electronmicroscope 1931. In 1933, Ruska built thefirst electron microscope that was morepowerful than the light microscope and, in1986, was awarded the Nobel Prize in Physics for this achievement.

Real LifeThe most powerful compound light microscopes have a total

magnification of up to 2,0003, which is sufficient for viewing

objects as small as 0.5 µm in diameter. For you to see smaller

objects, the wavelength of the light beam must be shorter than the

wavelength of visible light. Electron beams have a much shorter

wavelength than that of visible light, so electron microscopes are

much more powerful than light microscopes.

Electron MicroscopesElectron microscopes can magnify an image up to 200,0003, and

they can be used to study very small structures inside cells or on cell

surfaces. In electron microscopes, both the electron beam and the

specimen must be placed in a vacuum chamber so that the electrons

in the beam will not bounce off gas molecules in the air. Because liv-

ing cells cannot survive in a vacuum, they cannot be viewed using

electron microscopes.

Transmission electron microscope In a transmission electron micro-

scope, shown in Figure 4, the electron beam is directed at a very thin

slice of a specimen stained with metal ions. Some structures in the

specimen become more heavily stained than others. The heavily

stained parts of the specimen absorb electrons, while those that are

lightly stained allow electrons to pass through. The electrons that

pass through the specimen strike a fluorescent screen, forming an

image on the screen. A transmission electron micrograph (TEM),

such as the one of sperm cells shown in Figure 4, can reveal a cell’s

internal structure in fine detail. TEM images are always in black

and white. However, with the help of computers, scientists often add

artificial colors to make certain structures more visible.

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


Thirty movies could be

stored on a disk the size

of a penny.

Using the scanning inter-

ferometric apertureless

microscope (SIAM)

researchers have viewed

features that are about

four atoms (1 nm) in diam-

eter. The technology could

also be used to code

information on storage

disks.

Applying Information

Would the SIAM likely

be more useful in under-

standing the overall

structure of the cell

or the structure

of biological

compounds

in the cell?

53


Reteaching Ask students which kind of micro-scope would be most useful to highschool students studying the gen-eral structure of a leaf. (The lightmicroscope shows good detail forgeneral observations.) Ask studentswhich kind of microscope would be most useful for studying three-dimensional details of a smallorganism, such as a unicellular protist. (The SEM shows the surfacesof objects in great detail.) Ask themwhich microscope would be mostuseful in research on the internalstructures of a bacterial cell. (TheTEM shows internal detail with greatmagnification.)

Quiz1. How many centimeters are in a

meter? How do these metric sys-tem terms tell you the answer?(There are 100 centimeters in ameter. The prefix centi- means100, so a centimeter is 1/100th ofa meter.)

2.What is the difference betweenmagnification and resolution?(Magnification means to make animage larger, while resolution refersto the sharpness of the image.)

AlternativeAssessmentHave students prepare wet mountsof amoebas. Using compound lightmicroscopes, have students bringthe amoebas into focus. Using thincentimeter rulers placed on themicroscope stages within theirfields of view, have students esti-mate the sizes of the amoebas in SIunits and explain how they madetheir determinations. KinestheticLS

GENERAL

GENERAL

CloseClose

Answers to Section Review

1. A millimeter is equal to 0.001 m, or one-thousandth of a meter. A micrometer is equalto 0.000001 m, or one-millionth of a meter.

2. As the magnification increases, the resolutiondecreases; therefore the clarity of the image isreduced.

3. Light microscopes can magnify an object up to2,000X. Electron microscopes can magnify anobject up to 200,000X.

4. Specimens must be placed in a vacuum as partof the preparation for viewing with electronmicroscopes; living things cannot survive in avacuum.

5. You would use a transmission electron microscope because the electron beam passesthrough the bacterium, revealing the cell’sinternal structure in great detail.

6. A. Incorrect. A crude microscope cannot visualize electrons. B. Incorrect. A crude microscope cannot visualize electrons.C. Incorrect. Leeuwenhoek used a microscope to view single-celled organisms. D. Correct.Hooke studied cork and other plant cells with his crude microscope.


Scanning electron microscope In a scanning electron microscope,

shown in Figure 5, the electron beam is focused on a specimen

coated with a very thin layer of metal. The electrons that bounce off

the specimen form an image on a fluorescent screen. A scanning

electron micrograph (SEM) shows three-dimensional images of cell

surfaces, such as the image of sperm cells shown in Figure 5. As

with the transmission electron microscope, images produced by the

scanning electron microscope are black and white, but often they

are artificially colored.

Scanning Tunneling MicroscopeNew video and computer techniques are increasing the resolution and

magnification 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. A

computer tracks the movement of the probe across the object, enabling

objects as small as individual atoms to be viewed. The computer

generates a three-dimensional image of the specimen’s surface. The

scanning tunneling microscope can be used to study living organisms.

scanning tunneling microscope

In a scanning electron micro-

scope, electrons bounce off a

specimen, forming a three-

dimensional image of the

specimen on a fluorescent

screen.

Figure 5 Scanning

electron 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 Differences

Explain why electron microscopes cannot beused to view the structure of living cells.

Critical Thinking Comparing Functions

Assume 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 scientistRobert Hooke used a crude microscope to examine

A electrons C individual atoms

B cork cells D single-celled organisms

Standardized Test PrepStandardized Test Prep

Section 1 Review

54


Section 2

OverviewBefore beginning this sectionreview with your students theobjectives listed in the StudentEdition. This section introducesstudents to the cell theory. It con-tinues by discussing the similaritiesand differences between prokary-otic cells and eukaryotic cells, andcloses by describing the cytoskele-ton and cell membrane.

Have the following list written onthe chalkboard when students enterthe room: rocks, grass, soil, water,telephone poles, and grasshoppers.Have students work with a partnerto make two new lists: those itemson the list that are comprised ofcells and those items not comprisedof cells. Have them write a rationalefor each answer. (Grass, grasshoppers,and telephone poles are comprised ofcells. They are either living or wereonce living. Rocks, soil, and waterare not comprised of cells. They arenonliving.) Logical

ActivityBacteria in Yogurt Smear someyogurt with live cultures on amicroscope slide. Mix a drop ofwater into the smeared yogurt andadd a cover slip. Allow students toexamine the slide under high powerusing a compound light micro-scope. They should see bacteria ofthe genus Lactobacillus. Ask themto compare the size of these cellswith the sizes of cells on preparedslides of human and plant tissues.

KinestheticLS

GENERAL

MotivateMotivate

LS

Bellringer

FocusFocus



• Active Reading

• Data Sheet for Math Lab GENERAL

GENERAL




• Basic Skills Worksheets Length, Area, and Volume Measure-ment and Scale Drawings

• Problem Solving WorksheetRatios and Proportions GENERAL

GENERAL

Planner CD-ROM

Transparencies

TR Bellringer

TR B2 Relationship Between SurfaceArea and Volume

TR B4 Animal Cells

TR B8 Lipid Bilayers

TR B10 Membrane Proteins

The Cell TheoryIt took scientists more than 150 years to fully appreciate the discov-

eries of Hooke and Leeuwenhoek. In 1838, the German botanist

Mattias Schleiden concluded that cells make up not only the stems

and 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 that

cells 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 are

about 100 trillion cells in the human body, most ranging from 5 µm

to 20 µm in diameter. What is the advantage of having so many tiny

cells instead of fewer large ones? All substances that enter or leave

a 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 in

numbers large enough to meet the cell’s needs. Small cells can

exchange substances more readily than large cells because small

objects have a higher surface area–to-volume ratio than larger

objects, as shown in Table 2. As a result, substances do not need to

travel as far to reach the center of a smaller cell.

cell theory

Cell Features Section 2

Objectives

● List the three parts of the

cell theory.

● Determine why cells must

be relatively small.

● Compare the structure of

prokaryotic cells with that

of eukaryotic cells.

● Describe the structure of

cell membranes.

Key Terms

cell theory

cell membrane

cytoplasm

cytoskeleton

ribosome

prokaryote

cell wall

flagellum

eukaryote

nucleus

organelle

cilium

phospholipid

lipid 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

55


Demonstration Cut three cubes of varying sizesfrom a potato. Pour food coloringinto three beakers, and place a cubein each beaker. After about 5 min.,remove each cube and cut it in half.Have students observe the results.Ask them to determine how thesurface area-to-volume ratio ofeach cube affected the penetrationof the food coloring into the cube.(The food coloring traveled the samedistance in each cube. However, thesmallest cube will be more completelycolored than others because it has thegreatest surface area-to-volume ratio.)

VisualLS

GENERAL

TeachTeach


CalculatingSurface Area and Volume

Skills AcquiredCalculating, analyzing,applying

Teacher’s NotesBring in cube-shaped game diceof three different sizes. Havestudents measure the sides ofthe dice. Then have them calcu-late the surface area-to-volumeratios of the dice and comparetheir results.

Answers to Analysis1. SA 5 24 mm2, V 5 8 mm3,

ratio is 3:1

2. SA 5 6 mm2, V 5 1 mm3,ratio is 6:1

3. By being flat, a Parameciumspreads its volume over a largearea. The surface area-to-volume ratio is increasedbecause there is more surfacearea per unit volume.

x + 6x - 7 - 0

76

0

52

MISCONCEPTION

ALERT

Shape vs. Size Large cells do not neces-sarily have small surface area-to-volumeratios. Many large cells have shapes thatallow them to maintain a large surface area-to-volume ratio. A cell could growlarge in one or two dimensions but remainsmall in others. For example, parts of neurons are very long cylinders with smalldiameters, whereas epithelial cells are broadand flat. In both cases, the surface area-to-volume ratio is quite large.

Besides requiring a relatively high surface area-to-volume ratio in order to live, cells also have a totalcellular density that permits them to function efficientlyin their surrounding environments. Given that cellsmust have water to survive and are primarily com-posed of water, ask students if they would expectthem to have a density less than, about equal to, ormore than that of water. Ask students to defend theirresponses. If a cell’s density is different from that of its nearby habitat, what might account for this densitydifference?


www.scilinks.org

Topic: Cell Features

Keyword: HX4034

Common Features of CellsCells share common structural features, including an outer bound-

ary called the . The cell membrane encloses the cell

and separates the cell interior, called the (SITE oh plaz

uhm), from its surroundings. The cell membrane also regulates

what enters and leaves a cell—including gases, nutrients, and

wastes. Within the cytoplasm are many structures, often suspended

in a system of microscopic fibers called the . Most

cells have ribosomes. (RIE buh sohmz) are the cellular

structures on which proteins are made. All cells also have DNA,

which provides instructions for making proteins, regulates cellular

activities, and enables cells to reproduce. Some specialized cells

such as red blood cells, however, later lose their DNA.

Ribosomes

cytoskeleton

cytoplasm

cell membrane

Calculating Surface Area and Volume Background

You can improve your understanding of the relationship

between a cell’s surface area and its volume by practicing

with the large cube in Table 2.

<

x + 6x - 7 - 0

2

8

4930

52

Paramecium (SEM)

Magnification: 2303

1. Find the total surface area of the cube.

• side length sld 5 4 mm

• surface area of one side 5 l 3 l 5 l2

• surface area of one side sl2d 5 4 mm 3 4 mm 5 16 mm2

• total surface area 5 6 3 l25 6 3 16 mm

25 96 mm

2

2. Calculate the volume of the cube.

• height (h) 5 l 5 4 mm

• volume 5 l23 h 5 16 mm

23 4 mm 5 64 mm

3

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 colon

x to y x_y x:y

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

5

Divide both numbers by their greatest common factor:

53

2

s96 4 32d

s64 4 32d

96

64

total surface area

volume

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 cube

with a side length of 1 mm

in Table 2.

3. Critical Thinking

Relating Concepts How

does the flatness of the

single-celled Paramecium

shown above affect the cell’s

surface area–to-volume ratio?

56


Teaching Tip Cell Walls Tell students that thecell wall varies among differenttypes of organisms. In plants, thecell wall is composed of cellulosefibers embedded in a polysaccharideand protein matrix. In eubacteria,the cell wall is composed of pepti-doglycan, a polymer of sugarscross-linked by a short polypeptide.In fungi, the cell wall is composedof chitin, a polysaccharide.

Interpreting Visuals Draw stu-dents’ attention to Figure 6, whichshows colorized electron micro-graphs of bacterial cells. Electronmicrographs are always black andwhite. Tell students that any colorshown in electron micrographs is usually digitally applied; that is,the colors are “painted on” bycomputers. These colors are artifi-cial; the living cell is not colored asis shown in the micrograph. Askstudents why colorized electronmicrographs might be useful. (To highlight structures and “pointthem out.”) Have students deter-mine which structures are beingpointed out in Figure 6. (Uppermicrograph from outside to inside:capsule, cell wall, cell membrane,cytoplasm with ribosomes, DNA.Lower micrograph: the flagella areyellow with a red “halo” and the“body” of the bacterium is green.)

GENERALBUILDERSKILL


did you know?

Prokaryotes Modern prokaryotesinclude archaebacteria and eubacte-ria. They can be found in nearlyany environment on Earth, includ-ing volcanic vents at the bottom ofthe ocean and ice in Arctic andAntarctic regions.

Trends in MicrobiologyBacteria Classification This textbook usesthe six-kingdom system of classification.However, the research of Carl Woese, Professorof Microbiology at University of IllinoisUrbana-Champaign, and his colleagues, sug-gests that archaebacteria and bacteria areuniquely distinct. In 1980, Woese suggested a three-domain classification. Bacteria andArchaea consist of prokaryotes, while Eukaryaconsists of all eukaryotes. The numerous king-doms within each domain correspond to phylain the six-kingdom system. Today, the three-domain system of classification is accepted andused widely in the field of microbiology.

Prokaryotes The smallest and simplest cells are prokaryotes. A (proh

KAIR ee oht) is a single-celled organism that lacks a nucleus and other

internal compartments. Without separate compartments to isolate

materials, prokaryotic cells cannot carry out many specialized func-

tions. Early prokaryotes lived at least 3.5 billion years ago. For nearly

2 billion years, prokaryotes were the only organisms on Earth. They

were very simple and small (1–2 µm in diameter). Like their ancestors,

modern prokaryotes are also very small (1–15 µm). The familiar

prokaryotes that cause infection and cause food to spoil belong to

a 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 in

humans, grow and divide very rapidly. Some prokaryotes do not need

oxygen to survive. Other prokaryotes cannot survive in the presence

of oxygen. Some prokaryotes can even make their own food.

The cytoplasm of a prokaryotic cell includes everything inside the

cell 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. In

prokaryotes, the genetic material is a single,

circular molecule of DNA. This loop of pro-

karyotic DNA is often located near the center

of the cell, suspended within the cytoplasm.

Prokaryotic cells have a sur-

rounding the cell membrane that provides

structure and support. The cells of fungi and

plants also have cell walls; only animal cells

and some protists lack cell walls. Prokaryotes

lack an internal supporting skeleton, so they

depend on a strong cell wall to give the cell

shape. A prokaryotic cell wall is made of

strands of polysaccharides connected by

short chains of amino acids. Some prokary-

otic cell walls are surrounded by a structure

called a capsule, which is also composed

of polysaccharides. The capsule enables

prokaryotes to cling to almost anything,

including teeth, skin, and food.

Many prokaryotes have (fluh JEL

uh), which are long, threadlike structures

that protrude from the cell’s surface and

enable movement. Prokaryotic flagella

rotate, propelling the organism through its

environment at speeds of up to 20 cell

lengths per second. Figure 6 shows a

prokaryote with several flagella.

flagella

cell wall

prokaryote


Figure 6 Prokaryotes.

Prokaryotic cells have little

internal structure. Many also

have a capsule and flagella.

Reading Effectively

For many words ending in

-um, the plural is formed by

changing the -um to -a. For

example, the plural of bac-

terium is bacteria, and the

plural of flagellum is flagella.

57


Using the Figure Draw students’ attention to Figure 7.This figure shows most of theorganelles that are commonlyfound in animal cells. Explain thatfor the sake of clarity only a few of the microtubules and micro-filaments have been drawn. In reality, these structures form adense network (the cytoskeleton)inside the cell. Point out that thenuclear envelope actually consistsof two lipid bilayers. Visual

Reading Organizer As studentsread about prokaryotes andeukaryotes, have them design achart that identifies and describesall of the differences betweenprokaryotic and eukaryotic cells.Encourage ELL students toexchange their organizer withnative English speakers to check for accuracy.

Logical

Teaching TipEukaryotic Cells Because all multicellular plants and animals arecomposed of eukaryotic cells, stressthat the eukaryotic cell can be anentity in itself, not just a compo-nent of a larger organism. Havestudents research one-celledeukaryotic organisms, like a yeastor a one-celled protist, and comparethem with eukaryotic cells that arepart of a multicellular plant or animal. Students should includedrawings, and record their findingson a one-page fact sheet.

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The Largest Cell Students may be surprisedto learn that the largest eukaryotic cell in theworld is about the size of a baseball! This cellis the yolk of an ostrich egg.


CareerCareer

Cytotechnologist Cytotechnologists stain,mount, and study cells to detect malignanciesor other abnormal conditions. They then reportall findings to a pathologist. A cytotechnologistmust be able to obtain precise results and workunder stress during emergency situations. Anundergraduate degree in cytotechnology ormedical technology is usually needed for entry-level positions. Invite a cytotechnologist from alocal hospital to talk to your class. Ask them todiscuss recommended high school courses,working conditions, employment opportunities,and salary ranges during their talk.



Mitochondrion

Microfilaments

Lysosome

Ribosomes

Golgi

apparatus

Smooth ER

Rough ER

Cell membrane

Microtubules

Nuclear pore

Nuclear envelope

Nucleus

Nucleolus

Figure 7 Animal cell. Like

all eukaryotic cells, animal cells

contain a cell membrane, a

nucleus, and other organelles.

Eukaryotic CellsThe first cells with internal compartments were primitive eukaryotic

cells, which evolved about 2.5 billion years ago. A (yoo

KAIR ee oht) is an organism whose cells have a nucleus. The

(NOO klee uhs) is an internal compartment that houses the

cell’s DNA. Other internal compartments, or organelles, enable

eukaryotic 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 outside

the nucleus. A complex system of internal membranes connects

some organelles within the cytoplasm. These membranes provide

channels that guide the distribution of substances within the cell.

The membranes also form envelopes called vesicles that move pro-

teins and other molecules from one organelle to another.

Many single-celled eukaryotes use flagella for movement. Short

hairlike structures called (SIL ee uh) protrude from the surface

of some eukaryotic cells. Flagella or cilia propel some cells through

their environment. In other cells, cilia and flagella move substances

across the cell’s surface. For example, cilia on cells of the human res-

piratory system, shown in Figure 8, sweep mucus and other debris

out of the lungs.

A web of protein fibers, shown in Figure 9, makes up the

cytoskeleton. The cytoskeleton holds the cell together and keeps the

cell’s membranes from collapsing. The fluid surrounding the cyto-

plasm’s organelles, internal membranes, and cytoskeleton fibers is

called the cytosol.

cilia

organelle

nucleus

eukaryote

Figure 8 Cilia. Cilia on

cells lining the respiratory

system remove debris from

air passages.

58


Teaching Tip Prokaryotic Cells vs. EukaryoticCells Have students make aGraphic Organizer similar to theone on the bottom of this page thatcompares prokaryotic cells witheukaryotic cells.

ActivityAmoeboid Movement Have students make wet mounts of liveamoeba. These protists move andfeed by means of pseudopods, or“false feet.” Pseudopods are cellextensions formed by actin fiberscontinuously contracting andexpanding, pulling the cell mem-brane in one place and pushing itout in another, as described on thispage. In fact, amoeboid movementis a dramatic example of actin-based cell movement. Although students will not be able to see theactin fibers, they will be able toobserve the granular cytoplasmstreaming into the projection as itbegins to form. Explain to studentsthat their own white blood cellsexhibit such movement to catchand ingest foreign invaders such as bacteria. Visual

Paired Reading Assign studentsto cooperative pairs, and have eachstudent read this page silently. Asthey read, have students indicatepassages they understand with acheck mark and passages they donot understand with a questionmark. Then have the partners discuss what they did or did notunderstand. Ask each pair to writea short paragraph summarizing the structure and function of the cytoskeleton. You may wish to pair ELL students with nativeEnglish speakers.

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The CytoskeletonThe cytoskeleton provides the inte-

rior framework of an animal cell,

much as your skeleton provides the

interior framework of your body. The

cytoskeleton is composed of an intri-

cate network of protein fibers

anchored to the inside of the plasma

membrane. By linking one region to

another, they support the shape of

the cell, much as steel beams anchor

the sides of a building to one another.

Other fibers attach the nucleus and

other organelles to fixed locations in

the cell. Because protein fibers are

too small for a light microscope to

reveal, biologists visualize the

cytoskeleton by attaching fluorescent dyes to antibodies. An anti-

body is an immune system protein specialized to bind to one

particular kind of molecule—in this case—cytoskeleton proteins.

When the cell is examined under fluorescent light, the fibers glow

because 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 tubes

called microtubules made of the protein tubulin, and (3) thick ropes

of protein called intermediate fibers.

Actin Fibers The actin fibers of the cytoskeleton form a network

just beneath the cell surface that is anchored to membrane proteins

embedded 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 and

pushing it out in others. If you examine the surface of a protist such

as the one shown in Figure 10, you will find it alive with motion.

Tiny projections extend out from the surface like fingers. Each is a

temporary projection of the plasma membrane that shoots out and

then retracts.

Microtubules Microtubules within the cytoskeleton act as a highway

system for the transportation of information from the nucleus to

different parts of the cell. RNA molecules are transported along

microtubular “rails” that extend through the interior of the cell like

train tracks. The RNA molecules, in complexes with proteins, are

attached to so-called motor proteins that chug along microtubules

like locomotives on tracks. The motor proteins drag the RNA-protein

complexes along with them like freight cars.

Intermediate Fibers The intermediate fibers of the cytoskeleton

provide a frame on which ribosomes and enzymes can be confined

to particular regions of the cell. The cell can organize complex

metabolic activities efficiently by anchoring particular enzymes

near one another.

Microtubules

Nucleus

Endoplasmicreticulum

Mitochondrion

Ribosomes

Figure 9 The cytoskeleton.

The cytoskeleton’s network of

protein fibers anchors cells

organelles and other compo-

nents of the cytoplasm.

Figure 10 Cytoskeletal

projections. The multiple

spikes on the surface of this

marine amoeba are projections

of the cytoskeleton stretching

the cell membrane outward.

59

Graphic Organizer

Use this graphic organizer with

Teaching Tip on this page.

can be

bacteria

prokaryotic

organelles

animals

plants

fungi

protists

which are found in

which containwhich lackwhich are known as

eukaryotic

Cells



AnswerThe cells are frozen very quickly,which prevents the formation oflarge ice crystals that woulddamage the cells.

Real Life

Group Activity Cell Membrane Model Organizethe class into groups. Tell eachgroup to make a model of the cellmembrane to help visualize itsstructure. Suggest that students cutoff half of one leg of several woodenclothespins. Different colors can beused to represent the hydrophobic(nonpolar) and hydrophilic (polar)ends. Proteins can be representedby buttons, bottle caps, and card-board tubes. Place completed models around the classroom.

Kinesthetic

Using the FigureDraw students’ attention to Figure 11, and then have studentsreview the terms polar and non-polar. Be certain students under-stand that the nonpolar interior ofthe lipid bilayer repels ions andpolar substances. Remind themthat even though water moleculesare polar, they are small enough tomove through the lipid bilayer.Make sure students understandthat membranes are found in manyplaces in cells, such as around theentire cell, mitochondria, the Golgiapparatus, lysosomes, and thenucleus. Ask students to list cellstructures that lack membranes.(ribosomes, microfilaments, micro-tubules, and cell walls) VisualLS

GENERAL

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GENERAL

did you know?

Phospholipids Phospholipids in a lipidbilayer can move about 2 µm in one second.Proteins also move, but because of their greatersize, their movement is slower. The membraneremains fluid unless the temperature decreasesto the point at which the membrane solidifies.If the phospholipids are rich in unsaturatedfatty acids, the membrane will remain fluid atlower temperatures. If the phospholipids arerich in saturated fatty acids, the membrane willbe more viscous at any given temperature.

Remind students that the cell membrane’s selectivepermeability is the result of how water, ions, and othersubstances interact with the polar and non-polar por-tions of the phospholipid bilayer. These differences inpolarity also determine how various proteins areembedded in the lipid bilayer.



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 a

soap bubble. The fluidity of cell membranes is caused by lipids,

which form the foundation of membranes. The lipids form a barrier

that separates the inside of the cell from the outside of the cell. This

barrier allows only certain substances in the cell’s environment to

pass through. This selective permeability of the cell membrane

determines which substances enter and leave the cell.

The Cell Membrane as a BarrierThe selective permeability of the cell membrane is caused mainly by

the way phospholipids interact with water. A is a lipid

made 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 will

form weak bonds with other polar substances. The head of a phos-

pholipid, which contains a phosphate group, is polar and is

attracted 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 double

layer called a , as shown in Figure 11. The nonpolar tails

of the phospholipids make up the interior of the lipid bilayer.

Because water both inside and outside the cell repels the nonpolar

tails, they are forced to the inside of the lipid bilayer. Ions and most

polar molecules, including sugars and some proteins, are repelled

by the nonpolar interior of the lipid bilayer. The lipid bilayer allows

lipids and substances that dissolve in lipids to pass through.

lipid bilayer

phospholipid

Real Life

Donated blood is frozen

in a special process

called cryopreservation.

Similar methods are used

to preserve human eggs,

embryos, and blood from

the umbilical cord, a rich

source of immune-system

cells.

Finding Information

Research how cryo-

preservation

methods enable

cells to with-

stand freezing.

Polar

Nonpolar

Polar

Lipid bilayer

Polar

head

Non-

polar

tails

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

60



1. If the surface area-to-volume ratio of a cell istoo small, substances cannot penetrate the cellquickly enough to meet the needs of the cell.

2. Prokaryotic cells lack organelles; eukaryoticcells have organelles, which enable them tocarry out unique, specialized functions inorganized ways.

3. Answers will vary, but may include markerproteins, which help cells recognize cells oftheir own type; receptor proteins, which bindto signal molecules; transport proteins, whichmove substances into and out of the cell; andenzymes, which aid in biochemical reactions.

4. Schleiden, Schwann, and Virchow observedthat plants as well as animals are composed of cells, which are the basic units of structureand function in living things and that cells ariseonly from other cells.

5. A. Incorrect. Flagella are not needed for celldivision. B. Correct. Flagella are structures ofmotility. C. Incorrect. Flagella play no role inmaintaining cell shape. D. Incorrect. Flagellaare not used to make cell proteins.

Reteaching Assign students to two cooperativegroups. Have group A represent aprokaryotic cell, and have group Brepresent a eukaryotic cell. Haveeach group take turns stating acharacteristic that its cell has orsomething that its cell is able to do.Write these characteristics on the board. Give the other group an opportunity to challenge theanswers. Award one point for eachcorrect answer, and deduct onepoint for each incorrect answer.

Quiz1. True or false: a spherical large

cell has a larger surface area-to-volume ratio than a sphericalsmall cell. (False. The sphericalsmall cell has the larger surface-area-to-volume ratio.)

2.Where is the DNA located in aeukaryotic cell? (in the nucleus)

3. What does the term “selectivelypermeable” mean in regards to acell membrane? (Some moleculescan pass through the membraneand some cannot.)

AlternativeAssessmentWorking in pairs, have studentsconstruct 3 “cells” out of thickpaper, using the measurementsgiven in Table 2. Tell students toseal the sides of the cells with cello-phane tape, and to leave the top of each cell open. Give the studentsenough sand to fill each of thecubes, and tell them to use thesematerials to show visually the sur-face area-to-volume ratio of thethree cells. Ask each pair to explainto you orally how the surface area-to-volume ratio differs among thecells. (Answers may vary dependingon the size and shape of the constructed cells.)

GENERAL

GENERAL

CloseClose


Carbohydrate

portion1. Cell-surface marker:

Identifies cell type

Phospholipid heads

Protein

portion

Phospholipid tails

2. Receptor protein:

Recognizes and

binds to substances

outside the cell

3. Enzyme:

Assists chemical

reactions inside

the cell

4. Transport protein:

Helps substances

move across

cell membraneLipid

bilayer

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 recall

that proteins are made of amino acids and that some amino acids

are polar, while others are nonpolar. The nonpolar part of a mem-

brane protein is attracted to the interior of the lipid bilayer but is

repelled by the water on either side of the lipid bilayer. In contrast,

the polar parts of the protein are attracted to the water on either

side of the lipid bilayer. This attraction helps to hold the protein in

the lipid bilayer. The motion and fluidity of phospholipids enable

some membrane proteins to move around within the lipid bilayer.

As shown in Figure 12, cell membranes contain different types of

proteins. Marker proteins attached to a carbohydrate on the cell’s sur-

face advertise cell type—such as a liver cell or a heart cell. Receptor

proteins bind specific substances, such as signal molecules, outside

the cell. Enzymes embedded in the cell membrane are involved in

important biochemical reactions in the cell. Transport proteins aid the

movement 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.

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

Critical Thinking Comparing Functions

Describe the functions of two types of cell-membrane proteins.

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

A bacterium that lost itsflagella would be unable to

A move C make proteins

B divide D maintain its shape


61


OverviewBefore beginning this sectionreview with your students theobjectives listed in the StudentEdition. This section focuses on the organelles of eukaryotic cells,beginning with the nucleus. Next,this section explores how proteinsare packaged and distributed, andthe role of mitochondria in the production of ATP. Lastly, the sec-tion describes structures found inplant cells that are not found inanimal cells.

Provide students with microscopeslides of eukaryotic cells. Have the students find the nucleus usingcompound light microscopes. Havethe students give rationales for whythey identified the structure theydid as the nucleus. Visual

ActivityCell Structures Using a lightmicroscope, show students a prepared slide of a cheek cell. Then show students electron micro-graphs of various cell parts andorganelles. Have students comparethese micrographs with the illustra-tions in this book. Ask them toattempt to find these structures inthe cheek cell. Only the cytoplasm,nucleus, and cell membrane will beobserved easily. Remind studentsthat the other structures either aretoo small to be seen with the lightmicroscope or may need to bestained in a different way to be seen.

VisualLS

GENERAL

MotivateMotivate

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Bellringer

FocusFocus

Section 3


• Active Reading

• Data Sheet for Quick Lab GENERAL

GENERAL




• Supplemental ReadingThe Lives of a Cell

Planner CD-ROM


StrategiesStrategiesINCLUSIONINCLUSION

Ask students to develop a chart listing thedifferent organelles of a cell. The chartshould include the names of each organelle,the functions of each organelle, and threestructures found in plant cells that are notfound in animal cells. From the chart, thestudent can design a poster showing all ofthe cell’s organelles.

GENERAL

• Learning Disability• Attention Deficit Disorder

Nuclear

pores

Nuclear envelope

Nucleolus

Section 3 Cell Organelles

The Nucleus Most functions of a eukaryotic cell are controlled by the cell’s

nucleus. As shown in Figure 13, the nucleus is surrounded by a

double membrane called the nuclear envelope, also called the

nuclear membrane. The nuclear envelope is made of two lipid

bilayers that separate the nucleus from the cytoplasm.

Scattered over the surface of the nuclear envelope are many

small channels through the envelope called nuclear pores.

Substances that are made in the nucleus, including RNA-ribosomal

protein complexes, move into the cytoplasm by passing through

the nuclear pores. Ribosomes are partially assembled in a region

of the nucleus called the nucleolus, which is also shown in Figure

13. Recall from Section 2 that ribosomes are the structures on

which proteins are made.

The hereditary information of a eukaryotic cell is coded in the

cell’s DNA, most of which is stored in the nucleus. Eukaryotic

DNA is wound tightly around proteins. Most of the time, DNA

exists as elongated and thin strands, which appear as a dark mass

under magnification. When a cell is about to divide, however, the

DNA strands, called chromosomes, wind up into a more compact

form and appear as dense, rod-shaped structures. The number of

chromosomes in a eukaryotic cell differs between species.

Human body cells have 46 chromosomes, while the cells of gar-

den peas have 14 chromosomes. You will learn more about DNA

and chromosomes later in this book.

Objectives

● Describe the role of

the nucleus in cell

activities.

● Analyze the role of internal

membranes in protein

production.

● Summarize the importance

of mitochondria in eukaryotic

cells.

● Identify three structures in

plant cells that are absent

from animal cells.

Key Terms

endoplasmic reticulum

vesicle

Golgi apparatus

lysosome

mitochondrion

chloroplast

central vacuole

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

Figure 13 Nucleus

62


Group Activity Selling Cells Have students workin groups of two or three to prepareadvertisements for cell organelles.Have each group choose anorganelle they want to “sell,” andhave them prepare print, audio, or video ads. Interpersonal

Using the FigureDraw students’ attention to Figure 14. Tell students that the figure shows an increasing level ofdetail from left to right. On the left,the entire cell is depicted, with theER circled. To the right of the cell,the ER in the circle is magnified toshow a greater level of detail. Andto the right of the circle is an elec-tron micrograph of ER. Labels pointout the same structures in the diagram as in the ER. However, the diagram shows the ER three-dimensionally, emphasizing itssheet-like nature. The electronmicrograph is a thin slice of a cell,so it has a two-dimensional qualityshowing a cross-sectional view.

VisualLS

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GENERAL

TeachTeach


Enzymes found in the abundant smooth ERof the liver help detoxify drugs and environ-mental pollutants. Detoxification usuallyinvolves a series of chemical reactions withinthe smooth ER of the liver that make the drugmore water soluble, allowing it to be excretedin the urine. The ingestion of many drugs,including barbiturates and alcohol, trigger anincrease in the amount of smooth ER andaccompanying enzymes in liver cells.

MEDICINEMEDICINECONNECTIONCONNECTION

Transparencies

TR Bellringer

TR B13 Processing of Proteins

TR B12 Mitochondrion

TR B9 Organelles

Ribosomes and the Endoplasmic Reticulum

Unlike prokaryotic cells, eukaryotic cells have a system of internal

membranes that play an essential role in the processing of proteins.

Cells make proteins on ribosomes. Each ribosome is made of dozens

of different proteins as well as RNA. Some of the ribosomes in a

eukaryotic cell are suspended in the cytosol, as they are in prokary-

otic cells. These “free” ribosomes make proteins that remain inside

the cell, such as proteins used to build new organelles.

Production of Proteins

Proteins that are exported from the cell, such as some signal

molecules, 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), or

ER, 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 with

embedded proteins.

The part of the ER with attached ribosomes is called rough ER

because it has a rough appearance when viewed in the electron

microscope. The rough ER helps transport the proteins that are

made by its attached ribosomes. As each protein is made, it crosses

the ER membrane and enters the ER. The portion of the ER that

contains the completed protein then pinches off to form a vesicle. A

is a small, membrane-bound sac that transports substances

in cells. Because certain proteins are enclosed inside vesicles, these

proteins are kept separate from proteins that are produced by free

ribosomes in the cytoplasm.

The rest of the ER is called smooth ER because it lacks ribosomes

and thus appears smooth when viewed in the electron microscope.

The smooth ER performs various functions, such as making lipids

and breaking down toxic substances.

vesicle

endoplasmic reticulum

www.scilinks.org

Topic: Proteins

Keyword: HX4151

Smooth ER

Ribosomes

Rough ER

The ER moves proteins and other substances within eukaryotic cells.

Figure 14 Endoplasmic reticulum

63


Reading Hint As students read thesection, ask them to list questionsthey have from the reading. Whenstudents have compiled their lists,ask students to share their ques-tions orally with the class. Make alist of questions on the board, andthen assign the questions to pairs ofstudents. Ask students to answerthe questions by pointing out pas-sages in the book that explain theanswers. Have students researchanswers to the questions unan-swered by this book.

Teaching TipWhat Am I? Have each studentchoose one cell part or organelleand write a “What Am I?” essay inwhich the student pretends to bethat structure. The essay should be written in the first person andinclude the function and structureof that cell part. Encourage stu-dents to be creative in their writingstyles. Have each student read hisor her essay aloud in class. Otherstudents should try to guess thestructure or organelle presented in the essay. Verbal

Co-op Learning

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Co-op Learning

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In 1945, Belgian-American cytologist AlbertClaude began electron microscope studies of the cell and discovered the endoplasmicreticulum and the detailed structure of mito-chondria. Along with Christian de Duve andGeorge E. Palade, Claude was awarded theNobel Prize in Physiology or Medicine in1974 for discoveries of the structural andfunctional organization of the cell.

Several diseases have been attributed toimproperly functioning lysosomes. Tay-Sachsdisease is caused by the deficiency of a lysoso-mal enzyme that digests lipids. As a result,cells in the brain become filled with lipids,causing impairment. Pompe’s disease resultswhen a lysosomal enzyme that breaks downglycogen is absent, causing liver damage.

REAL WORLDREAL WORLDCONNECTIONCONNECTION

Packaging and Distribution of ProteinsVesicles that contain newly made proteins move through the cytoplasm

from the ER to an organelle called the Golgi apparatus. The

(GOHL jee) is a set of flattened, membrane-bound sacs that

serves as the packaging and distribution center of the cell. Enzymes

inside the Golgi apparatus modify the proteins that are received in

vesicles from the ER. The modified proteins are then enclosed in new

vesicles that bud from the surface of the Golgi apparatus. Other vesi-

cles 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 proteins

are packaged into vesicles.

Step The vesicles transport the newly made proteins from the

rough ER to the Golgi apparatus.

Step In the Golgi apparatus, proteins are processed and then

packaged into new vesicles.

Step Many of these vesicles move to the cell membrane and

release their contents outside the cell.

Step Other vesicles, including lysosomes, remain within the

cytoplasm. Lysosomes digest and recycle the cell’s used

components by breaking down proteins, nucleic acids,

lipids, and carbohydrates.

lysosomes

apparatus

Golgi

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 outside

the cell.

4

Other vesicles remain in

the cell and become

lysosomes.

5

Nucleus

Proteins are processed by an internal system of membranes.

Figure 15

Interpreting Graphics

As you read, use Steps 1–5

in the text, shown in red, to

help you follow the same

numbered steps shown in

Figure 15.

64



MISCONCEPTION

ALERT

Mitochondria in Plant CellsMake sure students understandthat plant cells, as well as ani-mal cells and almost all othereukaryotic cells, contain mito-chondria. Many students thinkthat because plants performphotosynthesis, which makestheir own food as well as ATP,that they do not contain mito-chondria. However, it is impor-tant for students to understandthat plants use the products of photosynthesis to fuel themanufacture of ATP in themitochondria. MitochondrialATP synthesis takes place inparts of plants that do notundergo photosynthesis, such as the roots, and when it is dark in parts of plants that doundergo photosynthesis, such as the leaves.

Using the FigurePoint out the infolding of the innermembrane of the mitochondrion.Ask students to suggest a purposefor this infolding of the membrane.(Enzymes for the reactions that takeplace here are located on these mem-branes, and the infolding increasesthe surface area.) Ask students tospeculate on how effective a mito-chondrion without the folds wouldbe. (With less surface area to carryout its chemical reactions, it mightnot be able to carry out its series ofreactions, or might produce muchless ATP.) Next, have students predict what might happen to anorganism if the mitochondria in itscells suddenly became only half asefficient. (Life processes that requireenergy might not be able to takeplace, might slow down, or might not be able to sustain themselves for as long a time.)

did you know?

Endosymbiosis The theory of endosymbiosisis a widely accepted explanation of the originof various cell organelles. The theory holds thatprokaryotes lived in association with othercells but lost their ability to reproduce inde-pendently, becoming organelles of today’seukaryotes. Scientists think that chloroplastsevolved from endosymbiotic photosynthetic bac-teria and mitochondria evolved from endosym-biotic aerobic bacteria. This theory is supportedby the fact that both chloroplasts and mito-chondria have their own DNA and can be inac-tivated with antibiotics that affect prokaryotesbut not eukaryotes.

MitochondriaNearly all eukaryotic cells contain many (miet uh

KAHN dree uh), like the one shown in Figure 16. A mitochondrion is

an organelle that harvests energy from organic compounds to make

ATP, the main energy currency of cells. Although some ATP is made

in 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 16

shows 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 inner

membrane. It is here that the chemical reactions that produce ATP

during cell metabolism take place.

Mitochondrial DNAThe nucleus is not the only organelle in the cell that contains nucleic

acids. Mitochondria also have DNA and ribosomes, and mitochon-

dria make some of their own proteins. However, most mitochondrial

proteins are made by free ribosomes in the cytosol. Mitochondrial

DNA is independent of nuclear DNA and similar to the circular DNA

of prokaryotic cells. This fact supports the widely accepted theory

that primitive prokaryotes are the ancestors of mitochondria. You

will learn more about the origin of mitochondrial DNA later in

this book.

mitochondria

Inner

membrane

Outer

membrane

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

Figure 16 Mitochondrion

The word mitochondrion is

from the Greek words

mitos, meaning “a thread”

and chrondros, meaning

“grain.” Knowing this makes

it easier to remember that a

mitonchondrion is a small,

elongated cell organelle.

65


Reteaching Pass out copies of electron micro-graphs of cells to students. Askeach student to color in 3 cellorganelles of their choosing, eachwith a different color. Then ask students to name the 3 organelles,identifying them by their color, and to describe the job of those 3 organelles within the cell.

Quiz1. The mitochondrion has the nick-

name “the powerhouse of thecell.” Explain why this is a goodnickname. (The mitochondrion isthe organelle [house] in which ATPis synthesized. ATP is the energycurrency that powers the cell.)

2.Name two organelles withinplant cells that are not presentwithin animal cells. (chloroplastsand central vacuoles)

AlternativeAssessmentHave students use food to make amodel of a eukaryotic cell. Gelatin(cytoplasm) could be poured into a large, clear cellophane bag (cellmembrane). Organelles could be represented by kidney beans(mitochondria), poppy seeds (ribosomes), sections of lasagnanoodles (ER and Golgi apparatus),peppercorns (lysosomes), spaghettinoodles (cytoskeleton), and anorange (nucleus). KinestheticLS

GENERAL

GENERAL

CloseClose


1. The nucleus contains DNA, directs cell activities,and assembles ribosomal proteins and RNA.

2. The proteins, made at the ribosomes attachedto the ER, pass through the ER. The proteinsare packaged into vesicles and transported tothe Golgi apparatus, where they are modifiedand repackaged. Some of these vesicles releasetheir contents from the cell.

3. Mitochondria produce most of the ATP usedfor chemical reactions in eukaryotic cells.

4. The cell wall contains cellulose and is thick andsomewhat rigid. When the central vacuole is

full, it pushes the other cell contents against thecell wall, providing added rigidity.

5. The digestive enzymes of the cell are storedwithin membranous vesicles so that they donot digest other cell structures.

6. A. Incorrect. The nucleus directs the activitiesof the cell. B. Incorrect. The lysosome is a stor-age vacuole for digestive enzymes. C. Incorrect.The mitochondrion generates ATP. D. Correct.The Golgi apparatus is the packaging and distri-bution center of a eukaryotic cell.


Structures of Plant Cells The organelles described in this section are found in both animal

cells and plant cells. However, plant cells have three additional

structures 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 thick

cell wall, composed of proteins and carbohydrates, including the

polysaccharide cellulose. The cell wall helps support and maintain the

shape of the cell, protects the cell from damage, and connects it with

adjacent cells.

Chloroplasts Plant cells contain one or

more . Chloroplasts are

organelles that use light energy to make

carbohydrates from carbon dioxide and

water. Chloroplasts are found not only in

plants but also in a wide variety of eukary-

otic algae, such as seaweed. Chloroplasts,

along with mitochondria, supply much of

the energy needed to power the activities

of plant cells. Like mitochondria, chloro-

plasts are surrounded by two membranes,

contain their own DNA, and are thought

to be the descendents of ancient prokary-

otic cells.

Central vacuole As shown in Figure 17,

much of a plant cell’s volume is taken up

by a large, membrane-bound space called

the (VAK yoo ohl). The

central vacuole stores water and may

contain many substances, including ions,

nutrients, and wastes. When the central

vacuole is full, it makes the cell rigid. This

rigidity enables a plant to stand upright.

central vacuole

chloroplasts

Describe the role of the nucleus in cellactivities.

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

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

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

Critical Thinking Inferring Relationships

What is the importance of a cell enclosing itsdigestive enzymes inside lysosomes?

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

A nucleus C mitochondrion

B lysosome D Golgi apparatus


Section 3 Review

Chloroplast

Central vacuole

Cell wall

Figure 17 Plant cell. Plant cells have a cell

wall, chloroplasts, and a large central vacuole

(shown in blue).

66


AlternativeAssessmentAssign students to cooperativepairs. Have each pair devise a testthat includes the material coveredin this chapter. Each pair must also provide an answer key. Set theparameters for the test. For exam-ple, designate the number and typeof questions allowed. After the testsare collected, exchange them withthe other pairs, and have the pairstake the tests. Let the pairs checktheir own answers when they havecompleted the test. Co-op Learning

GENERAL

Answer to Concept Map

The following is one possible answer toPerformance Zone item 15.


include

which containwhich contain

Eukaryotic cells

cell wall

animal cells

cell membrane

plant cells

mitochondriachloroplasts

central vacuole

Key Concepts

Study CHAPTER HIGHLIGHTS

ZONE

Key Terms

Section 1

light microscope (51)

electron microscope (51)

magnification (51)

resolution (51)

scanning tunneling

microscope (54)

Section 2

cell 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 3

endoplasmic reticulum (63)

vesicle (63)

Golgi apparatus (64)

lysosome (64)

mitochondrion (65)

chloroplast (66)

central vacuole (66)

BIOLOGYBIOLOGY

Unit 1—Use this unit to review the key

concepts and terms in this chapter.

Looking at Cells

● Microscopes enable biologists to examine the details of cell

structure 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 than

large 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

67


ANSWERS

Understanding Key Ideas

1. c

2. a

3. d

4. d

5. c

6. a

7. The cell membrane regulateswhat substances enter and leavethe cell, thus maintaining theintegrity of the cell.

8. A scanning electron microscopeproduced the image.

9. Because of the presence of hydro-gen ions in lysosomes, one caninfer that the inside of the lyso-some is acidic. Digestive enzymesin a lysosome therefore work bestin an acidic environment.

10. One possible answer to the con-cept map is found at the bottomof the Study Zone page.

Critical Thinking

11. The arrangement of phospholipidsin the lipid bilayer influences thepermeability of this bilayerbecause the nonpolar lipidtails repel water (and otherpolar/water-soluble molecules).

12. Muscles cells use ATP when they contract. Having manymitochondria would help providethe large amount of ATP neededfor contraction.

13. Antibiotics can kill bacterial cells,while not harming human cells.Drugs that kill eukaryotic para-sites may also harm eukaryotichuman cells.

Alternative Assessment

14. Answers will vary. Have students access theInternet and other resources for this informa-tion. Students may focus on the role of vari-ous types of transport proteins in maintainingthe organs.

Section Questions

1 1, 2, 8

2 3, 4, 7, 11, 14

3 5, 6, 9, 10, 12, 13

Assignment Guide


CHAPTER 3

Understanding Key Ideas

1. The main advantage of the transmissionelectron microscope is that it allows theviewer to seea. 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.

2. 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.

3. 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.

4. 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.

5. 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.

6. 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.

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

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

9. 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.)

10. 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.

Critical Thinking

11. Recognizing Relationships How does thearrangement of phospholipids influencethe permeability of the lipid bilayer?

12. Inferring Relationships Muscle cells havemore mitochondria than some other kindsof eukaryotic cells. In what way wouldhaving many mitochondria be beneficial tomuscle cells?

13. Applying Information Drugs that rid thebody of eukaryotic parasites often havemore side effects and are harder on thebody than drugs that act on bacterial para-sites. Suggest a reason for this difference.

Alternative Assessment

14. Interactive Tutor Unit 1 Cell Transport and

Homeostasis Write a report summarizing therole of the cell membrane in the preservationof body organs donated for transplant.

PerformanceZONE

CHAPTER REVIEW

68



C

D

E

F

B

A

Standardized Test Prep

Understanding ConceptsDirections (1–4): For each question, write ona separate sheet of paper the letter of thecorrect answer.

1 What structure houses a eukaryotic cell’sDNA? A. cell wallB. cytoskeletonC. ERD. nucleus

2 Which of the following is not a proteinthat might be found in cell membranes?F. enzymeG. lipidH. markerI. transporter

3 What structures are found in both eukaryotic cells and prokaryotic cells?A. mitochondrion and ERB. nucleus and cell membraneC. mitochondrion and ribosomeD. cell membrane and ribosome

4 What property of phospholipids makesthem ideal for making up the selectivelypermeable cell membrane? F. They repel small ions.G. They react readily with water molecules.

H. They form triple layers that insulate the cell.

I. They have a nonpolar region and apolar region.

Directions (5): For the following question,write a short response.

5 Analyze why smaller pieces of food cookfaster than larger pieces of food, based onthe relationship between surface area andvolume.

Reading SkillsDirections (6): Read the passage below.Then answer the question.

Microbiologists study the growth, structure, development, and many othercharacteristics of bacteria and other microorganisms. Some microbiologists alsostudy the action of microorganisms on livingand dead tissue. This career requires at leasta two-year technical training degree from acommunity college or technical institution.Many microbiologists have a four-year bachelor’s degree plus a master’s or doctoraldegree.

6 What tools would be most useful for amicrobiologist? A. centrifuges and syringesB. nets and specimen cagesC. electron and light microscopesD. balances and graduated cylinders

Interpreting GraphicsDirections (7): Base your answer to question7 on the diagram below.

Animal Cell

7 What is the function of the structurelabeled A?F. making ATPG. making carbohydratesH. making proteins I. moving proteins through the cell

Test

When possible, use the text in the test to answer

other questions. For example, use a multiple-choice

answer to “jump start” your thinking about another

question.

69

Question 4 Answer I is the correctchoice. The nonpolar tails repelwater, while the polar headsattract water. This allows phos-pholipids to form double layersthat keep many molecules fromentering or exiting the cell. AnswerF is incorrect because the polarheads of phospholipids attractsmall ions. Answer G is incorrectbecause the polar heads interact,not react, with water molecules.Answer H is incorrect because theyform a double layer, not a triplelayer.

Question 5 Smaller pieces of foodhave significantly less volume thanlarger pieces, yet their surface areais only slightly smaller; thus, theyhave a larger surface-area-to-vol-ume ratio, resulting in more sur-face area exposed to heat whilecooking.

Question 6 Answer C is the cor-rect choice. Answer A is incorrectbecause these tools allow scientiststo examine tissues and blood sam-ples, not as useful to microbiolo-gists. Answer B is incorrectbecause these tools are used tocapture and contain larger organ-isms. Answer D is incorrectbecause these tools allow scientiststo measure larger samples, whichis less useful to microbiologists.

Question 7 Answer F is the cor-rect choice. The mitochondriamake ATP, which is important forenergy transfer in living things.Answer G is incorrect because thisstructure is not a chloroplast,which makes carbohydrates.Answer H is incorrect because thisstructure is not a ribosome or therough endoplasmic reticulum,which make proteins. Answer I isincorrect because this structure isnot the rough endoplasmic reticu-lum, the Golgi apparatus, or avesicle, which move proteinsthrough the cell.

Answers

1. D

2. G

3. D

4. I

5. Smaller pieces of food have a larger surface-area-to-volume ratio than larger pieces.

6. C

7. F

Standardized Test Prep


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