The Life of a Cell - images.pcmac.orgimages.pcmac.org/SiSFiles/Schools/AL/FayetteCounty/FayetteCounty... · The Life of a Cell A cell is the most basic unit of living organisms. No

The Life of a CellThe Life of a Cell

A cell is the most basic unit of livingorganisms. No matter how complex anorganism becomes, at its core is a collectionof cells. As a cell grows in size, it eventuallydivides to form two identical cells. In manyorganisms, cells work together, formingmore complex structures.

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UNIT CONTENTSUNIT CONTENTS

The Chemistry of Life

A View of the Cell

Cellular Transport and the Cell Cycle

Energy in a Cell

Unit 3Unit 3

The Life of a CellBIODIGESTBIODIGEST

UNIT PROJECTUNIT PROJECT

Use the Glencoe Science Web site for more project

activities that are connected to this unit.science.glencoe.com

BIOLOGY

http://science.glencoe.com

144 THE CHEMISTRY OF LIFE

The Chemistry of Life

What You’ll Learn■ You will relate the structure

of an atom to how it interacts with other atoms.

■ You will explain how water is important to life.

■ You will compare the role of carbon compounds inorganisms.

Why It’s ImportantLiving organisms are made ofsimple elements as well as com-plex carbon compounds. Withan understanding of these ele-ments and compounds, you willbe able to relate them to howliving organisms function.

Go through the chapter andobserve the structure diagrams.Write down any questions youhave. As you read the chapter,try to answer the questionsfrom the reading material.

To find out more about cellchemistry, visit the GlencoeScience Web site.science.glencoe.com

READING BIOLOGYREADING BIOLOGY

6

The gold shown in this scanning-tunneling microscopepicture is made ofatoms. Atoms make up the colorful frogs andthe element gold.

ChapterChapter

BIOLOGY

Magnification:32 900�


Section

Elements Everything—whether it is a rock,

frog, or flower—is made of sub-stances called elements. Suppose youfind a nugget of pure gold. You couldgrind it into a billion bits of powderand every particle would still be gold.You could treat the gold with everyknown chemical, but you could neverbreak it down into simpler sub-stances. That’s because gold is an ele-ment. An element is a substance thatcan’t be broken down into simplerchemical substances. On Earth, 90elements occur naturally.

Natural elements in living things

Of the 90 naturally occurring ele-ments, only about 25 are essential toliving organisms. Table 6.1 lists someelements found in the human body.Notice that only four of the 90 ele-ments—carbon, hydrogen, oxygen,and nitrogen—make up more than96 percent of the mass of a human.Each element is identified by a one-or two-letter abbreviation called asymbol. For example, the symbol Crepresents the element carbon, Carepresents calcium, and Cl representschlorine.

6.1 ATOMS AND THEIR INTERACTIONS 145

What makes a living thingdifferent from a nonlivingthing? Are the particles

that make up a rock different fromthose of a frog or a clam? The differ-ence between living and nonlivingthings may be readily apparent toyou. For example, you may haveplayed a game of basketball yesterday,something you would not expect arock to do. We know, however,that living things have a greatdeal in common with rocks,CDs, computer chips, andother nonliving objects.Both living and nonliv-ing things are composedof the same basic buildingblocks called atoms.

SECTION PREVIEW

ObjectivesRelate the particlestructure of an atom to the identity of elements.Relate the formation of covalent and ionicchemical bonds to thestability of atoms.Distinguish mixturesand solutions.Define acids and bases and relate theirimportance to biologicalsystems.

Vocabularyelementatomnucleusisotopecompoundcovalent bondmoleculeionionic bondmetabolismmixturesolutionpHacidbase

6.1 Atoms and TheirInteractions

All things are similarat the atomic level.


Figure 6.1Trace elements areneeded in smallamounts to controlcell metabolism.

Percent by Percent by mass in mass in

Element Symbol human body Element Symbol human body

Oxygen O 65.0 Iron Fe trace

Carbon C 18.5 Zinc Zn trace

Hydrogen H 9.5 Copper Cu trace

Nitrogen N 3.3 Iodine I trace

Calcium Ca 1.5 Manganese Mn trace

Phosphorus P 1.0 Boron B trace

Potassium K 0.4 Chromium Cr trace

Sulfur S 0.3 Molybdenum Mo trace

Sodium Na 0.2 Cobalt Co trace

Chlorine Cl 0.2 Selenium Se trace

Magnesium Mg 0.1 Fluorine F trace

Table 6.1 Elements that make up the human body

Trace elementsNotice that some of the elements

listed in Table 6.1, such as iron andmagnesium, are present in livingthings in very small amounts. Suchelements are known as trace ele-ments. They play a vital role in main-taining healthy cells in all organisms,as shown by the examples in Figure 6.1. Plants obtain trace ele-ments by absorbing them through

their roots. Animals get these impor-tant elements from the foods they eat.

Atoms: The BuildingBlocks of Elements

Elements, whether they are foundin living things or not, are made upof atoms. An atom is the smallestparticle of an element that has thecharacteristics of that element.

Mammals use iodine (I), anessential element, to producesubstances that affect the ratesof growth, development, andchemical activities in the body.

AA Plants must have magnesium(Mg) in order to form thegreen pigment chlorophyll thatcaptures light energy for theproduction of sugars.

BB A trace of fluorine, anotheressential element, binds withthe surface structure of teeth,making them resistant todecay.

CC

Mg++ ions

Electron energy levels

Nucleus

e – Nucleus8 protons (p )8 neutrons (n )

Oxygen atom

Hydrogen atom

p+

e –

e –

e –e –

e – e –

e –

+

0

e –


Figure 6.2Electrons move rapidly around atoms, forming electronclouds that have several energy levels.

The structure of an atomEach element has distinct charac-

teristics that result from the structureof the atoms that compose the element. For example, iron differsfrom aluminum because the structureof iron atoms differs from that of alu-minum atoms. Still, all atoms havethe same general arrangement. Thecenter of an atom is called thenucleus (NEW klee us) (plural,nuclei). It is made of positivelycharged particles called protons (p+)and particles that have no charge,called neutrons ( n0). All nuclei arepositively charged because of thepresence of protons.

Forming a cloud around thenucleus are even smaller, negativelycharged particles called electrons ( e–).If you’ve ever looked at a spinningfan, you’ve probably noticed that asthe fan blades turn, they appear to

form a blurry disk that occupies aspace around the center of the fan.Similarly, an electron cloud is thespace around the atom’s nucleus thatis occupied by these fast-movingelectrons. Although it is impossibleto pinpoint the exact location of anelectron because it is moving soquickly, the electron cloud is an areawhere it is most likely to be found.

Electron energy levelsElectrons travel around the nucleus

in certain regions known as energylevels, as indicated by Figure 6.2.Each energy level has a limited capac-ity for electrons. Because the firstenergy level is the smallest, it can holda maximum of only two electrons.The second level is larger and canhold a maximum of eight electrons.The third level is larger yet and canhold 18 electrons. For example,

Oxygen has two electrons in its first energylevel and six electrons in the second level.

BB

Hydrogen, the simplest atom, has justone electron in its first energy level andone proton in its nucleus.

CCAn atom has a nucleus and electrons incloudlike energy levels.

AA

the oxygen atom in Figure 6.2 has atotal of eight electrons. Two elec-trons fill the first energy level. Theremaining six electrons occupy thesecond level.

Atoms contain equal numbers ofelectrons and protons; therefore,they have no net charge. The hydro-gen (H) atom in Figure 6.2 has justone electron and one proton. Oxygen(O) has eight electrons and eight pro-tons. Figure 6.3 shows three otherelements whose properties differbecause of their atomic structure.

Isotopes of an ElementAtoms of an element sometimes

contain different numbers of neutronsin the nucleus. Atoms of the sameelement that have different numbersof neutrons are called isotopes (I suhtohps) of that element. For example,most carbon nuclei contain six neu-trons. However, some have seven oreight neutrons. Each of these atomsis an isotope of the element carbon.Scientists refer to isotopes by givingthe combined total of protons andneutrons in the nucleus. Thus, themost common carbon atom isreferred to as carbon-12 because ithas six protons and six neutrons.Other isotopes of carbon include car-bon-13 and carbon-14.

Isotopes are often useful to scien-tists. The nuclei of some isotopes,such as carbon-14, are unstable andtend to break apart. As nuclei break,they give off radiation. These iso-topes are said to be radioactive.Because radiation is detectable and


Figure 6.3The properties of anelement are deter-mined by the struc-ture of its atoms. Asyou can see, copper,gold, and carbonhave very differentproperties.

Figure 6.4Radioactive isotopes are used in medicine todiagnose and/or treat some diseases.

Radioactive iodine (I) introduced into thebody is absorbed by the thyroid gland. Bydetecting the radioactive iodine taken up, thefunction of the thyroid gland can be mea-sured. The thyroid scan on the left is normaland the scan on the right is overactive.

AA

Radiation given off when radioactive isotopesbreak apart is deadly to rapidly growing cancercells. The patient is being treated with radiationfrom a radioactive isotope of cobalt (Co).

BB

What information can be gained from seeing thenucleus of an atom? Looking at a model of an atom’snucleus can reveal certain information about that particularatom. Models may help predict electron number, position ofelectrons in energy levels, and how isotopes of an elementdiffer from each other.

AnalysisExamine diagrams A and B. Both are models of an atom

of beryllium. Only the nucleus of each atom is shown.

Thinking Critically1. What is the neutron number for A? For B?2. Which diagram represents an isotope of beryllium?

Explain how you were able to tell.3. How many electrons are present in atoms A and B?

Explain how you were able to tell.4. How many energy levels would be present for A and B?

How might the electrons in A and B be distributed inthese levels?

Problem-Solving Lab 6-1Problem-Solving Lab 6-1 Interpreting ScientificIllustrations

can damage or kill cells, scientistshave developed some useful applica-tions for radioactive isotopes, asdescribed in Figure 6.4.

Atomic models like those discussedin the Problem-Solving Lab on thispage help scientists and studentsvisualize the structure of atoms andunderstand complex intermolecularinteractions.

Compounds and Bonding

Water is a substance that everyoneis familiar with; however, water is notan element. Rather, water is a type ofsubstance called a compound. Acompound is a substance that iscomposed of atoms of two or moredifferent elements that are chemicallycombined. Water(H2O) is a com-pound composedof the elementshydrogen andoxygen. If youpass an electriccurrent throughwater, it breaks

down into these elements. Just as thecombined ingredients in a pizzaresult in a tasty meal, you can see inFigure 6.5 that the properties of acompound are different from thoseof its individual elements.

How covalent bonds formMost matter is in the form of com-

pounds. But how and why do atomscombine, and what is it that holds theatoms of unlike components togetherin a compound? Atoms combine withother atoms only when conditionsare right, and they do so to becomemore stable.


Figure 6.5Table salt is made from the elementssodium (Na) and chlorine (Cl). The flaskcontains the poisonous, yellow-greenchlorine gas. The lump of silver-whitemetal is the element sodium. The whitecrystals of table salt no longer resembleeither sodium or chlorine.

AA BBBeryllium nucleus Beryllium nucleus

Most common form

Proton

Remember electron energy levels?For most elements, an atom becomesstable when its outermost energylevel is full, such as having eight elec-trons in the second level. An excep-tion is hydrogen, which becomes sta-ble when its first energy level is full(two electrons). How do elements fillthe energy levels and become stable?One way is to share electrons withother atoms.

For example, two hydrogen atomscan combine with each other by shar-ing their electrons, as shown inFigure 6.6. As you know, individualatoms of hydrogen contain only oneelectron. Each atom becomes stableby sharing its electron with the otheratom. The two shared electronsmove about the first energy level ofboth atoms. The attraction of the

positively charged nuclei for theshared, negatively charged electronsholds the atoms together. When twoatoms share electrons, such as hydro-gen sharing with oxygen in water, theforce that holds them together iscalled a covalent bond (koh VAY

lunt). Most compounds in organismshave covalent bonds. Examplesinclude sugars, fats, proteins, andwater.

A molecule is a group of atomsheld together by covalent bonds andhaving no overall charge. A moleculeof water is represented by the chemi-cal formula H2O. The subscript 2represents two atoms of hydrogen(H) combined with one atom of oxy-gen (O). As you will see, many com-pounds in living things have morecomplex formulas.


Figure 6.6Sometimes atoms combine by sharing electronsto form covalent bonds.

Hydrogen gas (H2) exists as two hydrogenatoms sharing electrons with each other.The electrons move around the nuclei ofboth atoms.

AA

When two hydrogensshare electrons withoxygen, they formcovalent bonds toproduce a moleculeof water.

BB

View an ani-mation of covalentbonding in thePresentation Builderof the InteractiveCD-ROM.

CD-ROM

+ +

–

–

p p

e

e

Hydrogen molecule

p+

p+

e–e–

e–

e–

e–

e–

e–

e–

e–e

Watermolecule

+

n088

p

–

How ionic bonds formNot all atoms bond with each

other by sharing electrons.Sometimes atoms combine with eachother by gaining or losing electronsin their outer energy levels. An atom(or group of atoms) that gains orloses electrons has an electricalcharge and is called an ion. An ion isa charged particle.

A different type of chemical bondholds ions together. The bondformed between a sodium atom (Na)and chlorine atom (Cl) is a goodexample of this. A sodium atom con-tains 11 electrons, including one inthe third energy level. A chlorineatom has 17 electrons, with the outerlevel holding seven electrons. Whensodium and chlorine combine, thesodium atom loses one electron, andthe chlorine atom gains it. Thus,with eight electrons in its outer level,the chlorine ion formed is stable andhas a negative charge. Sodium haslost the one electron that was in itsthird energy level. Thus, the sodiumion is stable and has a positivecharge. The attractive force betweentwo ions of opposite charge is knownas an ionic bond. The bond betweensodium and chlorine is an ionic bond,as shown in Figure 6.7.

Ionic compounds are less abundantin living things than are covalent

molecules, but ions are important inbiological processes. For example,sodium and potassium ions arerequired for transmission of nerveimpulses. Calcium ions are necessaryfor muscles to contract. Plant rootsabsorb essential minerals in the formof ions.

Chemical ReactionsWhen chemical reactions occur,

bonds between atoms are formed orbroken, causing substances to com-bine and recombine as different mole-cules. In organisms, chemical reac-tions occur over and over inside cells.All of the chemical reactions thatoccur within an organism are referredto as that organism’s metabolism.These reactions break down andbuild molecules that are important


Figure 6.7The positive chargeof a sodium ionattracts the negativecharge of a chlorineion, and the elementscombine with anionic bond that formsexplosively, as shownin the photograph.

OriginWORDWORD

metabolism From the Greekword metabole,meaning “change.”Metabolisminvolves manychemical changes.

View an animation of ionicbonding in thePresentation Builderof the InteractiveCD-ROM.

CD-ROM

+

+

+

11p +

12 n 0

2e –8e –

e – 7e –

+Sodium atom Chlorine atom

Na Cl NaCl

Sodium + ion Chloride – ion

Ionic bond

17 p +

18 n 0

2e –8e –

11p +

12 n 0

2e –8e –

8e –

17 p +

18 n 0

2e –8e –

for the functioning of organisms.Scientists represent chemical reac-tions by writing chemical equations.Chemical equations use symbols andformulas to represent each elementor substance.

Writing chemical equationsThe events that take place when

hydrogen gas combines with oxygengas are shown in Figure 6.8. Sub-stances that undergo chemical reac-tions, such as hydrogen and oxygen,are called reactants. Substancesformed by chemical reactions, such aswater, are called products.

It’s easy to tell how many moleculesare involved in a reaction because thenumber before each chemical formulaindicates the number of molecules ofeach substance. The subscript num-bers in a formula indicate the numberof atoms of each element in a mole-cule of the substance. A molecule oftable sugar can be represented by theformula C12H22O11. The lack of anumber before a formula or under asymbol indicates that only one atomor molecule is present.

Looking at the equation in Figure 6.8, you can see that eachmolecule of hydrogen gas is com-posed of two atoms of hydrogen.


Figure 6.8This balanced equation shows two molecules of hydro-gen gas reacting with one molecule of oxygen gas toproduce two molecules of water.

Likewise, a molecule of oxygen gas is made of two atoms. Perhaps theeasiest way to understand chemicalequations is to know that atoms areneither created nor destroyed inchemical reactions. They are simplyrearranged. Therefore, an equation iswritten so that the same numbers ofatoms of each element appear onboth sides of the arrow. In otherwords, equations must always bewritten so that they are balanced.

Mixtures and SolutionsWhen elements combine to form a

compound, the elements no longerhave their original properties. Whathappens if substances are just mixedtogether and do not combine chemi-cally? A mixture is a combination ofsubstances in which the individualcomponents retain their own proper-ties. Figure 6.9 shows a mixture ofsand and sugar. If you stirred sandand sugar together, you could still tellthe sand from the sugar. Neithercomponent of the mixture wouldchange, nor would they combinechemically. You could easily separatethem by adding water to dissolve thesugar and then filtering the mixtureto collect the sand.

+

+2H2 O2 2H2O

A solution is a mixture in whichone or more substances (solutes) aredistributed evenly in another sub-stance (solvent). In other words, onesubstance is dissolved in another andwill not settle out of solution. Youmay remember making Kool-Aidwhen you were younger. The sugarmolecules in Kool-Aid dissolve easilyin water to form a solution, as shownin Figure 6.10.

Solutions are important in livingthings. In organisms, many vital sub-stances, such as sugars and mineralions, are dissolved in water. The moresolute that is dissolved in a givenamount of solvent, the greater is thesolution’s concentration (strength).The concentration of a solute isimportant to organisms. Organismscan’t live unless the concentration ofdissolved substances stays within aspecific, narrow range. Organismshave many mechanisms to keep the


Figure 6.10The sugar molecules in the Kool-Aid dissolvein the water, making a solution. Here, sugaris the solute and water is the solvent.

Figure 6.9This combina-tion of sand and sugar illustrates

a mixture. Bothcomponentsretain theiroriginal properties.

concentrations of molecules and ionswithin this range. For example, thepancreas and other organs in yourbody produce substances that keepthe amount of sugar dissolved in yourbloodstream within a critical range.

Watermolecules

Sugarmolecules

Sugarcrystal


Figure 6.11The pH scale rangesfrom 0 to 14. Lemonand tomato are acidic(pH < 7). Pure water is neutral (pH 7).Ammonia (pH 11) is basic.

0 1 2 3 4 5 6 7

NeutralMore acidic

8 9 10

Acids and basesChemical reactions can occur only

when conditions are right. For exam-ple, a reaction might depend on tem-perature, the availability of energy, ora certain concentration of a substancedissolved in solution. Chemical reac-tions in organisms also depend onthe pH of the environment. The pHis a measure of how acidic or basic asolution is. A scale with values rang-ing from 0 to 14 is used to measurepH. Indicators like pH paper describethe pH of substances like thoseshown in Figure 6.11.

Substances with a pH below 7 areacidic. An acid is any substance thatforms hydrogen ions (H+) in water.When the compound hydrogen chlo-ride (HCl) is added to water, hydro-gen ions (H+) and chloride ions (Cl–)are formed. Thus, hydrogen chloridein solution is called hydrochloricacid. This acidic solution contains anabundance of H+ ions and has a pHbelow 7.

Substances with a pH above 7 arebasic. A base is any substance thatforms hydroxide ions (OH–) in water.For example, if sodium hydroxide(NaOH) is dissolved in water, it forms

CAREERS IN BIOLOGY

Weed/Pest ControlTechnician

A career working with chemicals

does not always require aPh.D. Weed and pestcontrol technicians usechemicals to get rid of unwanted weeds,insects, and other pests.

Skills for the JobAfter high school,

most technicians receive on-the-job training in pestcontrol or take correspondencecourses to earn a degree in this field. In many states, you must pass a test to become licensed.

As a technician, you may visit homes, office buildings,restaurants, hotels, and other places where insects, animals,or weeds have become a problem. You will choose the cor-rect chemical and form, such as a spray or gas, to get rid ofor prevent infestations of flies, roaches, termites, or othercreatures. You will select different chemicals to deal withweeds. You might also set traps to catch rats, mice, moles, or other animals.

To find out more about careers in related fields, visit the Glencoe Science Web site. science.glencoe.com

LemonpH 2

TomatopH 4

WaterpH 7

EggpH 8

AntacidpH 10

MilkpH 6

BIOLOGY


Determine pH The pH of a solu-tion is a measurement of how acidic or basic that solution is. An easy way to measure the pH of a solution is to use pH paper.

Procedure ! Pour a small amount (about 5 mL)

of each of the following into separate clean, small beakers or other small glass containers: lemon juice, householdammonia solution, liquid detergent, shampoo, and vinegar.

@ Dip a fresh strip of pH paper briefly into each solution andremove.

# Compare the color of the wet paper with the pH colorchart; record the pH of each material. CAUTION: Washyour hands after handling lab materials.

Analysis1. Which solutions are acids?2. Which solutions are bases?3. What ions in the solution caused the pH paper to change?

Which solution contained the highest concentration ofhydroxide ions? How do you know?

MiniLab 6-1MiniLab 6-1

sodium ions (Na+) and hydroxideions (OH–). This basic solution con-tains an abundance of OH– ions andhas a pH above 7.

Acids and bases are important toliving systems, but strong acids andbases can also be dangerous. Forexample, some plants grow well onlyin acidic soil, whereas others requiresoil that is basic. Another example,orange juice, is a common acid thatcan corrode immature teeth if theacid is not later rinsed away. The

MiniLab shown here describes howyou can investigate several householdsolutions to determine if they areacids or bases.


Household Solutions

More basic

11 12 13 14

Section AssessmentSection AssessmentUnderstanding Main Ideas1. Describe where the electrons are located in

an atom.2. A nitrogen atom contains seven protons,

seven neutrons, and seven electrons. Describe the structure of a nitrogen atom. Use a labeled drawing to help you explain this structure.

3. How does the formation of an ionic bond differ from the formation of a covalent bond?

4. What can you say about the proportion ofhydrogen ions and hydroxide ions in a solutionthat has a pH of 2?

Thinking Critically5. A fluorine atom has nine electrons. Make an

energy level diagram of fluorine. How manyelectrons would be needed to fill its outer level?

6. Interpreting Scientific Illustrations Study thediagram in Figure 6.10, which shows the processof a polar compound dissolving in water.Describe the process step-by-step. Tell what thewater molecules are doing and why. Describewhat is happening to the sugar molecules andwhy. Describe the nature of the mixture after thecompound dissolves. For more help, refer toThinking Critically in the Skill Handbook.

SKILL REVIEWSKILL REVIEW

Experimenting HouseholdammoniapH 11

Draincleaner

pH 13

Section


Water and ItsImportance

Water is perhaps the most impor-tant compound in living organisms.Most life processes can occur onlywhen molecules and ions are free tomove and collide with one another.This condition exists when they aredissolved in water. Water also servesas a means of material transportationin organisms. For example, blood andplant sap, which are mostly water,transport materials in animals andplants. In fact, water makes up 70 to95 percent of most organisms.

Water is polarSometimes, when atoms form

covalent bonds, they do not share theelectrons equally. The water mole-cule pictured in Figure 6.12 shows

that the shared electrons areattracted by the oxygen atom morestrongly than by the hydrogen atoms.As a result, the electrons spend moretime near the oxygen atom than theydo near the hydrogen atoms.

When atoms in a covalent bond do not share the electrons equally,they form a polar molecule. A polarmolecule is a molecule with anunequal distribution of charge; thatis, each molecule has a positive endand a negative end. Water is anexample of a polar molecule. Polarwater molecules attract each other aswell as ions and other polar mole-cules. Because of this attraction,water can dissolve many ionic com-pounds, such as salt, and many otherpolar molecules, such as sugar.

Water molecules also attract otherwater molecules. The positively

Most of us take water forgranted. We turn on thekitchen faucet at home to

get a drink and expect water tocome out of the faucet. When wehike in a forest, we expect water tobe flowing in the streams and fill-ing the lakes. Most of the time, wedon’t think about how importantwater is to our life and the life ofother organisms on Earth.

SECTION PREVIEW

ObjectivesRelate water’s uniquefeatures to polarity.Explain how theprocess of diffusionoccurs and why it isimportant to cells.

Vocabularypolar moleculehydrogen bonddiffusiondynamic equilibrium

6.2 Water and Diffusion

Water is vital to theliving world.

charged hydrogen atoms of onewater molecule attract the negativelycharged oxygen atoms of anotherwater molecule. This attraction ofopposite charges between hydrogenand oxygen forms a weak bond calleda hydrogen bond. Hydrogen bondsare important to organisms becausethey help hold many large molecules,such as proteins, together.

Also because of its polarity, waterhas the unique property of being ableto creep up thin tubes. Plants in par-ticular take advantage of this prop-erty, called capillary action, to getwater from the ground. Capillaryaction and the tension on the water’ssurface, which is also a result ofpolarity, play major roles in gettingwater from the soil to the tops ofeven the tallest trees.

Water resists temperature changes

Water resists changes in tempera-ture. Therefore, water requires moreheat to increase its temperature than

do most other common substances.Likewise, water loses a lot of heatwhen it cools. In fact, water is like aninsulator that helps maintain a steadyenvironment when conditions mayfluctuate. Because cells exist in anaqueous environment, this propertyof water is extremely important tocellular functions as it helps cellsmaintain an optimum environment.

Water expands when it freezesWater is one of the few substances

that expands when it freezes. Becauseof this property, ice is less dense thanliquid water and floats as it forms in apond. Use the Problem-Solving Lab onthe next page to investigate thisproperty. Water expands as it freezesinside the cracks of rocks. As itexpands, it often breaks apart therocks. Over long time periods, thisprocess forms soil.

The properties of water make it anexcellent vehicle for carrying sub-stances in living systems. Another wayto move substances is by diffusion.

6.2 WATER AND DIFFUSION 157

p+

Hydrogen atom

Oxygen atom

Hydrogen atom

p+

e–

e–

e–

e–

e–e–

e–

e–

e–

e–

8p+

8n0

Figure 6.12Because of water’sbent shape, the pro-truding oxygen end of the molecule has aslight negative charge,and the ends with protruding hydrogenatoms have a slight positive charge.

+

+

Negatively-charged end

Positively-charged end

Why does ice float? Most liquids con-tract when frozen. Water is different; it expands. Freezing changes the density of water to that of ice, which allows ice to float. Density refers to compactness and is often described as the mass of a substance per unit of volume. A mathematical expres-sion of density would read as follows:

AnalysisExamine the following table. It shows the volume and

mass for a sample of water and ice.

Thinking Critically1. How does the density of ice compare with the density of

water? Use specific values and proper units expressingdensity in your answer. Which of the two, ice or water, isless compact? Explain your answer.

2. Are the molecules of water moving closer togethertoward one another or farther apart as water freezes?Explain your answer.

3. Explain why a glass bottle filled with water will shatter ifplaced in a freezer.

4. Explain why ice forming within a living organism mayresult in its death.

Problem-Solving Lab 6-2Problem-Solving Lab 6-2 Using NumbersDiffusion

All objects in motion have energyof motion called kinetic energy. Amoving particle of matter moves in astraight line until it collides withanother particle, much like the Ping-Pong balls shown in Figure 6.13.After the collision, both particlesrebound. Particles of matter, like thePing-Pong balls, are in constantmotion, colliding with each other.

Early observations:Brownian motion

In 1827, Scottish scientist RobertBrown used a microscope to observepollen grains suspended in water. Henoticed that the grains moved con-stantly in little jerks, as if beingstruck by invisible objects. Thismotion, he thought, was the result ofa life force hidden within the pollengrains. However, when he repeatedhis experiment using dye particles,which are nonliving, he saw the sameerratic motion. This motion is nowcalled Brownian motion. Brown hadno explanation for the motion he saw,but today we know that Brown wasobserving evidence of the randommotion of molecules. This was theinvisible “life” that was moving thetiny visible particles. The random


Figure 6.13Molecules, like thesePing-Pong balls, havekinetic energy, theenergy of motion.

Source of sample Mass (g)Volume (cm3)

Water

Ice

126

126

126

140

Data Table

Density = MassVolume

Examine the Rate of Diffusion In this lab, you will place a small potato cube in a solu-tion of potassium permanganate and observe how far the dark purple color diffuses into the potato after a given length of time.

Procedure! Using a single-edge razor blade, cut a cube 1 cm on each

side from a raw, peeled potato. CAUTION: Be careful withsharp objects. Do not cut objects while holding them inyour hand.

@ Place the cube in a cup or beaker containing the purplesolution. The solution should cover the cube. Note andrecord the time. Let the cube stand in the solution forbetween 10 and 30 minutes.

# Using forceps, remove the cube from the solution andnote the time. Cut the cube in half.

$ Measure, in millimeters, how far the purple solution has diffused, and divide this number by the time youallowed your potato to remain in the solution. This is the diffusion rate.

Analysis1. How far did the purple solution diffuse?2. What was the rate of diffusion per minute?

MiniLab 6-2MiniLab 6-2 Applying Concepts

movement that Brown observed ischaracteristic of gas, liquid, and somesolid molecules.

The process of diffusionMolecules of different substances

that are in constant motion have aneffect on each other. For example, ifyou layer pure corn syrup on top ofcorn syrup colored with food color-ing in a beaker as illustrated inFigure 6.14, over time you willobserve the colored corn syrup mix-ing with the pure corn syrup. Thismixture is the result of the randommovement of corn syrup molecules.Diffusion is the net movement ofparticles from an area of higher con-centration to an area of lower con-centration. Diffusion results becauseof the random movement of particles(Brownian motion). The corn syrupin Figure 6.14 will begin to diffusein hours but will take months to mixcompletely.

Diffusion is a slow process becauseit relies on the random molecularmotion of atoms. Three key factors,concentration, temperature, andpressure, affect the rate of diffusion.The concentration of the substancesinvolved is the primary controllingfactor. The more concentrated thesubstances, the more rapidly diffu-sion occurs. For example, loose sugarplaced in water will diffuse morerapidly than will a more concentratedcube of sugar. Two external factors,temperature and pressure, can speedthe process of diffusion. An increasein temperature or agitation will cause more rapid molecular move-ment and will speed diffusion.Similarly, increasing pressure willaccelerate molecular movement and,therefore, diffusion. With commonmaterials, you can use the MiniLabshown here to learn more about dif-fusion in a cell.

6.2 WATER AND DIFFUSION 159

Figure 6.14The random movementof particles of corn syrupwill cause the coloredsample to diffuse intothe uncolored sample.

Material moving outof cell equals materialmoving into cell


Figure 6.15When a cell is in dynamic equilibriumwith its environment, materials moveinto and out of the cell at equal rates. As a result, there is no net change in concentration inside the cell.

Section AssessmentSection AssessmentUnderstanding Main Ideas1. Explain why water is a polar molecule.2. How does a hydrogen bond compare to a

covalent bond?3. What property of water explains why it can

travel to the tops of trees?4. What is the eventual result of diffusion? Describe

concentration prior to and at this point.

Thinking Critically5. Explain why water dissolves so many different

substances.

6. Inferring If a substance is known to enter a cellby diffusion, what effect would raising the tem-perature have on the cell? For more help, referto Thinking Critically in the Skill Handbook.


The results of diffusionAs the colored corn syrup contin-

ues to diffuse into the pure cornsyrup, the two will become evenlydistributed eventually. After thispoint, the atoms continue to moverandomly and collide with oneanother; however, no further change

in concentration will occur. Thiscondition, in which there is continu-ous movement but no overall con-centration change, is called dynamicequilibrium. Figure 6.15 illustratesdynamic equilibrium in a cell.

Diffusion in living systemsMost substances in and around a

cell are in water solutions where theions and molecules of the solute aredistributed evenly among water mole-cules, like the Kool-Aid and waterexample. The difference in concen-tration of a substance across space iscalled a concentration gradient.Because ions and molecules diffusefrom an area of higher concentrationto an area of lower concentration,they are said to move with the gradi-ent. If no other processes interfere,diffusion will continue until there isno concentration gradient. At thispoint, dynamic equilibrium occurs.Diffusion is one of the methods bywhich cells move substances in andout of the cell.

Diffusion in biological systems isalso evident outside of the cell andcan involve substances other thanmolecules in an aqueous environ-ment. For example, oxygen (a gas)diffuses into the capillaries of thelungs because there is a greater con-centration of oxygen in the air sacs ofthe lungs than in the capillaries.

Section

6.3 LIFE SUBSTANCES 161

Role of Carbon in Organisms

A carbon atom has four electronsavailable for bonding in its outerenergy level. In order to become sta-ble, a carbon atom forms four cova-lent bonds that fill its outer energylevel. Look at the illustration show-ing carbon atoms and bond types in Figure 6.16. Carbon can bond withother carbon atoms, as well as withmany other elements. When eachatom shares two electrons, a doublebond is formed. A double bond isrepresented by two bars between car-bon atoms. When each shares threeelectrons, a triple bond is formed.Triple bonds are represented by threebars drawn between carbon atoms.

Did you ever hear the saying,“You are what you eat”? It’sat least partially true because

the compounds that form the cells andtissues of your body are produced fromsimilar compounds in the foods youeat. Common to most of these foodsand to most substances in organisms isthe element carbon. The first carboncompounds that scientistsstudied came from organ-isms and were calledorganic compounds.

SECTION PREVIEW

ObjectivesClassify the variety oforganic compounds.Describe how polymersare formed and brokendown in organisms.Compare the chemicalstructures of carbohy-drates, lipids, proteins,and nucleic acids, andrelate their importanceto living things.

Vocabularyisomerpolymercarbohydratelipidproteinamino acidpeptide bondenzymenucleic acidnucleotide

6.3 Life Substances

Carbon defines living organisms.

Figure 6.16When two carbonatoms bond, they shareone, two, or three elec-trons each and form acovalent bond.

Double bond Triple bond

Single bond

When carbon atoms bond to eachother, they can form straight chains,branched chains, or rings. In addi-tion, these chains and rings can havealmost any number of carbon atomsand can include atoms of other ele-ments as well. This property makes ahuge number of carbon structurespossible. In addition, compoundswith the same simple formula oftendiffer in structure. Compounds thathave the same simple formula butdifferent three-dimensional struc-tures are called isomers (I suh murz).The glucose and fructose moleculesshown in Figure 6.17 have the samesimple formula, C6H12O6, but differ-ent structures.

Molecular chainsCarbon compounds also vary

greatly in size. Some compoundscontain just one or two carbonatoms, whereas others contain tens,hundreds, or even thousands of car-bon atoms. These large moleculesare called macromolecules. Proteinsare examples of macromolecules inorganisms. Cells build macromole-cules by bonding small molecules

together to form chains called poly-mers. A polymer is a large moleculeformed when many smaller mole-cules bond together.

Condensation is the chemical reac-tion by which polymers are formed.In condensation, the small moleculesthat are bonded together to make apolymer have an –H and an –OHgroup that can be removed to formH–O–H, a water molecule. The sub-units become bonded by a covalentbond as shown in Figure 6.18.

Hydrolysis is a method by whichpolymers can be broken apart.Hydrogen ions and hydroxide ionsfrom water attach to the bondsbetween the subunits that make upthe polymer, thus breaking the poly-mer as shown in Figure 6.18.

The structure of carbohydratesYou may have heard of runners

eating large quantities of spaghetti orother starchy foods the day before arace. This practice is called “carbohy-drate loading.” It works because car-bohydrates are used by cells to storeand release energy. A carbohydrateis an organic compound composed of


OriginWORDWORD

polymerFrom the Greekwords poly, meaning“many,” and meros,meaning “part.” Apolymer has manybonded subunits(parts).

hydrolysisFrom the Greekwords hydro, mean-ing “water,” andlysis, meaning “tosplit or loosen.” In hydrolysis, molecules are splitby water.

Figure 6.17The differentarrangement ofhydrogen and oxy-gen atoms aroundeach carbon atomgives glucose andfructose moleculesdifferent chemicalproperties. When glucose and fructosecombine, they formthe disaccharidesucrose, also knownas table sugar.

OH

OH

CH2OH CH2OH

Glucose

O

HOOH

OH

O HHOCH2

Fructose

Sucrose

O

HO

+ +

OH

O H2O

CH2OH

O

HOOH

OH

CH2OH

HOCH2O

HO

H OH

HOH

HOH

H

OH

H

OH H

OH

HOH

H2O

H OH

OH

H

H2O

Condensation

Subunits

Subunits

Hydrolysis

Polymer

Starch

Glucosesubunits

Glycogen

Glucosesubunits

carbon, hydrogen, and oxygen with aratio of about two hydrogen atomsand one oxygen atom for every car-bon atom.

The simplest type of carbohydrateis a simple sugar called a monosac-charide (mahn uh SAK uh ride).Common examples are the isomersglucose and fructose. Two monosac-charide molecules can link togetherto form a disaccharide, a two-sugarcarbohydrate. When glucose andfructose combine by a condensationreaction, a molecule of sucrose,known as table sugar, is formed.

The largest carbohydrate mole-cules are polysaccharides, polymerscomposed of many monosaccharidesubunits. The starch, glycogen, andcellulose pictured in Figure 6.19 areexamples of polysaccharides. Starchconsists of highly branched chains ofglucose units and is used as food stor-age by plants in food reservoirs suchas seeds and bulbs. Mammals storefood in the liver in the form of glyco-gen, a glucose polymer similar to


Figure 6.18Condensation and Hydrolysis

Figure 6.19Look at the structural differences among the polysaccha-rides starch, glycogen, and cellulose. Notice that all threeare polymers of glucose.

Polymers are formed by condensationand can be broken by hydrolysis, thereaction that occurs when water isintroduced to a polymer.

AA

The silk produced by thesecaterpillars is a protein, abiological polymer formedby condensation reactions.

BB

Glucosesubunits

Crosslinkbonds

Cellulose

Potato

Liver

Cotton

starch but more highly branched.Cellulose is another glucose polymerthat forms the cell walls of plants andgives plants structural support.Cellulose is also made of long chainsof glucose units hooked together inarrangements somewhat like a chain-link fence.

The structure of lipidsIf you’ve ever tried to lose weight,

you may have wished that lipids (fats)never existed. Lipids, however, areextremely important for the properfunctioning of organisms. Lipids areorganic compounds that have a largeproportion (much greater than 2 to1) of C–H bonds and less oxygenthan carbohydrates. For example, alipid found in beef fat has the for-mula C57H110O6.

Lipids are commonly called fats

and oils. They are insoluble in waterbecause their molecules are nonpolar.Recall that a nonpolar molecule isone in which there is no net electricalcharge and, therefore, lipids are notattracted by water molecules. Cellsuse lipids for energy storage, insula-tion, and protective coatings. In fact,lipids are the major components ofthe membranes that surround all liv-ing cells. Lipids are also used in foodpreparation as discussed in theBioTechnology feature. The most com-mon type of lipid, shown in Figure6.20, consists of three fatty acidsbound to a molecule of glycerol.

The structure of proteinsProteins are essential to all life.

They provide structure for tissues andorgans and carry out cell metabolism.A protein is a large, complex polymer


CH2 CH2O

O

C CH2 CH2 CH2 CH2 CH2 CH2 CH2 CH2 CH2 CH2 CH2 CH2 CH2 CH3CH CH

CH2 CH2O

O

C CH2 CH2 CH2 CH2 CH2 CH2 CH2 CH2 CH2 CH2 CH2 CH2 CH2 CH3CH CH

CH2 CH2CH CH2O

O

C CH2 CH2 CH2 CH2 CH2 CH2 CH2 CH2 CH2 CH2 CH2 CH2 CH2 CH3

Glycerol

Unsaturated fatty acid

Unsaturated fatty acid

Saturated fatty acid

Double bond

Double bond

Figure 6.20Glycerol is a three-carbon molecule thatserves as a backbonefor the lipid mole-cule. Attached to theglycerol are threefatty acids.

The carbon atoms in saturatedfatty acids cannot bond with anymore hydrogen atoms.

AA

The carbon atoms in unsaturatedfatty acids can bond with morehydrogen atoms.

BB

Meat

Butter

Peanut butter

composed of carbon, hydrogen, oxy-gen, nitrogen, and usually sulfur. Thebasic building blocks of proteins arecalled amino acids, shown in Figure6.21a. There are 20 common aminoacids. These 20 building blocks, invarious combinations, make literallythousands of proteins. Therefore,proteins come in a large variety ofshapes and sizes. In fact, proteins varymore in structure than any other classof organic molecules.

Amino acids are linked togetherwhen an –H from one amino acidand an –OH group from anotheramino acid are removed to form awater molecule. The covalent bondformed between the amino acids, likethe bond labeled in Figure 6.21b, iscalled a peptide bond. The numberand order of amino acids in proteinchains determine the kind of protein.

Many proteins consist of two ormore amino acid chains that are heldtogether by hydrogen bonds.

Proteins are the building blocks ofmany structural components oforganisms, as illustrated in Figure6.22. Proteins are also important inthe contracting of muscle tissue,transporting oxygen in the blood-stream, providing immunity, regulat-ing other proteins, and carrying outchemical reactions.

Enzymes are important proteinsfound in living things. An enzyme isa protein that changes the rate of achemical reaction. In some cases,


Figure 6.22Proteins make upmuch of the structureof organisms, such ashair, fingernails,horns, and hoofs.

Figure 6.21Each amino acid contains a central carbon atom to which areattached a carboxyl group, a hydrogen atom, an amino group(–NH2), and a group (–R) that makes each amino acid different (a).Amino acids are linked together by peptide bands (b).

H Hydrogen atom

CarboxylAmino group NH2 — C — COOH group

R Variable group

Peptidebond

H H H

NH2 — C — C ——— N — C — COOH

R O R

a b

After thereaction, the enzymereleased is in itsoriginal shapeand can go on tocarry out the samereaction again andagain. In doing so,enzymes increase thespeed at which chemicalreactions occur without beingaltered themselves by the reaction.

44

Each substrate fits into an area ofthe enzyme called the active site. Thisfitting together is often compared to a lock-and-key mechanism. However,

the enzyme changes shape a littleto fit with the substrate. The

enzyme-substrate complexputs stress on the sub-

strate due to the bind-ing of the substrate to

the enzyme.

22


Action of Enzymes

A n enzyme enables molecules, called substrates, to undergo a chemical change to

form new substances, called products. The enzyme works due to an area on its surface that fits the shape of the substrate, called an active site. When the substrate fits the active site, it causes the enzyme to alter its shape slightly as shown below.

Critical Thinking How can an enzyme participate over and over in chemical reactions?

Lysozyme structure

INSIDESSTORTORYY

INSIDESubstrate

Activesite

Enzymes act on specific substrates, such as sucrose, a disaccharide made up of glucose and fructose bonded together.

11

The products arereleased; in this casethe sucrose bonds arehydrolyzed, releasingglucose and fructose.

33

H2O

Glucose Fructose

Substrate (Sucrose)

Active sites

Active site

Enzyme

Enzyme

Products

enzymes increase the speed of reac-tions that would otherwise occur soslowly you might think they wouldn’toccur at all.

Enzymes are involved in nearly allmetabolic processes. They speed thereactions in digestion of food.Enzymes also affect synthesis of mole-cules, and storage and release ofenergy. How do enzymes act like alock and key to facilitate chemicalreactions within a cell? Read theInside Story to find out. The BioLab atthe end of this chapter also experi-ments with enzymes.

The structure of nucleic acidsNucleic acids are another impor-

tant type of organic compound that isnecessary for life. A nucleic acid(noo KLAY ihk) is a complex macro-molecule that stores cellular informa-tion in the form of a code. Nucleicacids are polymers made of smallersubunits called nucleotides.

Nucleotides consist of carbon,hydrogen, oxygen, nitrogen, andphosphorus atoms arranged in threegroups—a base, a simple sugar, and aphosphate group—as shown inFigure 6.23. You have probablyheard of the nucleic acid DNA,which stands for deoxyribonucleic

acid. DNA is the master copy of anorganism’s information code. Theinformation coded in DNA containsthe instructions used to form all of anorganism’s enzymes and structuralproteins. Thus, DNA forms thegenetic code that determines how anorganism looks and acts. DNA’sinstructions are passed on every timea cell divides and from one genera-tion of an organism to the next.

Another important nucleic acid isRNA, which stands for ribonucleicacid. RNA is a nucleic acid thatforms a copy of DNA for use in mak-ing proteins. The chemical differ-ences between RNA and DNA areminor but important. A later chapterdiscusses how DNA and RNA worktogether to produce proteins.


Figure 6.23Each nucleic acid isbuilt of subunitscalled nucleotidesthat are formed froma sugar moleculebonded to a phos-phate group and anitrogen base.

Section AssessmentSection AssessmentUnderstanding Main Ideas1. List three important functions of lipids in living

organisms.2. Describe the process by which polymers in

living things are formed from smaller molecules.

3. How does a monosaccharide differ from a disaccharide?

Thinking Critically4. Enzymes are proteins that facilitate

chemical reactions. Based on your knowledge of enzymes, what might the result be if

one particular enzyme malfunctioned or was not present?

5. Making and Using Tables Make a table com-paring polysaccharides, lipids, proteins, andnucleic acids. List these four types of biologicalsubstances in the first column. In the next twocolumns, list the subunits that make up each substance and the functions of each in organ-isms. In the last column, provide some examplesof each from the chapter. For more help, refer toOrganizing Information in the Skill Handbook.


P

O

OHO

HO

OCH2

OH OH

NH2O N

NPhosphate Ribosesugar Nitrogen

base

View an animation of enzymeaction in the Presen-tation Builder of theInteractive CD-ROM.

CD-ROM

Does temperature affectan enzyme reaction?

T he compound hydrogen peroxide, H2O2, is a by-product of meta-bolic reactions in most living things. However, hydrogen peroxide is

damaging to delicate molecules inside cells. As a result, nearly all organ-isms contain the enzyme peroxidase, which breaks down H2O2 as it isformed. Potatoes are one source of peroxidase. Peroxidase speeds up thebreakdown of hydrogen peroxide into water and gaseous oxygen. Thisreaction can be detected by observing the oxygen bubbles generated.

YOUR OWNDESIGN

YOUR OWNDESIGN

ProblemDoes the enzyme peroxidase work

in cold temperatures? Does peroxi-dase work better at higher tem-

peratures? Does peroxidasework after being frozen orboiled?

HypothesesMake a hypothesis regarding how

you think temperature will affect therate at which the enzyme peroxidasebreaks down hydrogen peroxide. Con-sider both low and high temperatures.

ObjectivesIn this BioLab, you will:■ Observe the activity of an enzyme.■ Compare the activity of the enzyme

at various temperatures.

Possible Materialsclock or timer ice400-mL beaker hot platekitchen knife waxed papertongs or large forceps thermometer5-mm thick potato slices3% hydrogen peroxide

Safety PrecautionsBe sure to wash your hands before

and after handling the lab materials.Always wear goggles in the lab.

Skill HandbookUse the Skill Handbook if you need

additional help with this lab.

PREPARATIONPREPARATION

PLAN THE EXPERIMENTPLAN THE EXPERIMENT

1. Decide on a way to test yourgroup’s hypothesis. Keep theavailable materials in mind.

2. When testing the activity ofthe enzyme at a certain tem-perature, consider the lengthof time it will take for thepotato to reach that tempera-ture, and how the temperaturewill be measured.

3. To test for peroxidase activity,add 1 drop of hydrogen perox-ide to the potato slice andobserve what happens.

4. When heating a thin potatoslice, first place it in a smallamount of water in a beaker.Then heat the beaker slowly sothat the temperature of thewater and the temperature ofthe slice are always the same.Try to make observations atseveral temperatures between10°C and 100°C.

Check the PlanDiscuss the following points

with other groups to decide on thefinal procedure for your experiment.1. What data will you

collect? How will you record them?

2. What factors should be controlled?

3. What temperatures will you test?

4. How will you achieve those tem-peratures?

5. Make sure your teacher has approved your experimental plan before you proceed further.

6. Carry out your experiment. CAUTION: Be care-ful with chemicals and heat.Wash hands after lab.


Going FurtherGoing Further

Changing Variables To carry this experi-ment further, you may wish to use hydrogenperoxide to test for the presence of peroxi-dase in other materials, such as cut pieces of different vegetables. Also, test raw beefand diced bits of raw liver.

To find out more about enzymes, visit the

Glencoe Science Web site.science.glencoe.com

1. Checking Your HypothesisDo your data support or rejectyour hypothesis? Explain yourresults.

2. Analyzing Data At what temperature did peroxidase work best?

3. Identifying Variables What fac-tors did you need to control inyour tests?

4. Recognizing Cause and Effect Ifyou’ve ever used hydrogen perox-ide as an antiseptic to treat a cutor scrape, you know that it foamsas soon as it touches an open

wound. How can you account forthis observation?

ANALYZE AND CONCLUDEANALYZE AND CONCLUDE

BIOLOGY


In 1996, the Federal Food and DrugAdministration (FDA) approved the use of a

new fat substitute called Olestra. Other fat substi-tutes may contain fewer calories than fat, but theybreak down when exposed to high temperatures.Olestra can withstand the high temperaturesneeded to produce fried foods like potato chips.

If it isn’t fat, what is it? Most fat substitutesare molecules that are similar to fat but do nothave fat’s high calorie content. The oldest fatsubstitutes are based on carbohydrates and areused in salad dressings, dips, spreads, candy, andother foods.

Protein-based fat substitutes are also com-mon. The proteins go through a process calledmicroparticulation, in which they are formedinto microscopic round particles. This roundshape gives the substances a pleasing smoothness.Protein-based fat substitutes can be used in somecooked foods, but not fried foods.

A fat substitute made from fat Unlikeother fat substitutes, Olestra is based on actualfat molecules. It is made by surrounding a sugarmolecule, sucrose, with six to eight fatty acidmolecules. Naturally occurring dietary fats aremade of a glycerol molecule with three fattyacids attached. Because Olestra contains manyfatty acid molecules, the digestive system cannotbreak it down and it passes through the bodyundigested.

Olestra does have drawbacks. As Olestrapasses through the digestive system, it can absorband carry some fat-soluble vitamins and nutri-ents. These include vitamins A and E, plus betacarotene, which has been shown to help preventsome forms of cancer.

Fat substitutes will not replace natural fatsentirely, but products like Olestra give food

scientists some options when they are developingreduced-fat and reduced-calorie foods.


Are fake fatsfor real?

Most of us love snacks like chips, cookies, candy, fries, and ice cream. But these foods aretypically high in fat, and most of us also realize that limiting our consumption of fat is

one of the keys to a healthy lifestyle.

TechnologyTechnologyBIOBIO

1. What are some of the pros and cons ofincluding foods made with Olestra in yourdiet? Do you think it’s a good idea to eatfoods made only with fat substitutes ratherthan true fats? Why or why not?

2. Set up a blind taste test to compare chips orother snack foods made with naturally occur-ring fats to snacks made with different fatsubstitutes. Record class preferences. Howmany tasters could tell the differencebetween “fake” fats and “real” fats?

For more information on food technology, visit the Glencoe

Science Web site.science.glencoe.com

INVESTIGATING THE TECHNOLOGYINVESTIGATING THE TECHNOLOGY

Snack foods made with fat substitutes

BIOLOGY


Chapter 6 AssessmentChapter 6 Assessment

SUMMARYSUMMARY

Section 6.1

Section 6.2

Section 6.3

Vocabularyacid (p. 154)atom (p. 146)base (p. 154)compound (p. 149)covalent bond (p. 150)element (p. 145)ion (p. 151)ionic bond (p. 151)isotope (p. 148)metabolism (p. 151)mixture (p. 152)molecule (p. 150)nucleus (p. 147)pH (p. 154)solution (p. 153)

Atoms andTheirInteractions

Main Ideas■ Water is the most abundant compound in living

things.■ Water is an excellent solvent due to the polar

property of its molecules.■ Particles of matter are in constant motion.■ Diffusion occurs from areas of higher concen-

tration to areas of lower concentration.

Vocabularydiffusion (p. 159)dynamic equilibrium

(p. 160)hydrogen bond (p. 157)polar molecule (p. 156)

Water andDiffusion

Main Ideas■ All organic compounds contain carbon

atoms. ■ There are four principal types of organic

compounds that make up living things:carbohydrates, lipids, proteins, and nucleicacids.

Vocabularyamino acid (p. 165)carbohydrate (p. 162)enzyme (p. 165)isomer (p. 162)lipid (p. 164)nucleic acid (p. 167)nucleotide (p. 167)peptide bond (p. 165)polymer (p. 162)protein (p. 164)

Life Substances

CHAPTER 6 ASSESSMENT 171

1. What are the basic building blocks of all matter?a. electrons c. atomsb. protons d. molecules

UNDERSTANDING MAIN IDEASUNDERSTANDING MAIN IDEAS 2. Which feature of water explains why waterhas high surface tension?a. water diffuses into cellsb. water’s resistance to temperature changesc. water is a polar moleculed. water expands when it freezes

Main Ideas■ Atoms are the basic building blocks of all

matter.■ Atoms consist of a nucleus containing protons

and neutrons. The positively charged nucleus issurrounded by a cloud of rapidly moving, nega-tively charged electrons.

■ Atoms become stable by bonding toother atoms through covalent or ionicbonds.

■ Components of mixtures retain theirproperties—components of solutions do not.

e–

e–

e–

e–

e–

e–

e– e–e–

e–

e–

e–e–e–e–

e–

e–

e–

e–e–


3. Which of the following describes an isotope ofthe commonly occurring oxygen atom whichhas 8 electrons, 8 protons, and 8 neutrons?a. 8 electrons, 8 protons, and 9 neutronsb. 7 electrons, 8 protons, and 8 neutronsc. 8 electrons, 7 protons, and 8 neutronsd. 7 electrons, 7 protons, and 8 neutrons

4. Which of the following will form a solution?a. sand and water c. salt and waterb. oil and water d. salt and sand

5. Which of the following applies to a watermolecule?a. Water is a nonpolar molecule.b. The atoms in water are bonded by ionic

bonds.c. The weak bond between two water mole-

cules is a covalent bond.d. Water molecules have a negatively charged

end and a positively charged end.6. Which of the following carbohydrates is a

polysaccharide?a. glucose c. sucroseb. fructose d. starch

7. Which of the following pairs is unrelated?a. sugar—carbohydrateb. fat—lipidc. amino acid—proteind. starch—nucleic acid

8. An acid is any substance that forms ________in water.a. hydroxide ions c. hydrogen ionsb. oxygen ions d. sodium ions

9. Which of these is NOT made up of proteins?a. hair c. fingernailsb. enzymes d. cellulose

10. Which of the following is NOT a smallersubunit of a nucleotide?a. phosphate c. ribose sugarb. nitrogen base d. glycerol

11. An enzyme ________ chemical reactions.12. A calcium atom has

20 protons and ________ electrons.

13. A ________ bond involves sharing of electrons.14. The first energy level of an atom holds

________ electrons; the second energy levelholds 8 electrons.

15. In a water molecule, each ________ atomshares one electron with the single ________atom.

16. A substrate fits into an area of an enzymecalled the ________.

17. Hydrogen, chlorine, and sodium are examples of ________.

18. Long chains of amino acids connected toeach other by a ________ bond form a________.

19. Diffusion is the process in which moleculesmove from a ________ concentration to a________ concentration.

20. The positively charged ________ atoms ofone water molecule attract the negativelycharged ________ atom of another watermolecule to form a hydrogen bond.

21. A magnesium atom has 12 electrons. When itreacts, it usually loses two electrons. Howdoes this loss make magnesium more stable?

22. Explain why water and a sponge would notbe effective in cleaning up a grease spill.

23. Explain why carbon is the most critical ele-ment to living things.

24. If heating a white substance produces a vapor

APPLYING MAIN IDEASAPPLYING MAIN IDEAS

172 CHAPTER 6 ASSESSMENT

TEST–TAKING TIPTEST–TAKING TIP

When Eliminating, Cross It OutCross out choices you’ve eliminated with your pencil. List the answer choice letters on the scratch paper and cross them out there. You’ll stop yourself from choosing an answer you’vementally eliminated.

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

CD-ROM


CHAPTER 6 ASSESSMENT 173

and black material, how do you know thesubstance was not an element?

25. Interpreting Data The following graphcompares the abundance of four elements inliving things to their abundance in Earth’scrust, oceans, and atmosphere. Which ele-ment is the most abundant in organisms?What can you say about the general composi-tion of living things compared to nonlivingmatter near Earth’s surface?

26. Designing an Experiment The enzyme per-oxidase triggers the breakdown of hydrogenperoxide to form water and oxygen gas.Design an experiment that measures the rateof the reaction.

27. Concept Mapping Complete the conceptmap by using the following vocabulary terms:protein, amino acid, peptide bond.

THINKING CRITICALLYTHINKING CRITICALLY

ASSESSING KNOWLEDGE & SKILLSASSESSING KNOWLEDGE & SKILLS

Two students were studying the effect oftemperature on two naturally occurringenzymes. The graph below summarizestheir data.

Using a Graph Study the graph and answerthe following questions.1. At what temperature does the maximum

activity of enzyme B occur?a. 0° c. 60°b. 35° d. 70°

2. At what temperature do both enzymeshave an equal rate of reaction?a. 10° c. 45°b. 20° d. 60°

3. Which of the following descriptions bestexplains the patterns of temperatureeffects shown on this graph?a. Each enzyme has its own optimal

temperature range.b. Both enzymes have the same optimal

temperature ranges.c. Each enzyme will function at room

temperature.d. Both enzymes are inactivated by

freezing temperatures.4. Designing an Experiment Design an

experiment to test the optimal pH ofenzyme B.

Perc

ent a

bund

ance

(num

bers

of a

tom

s)

C H O N Others

10

0

20

30

40

50

Abundance of Four Elements

In living things

In Earth's crust, oceans, andatmosphere

Rate

s of r

eact

ion

Temperature (°C)

Enzyme BEnzyme A

0 20 40 60 80 100

Effects of Temperature on TwoNaturally Occuring Enzymes

1.A(n)is

made of

2. linkedby 3.


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