Energy and Metabolism

> Where do living things get energy?

> How do chemical reactions occur?

:> Why are enzymes important to livingthings?

Figure 1.4 This athlete uses energy to 'move the basketball. >What fonn ofenergy is used to move the basketball?

- ..' ,~ -Key Terms Why I.t Matters, I • '

" . ,:energy, enzyme":. react3lif> ...,.:~···!~substrate<,;;'~~R(!:~

All living thin need energy tosurvive.You ' that energy bybreaking co f moleculesfrOmfoodyo intosimpler,stabler mol es.

Changes constantly occur in living things. IiIfacti y *could say thata key feature of life is change. The ability to move 0 ~hange matteris calleden¢rgyl~ Energy exists in many forms-s-ine fling light heat,chemical energy, mechanical energy, and electri ~nergy-and canbe converted from one form to another. The athlet iP Rgure 14Isusing mechanical energy to move the basketball I

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Changing Matter ILiving things are made of matter, which consists 0ia ubstance witha form. A physical change occurs when only the fo I or shape ofthe matter changes. The substances that make up ~ matter do notchange into different substances. When you pour ar into iced teaand stir, the sugar crystals disappear. The sweet ;ijShOWS that thesugar is still there but has changed form. I

A chemical change occurs when a substance C I Iges into adifferent substance. In a chemical change, the identifY of the matterchanges. When wood bums, the carbohydrates in th~ wood fiberscombine with oxygen, 0Z' in the air. The wood fibers1change to differ-en~substances: carbon dioxide, CO2, and watervap I, ~O.

Conservation of Mass Matter is neither create 1 nor destroyedin any change. The same mass is present before and kiter the wood

bums and the sugar dissolves. This dhservation iscalled the law of conservation Of'i+' .Conservation of Energy Every change in matterrequires a change in energy. Energy inay change from.one form to another, but the total amount of energydoesnot change. This observation is called the lawof conservation of energy. In some changes, energyis taken in from the surroundings. In;others, energyis released into the surroundings. The total amountof usable energy decreases because some energy isgiven off to the surroundings as hear. >'Uving thingsuse different chemical reaCtions to get the energy neededfor life processes.

) Reading Check What is a chemical changer

64 CHAPTER 3 Chemistry of life

Chemical ReactionsChanging a substance requires a chemical reaction. During thisprocess, bonds between atoms are broken, and new ones are formed.A rea.ctantis a substance that is changed in a chemical reaction. Aproductis a new substance that is formed. Scientists summarizereactions by writing equations in the following form:

Reactants ~ Products

The arrow means "changes to" or "forms:' For example, carbondioxide and water can react to form carbonic acid, H2C03, in yourblood. The following equation represents this reaction:

CO2 +H20 ( ) H2C03

The double arrow indicates that the products can reform reactants.In this example, carbonic acid changes back to carbon dioxide andwater in your lungs.

Activation Energy Chemical reactions can occur only under theright conditions. To form new bonds, particles must get close enoughto share electron clouds. However, even though the particles moveconstantly, as they get close, their negatively charged electron cloudstend to push them apart. To react, the particles must collide fastenough to have kinetic energy to overcome the repulsion betweenthem. Thea.(Ztlya'ti()tlell,ergy of the reaction is the minimum kineticenergy that colliding particles need to start a chemical teaction.

Alignment Even if enough energy is available, the product still maynot form. When the reactant particles collide, the correct atoms mustbe brought close together in the proper orientation, as Figure 15shows. Otherwise, the product will not form. >Chemicalreactions canoccur only when the activation energy is available and the correct atomsare aligned. In living things, chemical reactions occur between large,complex biomolecules. Life can only exist if these molecules collidein the correct orientation.

:) Reading Check What causes particles to repel other particles?

Reaction Conditions

.Conditiof'ls ~esi.dt Products·

enoughenergy; wrongorientation

energy theca citytOd0WQrk.iteaCriuttaSu ~~Pflll(lleculetl1atpartiCi-patesina .cllel1li t reaction···

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.activation en rgy the minimum amount of .

.energy required to starta chemical reaction .

ACADEMICVOCABULARYorientation the relr1ativeposition or directlo ofsomething

Figure 15 Chemical reactions can occuronly under the right conditions. The correctatoms of reactants must be aligned, andthey must collide with enough energy.> What term describes the minimumamountof energy needed for a reactionto occur?

SECTION 4 Energy and Metabolisrn 65

...,'.--..• ...:, ..,',.....'.,.','.',., "".,",'> --.:---:,'.':,'::.".,,'."

e~e·.arnolecule.eRhefllroteino.r·HN~thatacts·asa.·cataly$t·.·in··.bioch~lTIit:alieactions.a~~<~it~onallllllZ}'rlle,~e·s~ethatattaches to asubStraresubstrate •.··the@ctantirlreactionscatalyzedbyenzyines

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wmJ\1,scmnks.OlllliTopic: EnzymesCode: HX80531

Figure16 Enzymes decrease the amountof energy neededto start a chemical reactionwithout changing the amount of energycontained in either the reactants or theproducts.

66 CHAPTER3 Chemistry of Life

BiOIOgi~al ReactionsLiving thin9s carry out many chemical reaction that help maintaina stable internal environment. Many of these re ctions require largeactivation e~ergies. Often, the reactants are lar e biomolecules thatmust collide in a very specific orientation. Man of these reactionswould not occur without the help of enzymes. ...··•.iEnzymes ~el:lZynIejs a molecule that incr ases the speedofbiochemical reactions. Enzymes hold molecul close togetherand in the correct orientation. This way, the m lecules do not haveto depend on random collisions to react. An e e lowers theactivation energy of a reaction, as Figure 16 sh ws.> By assisting innecessary biochemical reactions, enzymes help org nisms maintain homeo-stasis..Without enzymes, chemical reactions W0

1

d not occur quicklyand easily enough for life to go on. I

Enzyme Activity Enzymes fitwith reactants . e a lock fits a key.As Figure 17 shows, each enzyme has an~c;ij.Y$i~etthe regionwhere the reaction takes place. The shape of th active site deter-mines which reactants, orsu."b$tt~t~$)will bind 0 it. Each differentenzyme acts only on specific substrates.

Step 0 Two substrates bind to an enzyme's ctive site. The sub-strates fit in a specific position and location, lik. a key in a lock.

Step e Binding of the substrates causes the lnzyme's shape tochange slightly. The substrates fit more tightly ~ the enzyme's activesite. The change in shape causes some bonds in the substrates tobreak and new bonds to form.

Step 0 The chemical reaction is complete when the product;a new substance, is formed. The unchanged e.e releases the

I .product, and can then be reused for another reaction.Conditions Many enzymes are proteins. Changes in temperature andpH can change a protein's shape. If an enzyme changes shape, it won'twork well. Most enzymes need a certain range of temperatures and pH.

prOducts

- _~~ _~_. __ I _ .. ~ ~ • _ --~~-.--- ---~-~--~--------

o The substrates bind to theenzyme's active site.

Substrates

o The enzyme fhanges shape,which catalr.[es the chemicalreaction be een the substrates.

Metabolism Your cells get most of the energy needed for metabo-lism from the food you eat. When food is digested, itis broken intosmall molecules that can enter the blood, which delivers them tocells. Here, chemical reactions release energy by breaking downthese food molecules so that cells can use it. The release of energyfrom food molecules involves many steps and many enzymes.

Consider the breakdown of sugar to release energy. You can usea match to supply enough activation energy to set fire to cellulose, apolysaccharide, when you burn wood. However, the match flame ishot, and the reaction of glucose with oxygen gets even hotter becauseit is so fast. Living things also "bum" sugars, but they use enzymes todo so. The enzymes reduce the activation energy so much that only alittle energy is needed to start the reaction. Then, a series of enzymescarries out the reaction in a slower, step-by-step manner so that theenergy can be captured in the form of ATPmolecules. In this process,very little heat is produced.

} Reading Check Why is the shape of an enzyme important?

"ReviewKEY IDEAS:I.. Explain the importance of chemical

reactions in living things.2. Describe two conditions necessary

for a chemical reaction to occur.3. Relate enzymes and homeostasis.

CRITICAL THINKING4. Applying Information Explain the

difference between usable energyand the total amount of energy.How is this difference accountedfor in living organisms?

5. Recognizing Relationships Yourbody breaks down starch intoglucose molecules. In this reaction,which substance is the reactant,and which is the product?

Fii~ure17 EnzY/llE!S catalyze specificreactions specific reactants.) Howwould a in the shapeof the active site the enzyme'sactivity?

6. Predicting Outcomes Research. the enzyme carbonic anhydrase.

Explain its role in maintaininghomeostasis in the humanbody. How might a moleculethat interferes with the actionof carbonic anhydrase affectyour body?

SfCTION·4 Energy and Metabolism 67