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For this reason, eachreaction requires a specific boost in chemical reactivity.This requirement is crucial, because it allows the cell to control each reaction. The control is exertedthrough the specialized proteins called enzymes,each of which accelerates,orcatalyzes,just one of the many possible kinds of reactions that a particularmolecule might undergo. Enzyme-catalyzedreactions are usually connected inseries,so that the product of one reaction becomes the starting material, or substrate,for the next (Figure 2-34).
These long linear reaction pathways are in turnlinked to one another, forming a maze of interconnected reactions that enablethe cell to survive, grow, and reproduce (Figure 2-35).TWoopposing streams of chemical reactions occur in cells: (l) Ihe catabolicpathways break down foodstuffs into smaller molecules, thereby generatingboth a useful form of energy for the cell and some of the small molecules that thecell needs as building blocks, and (2) the anabolic, or biosynthellq pathways usethe energy harnessed by catabolism to drive the synthesis of the many othermolecules that form the cell. Together these two sets of reactions constitute themetabolism of the cell (Figure 2-36).Many of the details of cell metabolism form the traditional subject of biochemistry and need not concern us here.
But the general principles by whichcells obtain energy from their environment and use it to create order are centralto cell biology. We begin with a discussion of why a constant input of energy isneeded to sustain living organisms.BiologicalOrderls MadePossibleby the Releaseof HeatEnergyfrom CellsThe universal tendency of things to become disordered is a fundamental law ofphysics-the secondlaw of thermodynamics-which states that in the universe,or in any isolated system (a collection of matter that is completely isolated fromthe rest of the universe), the degreeof disorder only increases.This law has suchprofound implications for all living things that we restate it in severalways.For example,we can present the second law in terms of probability and statethat systems will change spontaneously toward those arrangements that havethe greatest probability.
If we consider, for example, a box of 100 coins all lyingheads up, a series of accidents that disturbs the box will tend to move thearrangement toward a mixture of 50 heads and 50 tails. The reason is simple:there is a huge number of possible arrangements of the individual coins in themixture that can achieve the 50-50 result, but only one possible arrangementthat keeps all of the coins oriented heads up. Becausethe 50-50 mixture is therefore the most probable, we say that it is more "disordered." For the same reason,Figure2-35 Someof the metabolicpathwaysand their interconnectionsin a typicalcell,About500commonmetabolicreactionsareshowndiagrammatically,with eachmoleculein a metabolicpathwayrepresentedby a filledcircle,as in the yel/owbox in Figure2-34.The pathwaythat ishighlightedin this diagramwith largercirclesand connectinglinesisthecentralpathwayof sugarmetabolism,whichwill be discussedshortly.CATALYSISANDTHEUSEOF ENERGYBYCELLSFigure2-36 Schematicrepresentationof the relationshipbetweencatabolicand anabolicpathwaysin metabolism.As suggestedhere,sincea major portion of the energystoredin the chemicalbondsof foodmoleculesis dissipatedas heat,the massof food requiredby any organismthat derivesall of its energyfrom catabolismis muchgreaterthan the massof the moleculesthat can be oroducedbv anabolism.67it is a common experience that one's living space will become increasingly disordered without intentional effort: the movement toward disorder is a sponta-neous process,requiring a periodic effort to reverse it (Figure 2-37).The amount of disorder in a system can be quantified and expressedas theentropy of the system: the greater the disorder, the greater the entropy.
Thus,another way to express the second law of thermodynamics is to say that systemswill change spontaneously toward arrangements with greater entropy.Living cells-by surviving, growing, and forming complex organisms-aregenerating order and thus might appear to defu the second law of thermodynamics. How is this possible?The answer is that a cell is not an isolated system:it takes in energy from its environment in the form of food, or as photons fromthe sun (or even, as in some chemosynthetic bacteria, from inorganic moleculesalone), and it then uses this energy to generate order within itself. In the courseof the chemical reactions that generateorder, the cell converts part of the energyit usesinto heat.
The heat is dischargedinto the cell'senvironment and disordersit, so that the total entropy-that of the cell plus its surroundings-increases, asdemanded by the laws of thermodlmamics.To understand the principles governing these energy conversions, think ofa cell surrounded by a sea of matter representing the rest of the universe. As thecell lives and grows, it creates internal order. But it constantly releases heatenergy as it synthesizes molecules and assembles them into cell structures.Heat is energy in its most disordered form-the random jostling of molecules.\iVhen the cell releasesheat to the sea, it increases the intensity of molecularmotions there (thermal motion)-thereby increasing the randomness, or disorder, of the sea.
The second law of thermodynamics is satisfied because theincrease in the amount of order inside the cell is more than compensated for byan even greater decreasein order (increase in entropy) in the surrounding seaof matter (Figure 2-38).\Mhere does the heat that the cell releases come from? Here we encounteranother important law of thermodynamics.
The first law of thermodynamicsstatesthat energy can be converted from one form to another, but that it cannott h e m a n ym o l e c u l e sthat form the cellfoodmoleculesCATABOLICPATHWAYSusefulforms ofenergy+lostheatt h e m a n y b u i l d i n gb l o c k sfor biosynthesis"SPONTANEOUS"REACTIONa st i m e e l a p s e sO R G A N I Z EEDF F O RRT E Q U I R I NEGN E R G YINPUTFigure2-37 An everydayillustrationofthe spontaneousdrive toward disorder.Reversingthis tendencytoward disorderreouiresan intentionaleffort and anIninputof energy:it is not spontaneous.fact.from the secondlaw ofwe can be certainthatthermodynamics,the humaninterventionrequiredwillreleaseenoughheatto the environmentfor theto morethan compensatereorderingof the items in this room.68Chapter2: CellChemistryand Biosynthesissea of matteroIJ\atoJ-Oo.'.{toa\o.*increaseddisorderincreasedorderFigure2-38A simplethermodynamicanalysisof a livingcell.Inthediagramon theleftthe(theseaof matter)moleculesof boththecellandtherestof theuniversein aaredepictedrelativelydisorderedInthediagramstate.on therightthecellhastakenin energyfromfoodmoleculesandreleasedheatbya reactionthatordersthemoleculesthecellcontains.Becausetheheatincreasesthedisorderin theenvironmentaroundthecell(depictedbythejaggedarrowsanddistortedmolecules,indicatingmoleculartheincreasedmotionscausedby heat),thesecondlawof thermodynamics-whichstatesthattheamountof disorderin theuniversemustalwaysincrease-issatisfiedasthecellgrowsanddivides.Fora detaileddiscussion,see(pp.118-119).Panel2-7be created or destroyed.Figure 2-39 illustrates some interconversions betweendifferent forms of energy.
The amount of energy in different forms will change asa result of the chemical reactions inside the cell, but the first law tells us that thetotal amount of energy must always be the same. For example, an animal celltakes in foodstuffs and converts some of the energy present in the chemicalbonds between the atoms of these food molecules (chemical bond energy) intothe random thermal motion of molecules (heat energy).As described above,thisconversion of chemical energy into heat energy is essentialif the reactions thatcreate order inside the cell are to cause the universe as a whole to become moredisordered.The cell cannot derive any benefit from the heat energy it releasesunless theheat-generating reactions inside the cell are directly linked to the processesthatgenerate molecular order.
It is the tight coupling of heat production to anincrease in order that distinguishes the metabolism of a cell from the wastefulburning of fuel in a fire. Later, we shall illustrate how this coupling occurs. Fornow it is sufficient to recognize that a direct linkage of the "burning" of foodmolecules to the generation of biological order is required for cells to create andmaintain an island of order in a universe tending toward chaos.PhotosyntheticOrganismsUseSunlightto SynthesizeOrganicMoleculesAll animals live on energy stored in the chemical bonds of organic moleculesmade by other organisms,which they take in as food. The molecules in food alsoprovide the atoms that animals need to construct new living matter.
Some animals obtain their food by eating other animals. But at the bottom of the animalfood chain are animals that eat plants. The plants, in turn, trap energy directlyfrom sunlight. As a result, the sun is the ultimate source of the energy used byanimal cells.Solar energy enters the living world through photosynthesis in plants andphotosynthetic bacteria. Photosynthesis converts the electromagnetic energyin sunlight into chemical bond energy in the cell.
Plants obtain all the atomsthey need from inorganic sources: carbon from atmospheric carbon dioxide,hydrogen and oxygen from water, nitrogen from ammonia and nitrates in the, :,CATALYSISANDTHEUSEOF ENERGYBYCELLSf a l l i n g b r i c kh a skinetic energyr a i s e db r i c kh a sp o t e n t i a le n e r g yo u eto pull ofgravity/'/'\heat isreleasedw h e n b r i c kh i t sthe floor,,iri:iitirililrliii1potential energy due to position--+.Tkinetic energy.
/c,3G\\&",\)\\lla&two hydrogen oxygen gasg a sm o l e c u l e s m o l e c u l e\r*r,c h e m i c abl o n d e n e r g y4heat dispersedtos ur r o u n di n g sheat energyliSli.ilr1';'#]trkinetic energyelectricalenergy+-sunlightu,.' j; G&'r a p i dv i b r a t i o n sa n drotations of two newlyf o r m e dw a t e r m o l e c u l e schemicatbondenersyinH2and02 +3heat energychlorophyllmoleculeelectromagnetic(light) energ! --+c h l o r o p h y lml oleculein excitedstatehigh energy electrons --+photosynthesischemicalbond energysoil, and other elements needed in smaller amounts from inorganic salts in thesoil.
They use the energy they derive from sunlight to build these atoms intosugars, amino acids, nucleotides, and fatty acids. These small molecules inturn are converted into the proteins, nucleic acids, polysaccharides, and lipidsthat form the plant. All of these substances serve as food molecules for animals, if the plants are later eaten.The reactions of photosynthesis take place in two stages (Figure 2-4O).lnthe first stage, energy from sunlight is captured and transiently stored as chemical bond energy in specializedsmall molecules that act as carriers of energy andreactive chemical groups. (We discussthese "activated carrier" molecules later.)Molecular oxygen (Oz gas) derived from the splitting of water by light is releasedas a waste product of this first stage.In the second stage,the molecules that serve as energy carriers are used tohelp drive a carbonfixallon process in which sugars are manufactured from carbon dioxide Bas (COz) and water (HzO), thereby providing a useful source ofstored chemical bond energy and materials-both for the plant itself and for anyanimals that eat it.
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