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We describe the elegant mechanisms that underlie these twostagesofphotosynthesis in Chapter 14.69Figure2-39 Someinterconversionsbetween different forms of energy.All energyformsare,in principle,interconvertible.ln all theseorocessesthe total amountof energyis conserved.Thus,for example,from the heightandweightof the brickin (1),we can predictexactlyhow much heatwill be releasedwhen it hitsthe floor.In (2),notethat thelargeamountof chemicalbond energyreleasedwhen wateris formedis initiallyconvertedto very rapidthermal motionsin the two new watermolecules;butwith other moleculesalmostcollisionsinstantaneouslyspreadthis kineticenergyevenlythroughoutthe(heattransfer),makingthesurroundingsfrom allnew moleculesindistinguishablethe rest.70Chapter2: CellChemistryand Biosynthesisf.^..^>\1'11capture o{light energyI---H2O+ CO2energycarflersSUGAR)()heatheatSTAGE2STAGE1Figure2-40Photosynthesis.Theenergycarrierscreatedin thefirstThetwo stagesof photosynthesis.stagearetwo moleculesthatwediscussshortly-ATPandNADPH.The net result of the entire process of photosynthesis, so far as the greenplant is concerned, can be summarized simply in the equationlight energy + CO2+ H2O -+ sugars + 02 + heat energyThe sugarsproduced are then used both as a source of chemical bond energyand as a source of materials to make the many other small and large organicmolecules that are essentialto the Dlant cell.CellsObtainEnergyby the Oxidationof OrganicMoleculesAll animal and plant cells are powered by energy stored in the chemical bonds oforganic molecules, whether they are sugarsthat a plant has photosynthesized asfood for itself or the mkture of large and small molecules that an animal haseaten.
Organisms must extract this energy in usable form to live, grow, andreproduce. In both plants and animals, energy is extracted from food moleculesby a process ofgradual oxidation, or controlled burning.The Earth'satmosphere contains a great deal of oxygen, and in the presenceof oxygen the most energeticallystable form of carbon is CO2and that of hydrogen is H2O.A cell is therefore able to obtain energy from sugarsor other organicmolecules by allowing their carbon and hydrogen atoms to combine with oxygento produce COz and H2O,respectively-a processcalled respiration.Photosynthesisand respiration are complementary processes(Figure 2-41).This means that the transactions between plants and animals are not all oneway.
Plants, animals, and microorganisms have existed together on this planetfor so long that many of them have become an essentialpart of the others' environments. The oxygen releasedby photosynthesis is consumed in the combustion of organic molecules by nearly all organisms. And some of the COzmolecules that are fixed today into organic molecules by photosynthesis in agreen leaf were yesterday releasedinto the atmosphere by the respiration of ananimal-or by that of a fungus or bacterium decomposing dead organic matter.We therefore seethat carbon utilization forms a huge cycle that involves the biosphere (all of the living organisms on Earth) as a whole, crossing boundariesPHOTOSYNTEHSISCOr+HrO+02+SUGARS02co,RESPIRATIONS U G A R+S Or+ HrO+ CO,co,orFigure2-41 Photosynthesisandrespirationas complementaryprocessesin the livingworld.
Photosynthesisusesthe energyof sunlightto producesugarsand otherorganicmolecules.Thesemoleculesin turn serveasfood for otherorganisms.Manyof theseorganismscarryout respiration,a processthat uses02 to form CO2from the samecarbonatomsthat had beentakenup asCO2andconvertedinto sugarsby photosynthesis.In the process,the organismsthat respireobtainthe chemicalbond energythatThefirstcellson thethey needto survive.Eartharethoughtto havebeencapableof neitherphotosynthesisnor respiration(discussedin Chapter14).However,photosynthesismusthaveprecededrespirationon the Earth,sincethereisstrongevidencethat billionsof yearsofphotosynthesiswere requiredbefore02had beenreleasedin sufficientquantityto createan atmosphererichin this gas,(TheEarth'satmospherecurrentlycontains20o/oOt.)CATALYSISANDTHEUSEOF ENERGYBYCELLS71co2 tN ATMOSPHEREANDWATER\\RESPIRATION\PHOTOSYNTHESISIPLANTS,ALGAEBACTERIAFigure2-42 The carbon cycle.Individualintocarbonatomsareincorporatedof the livingworld byorganicmoleculesactivityof bacteriathe photosyntheticTheypasstoand plants(includingalgae).and organicanimals,microorganisms,materialin soiland oceansin cyclicpaths.whenCO2is restoredto the atmosphereorganicmoleculesareoxidizedby cellsorburnedby humansasfuels.H U M U SA N D D I S S O L V E DORGANICMATTERbetween individual organisms (Figare2-42).
Similarly, atoms of nitrogen, phosphorus, and sulfur move between the living and nonliving worlds in cycles thatinvolve plants, animals, fungi, and bacteria.Oxidationand ReductionInvolveElectronTransfersThe cell does not oxidize organic molecules in one step, as occurs when organicmaterial is burned in a fire. Through the use of enzyme catalysts,metabolismtakes the molecules through a large number of reactions that only rarely involvethe direct addition of oxygen. Before we consider some of these reactions andtheir purpose, we discusswhat is meant by the process of oxidation,Oxidation does not mean only the addition of oxygen atoms; rather, itapplies more generally to any reaction in which electrons are transferred fromone atom to another.
Oxidation in this senserefers to the removal of electrons,and reduction-the converse of oxidation-means the addition of electrons.Thus, Fe2*is oxidized if it loses an electron to become Fe3*,and a chlorine atomis reduced if it gains an electron to become Cl-. Since the number of electrons isconserved (no loss or gain) in a chemical reaction, oxidation and reductionalways occur simultaneously: that is, if one molecule gains an electron in a reaction (reduction), a second molecule loses the electron (oxidation). \Mhen a sugarmolecule is oxidized to CO2and HzO, for example, the 02 molecules involved informing H2O gain electrons and thus are said to have been reduced.The terms "oxidation" and "reduction" apply even when there is only a partial shift of electrons between atoms linked by a covalent bond (Figure 2-43).(A)e-€p+e_F O R M A T I OONFA POLARCOVALENTgOttgry'TryH€-e-IH*C-HIHI./\*)ATOM 1ATOM 2methaneuMoLEcuLEmethanolIH _ C" -' O HIHgFigure2-43 Oxidation and reduction.(A)Whentwo atoms form a polar covalentbond(seep.
50),the atom endingup with a greatershareof electronsis saidto be reduced,Thewhilethe otheratom acquiresa lessershareof electronsand is saidto be oxidized.reducedatom hasacquireda partialnegativecharge(6-)asthe positivechargeon theatomicnucleusis now morethan equaledby the total chargeof the electronssurroundingit, and conversely,the oxidizedatom hasacquireda partialpositivecharge(6+).(B)Thesinglecarbonatom of methanecan be convertedto that of carbondioxideby the successivereplacementof its covalentlybondedhydrogenatomswith oxygenatoms.With eachstep,electronsareshiftedawayfrom the carbon(asindicatedby theb/ueshading),and the carbonatom becomesprogressivelymoreoxidized.Eachofthesestepsis energeticallyfavorableunderthe conditionspresentinsidea cell.I formaldehydec:oHlII f o r m i ca c i dHC:OHOIIIn---nc a r b o nd i o x i d e72Chapter2:CellChemistryand Biosynthesis\Mhen a carbon atom becomes covalently bonded to an atom with a strong affinity for electrons, such as oxygen,chlorine, or sulfur, for example, it givesup morethan its equal share of electrons and forms a polar covalent bond: the positivecharge of the carbon nucleus is now somewhat greater than the negative chargeofits electrons, and the atom therefore acquires a partial positive charge and issaid to be oxidized.
Conversely,a carbon atom in a C-H linkage has slightly morethan its share ofelectrons, and so it is said to be reduced (seeFigure 2-43).'W/hena molecule ina cell picks up an electron (e), it often picks up a proton (H+) at the same time (protons being freely available in water). The net effectin this caseis to add a hydrogen atom to the moleculeA+o+H+-+AHEven though a proton plus an electron is involved (instead ofjust an electron),such hydrogenation reactions are reductions, and the reverse, dehydrogenationreactions, are oxidations. It is especiallyeasyto tell whether an organic moleculeis being oxidized or reduced: reduction is occurring if its number of C-H bondsincreases,whereas oxidation is occurring if its number of C-H bonds decreases(see Figure 2-438).Cells use enzymes to catalyze the oxidation of organic molecules in smallsteps,through a sequenceof reactions that allows useful energy to be harvested.We now need to explain how enzymes work and some of the constraints underwhich they operate.EnzymesLowerthe BarriersThatBlockChemicalReactionsConsiderthe reactionpaper + 02 -+ smoke + ashes+ heat + CO2+ H2OThe paper burns readily, releasing to the atmosphere both energy as heat andwater and carbon dioxide as gases,but the smoke and ashesnever spontaneouslyretrieve these entities from the heated atmosphere and reconstitute themselvesinto paper.\Ahen the paper burns, its chemical energy is dissipated as heat-notlost from the universe, since energy can never be created or destroyed,but irretrievably dispersed in the chaotic random thermal motions of molecules.At thesame time, the atoms and molecules of the paper become dispersed and disordered.
In the language of thermodlmamics, there has been aloss of free energJ,that is, of energy that can be harnessedto do work or drive chemical reactions.This loss reflects a loss of orderliness in the way the energy and molecules werestored in the paper. We shall discuss free energy in more detail shortly, but thegeneral principle is clear enough intuitively: chemical reactions proceed spontaneously only in the direction that leads to a loss of free energy;in other words, thespontaneous direction for any reaction is the direction that goes "dor.rmhill."A"doumhill" reaction in this senseis often said tobe energeticallyfauorable,Although the most energetically favorable form of carbon under ordinaryconditions is COz, and that of hydrogen is HzO, a living organism does not disappear in a puff of smoke, and the book in your hands does not burst intoflames.
This is because the molecules both in the living organism and in thebook are in a relatively stable state, and they cannot be changed to a state ofIower energy without an input of energy: in other words, a molecule requiresactivation energy-a kick over an energy barrier-before it can undergo achemical reaction that leaves it in a more stable state (Figure 244).In the caseof a burning book, the activation energy is provided by the heat of a lightedmatch. For the molecules in the watery solution inside a cell, the kick is delivered by an unusually energetic random collision with surrounding moleculescollisions that become more violent as the temperature is raised.In a living cell, the kick over the energy barrier is greatly aided by a specialized class of proteins-the enzymes.
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