B. Alberts, A. Johnson, J. Lewis и др. - Molecular Biology of The Cell (6th edition) (1120996), страница 27
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Organisms must extract this energy in usable form to live, grow, and reproduce. In both plants and animals, energy is extracted from food molecules by aprocess of gradual oxidation, or controlled burning.The Earth’s atmosphere contains a great deal of oxygen, and in the presence ofoxygen the most energetically stable form of carbon is CO2 and that of hydrogenFigure 2–17 Some interconversionsbetween different forms of energy.All energy forms are, in principle,interconvertible. In all these processes thetotal amount of energy is conserved. Thus,for example, from the height and weightof the brick in (1), we can predict exactlyhow much heat will be released when it hitsthe floor.
In (2), note that the large amountof chemical-bond energy released whenwater is formed is initially converted tovery rapid thermal motions in the two newwater molecules; but collisions with othermolecules almost instantaneously spreadthis kinetic energy evenly throughout thesurroundings (heat transfer), making thenew molecules indistinguishable from allthe rest.CATALYSIS AND THE USE OF ENERGY BY CELLSPHOTOSYNTHESISCO2 + H2OO2H2O55CELLULAR RESPIRATIONO2 + SUGARSSUGARS + O2CO2CO2PLANTSALGAESOME BACTERIASUGARS ANDOTHER ORGANICMOLECULESH2O + CO2O2MOSTLIVINGORGANISMSH2OUSEFULCHEMICALBONDENERGYENERGYOFSUNLIGHTis H2O. A cell is therefore able to obtain energy from sugars or other organic molecules by allowing their carbon and hydrogen atoms to combine with oxygen toproduce CO2 and H2O, respectively—aMBoC6processcalled aerobic respiration.m2.41/2.18Photosynthesis (discussed in detail in Chapter 14) and respiration are complementary processes (Figure 2–18).
This means that the transactions betweenplants and animals are not all one way. Plants, animals, and microorganisms haveexisted together on this planet for so long that many of them have become anessential part of the others’ environments. The oxygen released by photosynthesis is consumed in the combustion of organic molecules during aerobic respiration.
And some of the CO2 molecules that are fixed today into organic moleculesby photosynthesis in a green leaf were yesterday released into the atmosphereby the respiration of an animal—or by the respiration of a fungus or bacteriumdecomposing dead organic matter. We therefore see that carbon utilization formsa huge cycle that involves the biosphere (all of the living organisms on Earth) as awhole (Figure 2–19).
Similarly, atoms of nitrogen, phosphorus, and sulfur movebetween the living and nonliving worlds in cycles that involve plants, animals,fungi, and bacteria.Oxidation and Reduction Involve Electron TransfersThe cell does not oxidize organic molecules in one step, as occurs when organicmaterial is burned in a fire. Through the use of enzyme catalysts, metabolism takesthese molecules through a large number of reactions that only rarely involve thedirect addition of oxygen.
Before we consider some of these reactions and theirpurpose, we discuss what is meant by the process of oxidation.CO2 IN ATMOSPHERE AND WATERRESPIRATIONPHOTOSYNTHESISPLANTS, ALGAE,BACTERIAANIMALSFOODCHAINHUMUS AND DISSOLVEDORGANIC MATTERSEDIMENTS ANDFOSSIL FUELSFigure 2–19 The carbon cycle. Individual carbon atoms are incorporated into organic molecules ofthe living world by the photosynthetic activity of bacteria, algae, and plants. They pass to animals,microorganisms, and organic material in soil and oceans in cyclic paths. CO2 is restored to theatmosphere when organic molecules are oxidized by cells or burned by humans as fuels.MBoC6 m2.42/2.19Figure 2–18 Photosynthesis andrespiration as complementary processesin the living world.
Photosynthesisconverts the electromagnetic energy insunlight into chemical-bond energy insugars and other organic molecules.Plants, algae, and cyanobacteria obtainthe carbon atoms that they need for thispurpose from atmospheric CO2 and thehydrogen from water, releasing O2 gasas a by-product.
The organic moleculesproduced by photosynthesis in turn serveas food for other organisms. Many of theseorganisms carry out aerobic respiration,a process that uses O2 to form CO2 fromthe same carbon atoms that had beentaken up as CO2 and converted into sugarsby photosynthesis. In the process, theorganisms that respire obtain the chemicalbond energy that they need to survive.The first cells on the Earth arethought to have been capable of neitherphotosynthesis nor respiration (discussedin Chapter 14). However, photosynthesismust have preceded respiration on theEarth, since there is strong evidence thatbillions of years of photosynthesis wererequired before O2 had been released insufficient quantity to create an atmosphererich in this gas.
(The Earth’s atmospherecurrently contains 20% O2.)Chapter 2: Cell Chemistry and Bioenergetics56Oxidation refers to more than the addition of oxygen atoms; the term appliesmore generally to any reaction in which electrons are transferred from one atomto another. Oxidation in this sense refers to the removal of electrons, and reduction—the converse of oxidation—means the addition of electrons. Thus, Fe2+ isoxidized if it loses an electron to become Fe3+, and a chlorine atom is reducedif it gains an electron to become Cl–. Since the number of electrons is conserved(no loss or gain) in a chemical reaction, oxidation and reduction always occursimultaneously: that is, if one molecule gains an electron in a reaction (reduction), a second molecule loses the electron (oxidation).
When a sugar molecule isoxidized to CO2 and H2O, for example, the O2 molecules involved in forming H2Ogain electrons and thus are said to have been reduced.The terms “oxidation” and “reduction” apply even when there is only a partialshift of electrons between atoms linked by a covalent bond (Figure 2–20). Whena carbon atom becomes covalently bonded to an atom with a strong affinity forelectrons, such as oxygen, chlorine, or sulfur, for example, it gives up more thanits equal share of electrons and forms a polar covalent bond.
Because the positivecharge of the carbon nucleus is now somewhat greater than the negative charge ofits electrons, the atom acquires a partial positive charge and is said to be oxidized.Conversely, a carbon atom in a C–H linkage has slightly more than its share ofelectrons, and so it is said to be reduced.When a molecule in a 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 effect inthis case is to add a hydrogen atom to the molecule.A + e– + H+ → AHEven though a proton plus an electron is involved (instead of just an electron),such hydrogenation reactions are reductions, and the reverse, dehydrogenationreactions are oxidations. It is especially easy to 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 2–20B).Cells use enzymes to catalyze the oxidation of organic molecules in smallsteps, through a sequence of reactions that allows useful energy to be harvested.We now need to explain how enzymes work and some of the constraints underwhich they operate.Figure 2–20 Oxidation and reduction.
(A) When two atoms form a polarcovalent bond, the atom ending up with a greater share of electrons is saidto be reduced, while the other atom acquires a lesser share of electrons andis said to be oxidized. The reduced atom has acquired a partial negativecharge (δ–) as the positive charge on the atomic nucleus is now more thanequaled by the total charge of the electrons surrounding it, and conversely,the oxidized atom has acquired a partial positive charge (δ+). (B) The singlecarbon atom of methane can be converted to that of carbon dioxide bythe successive replacement of its covalently bonded hydrogen atomswith oxygen atoms. With each step, electrons are shifted away from thecarbon (as indicated by the blue shading), and the carbon atom becomesprogressively more oxidized.
Each of these steps is energetically favorableunder the conditions present inside a cell.H methaneHOH+(A)ATOM 1_+_e_eeDHT+_eATOM 2partialpositivecharge (δ+)oxidized+ _eN_+MOLECULECOHHAOeRH methanolIformaldehydeHCOHformic acidCOCOHOpartialnegativecharge (δ–)reduced(B)EDUCTIHO_eHXIFORMATION OFA POLARCOVALENTBONDCcarbon dioxideONCATALYSIS AND THE USE OF ENERGY BY CELLS57Enzymes Lower the Activation-Energy Barriers That BlockChemical Reactionsaactivationenergy forreactionY XYbreactanttotal energytotal energyConsider the reactionpaper + O2 → smoke + ashes + heat + CO2 + H2OOnce ignited, the paper burns readily, releasing to the atmosphere both energyas heat and water and carbon dioxide as gases.
The reaction is irreversible, sincethe smoke and ashes never spontaneously retrieve these entities from the heatedatmosphere and reconstitute themselves into paper. When the paper burns, itschemical energy is dissipated as heat—not lost from the universe, since energycan never be created or destroyed, but irretrievably dispersed in the chaotic random thermal motions of molecules. At the same time, the atoms and molecules ofthe paper become dispersed and disordered. In the language of thermodynamics,there has been a loss of free energy; that is, of energy that can be harnessed to dowork or drive chemical reactions. This loss reflects a reduction of orderliness inthe way the energy and molecules were stored in the paper.We shall discuss free energy in more detail shortly, but the general principleis clear enough intuitively: chemical reactions proceed spontaneously only inthe direction that leads to a loss of free energy.
In other words, the spontaneousdirection for any reaction is the direction that goes “downhill,” where a “downhill”reaction is one that is energetically favorable.Although the most energetically favorable form of carbon under ordinary conditions is CO2, and that of hydrogen is H2O, a living organism does not disappearin a puff of smoke, and the paper book in your hands does not burst into flames.This is because the molecules both in the living organism and in the book are in arelatively stable state, and they cannot be changed to a state of lower energy without an input of energy: in other words, a molecule requires activation energy—akick over an energy barrier—before it can undergo a chemical reaction that leavesit in a more stable state (Figure 2–21).
In the case of a burning book, the activationenergy can be provided by the heat of a lighted match. For the molecules in thewatery solution inside a cell, the kick is delivered by an unusually energetic random collision with surrounding molecules—collisions that become more violentas the temperature is raised.The chemistry in a living cell is tightly controlled, because the kick over energybarriers is greatly aided by a specialized class of proteins—the enzymes. Eachenzyme binds tightly to one or more molecules, called substrates, and holdsthem in a way that greatly reduces the activation energy of a particular chemicalreaction that the bound substrates can undergo.
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