4 Обмен энергией (1160073), страница 3
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Thesecond law of thermodynamics, which can be stated in several forms,says that the universe always tends toward more and more disorder: inall natural processes, the entropy of the universe increases.Living organisms consist of collections of molecules much morehighly organized than the surrounding materials from which they areconstructed, and they maintain and produce order, seemingly obliviousto the second law of thermodynamics. Living organisms do not violatethe second law; they operate strictly within it.
To discuss the application of the second law to biological systems, we must first define thosesystems and the universe in which they occur. The reacting system isthe collection of matter that is undergoing a particular chemical orphysical process; it may be an organism, a cell, or two reacting compounds. The reacting system and its surroundings together constitutethe universe.
Some chemical or physical processes can be made to takeplace in isolated or closed systems, in which no material or energyis exchanged with the surroundings. Living cells and organisms areopen systems, which exchange both material and energy with theirsurroundings; living systems are never at equilibrium with theirsurroundings.We have defined earlier in this text three thermodynamic quantities that describe the energy changes occurring in a chemical reaction.Gibbs free energy (G) expresses the amount of energy capable of doingwork during a reaction at constant temperature and pressure (p.
8).When a reaction proceeds with the release of free energy (i.e., when thesystem changes so as to possess less free energy), the free-energychange, AG, has a negative sign and the reaction is said to be exergonic. In endergonic reactions, the system gains free energy and AG ispositive. Enthalpy, H, is the heat content of the reacting system. Itreflects the number and kinds of chemical bonds in the reactants andNow, in the second law of thermodynamics365366Part HI Bioenergetics and MetabolismBOX 13-1Entropy: The Advantages of Being DisorganizedThe term entropy, which literally means "a changewithin," was first used in 1851 by Rudolf Clausius,one of the promulgators of the second law.
A rigorous quantitative definition of entropy involves statistical and probability considerations. However,its nature can be illustrated qualitatively by threesimple examples, each of which shows one aspect ofentropy. The key descriptors of entropy are randomness or disorder, manifested in different ways.Case 1: The Teakettle and theRandomization of HeatWe know that steam generated from boiling watercan do useful work. But suppose we turn off theburner under a teakettle full of water at 100 °C (the"system") in the kitchen (the "surroundings") andallow it to cool.
As it cools, no work will be done,but heat will pass from the teakettle to the surroundings, raising the temperature of the surroundings (the kitchen) by an infinitesimally smallamount until complete equilibrium is attained. Atthis point all parts of the teakettle and the kitchenwill be at precisely the same temperature. The freeenergy that was once concentrated in the teakettleof hot water at 100 °C, potentially capable of doingwork, has disappeared.
Its equivalent in heat energy is still present in the teakettle + kitchen (i.e.,the "universe") but has become completely randomized throughout. This energy is no longeravailable to do work because there is no temperature differential within the kitchen. Moreover, theincrease in entropy of the kitchen (the surround-Table 13-1 Some physical constants andunits frequently used in thermodynamicsBoltzmann constant, k = 1.381 x 10~23 J/KAvogadro's number, N = 6.022 x 1023 mol"1Faraday constant, J = 96,480 J/V • molGas constant, R = 8.315 J/mol • K( = 1.987 cal/mol • K)Units of AG and MI are J/mol (or cal/mol)Units of AS are J/mol • K (or cal/mol • K)1 cal = 4.184 JUnits of absolute temperature, T, are degreesKelvin, K25 °C = 298 KAt 25 °C, RT = 2.479 kJ/mol( = 0.592 kcal/mol)In* = 2.303 logxings) is irreversible.
We know from everyday experience that heat will never spontaneously passback from the kitchen into the teakettle to raisethe temperature of the water to 100 °C again.Case 2: The Oxidation of GlucoseEntropy is a state or condition not only of energybut also of matter. Aerobic organisms extract freeenergy from glucose obtained from their surroundings.
To extract this energy they oxidize the glucose with molecular oxygen, also obtained from thesurroundings. The end products of the oxidativemetabolism of glucose are CO2 and H2O, which arereturned to the surroundings. In this process thesurroundings undergo an increase in entropy,whereas the organism itself remains in a steadystate and undergoes no change in its internalorder. Although some of the entropy arises fromthe dissipation of heat, entropy also arises fromanother kind of disorder, illustrated by the equation for the oxidation of glucose by living organisms, which we can write asC6H12O6 + 6O2> 6CO2 + 6H2Oor represent schematically as7 molecules12 moleculesA(a gas)Glucose(a solid)! (a gas)L•+XX++1_H 2 O(a liquid)products.
When a chemical reaction releases heat, it is said to be exothermic; the heat content of the products is less than that of the reactants and AH has a negative value. Reacting systems that take up heatfrom their surroundings are endothermic and have positive values ofAH (p. 66). Entropy, S, is a quantitative expression for the randomnessor disorder in a system (Box 13-1). When the products of a reaction areless complex and more disordered than the reactants, the reaction issaid to proceed with a gain in entropy (p.
72). The units of AG and AHare joules/mole or calories/mole (recall that 1 cal equals 4.18 J); units ofentropy are joules/mole • degree Kelvin (J/mol • K) (Table 13-1).Under the conditions existing in biological systems (at constanttemperature and pressure), changes in free energy, enthalpy, and entropy are related to each other quantitatively by the equationAG = AH - T AS(13-1)in which AG is the change in Gibbs free energy of the reacting system,Chapter 13 Principles of BioenergeticsThe atoms contained in 1 molecule of glucose plus 6molecules of oxygen, a total of 7 molecules, aremore randomly dispersed by the oxidation reactionand are now present in a total of 12 molecules(6CO2 + 6H2O).Whenever a chemical reaction proceeds so thatthere is an increase in the number of molecules—or when a solid substance, such as glucose, is converted into liquid or gaseous products, which havemore freedom to move or fill space than a solid—there is an increase in molecular disorder and thusan increase in entropy.However, if the 125 letters making up this quotation were allowed to fall into a completely random, chaotic pattern, as shown in the followingbox, they would have no meaning whatsoever.Case 3: Information and EntropyThe following short passage from Julius Caesar,Act IV, Scene 3, is spoken by Brutus, when he realizes that he must face Mark Antony's army.
It is aninformation-rich nonrandom arrangement of 125letters of the English alphabet:There is a tide in the affairs of men,Which, taken at theflood,leads on to fortune;Omitted, all the voyage of their lifeIs bound in shallows and in miseries.In addition to what this quotation says overtly, ithas many hidden meanings. It not only reflects acomplex sequence of events in the play, it also echoes the play's ideas on conflict, ambition, and thedemands of leadership.
Permeated with Shakespeare's understanding of human nature, it is veryrich in information.In this form the 125 letters would contain little orno information, but would be very rich in entropy.Such considerations have led to the conclusion thatinformation is a form of energy; information hasbeen called "negative entropy." In fact, the branchof mathematics called information theory, which isbasic to the programming logic of computers, isclosely related to thermodynamic theory. Livingorganisms are highly ordered, nonrandom structures, immensely rich in information and thus entropy-poor.A// is the change in enthalpy of the system, T is the absolute temperature, and AS is the change in entropy of the reacting system. By convention AS has a positive sign when entropy increases and A// has anegative sign when heat is released by the system to its surroundings.Either of these conditions, which are typical of favorable processes, willtend to make AG negative.
In fact, AG of a spontaneously reactingsystem is always negative.The second law of thermodynamics states that the entropy of theuniverse increases during all chemical and physical processes, but itdoes not require that the entropy increase take place in the reactingsystem itself. The order produced within cells as they grow and divideis more than compensated for by the disorder they create in their surroundings in the course of growth and division (Box 13-1, case 2). Inshort, living organisms preserve their internal order by taking fromthe surroundings free energy in the form of nutrients or sunlight, andreturning to their surroundings an equal amount of energy as heat andentropy.368Part III Bioenergetics and MetabolismCells Require Sources of Free EnergyCells are isothermal systems—they function at essentially constanttemperature (and at constant pressure). Heat flow is not a source ofenergy for cells because heat can do work only as it passes from a zoneor object at one temperature to a zone or object at a lower temperature.The energy that cells can and must use is free energy, described by theGibbs free-energy function G, which allows prediction of the directionof chemical reactions, their exact equilibrium position, and the amountof work they can in theory perform at constant temperature and pressure.
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