490 Chapter 13 Principles of Bioenergetics energy exchanges. Finally, we consider the importance not violate the second law, they operate strictly within of oxidation-reduction reactions in living cells, the en- it. To discuss the application of the second law to bio rgetics of electron-transfer reactions, and the electron logical systems, we must first define those systems and carriers commonly employed as cofactors of the en- their surroundings zymes that catalyze these reactions The reacting system is the collection of matter that is undergoing a particular chemical or physical process: it may be an organism, a cell, or two reacting com- 13.1 Bioenergetics and Thermodynamics pounds. The reacting system and its surroundings to- gether constitute the universe. In the laboratory, some Bioenergetics is the quantitative study of the energy chemical or physical processes can be carried out in iso transductions that occur in living cells and of the nature lated or closed systems, in which no material or energy and function of the chemical processes underlying these is exchanged with the surroundings. Living cells and or- modynamics have been introduced in earlier chapters material and energy with their surroundings: living sys- and the constant transactions between system and sur- Biological Energy Transformations Obey the Laws roundings explain how organisms can create order within themselves while operating within the second law of Thermodynamics of thermodynamics Many quantitative observations made by physicists and In Chapter 1(p. 23)we defined three thermody chemists on the interconversion of different forms of namic quantities that describe the energy changes oc energy led, in the nineteenth century. to the formula- curring in a chemical reaction tion of two fundamental laws of thermodynamics. The first law is the principle of the conservation of energy. Gibbs free energy, G, expresses the amount of for any physical or chemical change, the tota energy capable of doing work during a reaction amount of energy in the universe remains constant, at constant temperature and pressure. When a energy may change form or it may be transported reaction proceeds with the release of free energy from one region to another, but it cannot be created (that is, when the system changes so as to or destroyed. The second law of thermodynamics, which possess less free energy), the free-energy change, can be stated in several forms, says that the universe AG, has a negative value and the reaction is said always tends toward increasing disorder: in all natu- exergonIc endergonic reactions, the ral processes, the entropy of the universe increases. system gains free energy and AG is positive Living organisms consist of collections of molecules Enthalpy, H, is the heat content of the reacting much more highly organized than the surrounding ma system. It reflects the number and kinds of terials from which they are constructed, and organisms chemical bonds in the reactants and products. maintain and produce order, seemingly oblivious to the When a chemical reaction releases heat, it is second law of thermodynamics. But living organisms do said to be exothermic. the heat content of the products is less than that of the reactants an AHhas, by convention, a negative value. Reacting systems that take up heat from their surroundings are endothermic and have positive values of AH. Entropy, S, is a quantitative expression for the randomness or disorder in a system(see Box 1-3) When the products of a reaction are less complex and more disordered than the reactants the reaction is said to proceed with a gain in entropy. The units of AG and aH are joules/mole or calories/mole (recall that 1 cal= 4.184 J); units of entropy ar joules/mole. Kelvin(J/mol K)(Table 13-1) Under the conditions existing in biological systen (including constant temperature and pressure changes in free energy, enthalpy, and entropy are re- "Now, in the second law of thermodynamics lated to each other quantitatively by the equation △G=△H-TAS (13-1)energy exchanges. Finally, we consider the importance of oxidation-reduction reactions in living cells, the energetics of electron-transfer reactions, and the electron carriers commonly employed as cofactors of the enzymes that catalyze these reactions. 13.1 Bioenergetics and Thermodynamics Bioenergetics is the quantitative study of the energy transductions that occur in living cells and of the nature and function of the chemical processes underlying these transductions. Although many of the principles of thermodynamics have been introduced in earlier chapters and may be familiar to you, a review of the quantitative aspects of these principles is useful here. Biological Energy Transformations Obey the Laws of Thermodynamics Many quantitative observations made by physicists and chemists on the interconversion of different forms of energy led, in the nineteenth century, to the formulation of two fundamental laws of thermodynamics. The first law is the principle of the conservation of energy: for any physical or chemical change, the total amount of energy in the universe remains constant; energy may change form or it may be transported from one region to another, but it cannot be created or destroyed. The second law of thermodynamics, which can be stated in several forms, says that the universe always tends toward increasing disorder: in all natural processes, the entropy of the universe increases. Living organisms consist of collections of molecules much more highly organized than the surrounding materials from which they are constructed, and organisms maintain and produce order, seemingly oblivious to the second law of thermodynamics. But living organisms do not violate the second law; they operate strictly within it. To discuss the application of the second law to biological systems, we must first define those systems and their surroundings. The reacting system is the collection of matter that is undergoing a particular chemical or physical process; it may be an organism, a cell, or two reacting compounds. The reacting system and its surroundings together constitute the universe. In the laboratory, some chemical or physical processes can be carried out in isolated or closed systems, in which no material or energy is exchanged with the surroundings. Living cells and organisms, however, are open systems, exchanging both material and energy with their surroundings; living systems are never at equilibrium with their surroundings, and the constant transactions between system and surroundings explain how organisms can create order within themselves while operating within the second law of thermodynamics. In Chapter 1 (p. 23) we defined three thermodynamic quantities that describe the energy changes occurring in a chemical reaction: Gibbs free energy, G, expresses the amount of energy capable of doing work during a reaction at constant temperature and pressure. When a reaction proceeds with the release of free energy (that is, when the system changes so as to possess less free energy), the free-energy change, G, has a negative value and the reaction is said to be exergonic. In endergonic reactions, the system gains free energy and G is positive. Enthalpy, H, is the heat content of the reacting system. It reflects the number and kinds of chemical bonds in the reactants and 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 H has, by convention, a negative value. Reacting systems that take up heat from their surroundings are endothermic and have positive values of H. Entropy, S, is a quantitative expression for the randomness or disorder in a system (see Box 1–3). When the products of a reaction are less complex and more disordered than the reactants, the reaction is said to proceed with a gain in entropy. The units of G and H are joules/mole or calories/mole (recall that 1 cal 4.184 J); units of entropy are joules/mole Kelvin (J/mol K) (Table 13–1). Under the conditions existing in biological systems (including constant temperature and pressure), changes in free energy, enthalpy, and entropy are related to each other quantitatively by the equation G H T S (13–1) 490 Chapter 13 Principles of Bioenergetics