13 - Thermodynamics: Beyond the First Law 13 Our discussion...

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E NTROPY AND THE S ECOND L AW OF T HERMODYNAMICS 331 J. Newman, Physics of the Life Sciences , DOI: 10.1007/978-0-387-77259-2_13, © Springer Science+Business Media, LLC 2008 Our discussion of thermodynamics in the last chapter was limited to energy consid- erations. Although energy conservation is a necessary requirement for any process to occur, it is not a sufficient condition. There are many energy-conserving processes that occur spontaneously, but that are not reversible even though that reversed process would also conserve energy. In this chapter we continue our introduction to thermo- dynamics with a discussion of entropy and the second law of thermodynamics. We relate entropy to the degree of disorder in an isolated system through a microscopic picture and we show that this disorder always increases with time. Life is a constant struggle to maintain a high degree of order. The corresponding reduction in entropy is accomplished at the expense of even more disorder in our environment in order to satisfy the second law of thermodynamics. We next discuss Gibbs free energy, related to chemical potential, the most important energy concept in biology. This thermody- namic state variable is a measure of the energy available for useful work at constant temperature and pressure, the usual conditions of life. The chapter concludes with several biological applications of these concepts, including ATP hydrolysis, photo- synthesis, and conformational changes in biomolecules. 1. ENTROPY AND THE SECOND LAW OF THERMODYNAMICS Many processes in nature that conserve energy and do not violate any of the other fun- damental principles we have introduced so far in our study of physics simply do not occur. Now when a basic physical process never happens even though it seems to sat- isfy all of the fundamentals in our theories of knowledge, there is something amiss. From many historical examples, it is usually the case that there is some new principle that would be violated by the occurrence of such a process. We begin this section with a brief discussion of some examples of processes in different areas of physics that never occur, leading to a qualitative presentation of the common principle that prohibits them. In mechanics, all sliding objects eventually come to rest because their kinetic energy has been lost due to what we call friction, the process by which mechanical energy is transferred to heat. Energy has not been lost, but the “useful” form of energy, which in mechanics is the sum of kinetic and potential energy, called mechanical energy, has been lost through its transfer to internal energy. Once a sliding object comes to rest, it is never the case that the internal energy of the object and surroundings spontaneously transfers back to the object in the form of mechanical energy making it move again. We conclude that although energy would be conserved in the reverse process, once “organized” energy, such as kinetic energy in which all molecules of the moving object translate together, is converted to random thermal motions of molecules, the process is irreversible. It is too
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13 - Thermodynamics: Beyond the First Law 13 Our discussion...

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