DNA, RNA, and proteins are informational macromole-cules. In addition to using chemical energy to form the covalent bonds between the subunits in these polymers, the cell must invest energy to order the subunits in their correct sequence. It is extremely improbable that amino acids in a mixture would spontaneously condense into a single type of protein, with a unique sequence. This would represent increased order in a population of molecules; but according to the second law of thermodynamics, the tendency in nature is toward ever-greater disorder in the universe: the total entropy of the universe is continually increasing. To bring about the synthesis of macro-molecules from their monomelic units, free energy must be supplied to the system (in this case, the cell).
The randomness or disorder of the components of a chemical system is expressed as entropy, S (Box 1-3).
Aiiy change in randomness of the system is expressed as entropy change, AS, which by convention has a positive value when randomness increases. J. Willard Gibbs, who developed the theory of energy changes during chemical reactions, showed that the free-energy content, G, of any closed system can be defined in terms of three quantities: enthalpy, H, reflecting the number and kinds of bonds; entropy, S; and the absolute temperature, T (in degrees Kelvin). The definition of free energy is G = H — TS. When a chemical reaction occurs at constant temperature, the free-energy change, AG, is determined by the enthalpy change, AH, reflecting the kinds and numbers of chemical bonds and noncovalent interactions broken and formed, and the entropy change, AS, describing the change in the system's randomness:
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