Physical Foundations-: Creating and Maintaining Order Requires Work and Energy
DNA, RNA, and proteins are informational macromolecules. 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 continu ally increasing. To bring about the synthesis of macro molecules from their monomeric 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). Any change in randomness of the system is expressed as entropy change, ΔS, 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 tempera ture, the free-energy change, ΔG, is determined by the enthalpy change, ΔH, reflecting the kinds and num bers of chemical bonds and noncovalent interactions broken and formed, and the entropy change, ΔS, de scribing the change in the system’s randomness:
ΔG=ΔH-TΔS
FIGURE 1–25 Adenosine triphosphate (ATP). The removal of the terminal phosphoryl group (shaded pink) of ATP, by breakage of a Phos Pho anhydride bond, is highly exergonic, and this reaction is coupled to many endergonic reactions in the cell (as in the example in Fig. 1–26b).
A process tends to occur spontaneously only if G is negative. Yet cell function depends largely on molecules, such as proteins and nucleic acids, for which the free energy of formation is positive: the molecules are less stable and more highly ordered than a mixture of their monomeric components. To carry out these ther modynamically unfavorable, energy-requiring (endergonic) reactions, cells couple them to other reactions that liberate free energy (exergonic reactions), so that the overall process is exergonic: the sum of the free energy changes is negative. The usual source of free energy in coupled biological reactions is the energy released by hydrolysis of phosphoanhydride bonds such as those in adenosine triphosphate (ATP; Fig. 1–25; see also Fig. 1–15). Here, each P represents a phosphoryl group:
When these reactions are coupled, the sum of ΔG1 and ΔG2 is negative—the overall process is exergonic. By this coupling strategy, cells are able to synthesize and maintain the information-rich polymers essential to life.
