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FIGURE 1-25 Adenosine triphosphate (ATP). The removal of the terminal phosphoryl group (shaded pink) of ATP, by breakage of a phos-phoanhydride 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 AG 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 (ender-gonic) 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 ( represents a phosphoryl group:

Amino acids-> polymer AG1 is positive (endergonic)

—(P)—(PP-> —(PP + ® AG2 is negative (exergonic)

When these reactions are coupled, the sum of AG1 and AG2 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.

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