The adenine nucleotides participate in every extended metabolic sequence, and thus serve as stoichiometric coupling agents linking all of the chemical activities that constitute life. Each metabolic sequence may be assigned an ATP coupling coefficient—the number of molecules of ATP regenerated (or used) when a mole of substrate is degraded or a mole of product formed. The ATP coupling coefficient for any metabolic conversion is an evolved phenotypic characteristic of the organism, and it determines the thermodynamically feasible direction of conversion under physiological conditions. Thus the unidirectionality of metabolic sequences is a direct consequence of the values of their ATP coupling coefficients. Using these coupling coefficients, “prices,” expressed in ATP equivalents, may be assigned to metabolites. Metabolism may thus be viewed as a problem in the allocation of resources. This approach may be pedagogically useful in focusing attention on metabolic interrelation-ships, rather than on individual sequences for their own sakes. It also explains the need for universal kinetic control of metabolic sequences by the energy charge of the adenine nucleotide pool.