Oxidative phosphorylation: thermodynamic criteria for the chemical and chemiosmotic hypotheses.

Oxidative phosphorylation is analyzed by means of nonequilibrium thermodynamics. It is shown that a mitochondrial system may be characterized in terms of the externally fixed affinities (negative free energies) for oxidation and phosphorylation A(o) (ex) and A(p) (ex), without knowledge of internal activities. If the electrochemical potential difference of H(+) is also fixed, a decision can be made between the chemiosmotic and chemical hypotheses. The chemiosmotic hypothesis is shown to be a limiting case of the chemical hypothesis. In general the P/O ratio cannot be expected to be constant, but will vary with A(p) (ex)/A(o) (ex); for certain ranges the variation may be marked.