Electron-transfer reactions provide the energetic basis for the life process, and through this, play a
crucial role in the geochemical cycle of the elements. It all begins, of course, with photosynthesis
CO2 + H2O ¡! fCH2Og + O2 ¢H± = +478 kJ mol¡1 (1)
in which fCH2Og represents the reduced carbon of glucose and its polymers. Living organisms are
thermodynamically unstable and require a continual source of free energy merely to survive. This they
obtain either from sunlight or from chemical free energy sources such as the carbohydrate products
of the above reaction. The metabolic activities of organisms are largely centered around the stepwise
breakdown of these foodstu®s in a way that transfers smaller increments of free energy to intermediates
(mainly ADP) that are able to e±ciently convey it to sites of synthesis or mechanical movement where
it can be utilized. The overall e®ect can be summarized as the transfer of electrons from the reduced
carbon of fCH2Og to an electron acceptor such as O2 (this is the reverse of photosynthesis) or to SO2¡
3
or NO¡
3 , accompanied by the release of CO2 to the environment.