Metabolic energy derives from processes of oxidation and reduction. When energy is consumed in a process, chemical energy is made available for synthesis of ATP as one atom gives up electrons (becomes oxidized) and another atom accepts electrons (becomes reduced). For example, observe the following aerobic metabolism of glucose.
The carbon in glucose moves from an oxidation state of zero to an oxidation state of +4. Concurrently, elemental oxygen moves from its oxidation state of zero to an oxidation state of −2 during the process.
Anaerobic catabolic reactions are similar, although the electron acceptor isn’t oxygen. The next example shows the fermentation of glucose to lactic acid.
In this case, one carbon (the methyl carbon of lactic acid) is reduced from the zero oxidation state to –3 while another carbon (the carboxyl carbon of lactic acid) gives up electrons and goes from an oxidation state of zero to +3. In this example, the electron acceptor and electron donor are located on the same molecule, but the principle remains the same: One component is oxidized and one is reduced at the same time.
Reactions that run in the opposite direction of the preceding ones, particularly the first, must exist. Glucose must be made from inorganic carbon—that is, CO 2. More generally, reducing equivalents and energy must be available to carry out the synthetic reaction.
The general reaction accounts for the fact that in some systems, something other than water supplies the reducing equivalents. For example, bacteria living in deep‐sea thermal vents can apparently use hydrogen sulfide (H 2S) as a source of reducing equivalents to synthesize glucose from carbon dioxide dissolved in the seawater.