Standard Reduction Potential E 0'

Movement of electrons through the electron transport system (ETS) results in an oxidation (the complex losing electrons) and a simultaneous reduction (the complex accepting electrons). We use the term "redox" to describe these joint reactions:

Reductant + oxidant <=> Oxididized reductant (has lost electrons) + Reduced oxidant (has gained electrons)

In order for electrons to be transferred from the reductant to the oxidant, the oxidant must have a greater affinity for electrons than the reductant does. The standard reduction potential, E 0' is a measure of the affinity of a molecule (or partial reaction) for electrons measured at pH 7 and 25C. Table 15.1 shows standard reduction potentials for several reactions/species of interest in biochemistry. Species with a higher standard reduction potential tend to accept electrons from molecules with a lower standard reduction potential. Thus, electrons tend to move from cytochrome c(+2) to Cytochrome a (+3), because E 0' = 0.25 volts for the cytochrome c(+2) half-reaction whereas E 0' = 0.29 volts for the cytochrome a(+3) half-reaction. Keep in mind, though, that the movement of electrons from one compound to another based on E 0' values is only a tendency. If equal concentrations of the various species are present, the E 0' tells the direction electrons will flow. At other concentrations, the free energy change for the process (see below) must be calculated to determine the direction of electron movement.

See also: Free Energy Changes from Oxidation/Reduction, Oxidations and Energy Generation, Electron Transport, Cytochromes