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, E0' 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 E0' = 0.25 volts for the cytochrome c(+2) half-reaction whereas E0' = 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 E0' values is only a tendency. If equal concentrations of the various species are present, the E0' 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.