As we saw in Chapter 2, the reduction potential (E) for a partial reduction reaction 
is a measure of the equilibrium constant of that partial reaction. With the exception of the b cytochromes in complex III (CoQH2–cytochrome c reductase), the standard reduction potential E°′ of the electron carriers in the electron-transport chain increases steadily from NADH to O2. For instance, for the partial reaction 
the value of the standard reduction potential is −320 mV, which is equivalent to a ΔG°′ of +14.8 kcal/mol for trans fer of two electrons. Thus this partial reaction tends to proceed toward the left; that is, toward the oxidation of NADH to NAD+.
In contrast, the standard reduction potential for the partial reaction
is +220 mV (ΔG°′ = −5.1 kcal/mol) for transfer of one electron. Thus this partial reaction tends to proceed toward the right; that is, toward the reduction of cytochrome c (Fe3+) to cytochrome c (Fe2+).
The final reaction in the electron-transport chain, the reduction of O2 to H2O
has a standard reduction potential of +816 mV (ΔG°′ = −37.8 kcal/mol for transfer of two electrons), the most positive in the whole series; thus this reaction also tends to proceed toward the right.
As illustrated in Figure 1, the steady increase in E°′ values, and the corresponding decrease in ΔG°′ values, of the carriers in the electron-transport chain favors the flow of electrons from NADH and FADH2 (generated from succinate) to O2. The energy released as electrons flow energetically “downhill” through the electron-transport chain complexes drives the pumping of protons against their concentration gradient across the inner mitochondrial membrane.
Fig1. Changes in reduction potential and free energy during the stepwise flow of electrons through the electron-transport chain. Blue arrows indicate electron flow; red arrows, translocation of protons across the inner mitochondrial membrane. Electrons pass through the multiprotein complexes from those with a lower reduction potential to those with a higher (more positive) reduction potential (left scale), with a corresponding reduction in free energy (right scale). The energy released as electrons flow through three of the complexes is sufficient to power the pumping of H+ ions across the membrane, establishing a proton-motive force.
