FIGURE 12-25 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