ATP synthesis is not an energetically favorable reaction: energy is needed in order for it to occur. This energy is derived from the oxidation of NADH and FADH2 by the four protein complexes of the electron transport chain (ETC). The ten NADH that enter the electron transport originate from each of the earlier processes of respiration: two from glycolysis, two from the transformation of pyruvate into acetyl-CoA, and six from the citric acid cycle. The two FADH2 originate in the citric acid cycle.
The events of the electron transport chain involve NADH and FADH, which act as electron transporters as they flow through the inner membrane space. In complex I, electrons are passed from NADH to the electron transport chain, where they flow through the remaining complexes. NADH is oxidized to NAD in this process. Complex II oxidizes FADH, garnering still more electrons for the chain. At complex III, no additional electrons enter the chain, but electrons from complexes I and II flow through it. When electrons arrive at complex IV, they are transferred to a molecule of oxygen. Since the oxygen gains electrons, it is reduced to water.
While these oxidation and reduction reactions take place, another, connected event occurs in the electron transport chain. The movement of electrons through complexes I-IV causes protons (hydrogen atoms) to be pumped out of the intermembrane space into the cell cytosol. As a result, a net negative charge (from the electrons) builds up in the matrix space while a net positive charge (from the proton pumping) builds up in the intermembrane space. This differential electrical charge establishes an electrochemical gradient. As we will see in the next section, it is this gradient that drives ATP synthesis in oxidative phosphorylation.