Most of the energy released during the oxidation of glucose and fatty acids to CO2 (stages I and II) is converted into high-energy electrons in the reduced coenzymes NADH and FADH2. We now turn to stage III, in which the energy transiently stored in these reduced coenzymes is converted by an electron-transport chain, also known as the respiratory chain, into the proton-motive force. We first describe the logic and components of the electron-transport chain. Next we follow the path of electrons as they flow through the chain and describe the mechanism of proton pumping across the inner mitochondrial membrane. We conclude this section with a discussion of the magnitude of the proton-motive force produced by electron transport and proton pumping. In Section 12.5, we will see how the proton-motive force is used to synthesize ATP.