When a compound is oxidized, it loses electrons. In biological systems, electrons from oxidation are generally transferred to electron-carrying molecules, such as  NAD⁺ or FAD to form NADH and FADH2, respectively. Note that  NAD⁺ and FAD are the oxidized forms of the molecules and NADH and FADH2 are the reduced forms. Thus, biological oxidations generate reduced electron carriers. Reduced electron carriers donate their electrons to acceptor molecules and become reoxidized in the process. The acceptor molecules are reduced because the oxidation of one species (e.g., the reduced electron carrier) cannot occur without the simultaneous reduction of another species (e.g., the acceptor molecule).

The inner mitochondrial membrane contains a complex called Complex I that accepts electrons from NADH. In the process, the complex is reduced and  NAD⁺ is re-formed. Electrons from FADH2 are transferred to a different mitochondrial complex, called Complex II. Figure 15.3 and Figure 15.2b depicts the movement of electrons from Complex I and Complex II through the other electron carriers in the inner mitochondrial membrane.

The complexes of the inner mitochondrial membrane that shuttle electrons are called the electron transport system (ETS). After electrons pass through Complex IV, they are donated to oxygen along with protons to form water Figure 15.2b. As electrons move through complexes I, III, and IV of the ETS, protons are "pumped" from the mitochondrial matrix to the intermembrane space. This creates a potential energy source, with a high concentration of protons in the intermembrane space and a relatively low concentration of protons in the mitochondrial matrix.

Pumps require energy to function. The "pumps" of the ETS chain derive their energy to transport protons from oxidation/reduction (called redox) reactions that occur as electrons move from one complex to another. To quantitate the amount of energy in these transfers, the standard reduction potential can be used.