NADH is a carrier of electrons produced in biological oxidations. The molecule exists in two forms that vary in whether or not they are carrying electrons. NADH is the reduced form of the molecule (carries electrons) and NAD⁺ is the oxidized form of the molecule (lacks electrons). NADH is produced from NAD⁺ in reactions such as conversion of acetaldehyde to ethanol by alcohol dehydrogenase (Figure). NADH is converted back to NAD⁺ by donating electrons (such as in the conversion of pyruvate to lactate) or by depositing electrons into the electron transport system.

NADH carries electrons to the electron transport system inside the mitochondrion via a shuttle system (Figure 15.11). Electrons that enter via the shuttle in Figure 15.11a bypass complex I of the electron transport system, whereas electrons that enter via the shuttle in Figure 15.11b enter at complex I.

In contrast to the reduced related compound, NADPH, which donates electrons primarily for biosynthetic reactions, NADH primarily donates electrons to the electron transport system for energy generation.