Electron carriers can be thought of as electron shuttles. These compounds can easily accept electrons (i.e., be reduced) or lose them (i.e., be oxidized). They, therefore, play an essential role in energy production because cellular respiration is contingent on the flow of electrons.

Over the many stages of cellular respiration, glucose breaks down into carbon dioxide and water. Electron carriers pick up electrons lost by glucose in these reactions, temporarily store the electrons and input them into the electron transport chain.

Two such electron carriers are NAD⁺ and FAD, which are both derived from B vitamins. The reduced forms of NAD⁺ and FAD, NADH and FADH₂, respectively, are produced during earlier stages of cellular respiration (glycolysis, pyruvate oxidation, and the citric acid cycle).

The reduced electron carriers NADH and FADH₂ pass electrons into complexes I and II of the electron transport chain, respectively. In the process, they are oxidized to form NAD⁺ and FAD.

Additional electron carriers in the electron transport chain are flavoproteins, iron-sulfur clusters, quinones, and cytochromes. With the assistance of enzymes, these electron carriers eventually transfer the electrons to oxygen molecules. The electron carriers become oxidized as they donate electrons and reduced as they accept them, and thus alternate between their oxidized and reduced forms.

Electron carriers provide a controlled flow of electrons that enables the production of ATP. Without them, the cell would cease to function.