The final stage of cellular respiration is oxidative phosphorylation that consists of two steps: the electron transport chain and chemiosmosis. The electron transport chain is a set of proteins found in the inner mitochondrial membrane in eukaryotic cells. Its primary function is to establish a proton gradient that can be used during chemiosmosis to produce ATP and generate electron carriers, such as NAD+ and FAD, that are used in glycolysis and the citric acid cycle.

The ETC is comprised of protein complex I, II, III, and IV. NADH and FADH₂ are reduced electron carriers that donate electrons to the ETC complexes. NADH can directly donate electrons into complex I, while FADH₂ donates electrons into complex II. Upon donation of electrons, NADH and FADH₂ are converted back to their oxidized forms NAD+ and FAD, respectively.

These ETC complexes pass electrons to one another through multiple redox reactions in an energetically downhill sequence. These reactions release energy that is used to pump H⁺ across the inner membrane from the matrix into the intermembrane space, establishing a proton gradient across the inner membrane. The flow of H⁺ ions down their electrochemical gradient back into the matrix through ATP synthase enables the conversion of ADP to ATP.