8.9: Electron Transport Chains
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.