Oxidative phosphorylation (OXPHOS) is the coordinated transfer of electrons and protons leading to the production of ATP. The OXPHOS system is composed of five mitochondrial inner membrane complexes, Complexes I-IV comprise the electron respiratory chain and Complex V the ATP synthase. Complexes I and II oxidize the electron donors NADH and FADH2, respectively, derived from the tricarboxylic acid (TCA) cycle, and the electrons are transferred to the electron carrier ubiquinone which shuttles them to Complex III. Cytochrome c transfers electrons between Complex III and the terminal complex in the respiratory chain, Complex IV. Complex IV catalyzes electron transfer from reduced cytochrome c to oxygen. Complex I, II and IV couple electron transfer to proton transport across the inner mitochondrial membrane into the inner membrane space. This generates a proton electrochemical gradient which is used to drive the synthesis of ATP from ADP and inorganic phosphate by ATP synthase (Complex V). (Adapted from PMID:23356252 and PMID:11331900.)