The supercomplexes of the mitochondrial electron transport chain complexes (modified from ).
Two electrons from NADH (top right) are passed through complex C I (blue), reducing Q to QH2. Diffusing from the Q-tunnel, QH2 may enter the proximal Q cavity of C III (the oxidation cavity) or may diffuse into the membrane pool. QH2 passes one electron to cyt c in the intermembrane space and one to Q in the distal cavity (the reduction cavity), creating the Q• intermediate. Oxidation of a second QH2 in the proximal cavity will lead to reduction of Q• in the distal cavity to QH2, which can then leave and join the membrane pool. CI, Complex I (NADH:ubiquinone oxidoreductase); CII, Complex II (succinate dehydrogenase); CIII, Complex III (cytochrome bc1); CIV, Complex IV (cytochrome c oxidase); cyt c, cytochrome c.