Complexes I–IV pass electrons from NADH/FADH₂ to O₂. Each transfer pumps H⁺ across the inner membrane, building a proton gradient (proton-motive force).
Fo-F1 ATP synthase is a true rotary motor — proton flow through Fo spins the c-ring at ~150 rpm, driving the F1 catalytic head to synthesize ATP from ADP+Pi.
Peter Mitchell's chemiosmotic hypothesis (Nobel 1978): proton gradient IS the energy currency. ~2.5 ATP per NADH, ~1.5 per FADH₂ — ~30 ATP per glucose molecule.