Electron transport chain, proton gradient, and ATP synthase rotation
The electron transport chain (Complexes I–IV) uses electrons from NADH/FADH₂ to pump protons across the inner mitochondrial membrane, generating an electrochemical gradient (proton-motive force ~200 mV). ATP synthase (Complex V) harnesses this gradient: proton flow through the Fₒ subunit drives rotation of the γ-subunit within the F₁ catalytic head, causing conformational changes in three β-subunits that cycle through open, loose, and tight states — synthesizing ATP with each rotation. The chemiosmotic mechanism earned Peter Mitchell the 1978 Nobel Prize.