The electron transport chain (Complexes I-IV) pumps H⁺ from the matrix into the intermembrane space as electrons flow from NADH to O₂, building an electrochemical gradient (the proton-motive force) that ATP synthase uses to make ATP:
ΔΨ ∝ ∫(pump rate − leak rate) dt
ATP rate ∝ proton flux through synthase × ADP availability
proton flux ∝ ΔΨ (down-gradient) − uncoupler leak
Chemiosmotic coupling means ATP synthase's rotary motor only turns — and only makes ATP — when protons flow back through it down the gradient built by the ETC. Uncoupling proteins (or drugs like DNP) let protons leak back across the membrane without going through the synthase, wasting the gradient as heat instead of ATP — exactly what brown fat does for thermogenesis.
- Substrate supply — more NADH/FADH₂ feeds electrons into the chain, driving pump rate.
- ADP availability — ATP synthase needs ADP + Pi as substrate; without it, protons back up and the gradient saturates (respiratory control).
- Uncoupler — protons leak straight back across the inner membrane, collapsing the gradient and cutting ATP output even with plenty of substrate.
- ETC inhibitor — blocks electron flow entirely (models cyanide poisoning), stopping proton pumping and ATP production.
This is literally how every aerobic cell in your body makes usable energy — dysfunction in this system underlies mitochondrial disease, ischemia-reperfusion injury, and ageing-related energy decline.