Figure 1. The proton circuit across the mitochondrial inner membrane and an equivalent electrical circuit.

(A) The primary, ATP-generating, proton circuit is shown as bold lines/boxes and the pathway of electron flow as a dashed vertical line. Secondary pathways of proton re-entry include metabolite transport [represented in the present Figure by the phosphate carrier, the transhydrogenase for the reduction of NADP⁺ to NADPH, and the endogenous and UCP (uncoupling protein)-mediated proton leaks]. Ca²⁺ cycles between the uniport and the Na⁺/Ca²⁺ antiport, and a sodium circuit links the proton and calcium circuits. (B) The equivalent electrical circuit modelling the primary proton circuit. The resistances associated with each ‘complex’ reflect the observation that membrane potential drops as the proton current drawn increases.