Fig. 1. Action of the electron transport chain leading to the build-up of a proton gradient across the inner mitochondrial membrane. Protons can cross the inner mitochondrial membrane via the F₀F₁ complex, thereby providing energy for the conversion of ADP to ATP. Alternatively, the proton gradient can be reduced by uncoupling proteins (UCP), either by direct transport of protons from the intermembrane space to the matrix or by transport of fatty acid anions in the opposite direction. Cyt c, cytochrome c. Q, coenzyme Q.