Fig. 1.

Schematic of mitochondrial electron transport chain (ETC). Dashed lines indicate the flow of electrons donated from either NADH or FADH2 to oxidative phosphorylation complexes I–IV (CI–CIV). As a result of electron transport, protons (H⁺) are translocated into the intermembrane space of the mitochondria creating a proton gradient across the inner mitochondrial membrane. The proton gradient is necessary to drive ATP production via ATP synthase, but under certain conditions, uncoupling proteins (UCP) are used to dissipate the proton gradient. Uncoupling proteins may be associated with cell protection, the prevention of free radical production, and lifespan.