The essential role of coenzyme Q in biological energy transduction is well established. Coenzyme Q is a unique carrier for two-electron transfer within the lipid phase of the mitochondrial membrane. The function is essential for proton-based energy coupling. The sites of entry and exit of electrons into the quinone are at specific quinone-binding sites which are constructed to allow only two-electron transfer and thus prevent damaging free radical formation by direct reaction of oxygen with the semiquinone. Failure of proper function with diminished energy supply can be related to insufficient quinone, modification of lipid fluidity, or lipid protein interaction and damage or poisoning in binding sites. Supplementation with coenzyme Q can act by reversal of deficiency or decreased mobility, or by overcoming binding site modification. Coenzyme Q has also been shown to increase antioxidant protection in membranes. New sites for coenzyme Q function in Golgi and plasma membrane show evidence for a role in growth control and secretion-related membrane flow.