The electron transport chain relies on the creation of a proton gradient across a membrane via electron transfer between an electron donor and electron acceptor. The proton gradient is coupled to the production of chemical energy usable by the host(usually in the form of ATP).In eukaryotes it occurs in the mitochondria and is the site of oxidative phosphorylation. This pathway is also the mechanism by which plants harness the sun’s energy.
Reference : Champe, Pamela C. and Richard A. Harvey. 1994. Lippincott’s Ilustrated Reviews : Biochemistry. Philadelphia. J.B. Lippincott and Company.