A process of biological oxidation in which electrons are transferred from a reduced substrate through a series of compounds to oxygen, the energy of the process being conserved by the formation of high-energy bonds in the form of adenosine triphosphate (an organic compound composed of adenine or base, the sugar ribose, and three phosphate groups) which serves as the major energy source within a cell to drive a number of biological processes; such as, photosynthesis, muscle contraction, and the synthesis of proteins.