<P> The electron transport chain consists of a spatially separated series of redox reactions in which electrons are transferred from a donor molecule to an acceptor molecule . The underlying force driving these reactions is the Gibbs free energy of the reactants and products . The Gibbs free energy is the energy available ("free") to do work . Any reaction that decreases the overall Gibbs free energy of a system is thermodynamically spontaneous . </P> <P> The function of the electron transport chain is to produce a transmembrane proton electrochemical gradient as a result of the redox reactions . If protons flow back through the membrane, they enable mechanical work, such as rotating bacterial flagella . ATP synthase, an enzyme highly conserved among all domains of life, converts this mechanical work into chemical energy by producing ATP, which powers most cellular reactions . A small amount of ATP is available from substrate - level phosphorylation, for example, in glycolysis . In most organisms the majority of ATP is generated in electron transport chains, while only some obtain ATP by fermentation . </P> <P> Most eukaryotic cells have mitochondria, which produce ATP from products of the citric acid cycle, fatty acid oxidation, and amino acid oxidation . At the mitochondrial inner membrane, electrons from NADH and FADH2 pass through the electron transport chain to oxygen, which is reduced to water . The electron transport chain comprises an enzymatic series of electron donors and acceptors . Each electron donor will pass electrons to a more electronegative acceptor, which in turn donates these electrons to another acceptor, a process that continues down the series until electrons are passed to oxygen, the most electronegative and terminal electron acceptor in the chain . Passage of electrons between donor and acceptor releases energy, which is used to generate a proton gradient across the mitochondrial membrane by actively "pumping" protons into the intermembrane space, producing a thermodynamic state that has the potential to do work . The entire process is called oxidative phosphorylation, since ADP is phosphorylated to ATP using the energy of hydrogen oxidation in many steps . </P> <P> A small percentage of electrons do not complete the whole series and instead directly leak to oxygen, resulting in the formation of the free - radical superoxide, a highly reactive molecule that contributes to oxidative stress and has been implicated in a number of diseases and aging . </P>

Where do the electrons in the etc come from