<P> In chloroplasts, light drives the conversion of water to oxygen and NADP to NADPH with transfer of H ions across chloroplast membranes . In mitochondria, it is the conversion of oxygen to water, NADH to NAD and succinate to fumarate that are required to generate the proton gradient . </P> <P> Electron transport chains are major sites of premature electron leakage to oxygen, generating superoxide and potentially resulting in increased oxidative stress . </P> <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>

What are the reactants and products of electron transport
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