<P> In the mitochondrial electron transport chain electrons move from an electron donor (NADH or QH) to a terminal electron acceptor (O) via a series of redox reactions . These reactions are coupled to the creation of a proton gradient across the mitochondrial inner membrane . There are three proton pumps: I, III, and IV . The resulting transmembrane proton gradient is used to make ATP via ATP synthase . </P> <P> The reactions catalyzed by Complex I and Complex III work roughly at equilibrium . This means that these reactions are readily reversible, by increasing the concentration of the products relative to the concentration of the reactants (for example, by increasing the proton gradient). ATP synthase is also readily reversible . Thus ATP can be used to build a proton gradient, which in turn can be used to make NADH . This process of reverse electron transport is important in many prokaryotic electron transport chains . </P> <P> In eukaryotes, NADH is the most important electron donor . The associated electron transport chain is </P> <P> NADH → Complex I → Q → Complex III → cytochrome c → Complex IV → O where Complexes I, III and IV are proton pumps, while Q and cytochrome c are mobile electron carriers . The electron acceptor is molecular oxygen . </P>

Where does etc take place in the cell