<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> <P> Energy obtained through the transfer of electrons down the ETC is used to pump protons from the mitochondrial matrix into the intermembrane space, creating an electrochemical proton gradient (ΔpH) across the inner mitochondrial membrane (IMM). This proton gradient is largely but not exclusively responsible for the mitochondrial membrane potential (ΔΨ). It allows ATP synthase to use the flow of H through the enzyme back into the matrix to generate ATP from adenosine diphosphate (ADP) and inorganic phosphate . Complex I (NADH coenzyme Q reductase; labeled I) accepts electrons from the Krebs cycle electron carrier nicotinamide adenine dinucleotide (NADH), and passes them to coenzyme Q (ubiquinone; labeled Q), which also receives electrons from complex II (succinate dehydrogenase; labeled II). Q passes electrons to complex III (cytochrome bc complex; labeled III), which passes them to cytochrome c (cyt c). Cyt c passes electrons to Complex IV (cytochrome c oxidase; labeled IV), which uses the electrons and hydrogen ions to reduce molecular oxygen to water . </P> <P> Four membrane - bound complexes have been identified in mitochondria . Each is an extremely complex transmembrane structure that is embedded in the inner membrane . Three of them are proton pumps . The structures are electrically connected by lipid - soluble electron carriers and water - soluble electron carriers . The overall electron transport chain: </P> <P> In Complex I (NADH: ubiquinone oxidoreductase, NADH - CoQ reductase, or NADH dehydrogenase; EC 1.6. 5.3), two electrons are removed from NADH and ultimately transferred to a lipid - soluble carrier, ubiquinone (UQ). The reduced product, ubiquinol (UQH), freely diffuses within the membrane, and Complex I translocates four protons (H) across the membrane, thus producing a proton gradient . Complex I is one of the main sites at which premature electron leakage to oxygen occurs, thus being one of the main sites of production of superoxide . </P>

Where do hydrogen ions come from in the electron transport chain