<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> <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>

Where do krebs (step 2) and the etc (step 3) occur