<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> <P> The pathway of electrons is as follows: </P> <P> NADH is oxidized to NAD, by reducing Flavin mononucleotide to FMNH in one two - electron step . FMNH is then oxidized in two one - electron steps, through a semiquinone intermediate . Each electron thus transfers from the FMNH to an Fe - S cluster, from the Fe - S cluster to ubiquinone (Q). Transfer of the first electron results in the free - radical (semiquinone) form of Q, and transfer of the second electron reduces the semiquinone form to the ubiquinol form, QH . During this process, four protons are translocated from the mitochondrial matrix to the intermembrane space . As the electrons become continuously oxidized and reduced throughout the complex an electron current is produced along the 180 Angstrom width of the complex within the membrane . This current powers the active transport of four protons to the intermembrane space per two electrons from NADH . </P>

Where is the electron transport chain located in the diagram of a mitochondrion below