Electron transport chain in mitochondria, animation. The electron transport chain is a series of reactions that create energy, in the form of ATP (adenosine triphosphate), for cellular processes. In eukaryotic cells it takes place on the inner membrane of mitochondria. A pair of electrons (blue dots) can be donated from either NADH (top right) or, as here, succinate, an intermediate product of the citric acid cycle. The electrons transfer to the FADH2 subunit of succinate dehydrogenase (complex II, large white oval). They then flow to coenzyme Q (small white oval), which carries them through the membrane to the cytochrome bc1 complex (complex III, red). The electrons are transferred to cytochrome c (yellow), a peripheral membrane protein bound to the outer surface of the inner membrane, which gives the electrons to cytochrome c oxidase (complex IV). This complex adds four electrons to oxygen, yielding two water molecules (blue). At complex III and IV the transfer of electrons causes protons to be pumped from the matrix to the intermembrane space, establishing a proton gradient across the inner membrane. The protons flow down this gradient through ATP synthase (complex V, right), which synthesises adenosine triphosphate (ATP) from adenosine diphosphate (ADP) and inorganic phosphate (Pi).
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