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Cytochromes are generally membrane-bound proteins that contain heme groups and carry out electron transport. They are found in the mitochondrial inner membrane of eukaryotes, in the chloroplasts of plants, in photosynthetic microorganisms, and in bacteria.

The heme group is a highly conjugated ring system (which means its electrons are very mobile) surrounding an iron ion, which readily interconverts between the Fe2+ (reduced) and Fe3+ (oxidized) states. Cytochromes are thus capable of performing oxidation and reduction. Because the cytochromes (as well as other complexes) are held within membranes in an organized way, the redox reactions are carried out in the proper sequence for maximum efficiency.

Other membrane-bound and soluble complexes and cofactors are involved in the chain of redox reactions, with the additional net effect that protons are transported across the membrane. The resulting transmembrane proton gradient (protonmotive force) is used to power cellular processes that require energy (such as rotation of flagella, transport of molecules across the membrane, and synthesis of ATP).

Several kinds of cytochrome exist and can be distinguished by spectroscopy, exact structure of the heme group, inhibitor sensitivity, and reduction potential:

See the articles on mitochondria and chloroplasts for more information on electron transport and related metabolic pathways.