any of a group of hemoprotein cell components that, by readily undergoing reduction and oxidation (gain and loss of electrons) with the aid of enzymes, serve a vital function in the transfer of energy within cells. Hemoproteins are proteins linked to a nonprotein, iron-bearing component. It is the iron (heme) group attached to the protein that can undergo reversible oxidation and reduction reactions, thereby functioning as electron carriers within the mitochondria (the organelles that produce energy for the cell through cellular respiration).

Cytochromes are subdivided into three classes (a, b, c) depending on their light-absorption spectra. At least 30 different cytochromes have been identified; they are designated by letters or combinations of letters and numbers, such as cytochrome a₃, cytochrome c, and cytochrome B562. Cytochrome c is the most stable and abundant member of the class, and it has been the most thoroughly studied. See also cellular respiration.