Any of a group of proteins, each with an iron-containing haem group, that form part of the electron transport chain in mitochondria and chloroplasts. Electrons are transferred by reversible changes in the iron atom between the reduced Fe(II) and oxidized Fe(III) states. There are four subclasses, designated a to d, with different chemical configurations of the haem group. Members of each group can be denoted by a suffixed number (e.g. cytochrome b₂) or by the characteristic wavelength (in nanometres) at which they maximally absorb light (e.g. cytochrome c₅₅₅). See also cytochrome oxidase.