Cytochromes are proteins with characteristic strong absorption of visible light due to their iron-containing heme prosthetic groups. Mitochondria contain three classes of cytochromes designated a, b and c distinguished by differences in their light absorption spectra. The cytochrome of type a and b and some of type c are integral proteins of inner mitochondrial membrane. The sequence of electron carriers in ETS is:
NADH → Q → Cyt b → Cyt c1 →Cyt c → Cyt a → Cyt a3 → O2