Cytochrome is 
(a) Metallo flavoproteins
(b) Fe containing porphyrin pigment
(b) Cytochrome is the respiratory pigment. It is composed of protein, iron and porphyrin ring. It functions as an enzyme in the respiratory chain. Unlike in haemoglobin the metal atom in the porphyrin ring must change it's valency for the molecule to function. Cytochromes are basically located in inner mitochondrial membranes and thylakoids of chloroplasts.