(d) : Cytochromes are generally membranebound hemoproteins that contain heme groups and carry out electron transport. The heme group is a highly conjugated ring system (which means its electrons are very mobile) surrounding a metal ion, which readily interconverts between the oxidation states. For many cytochromes the metal ion present is that of iron, which interconverts between Fe2+ (reduced) and Fe3+ (oxidized) states (electron-transfer processes) or between Fe2+ (reduced) and Fe3+ (formal, oxidized) states (oxidative processes). Cytochromes are thus capable of performing oxidation and reduction.