It is distinguished between ubiquinones and plastoquinones. The two groups differ in their different side chains. Coenzyme Q, an unbiquinone, has a key position as a primary electron acceptor in photosystem II of photosynthesis. The best-known and also most important example for cells is iron, that occurs as trivalent Fe III and as bivalent Fe II . In cytochromes, changes of valency occur.
Cytochromes are members of both the respiratory chain and photosynthesis.
The molecules differ mainly in their protein parts. The best-known metalloprotein without a heme group taking part in photosynthesis is ferredoxin.