Two classes of reaction centres are recognized. Type I, found in green-sulfur bacteria, Heliobacteria, and plant/cyanobacterial PS-I, use iron sulfur clusters as electron acceptors. Type II, found in chloroflexus, purple bacteria, and plant/cyanobacterial PS-II, use quinones. Not only do all members inside each class share common ancestry, but the two classes also, by means of common structure, appear related.
Cyanobacteria, the precursor to chloroplasts found in green plants, have both photosystems with both types of reaction centers. Combining the two systems allows for producing oxygen.