<P> Cyanobacteria are the only bacteria that produce oxygen during photosynthesis . Earth's primordial atmosphere was anoxic . Organisms like cyanobacteria produced our present - day oxygen - containing atmosphere . </P> <P> The other two major groups of photosynthetic bacteria, purple bacteria and green sulfur bacteria, contain only a single photosystem and do not produce oxygen . </P> <P> Purple bacteria contain a single photosystem that is structurally related to PS II in cyanobacteria and chloroplasts: </P> <Dl> <Dd> P870 → P870 → ubiquinone → bc 1 → cytochrome c 2 → P870 </Dd> </Dl>

Where do all the electrons for the light reactions of photosynthesis come from