As noted in the previous section, photosynthesis in the green and purple bacteria does not generate oxygen, whereas photosynthesis in cyanobacteria, algae, and plants does.* This difference is attributable to the presence of two types of photosystems (PS) in the latter organisms: PSI reduces NADP⁺ to NADPH, and PSII forms O₂ from H₂O. In contrast, the green and purple bacteria have only one type of photosystem, which cannot form O₂. We first discuss the simpler photosystem of purple bacteria and then consider the more complicated photosynthetic machinery in chloroplasts.