Molecular model of the photosystem II from Thermosynechococcus vestitus BP-1. Photosystem II is a hetero 36-mer multi-protein complex embedded in the thylakoid membrane of plants, algae, and cyanobacteria. It plays a vital role in the light-dependent reactions of photosynthesis, where light energy is used to convert water into molecular oxygen and generate a proton gradient for ATP synthesis.