Photosystem II

Photosystem II is a large protein complex. It is the first protein complex in the light-dependent reactions of oxygenic photosynthesis. 
The complex is in the thylakoid membrane of plants, algae, and cyanobacteria. 
In the photosystem, enzymes capture photons of light to energize electrons. The electrons are then transferred by coenzymes and cofactors. The energized electrons are replaced by oxidizing water to form hydrogen ions and molecular oxygen.
By replenishing lost electrons with electrons from the splitting of water, photosystem II provides the electrons for photosynthesis to occur.