<P> Photosystem II (or water - plastoquinone oxidoreductase) is the first protein complex in the light - dependent reactions of oxygenic photosynthesis . It is located in the thylakoid membrane of plants, algae, and cyanobacteria . Within the photosystem, enzymes capture photons of light to energize electrons that are then transferred through a variety of coenzymes and cofactors to reduce plastoquinone to plastoquinol . The energized electrons are replaced by oxidizing water to form hydrogen ions and molecular oxygen . </P> <P> By replenishing lost electrons with electrons from the splitting of water, photosystem II provides the electrons for all of photosynthesis to occur . The hydrogen ions (protons) generated by the oxidation of water help to create a proton gradient that is used by ATP synthase to generate ATP . The energized electrons transferred to plastoquinone are ultimately used to reduce NADP to NADPH or are used in Non cyclic photophosphorylation . </P> <P> The core of PSII consists of a pseudo-symmetric heterodimer of two homologous proteins D1 and D2 . Unlike the reaction centers of all other photosystems in which the positive charge sitting on the chlorophyll dimer that undergoes the initial photoinduced charge separation is equally shared by the two monomers, in intact PSII the charge is mostly localized on one chlorophyll molecule (70 − 80%). Because of this, P680 is highly oxidizing and can take part in the splitting of water . </P>

Where do the electrons needed by photosystem 2 originate