<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 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 which have a special pair of closely spaced chlorophyll molecules, the pigment that undergoes the initial photoinduced charge separation in PSII is a chlorophyll monomer . Because the positive charge is not shared across two molecules, the ionised pigment is highly oxidizing and can take part in the splitting of water . </P>

Where do the electrons needed by photosystem two originate