The overall reaction catalysed by photosystem II is:

Q represents plastoquinone (shown left), the oxidized form of Q. QH₂ represents plastoquinol, the reduced form of Q. This process of reducing quinone is comparable to that which takes place in the bacterial reaction centre. Photosystem II obtains electrons by oxidizing water in a process called photolysis. Molecular oxygen is a byproduct of this process and it is this reaction that supplies the atmosphere with oxygen. This reaction is catalyzed by a reactive centre in photosystem II containing four manganese ions.

oxygen evolving complex - Manganese is shown in purple, oxygen in red, and calcium in green

location of OEC within PSII (Note: Very large molecule! Please wait for it to load).

Every time the P680 absorbs a photon, it emits an electron, gaining a positive charge. This charge is neutralized by the extraction of an electron from the manganese centre which sits directly below it. The process of oxidizing two molecules of water requires four electrons. The water molecules which are oxidized in the manganese centre are the source of the electrons which reduce the two molecules of Q to QH₂.

After the electron has left photosystem II it is transferred to a cytochrome b6f complex and then to , a blue copper protein and electron carrier. The plastocyanin complex carries the electron that will neutralize the pair in the next reaction centre, photosystem I.