text: 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 center (70−80%). Because of this, P680 is highly oxidizing and can take part in the splitting of water.
  Photosystem II (of cyanobacteria and green plants) is composed of around 20 subunits (depending on the organism) as well as other accessory, light-harvesting proteins.  Each photosystem II contains at least 99 cofactors: 35 chlorophyll a, 12 beta-carotene, two pheophytin, two plastoquinone, two heme, one bicarbonate, 20 lipids, the  cluster (including two chloride ions), one non heme  and two putative  ions per monomer.  There are several crystal structures of photosystem II. The PDB accession codes for this protein are , ,  (3BZ1 and 3BZ2 are monomeric structures of the Photosystem II dimer), , , , , , .