PSII is a protein complex located in the thylakoid membrane of oxygenic photosynthetic organisms. It is composed of two protein sub-units D1 and D2 (see Fig. ). The membrane of PSII contains 36 trans-membrane α-helices. Five of these helices belong to the proteins D1 and D2 each. Six of those belong to the proteins CP43 and CP47. CP43 and CP47 are called the core antenna proteins that bind 13 and 16 Chl a molecules, respectively. Overall 11 β-carotene units can be found in the antenna proteins. The central magnesium atom of the majority of the Chl a molecules in the antenna proteins is coordinated by a histidine (His) residue. Additionally, the tetrapyrrole macrocycle of these Chl a molecules is not exactly planar, but slightly bent out of the macrocycle plane. The proteins D1 and D2 contain the co-factors that are associated with the RC of PSII. The D1 and D2 sub-units are distinguished by the presence of the so-called Oxygen Evolving Complex (OEC) near the sub-unit D1 (not shown in Fig. ) and other structural aspects. The role of this manganese cluster is the oxidation of water and formation of oxygen .