Fig. 2.

Schematic diagram of the electron transfer reactions occurring in the membrane-bound PSII RC and the formation of singlet oxygen at the site of ³P680. The purified complex (D1, D2, α and β subunits of Cyt b₅₅₉ and the PsbI protein) has lost both of the secondary electron acceptors, Q_A and Q_B, the non-heme iron (Fe) and also the water-splitting Mn cluster, Mn₄CaO₅. The figure shows that if the triplet state of P₆₈₀ is formed it will be quenched by ground state oxygen to form ¹O₂ which can damage either the pigment–protein complex or the lipid membrane. The cofactor arrangement in the RC complex shows the distance of the two β-carotenes from the four central Chl cofactors, based on Umena et al. (2011).