FIGURE 1.

(A) Scheme of the sequence of light-induced reactions in PSII (B) Energy-level diagram of radical-pair states in PSII. The zero-level of the Gibbs free-energy scale is chosen to correspond to the excited state of the chlorophyll antenna, which is lower than the P680^∗ level due to the antenna entropy contribution (Dau, 1994; Dau and Sauer, 1996). Modified from Buchta et al. (2007). (C) S-state cycle of the photosynthetic water oxidation. Inner circle: The classical Kok model (Kok et al., 1970). Outer circle: Extended S-cycle describing both oxidation of the Mn complex by electron transfer to the Y_Z^•+ radical and proton removal from the Mn complex or its ligand environment (Based on Klauss et al., 2012).