Fig. 3.

Highly simplified scheme of Kok’s oxygen clock model; misses and double hits are not shown. S_i (i = 0, 1, 2, 3, 4) represent the particular S-states of the manganese cluster of OEC. The S₄-state is assumed to be kinetically indistinguishable from the S₀-state. During an S-state transition, Y_Z⁺ (formed through PSII reactions) is reduced (with rate constants k₀₁, k₁₂, k₂₃ and k₃₀). Modified from Lazár and Schansker (2009). For a review, including the involvement of manganese, see Najafpour et al. (2012).