On the Distribution of Excitation Energy to Two Photoreactions of Photosynthesis

From kinetic analysis of the Z‐scheme we have derived expressions for fractions of open reaction centers and throughput quantum yield. The fraction of absorbed quanta available to photoreaction II has been treated by the wavelength dependent function, α. However, for purposes of analysis in terms of throughput electron flow it is necessary to introduce a related function, α', for actual processing of excitations by photoreaction II; α' differs from α if the individual quantum yields of the two photoreactions are not equal. ‘Included in our findings are the following. As a close approximation quantum yield is maximum at α’= 0.5 and a symmetrical function of α' around the line α' =0.5. As previously noted, lowering of the apparent equilibrium constant between the photoreactions flattens dependence of quantum yield on α' near α' = 0.5 but also lowers the attainable quantum yield. For any given equilibrium constant the effective fractions of open reaction centers are equal at α' = 0.5 and mirror images of each other around the line α' = 0.5. Slow changes in quantum yield following changes in wavelength (the State 1‐State 2 phenomenon) are explainable in terms of changes in α' which also have the effect of flattening dependence of quantum yield on wavelength. We have used these changes to estimate the neutral wavelength for α' = 0.5 as 681–682 nm in Chlorella.