A demonstration of the physiological role of membrane phosphorylation in chloroplasts, using the bipartite and tripartite models of photosynthesis

Using the equations derived from the bipartite and tripartite models for photosynthetic organization in green plants, we have been able to characterize the effect of membrane phosphorylation on energy transduction. Phosphorylation reversibly increases α (the proportion of absorbed quanta going directly to Photosystem (PS) I). This increase in α we believe to be due to a decrease in the coupling between the PS II core and its associated light-harvesting complex [ΨT(3,2)·ΨT(2,3)]. Phosphorylation also reversibly increases the transfer of energy from PS II to PS I [ψT_(II→I)]. We propose that membrane phosphorylation provides the in vivo control of α, ΨT(3,2)·ΨT(2,3) and ψT_(II→I). From the data we present it is clear that the changes caused in energy distribution as a result of phosphorylation are large enough to induce real changes in electron-transfer reactions. The effects of phosphorylation on these parameters are distinct from those of Mg²⁺ depletion. We have discussed changes in ΨT(3,2)·ΨT(2,3) (the coupling term) with respect to the 'connected package' model of photosynthetic units (Butler, W.L. (1980) Proc. Natl. Acad. Sci. U.S.A. 77, 4697-4701) and the proposed α- and β-centers of PS II (Melis, A. and Homann, P.H. (1976) Photochem. Photobiol. 23, 345-350). The demonstration of changes in reversible coupling [ΨT(3,2)·ΨT(2,3)] strongly supports a connected package model in which the degree of 'connectivity' is under physiological control.