Relative Activity of Photosystems I and II Is Regulated

In order for PSII and PSI to act in sequence during linear electron flow, the amount of light energy delivered to the two reaction centers must be controlled so that each center activates the same number of electrons. If the two photosystems are not equally excited, then cyclic electron flow occurs in PSI, and PSII becomes less active. One mechanism for regulating the relative contribution of linear and cyclic electron flow in chloroplasts entails the reversible phosphorylation and dephosphorylation of the proteins associated with the PSII light-harvesting complex (LHCII).

According to this mechanism, a membrane-bound protein kinase senses the relative activities of the two photosys-

PSII membrane domains (stacked)

Thylakoid membrane

Light STATE 1

PSI membrane domains (unstacked)

Stroma

Lumen

Carbohydrate

Light

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