Regulation of photosynthesis by reversible phosphorylation of the light-harvesting chlorophyll a/b protein

Photosynthetic CO2 fixation in chloroplasts is driven by ATP and NADPH generated by electron and proton transport within the thylakoid mem- brane. The movement of an electron from water to NADP+ along the electron transport chain requires two photons and involves two separate photosystems (PS I and PS II) operating in series. Each photosystem contains a reaction centre consisting of one or two chlorophyll a molecules capable of producing across the membrane a separation of reductant and oxidant that will initiate electron transport How- ever, less than 1% of chlorophyll molecules possess reaction centre activity. The remainder have a less direct connection with electron transport. Their role is to absorb as much light as possible and channel the resulting excitation energy to the reaction centres. Without these light-harvesting chlorophyll molecules, the electron transport rate would be severely limited under any light intensity less than that of full sunlight.