Electron Transport, Photophosphorylation and Thylakoid Stacking

Abstract

Lettuce chloroplasts in the stacked or unstacked state have been compared with respect to cyclic photophosphorylation mediated by phenazine metho-sulphate, and non-cyclic photophosphorylation and uncoupled electron transport mediated by methyl viologen. The same ionic medium was used to assay chloroplasts in either structural state, in order to equalize effects of ionic composition which may influence reaction rates. Over a wide range of irradiance (400–700 nm, 20°C), chloroplasts in the stacked state exhibited higher rates of non-cyclic photophosphorylation, but lower rates of cyclic photophosphorylation compared with unstacked thylakoids. Unstacked membranes gave higher rates of uncoupled non-cyclic electron flow at relatively high irradiance, but not at low irradiance. The results are discussed in terms of the lateral heterogeneous distribution of membrane components of chloroplasts in the stacked state.