Proton and Ion Transport Across the Thylakoid Membranes

Abstract

Upon illumination of a weakly buffered chloroplast suspension a prompt increase in the suspension pH is observed reaching a steady state after a short time (between 1/2 and 2 min). When the light is turned off the pH gradually decreases to its initial value (Jagendorf and Hind, 1963). This observation was interpreted as light-induced proton transport across the thylakoids membranes and opened the way for a vast number of studies. It was found that factors that stimulate electron transport greatly enhance both the rate and the extent of proton uptake, while uncouplers inhibit it (Neumann and Jagendorf, 1964). Mitchell (1961) has suggested earlier that proton transport is the process that couples electron transport to phosphorylation and the discovery of acid-base-induced ATP formation (Jagendorf and Uribe, 1966; Chap. III. 9), confirmed the existence of a link between light-induced electron transport, proton transport, and phosphorylation. Earlier Hind and Jagendorf (1963) discovered that when chloroplasts are illuminated at low pH (6.5) and then transferred in the dark to a medium of high pH (8.5) phosphorylation takes place. The close relationships between postillumination ATP synthesis (see Chap. III.6) and proton uptake was established much later, as it was found that various buffers that penetrate the thylakoids membranes and increase the internal buffer capacity and thus the extent of proton uptake also increase several-fold the yield of postillumination ATP synthesis (Nelson et al., 1971; Avron, 1972).