Active transport, ion movements, and pH changes

Abstract

Electron transport in chloroplasts takes place across the thylakoid membrane in such a way as to redistribute ions. This redistribution can cause transmembrane electric fields and transmembrane hydrogen ion activity differences, events that are often correlated with the ability of the membrane to phosphorylate ADP. Analysis of the chemistry responsible for the phosphorylation is difficult because there seems to be no single satisfactory description of energized state of the thylakoid vesicles responsible for the photophosphorylation, the state depending on the experimental protocol employed. Under some conditions, acidification of the lumen confers on the vesicles the ability to synthesize ATP. Thus, when electron transport or preincubation in acid causes exogenous protonated buffers to accumulate in the lumen, ATP can be made in amounts commensurate with the accumulation if the pH of the medium is raised. When permeant exogenous buffers are absent an ability to make ATP also develops during prior electron transport, presumably because of protonation of membrane components. The nature of the energized state responsible for post-illumination phosphorylation in the latter instance is unclear. The energized state probably cannot then be simply a general delocalized ionic disequilibrium because of the precisely exponential nature of its decay with time after the light is off. The nature of the energized state which drives prompt phosphorylation in single stage experiments is even more puzzling. It may not depend on the kind of ion fluxes that result in a reversible pH rise in the medium. Certainly phosphorylation can begin at high efficiency when any measurable acidification of the vesicle lumen is prevented, even under conditions where the presence of a membrane potential is unlikely.