The Distribution of Photosynthetic Reactions in the Chloroplast Lamellar System

Abstract

Chloroplast fragmentation, ATPase, Proton pump, Cyclic Phosphorylation, NH4Cl-uncoupling Cyclic phosphorylation and latent ATPase follow the distribution of photosystem I in the chloroplast membrane. Both activities are found higher in stroma lamellae than in grana. The extent of proton uptake is found to be higher in the grana. Since this uptake depends on internal volume and buffer capacity besides proton pump activity, the distribution of the proton pump proper remains to be elucidated. Any fragmentation of the large inner compartment of the chlorolast lamellar system into smaller vesicles results in decreased sensitivity of cyclic phosphorylation to uncoupling by ammonium chloride. Consequently both, isolated grana and stroma lamellae, show decreased uncoupling by ammonium chloride. The effect can be explained by the action of a membrane potential in photophosphorylation which builds up during illumination and might be more stable in a system with the larger number of individual compartments just for statistical reasons. No assumption on changes in specific ion permeabilities during fragmentation of chloroplasts are needed.