PHOTOSYNTHETIC PHOSPHORYLATION AS ENERGY SOURCE FOR PROTEIN SYNTHESIS AND CARBON DIOXIDE ASSIMILATION BY CHLOROPLASTS

Abstract

The evidence reported here shows that isolated whole spinach chloroplasts can incorporate amino acid solely at the expense of a light-induced, endogenous process which has the characteristics of cyclic photophosphorylation. By contrast, the photoassimilation of CO2 under the same experimental conditions shows a dependence on both cyclic and noncyclic photophosphorylation. Since cyclic photophosphorylation, unlike the noncyclic process, is not accompanied by oxygen evolution, its operation in intact cells is not readily demonstrable. However, evidence has recently been accumulating for the functioning of cyclic photophosphorylation in intact cells. The results obtained here with whole chloroplasts strengthen the view that endogenous cyclic photophosphorylation has a physiological role. This role may not be limited to supplying ATP for CO2 assimilation, over and above the ATP supplied by the noncyclic process. Cyclic photophosphorylation may also serve as the main source of ATP for those biosynthetic processes in chloroplasts which, like protein synthesis and the synthesis of DNA and RNA from mononucleotides, depend solely on ATP.