396 DANIEL I. ARNON 



cyclic and non-cyclic photophosphorylation are required for COg assimila- 

 tion, has recently been obtained by Trebst et al. [34]. 



Trebst et al. [34] have investigated CO2 assimilation by isolated 

 chloroplasts in a catalytic system, i.e. one in which, as in an intact cell, 

 TPNHo and ATP were present in catalytic amounts and COg fixation was 

 therefore possible only in the light while TPNHo and ATP were being 

 continuously regenerated at the expense of absorbed light energy. CO2 

 assimilation was then investigated, under three conditions : {a) when the 



/ 



PHOSPHOGLYCHriA'i'E 



PHEWL/ WATER - 



Fig. 32. Radioautograph of a chromatogram showing products of photo- 

 synthetic ^*C02 assimilation by illuminated chloroplasts in the absence of added 

 FMN (Trebst, Losada, and Arnon [34]). 



photochemical phase was limited to non-cyclic photophosphorylation, 

 [b) when the photochemical phase w^as limited to cyclic photophosphoryla- 

 tion, and (c) when the photochemical phase included both [a) and {b). 



Figures 32 and 33 show that under conditions {a) and (6) COo assimila- 

 tion was limited almost entirely to the formation of phosphoglycerate. As 

 shown in Fig. 34, the formation of sugar phosphates, which is taken as a 

 measure of a reductive (photosynthetic) COo assimilation in this recon- 

 stituted chloroplast system, was observed only in case (c) when a proper 

 balance was established between cyclic and non-cyclic photophosphorylation. 



In the experiments illustrated by Figs. 32, 33, and 34, the balance 



