DANIEL I. ARNON 



497 



cycle) will lead diiedly to a solution ol the unique proI)lcins of 

 photosynthesis, namely, the knowledge ol the act or acts by which 

 electromagnetic energy is transformed into chemical energy" (26, 

 p. 40) . 



The investigation of COo assimilation by isolated chloroplasts has 

 shown that the energy requirements for this j^rocess were satisfied 

 jointly by ATP and TPNHo (153, 151, 96, 152) . When experimental 

 conditions were so arranged that only TPNHo was formed in the 

 light, no formation of sugars occurred. In the absence of ATP the 

 photosynthetic "reducing power," TPNHo, was powerless to reduce 

 COo (96) . ^Vork with reconstituted chloroplast systems and inhibitors 

 demonstrated (96, 152) that both ATP and TPNHo must be supplied 

 for photosynthetic COo assimilation. ATP and TPNHo have there- 

 fore been jointly termed "assimilatory power" (18) . A general 

 scheme for COo assimilation by isolated chloroplasts is summarized 

 in Fig. 2. 



To recapitulate, the work with isolated chloroplasts provided no 

 evidence for postulating a photochemical formation of a special re- 



^Ru-5-P 



Hexose • 

 phosphates 



■^STARCH 



Carbohydrote synthesis by isoloted chloroplasts. 



Fig. 2. Diagram of the reductive carbohydrate cycle in chloroplasts. The cycle 

 consists of three phases. In the carboxylative phase (A), ribulose-5-phosphate 

 (Rii-.5-P) is phosphorylated to ribulose diphosphate (RuDP), which then accepts 

 a molecule of COo and is cleaved to 2 molecules of phosphoglyceric acid (PGA); 

 in the reductive phase (B) PGA is reduced and converted to hexose phosphates; 

 in the regenerative phase (C) liexose phosphate is converted into storage carboliy- 

 drates (starch) and into the pentose monophosphate needed for the carboxylative 

 phase. All the reactions of the cycle occur in the dark. The reactions of the 

 carboxylative and reductive phases are dri\en by All* and TPNHo formed in tlie 

 light. ' 



