PHOTOSYNTHETIC PHOSPHORYLATION AND THE ENERGY CONVERSION PROCESS 341 



specialized for carrying out the complete process of photosynthesis in 

 green plants. It seemed legitimate therefore to explore the component 

 photochemical reactions in isolated chloroplasts with the expectation that 

 they would also be relevant to photosynthetic events in intact cells. 



2. The role of light in COo assimilation 



From a biochemical point of view, the central problem of photo- 

 synthesis was the identification of those photochemical reactions that 

 provide the energy required for the conversion of COo to carbohydrates. 

 As for COo assimilation proper, it became evident by 1956 that the early 

 proposals of Thimann [24], IJpmann [25], and Ruben [26] about it being 

 a dark process, were correct. Their hypotheses that CO^ reduction in 

 photosynthesis is a dark reaction, a reversal of the well-known oxidative 

 reaction of glycolysis, received experimental support mainly from the work 

 of Calvin and his associates [27], who identified phosphoglyceric acid and 

 other well-known products of glycolysis among the early products of 

 photosynthesis. 



A special feature of CO., assimilation in photosynthesis was found to 

 be the carboxylation reaction that accounted for the appearance of phos- 

 phoglyceric acid as the first stable product of COo fixation. Work in the 

 laboratories of Calvin [zH], Horecker [29], Ochoa [30], and Racker [31] 

 established the presence in photosynthetic tissues of two special enzymes, 

 carboxydismutase and phosphoribulokinase, which accounted for the 

 entry of COo into the metabolism of photosynthetic cells by way of a five- 

 carbon phosphorylated sugar, ribulose diphosphate. Ribulose diphosphate on 

 combining with COo is split to gi\e two molecules of phosphoglyceric acid. 



However, even this special feature of carbon assimilation was soon 

 found in non-photosynthetic bacteria as well. In fact, Trudinger [32] and 

 Aubert ef a/. [t,t,] found the entire "photosynthetic carbon cycle" in the 

 non-photosynthetic sulphur bacterium Thiolnicilhis denitrificans. It thus 

 became clear that COo assimilation is fundamentally extraneous to the 

 photosynthetic process. All the reactions of COo assimilation in photo- 

 synthesis occur also in non-chlorophyllous cells. 



The carboxylation reaction resulting in the formation of phospho- 

 glyceric acid (PGA) requires ATP, and the reduction of PGA to the level 

 of carbohydrate requires both ATP and reduced pyridine nucleotide. The 

 distinction between photosynthetic and non-photosynthetic cells seems to 

 lie, therefore, in the manner in ^\hich ATP and reduced pyridine nucleo- 

 tide are formed. Photosynthetic cells form these compounds at the expense 

 of light energy whereas non-photosynthetic cells form them at the expense 

 of energy released by dark reactions. 



Before this biochemical interpretation of photosynthesis could be 

 accepted with confidence, it was necessary to determine whether the 



