498 LIGHT AND LIFE 



cluctant capable of direct reduction of COo to the level of carbo- 

 hydrate by some unknown reaction (s) peculiar to photosynthesis. 

 The investigation of CO^ assimilation by isolated chloroplasts re- 

 vealed that here also carbohydrate formation occurs by dark reactions 

 that are driven by reduced pyridine nucleotides and ATP. 



The "photosynthetic" carbohydrate cycle in green plants differed 

 from the reductive carbohydrate cycle in non-chlorophyllous cells in 

 having a TPN-dependent rather than the conventional DPN-depen- 

 dent triose phosphate dehydrogenase (96) . But this special feature of 

 photosynthetic COo assimilation did not alter the reducing potential 

 level of the needed reductant, which is the same for both TPNH2 

 and DPNHo. 



The investigation of CO^ assimilation by isolated chloroplasts has 

 thus completed the chain of evidence for the view that carbohydrate 

 synthesis is removed from the domain of photosynthetic reactions 

 proper in which light is converted into chemical energy. In green 

 plants the first stable, chemically defined products of this energy con- 

 version are not intermediates of COo assimilation but are the two 

 components of assimilatory power, TPNHo and ATP (9, 153) . 



5. Photosynthetic Phosphorylation 



The experiments with isolated chloroplasts have thus narrowed 

 down the problem of energy conversion in photosynthesis to the 

 identification of those reactions in -which TPNHo and ATP are 

 formed by light. With respect to TPNHo it had already been shown 

 independently by Vishniac and Ochoa (162), Tolmach (149), and 

 Arnon (5) that isolated chloroplasts can reduce pyridine nucleotides 

 and simultaneously evolve oxygen, if this photochemical reaction is 

 coupled with a "pulling reaction," i.e., an enzyme system capable of 

 utilizing at once the newly formed reduced pyridine nucleotide. 



A direct reduction of pyridine nucleotide by illuminated chloro- 

 plasts was first demonstrated by San Pietro and Lang (132), who 

 measured the accumulation of the reduced nucleotides, but not oxygen 

 evolution. In their experiments DPN was reduced in preference to 

 TPN and the reduction of pyridine nucleotides occurred only in the 

 presence of appreciable concentrations of chloroplast material. How- 

 ever, they have also noted that the reaction proceeded at lower 

 .:oncentrations of chloroiihyll if a soluble extract from chloroplasts 

 was added. 



The observations of San Pietro and Lang were confirmed in our 

 Mboratory except i/iat under our conditions TPN, insteatl of DPN, 



