THE PHOTOTROPHIC ASSIMILATION OF CARBON 47 



green cells and that, principally through a-ketoglutaric acid 

 and glutamine, which forms a reservoir of amino- groups, 

 the tricarboxylic acid cycle is also linked with protein 

 synthesis and breakdown (see Fig. 10). In the dark the 

 tricarboxylic acid cycle is maintained with oxaloacetic acid 

 derived by carboxylation from the pyruvic acid produced 

 in glycolysis. Under these conditions its intermediates 

 rapidly acquire radioactive carbon supplied in carbon 

 dioxide. In the light the carbon dioxide concentration in 

 the cell is reduced by photosynthetic fixation and this car- 

 boxylation is suppressed. The tricarboxylic acid cycle is 

 then evidently maintained by deamination of glutamic acid 

 to yield a-ketoglutaric acid, the ammonia liberated, together 

 with carbon skeletons provided by photosynthesis, being 

 used for protein synthesis. According to this view the tri- 

 carboxylic acid cycle of respiration continues unaffected 

 during photosynthesis, but is then maintained from different 

 sources to those which maintain it in the dark.^^^ This 

 explanation is consistent with the observation that a major 

 portion of the insoluble products formed by algae in short 

 periods of photosynthesis is protein and that glutamic acid 

 only acquires appreciable amounts of tracer after longer 

 times. ^^ 



There is thus no conclusive evidence to show that the 

 rate of respiration is altered to any great extent when 

 photosynthesis occurs. 



