THE PHOTOTROPHIC ASSIMILATION OF CARBON 4I 



of short-term photosynthesis there is an important quantita- 

 tive difference, phosphoglyceric acid in the former case 

 constituting a much greater proportion of the total. In 

 Scenedesmus the radioactive carbon fixed in the dark imme- 

 diately following illumination is found almost exclusively 

 in the carboxyl groups of phosphoglyceric and pyruvic 

 acids. Thus it appears that it is only the Cg acceptor that 

 survives after the cessation of illumination and that the 

 reductions concerned in fixation can only occur in the light. 

 This view is supported by the fact that reduction in the Hill 

 reaction likewise ceases immediately in the dark and by 

 the results of experiments with inhibitors. ^^^ This implies 

 an intimate association between the reduction required in 

 the fixation cycle and the photochemical reaction. It may 

 be that there is only one specific reduction coupled with 

 the photochemical reaction and that other reductions are 

 accomplished indirectly at the expense of a proportion of 

 the products of photosynthesis which are degraded to pro- 

 vide the necessary energy.^^^ Our knowledge of this most 

 important step in photosynthesis is thus still in an elemen- 

 tary stage. 



INTER-RELATIONS BETWEEN THE CARBON DIOXIDE 

 FIXATION CYCLE AND OTHER METABOLIC SYSTEMS 



The close connexion existing between the metabolic 

 cycles involving sugars, organic acids and proteins has been 

 recognized for some time and the concept has arisen of a 

 'metabolic pool' of intermediates common to these different 

 processes and through which they are mutually correlated. 

 The one substance now known with a degree of certainty 

 to be an intermediate in photosynthesis, phosphoglyceric 

 acid, is also an intermediate in glycolysis" readily trans- 

 formable into substances involved in other metabolic 

 sequences and may be regarded as a component of this 

 'pool'. This and other considerations (see ref. 199) make it 

 evident that the photosynthetic fixation of carbon dioxide 

 intermeshes with other metabolic processes at an early 

 stage. This idea, which is contrary to the concept of photo- 

 synthesis which prevailed until recently, has received ample 



