AUTOTROPHIC ASSIMILATION 79 



a-Ketoglutaric acid may be derived through the tricar- 

 boxylic acid cycle from carbohydrate or from the primary 

 products of photosynthesis by the pathways indicated in 

 Fig. 10. A corresponding mechanism for the formation of 

 the related aspartic acid from oxaloacetic acid is unknown'*^ 

 and aspartic acid does not appear to occupy a similar central 

 position in the metabolism of Chlorophyceae to that of 

 glutamic acid.^' ^ There is evidence that in algae other 

 amino-acids are formed from glutamic and aspartic acids 

 by transamination processes such as have been found to 

 occur in other organisms, ^^' ^^^" e.g.: 



COOH COOH COOH COOH 



CH.NH2 + C:0 



I I 



C^H2 Cri3 



COOH 



Aspartic 

 acid 



Pyruvic 

 acid 



C:0 



CH2 



I 

 COOH 



Oxaloacetic 

 acid 



+ CH.NH2 (22) 

 CH3 



Alanine 



The necessary keto-acid in this particular example may be 

 provided directly either by glycolysis or by photosynthesis. 

 Transaminase systems catalysing the transfer of the amino- 

 group from aspartic acid, alanine and leucine to a-keto- 

 glutaric acid and from aspartic acid, glutamic acid and 

 leucine to pyruvic acid have been demonstrated in Chlorella 

 vulgaris}^^^ The synthesis of the basic amino-acids, arginine, 

 lysine and ornithine, which appear to be of particular im- 

 portance in C vulgaris,"''^ has been studied in detail in 

 other organisms^^' ^^ but there is nothing to indicate 

 whether or not the mechanisms that have been found occur 

 in algae also. 



Amide formation is of general occurrence in plants^^ and 

 the observed increase in amide in Chlorelta supplied with 

 ammonia^'^ is in agreement with expectation. Glutamine 

 formation in animal tissues, higher plants and bacteria, has 

 been found to be dependent on aerobic respiration and 

 evidently involves a phosphorolytic reaction, the energy 

 necessary for the formation of the amide linkage being 

 derived from adenosine triphosphate:^^' *^ 



