224 PLANT BIOCHEMISTRY 



ammonium ion supplied by solution of this gas. It is probable that 

 ammonium ion entered the metabolic systems of the organisms con- 

 cerned by reaction with certain keto acids to form corresponding 

 amino acids. This reversible process is now known to occur in a 

 great variety of plants, animals, and microorganisms and may be 

 practically vmiversal. 



-OOCCH2CH2COCOO- + NH4+ t=^ -OOCCH2CH2CCOO- + H3O+ 



a-ketoglutarate | | 



NH 



a-imi noglutarate 



-OOCCH2CH2CCOO- + TPNH + 2H:,0+ ^ 



11 

 NH 



-OOCCH.CH.CHCOO- + TPN+ + 2H2O 



I 

 NH3+ 



glutaiiiate 



A large proportion of the glutamate required by plants is formed in 

 this way. Indeed, this process is felt to be the central one in the 

 utilization of ammonia. The a-ketoglutarate required is readily avail- 

 able from the tricarboxylic cycle, where it is formed during the normal 

 metabolism of carbohydrates and lipides. The first step may be spon- 

 taneous, the second requires an enzyme called glutamic dehydrogenase, 

 and the reduced coenzyme must be made available by means of energy 

 from either photosynthesis or respiration. 



Another mechanism for the incorporation of ammonia is represented 

 as 



-OOCCH=CHCOO- + NH4+ ' -OOCCH2CHCOO- 



aspartase 1 



NH3+ 

 aspartate 



Avherein this second amino acid is formed directly from inorganic 

 nitrogen, employing still another intermediate from the tricarboxylic 

 acid cycle. These two systems probably account for the principal 

 consimiption of ammonium ion. 



Conversely they may also participate in an important way in the 

 formation of ammonia <luring the decay of animal and vegetable 

 matter. The organisms ol decay convert amino acids into correspond- 

 ing acids and ammonium ion. The organic compounds are metab- 

 olized, supplying the energy needed for the growth and reproduction 

 of the organisms of decomposition. At the same time, some of the 

 ammonia may be used, the rest set free for other species. Probably 



