402 



PLANT METABOLISM 



ferred to an a-keto acid, and a new a-amino acid and a new a-keto 

 acid are formed (see Chap. 13, reactions 37 and 38) : 



NH2 NH, O 



II I I II 



R-C-COOH + R'-CH-COOH ^izzi R-CH-COOH +R'-C-COOII 



Free ammonia does not function in the reaction. Transaminations from 

 glutamic acid to pyruvic and oxalacetic acids to form alanine and aspartic 

 acid, respectively, have been demonstrated in plants, and other trans- 

 aminations occur but at considerably slower rates. 



Metabolism of amides by seedlings 



Germinating seeds may produce large quantities of the amides, aspara- 

 gine and glutamine. Asparagine is the yS-amide of aspartic acid (HOOC— 

 CHNH2 — CHo — CONHo), and glutamine has a comparable structure 

 (HOOC— CHNH2-(CHo)o— CONH,). When a high-protein seed such 

 as a lupine seed germinates, there is a demand upon the i^rotein as an 

 energy yielding material for growth before the seedling establishes its 

 synthetic capacity. AVhen protein is broken down and respired, ammonia 

 accumulates; high levels of ammonia would be toxic, but by forming 

 amides the plant detoxifies it. As much as 85 per cent of the protein 

 nitrogen disappearing in a germinating lupine seed may appear in the 

 single compound, asparagine. Later in the growth of the plant the 

 amide nitrogen is used in the resynthesis of protein, and when the plant 

 matures, its amide content is very low. 



Biological fixation of nitrogen 



The nitrogen cycle in nature is pictured in Fig. 15-7, in a form which 

 is simplified by omitting the marine cycle of nitrogen. Nitrogen is added 

 to the cycle from the vast reservoir of atmospheric N2 by chemical fixa- 

 tion,^ by the symbiotic nitrogen fixation of leguminous plants, and by 

 the nonsymbiotic nitrogen fixation of free-living bacteria and blue-green 

 algae. Nitrogen is lost to the sea by leaching, erosion, and sewage dis- 

 posal, and to the atmosphere by bacterial denitrification." Man's in- 

 stallations for the chemical fixation of No are impressive, but their con- 

 tribution is minor compared to that of biological nitrogen fixation in 

 maintaining the nitrogen cycle in balance. Chiefly because of improper 



^"Fixation" is the conversion of gaseous nitrogen (N„) into chemically combined 

 forms such as ammonia, nitric acid, protein, etc. 



* Denitrification is the release of free nitrogen (Ng) from nitrogenous compounds, 

 or the opposite of fixation. 



3 



