228 • PLANT BIOCHEMISTRY 



calcium cyanamide, and urea to the extent of approximately 2 times 

 10*5 tons of nitrogen. 



Estimates of the biological fixation of nitrogen in the United States 

 range from about 10*^ to 10^ tons per year. In either case chemical 

 fertilizers are becoming very significant to crop production in this 

 country, especially since fertilizers are applied intensively to the crop 

 lands, whereas biological fixation is distributed over the land surface 

 as a whole. Even so, there is probably still a nationwide net loss of 

 nitrogen from the agricultural areas, although this loss may be declin- 

 ing. Other countries are probably less fortunate. Biological nitrogen 

 fixation on land may be about evenly divided between the symbiotic 

 and non-symbiotic processes as occurring in the cultivated legumes 

 bearing Rliizobin and in the Azotobncter and Clostridia, respectively. 



Organic Nitrogen 



The metabolism of organic compounds of nitrogen has been studied 

 intensively in the last decade and much has been learned, particularly 

 concerning the metabolism of amino acids, purines, and pyrimidines. 

 Since higher plants and animals seem to employ similar pathways, 

 only one discussion at the biochemical level is really necessary. There- 

 fore, the pertinent details are assembled in Chapter 20 on nitrogen 

 metabolism, in the section on the animal. It so happens that most of 

 the work has been done with animals and microorganisms. 



As mentioned earlier, when ammoniiun ion is absorbed by root cells 

 much of the nitrogen appears first in glutamic and aspartic acids. 

 The nitrogen thus accumulated is transported throughout the plant 

 mainly but not entirely in the form of asparagine and glutamine, 

 which probably are synthesized from the corresponding amino acids. 



Next, all cells seem to transfer the amino groups to appropriate 

 «-keto acids, forming other amino acids as in the example below. The 



-OOCCH2CH2CHCOO- + CH3COCOO- ^ 



pyruvate 



NH3+ 

 glutaraate 



-OOGCH2CH2COCOO- + CH3CHCOO- 



1 

 NH3+ 



a-ketoglutarate alanine 



enzyme functioning is called a transaminase and requires a coenzyme 

 called pyridoxal phosphate, discussed on page 340 under vitamin 

 Bg. Glutamate ion or glutamic acid itself may be the amino donor 

 in all cases, serving as a central intermediate. Since transaminations 



