NITROGENOUS COMPOUNDS 93 



IV. NITROGENOUS COMPOUNDS 



82. Nitrates and Ammonia.- -There is always present in 

 growing plants a certain amount of nitrates that have been 

 absorbed by the plant roots. In some plants the amount 

 may be 1.5 to 3 per cent, of the dry weight, but this is excep- 

 tional. They are present only until the synthetic processes 

 are able to convert them into other compounds. This is 

 shown by the fact that as a rule most of the nitrates are in 

 the root, less in the stem and leaves, and none in the seed. 

 Nitrates are present only as they are absorbed by the roots. 

 Plants do not form nitrates from other compounds of nitro- 

 gen. Ammonia is sometimes present in small amounts as an 

 absorbed constituent, for some plants can use ammonium 

 salts as well as nitrates in the manufacture of proteins. 

 Moreover, it may occur as a decomposition product of pro- 

 tein hydrolysis (Section 9), or as an intermediate product 

 in the synthesis of proteins (Section 30). 



83. Amides and Amino-acids. — It has been shown in 

 discussing the hydrolysis of proteins to facilitate their 

 transportation, and also in describing the synthesis of 

 proteins from raw materials (Section 30), that some of 

 the intermediate compounds are amides or amino-acids. 

 Amides, sometimes called acid amides, are organic acids in 

 which the hydroxyl of the carboxyl group is replaced by 

 NH2, a common amide being acet-amide, CH3CONH2. 

 Acids having more than one carboxyl group may have one 

 or all of the hydroxyl groups replaced by NH2. They are, 

 as a rule, crvstalline substances and more or less soluble in 

 water. They are basic in character, forming salts with 

 acids. An amino-acid, sometimes called amido-acid, is an 

 organic acid in which one of the alkyl hydrogen atoms 

 is replaced by NH 2 . A common one is amino-acetic acid, 

 CH 2 (NH 2 )COOH, or glycocoll. The amino-acids as a group 

 are crystalline substances soluble in water. They will unite 

 with acids to some extent on account of their amine group 

 and with bases on account of their carboxyl group. 



Apparently the formation of these compounds is in most 

 cases transitory; they are merely intermediate products, 



