112 PHYSIOLOGY OF THE FUNGI 



greater number utilize ammonium nitrogen. All species are abletoutilize 

 some form of organic nitrogen. Other constituents in media, especially 

 the four-carbon dicarboxylic acids, modify the availability of ammonium 

 nitrogen and certain amino acids. Not all amino acids are of equal value 

 in fungus nutrition. The primary amino acids are those which enter 

 directly metabolic pathways, while secondary amino acids are deaminated 

 before the nitrogen is used. 



Most of the nitrogen utilized by fungi enters into the synthesis of pro- 

 teins. The primary amino acids are formed first, and the secondary 

 amino acids are formed from primary amino acids. Proteins are the 

 most complex compounds synthesized by living cells. Many of the 

 vitamins and other essential metabolites also contain nitrogen. 



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