Protein metabolism 



20 



Green plants and a variety of microorganisms utilize inorganic 

 forms of nitrogen in synthesizing their proteins. The higher animals 

 can also employ ammonium ion to some extent, but they all seem to 

 need a certain proportion of organic nitrogen for protein synthesis. 

 This requirement is at least partly satisfied by "essential amino acids" 

 in the diets. Again, in this group of compounds, as in those of the 

 earlier chapters, there is a major difference between the nutrition of 

 animals and that of higher plants. 



UTILIZATION OF PROTEINS 



Animals regularly ingest proteins as components of the plant and 

 animal materials eaten. In addition, the diet normally contains 

 smaller quantities of free amino acids and peptides which supply a 

 portion of the required organic nitrogen. Both the proteins and 

 larger peptides must be hydrolyzed to amino acids and small peptides 

 before absorption by the digestive tract. Only after these processes 

 of digestion can the components of proteins be distributed and put 

 to metabolic use by body tissues. 



The products of the digestion of proteins are used in several ways. 

 For example, certain amino acids may be converted into others needed 

 by the animal but not obtained in sufficient supply from the diet. 

 Or amino acids may be stripped of their nitrogen and oxidized to 

 carbon dioxide and water for the energy available from such trans- 

 formations. Still another fraction of the amino acids is condensed to 



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