PROTEINS 129 



sary in the food to maintain life, but after all it is really the 

 amino acids which are needed, for in digestion the proteins are 

 split up into amino acids, and these substances are supplied to 

 the living cells of the tissues. Some of these amino acids are 

 used to build up new cell proteins or repair old ones, whereas 

 others are transformed into various compounds some of which 

 undoubtedly have most vitally important roles to play in con- 

 trolling or instigating many of the vital processes. Others of 

 the amino acids undoubtedly are simply broken down and 

 burned in the body as fuel, just as are carbohydrates and fats 

 generally speaking. Some of the important roles of the amino 

 acids will be considered later in connection with the study of 

 metabolism. 



