118 



Comparative Animal Physiology 



curring amino acids, whereas animals in general (except flagellates) have lost 

 the ability to synthesize some of these. The amino acids that are required in 

 the diet of several kinds of animals are surprisingly similar (Table 21). Differ- 

 ent amino acids are apparently needed not specifically for the nitrogen they 

 provide but for their molecular skeletons. For each animal amino acids fall 

 into three groups: (1) the amino acids that are not synthesized and are, there- 

 fore, required in the diet; (2) the amino acids that can be synthesized from 

 ammonia and carbohydrate; and (3) those that are synthesized from specific 

 essential amino acid precursors. Using the growing rat as an example, the 

 amino acids required in the diet are: 



CH, 



I 

 CHOH 



I 

 CH-NH2 



COOH 

 threonine 



CH3 CzHs 



CH 



I 

 CH-NH2 



I 

 COOH 



isoleucine 



CH.-NH2 



(CH2)3 



I 

 CH-NH2 



I 



COOH 



lysine 



HN=C 



.NH2 



^NH 



I 

 CCHOs 



CH-NHi. 



I 

 COOH 



arginine* 



CH 



/ 



C— C-CHo-CH (NHO-COOH 



