168 



Comparative Animal Physiology 



in the starfish. ^^' In some insects the carboxypeptidase is relatively weak, 

 compared with the aminopeptidase and dipeptidase. ^^^ The dipeptidase ot 

 Amoeba proteus is found in the cytoplasmic matrix. "*^ The dipeptidase of 

 Didinium is similar to that of its prey Paramecium, and the amounts are addi- 

 tive when Didinium. is feeding; hence the dipeptidase of this animal is prob- 

 ably derived directly from its food. ^"^ In some trypanosomes the pH optimum 

 of carboxypeptidase is lower than the optimum of other peptidases. ''^ The 

 peptidases are relatively similar in all animals, and carboxypeptidases often 

 occur separately from aminopeptidases and dipeptidases. 



DIGESTION OF CARBOHYDRATES 



Plants synthesize a vast array of carbohydrates. Animals can break down 

 some of these and can modify a few of them synthetically. 



Nature of Carbohydrates and Carbohydrases. Carbohydrates are hydrolized 

 to monosaccharides, mostly hexoses (six-carbon sugars), less often pentoses, 

 and rarely tetroses, trioses, and dioses. Hexoses can be pictured as carbon- 

 oxygen rings with side groups attached in a variety of patterns. All the natural 

 sugars used by animals are D-sugars. However, a D-sugar may exist in two 

 isomeric forms, of which the a-form is more strongly dextrorotatory than the 

 yS-form. The structure of a- and (S-glucose differs in the position of the H and 

 OH on carbon No. 1 and is represented as follows: 



a-glucose (rotation -f- H 1 ^ ) 



/? -glucose (rotation -|-19") 



CH2OH 

 I 

 C 



H 



\ 

 ( 



OH 



H 



\T 



C— 

 I 

 H 



O 



-C 



I 

 OH 



H 



\ 



OH 



CH2OH 



I 



c o 



H /A \ ^OH 



^C C 



OH \oH h/ H 



c c 



I I 



H OH 



Monosaccharides are combined to form disaccharides and trisaccharides. 

 The structure of sucrose is given as: 



H. /h 



CH=OH 



I 



c- 



OH 





c- 



h 



o 



c 

 -c 



/ 



H 



OH 



CH2OH 



I 

 c 



H 



/ 



C- 



/ 

 OH 



O. 



OH 



\ 



— c- 



H 



H 



\ 



CH.OH 



a-glucose 



/J-tructose (fructofuranose) 



