176 Comparative Animal Physiology 



and insects (Table 29). It is not clear whether raffinose is digested in animals 

 by an a-galactosidase or by a /?-fructosidase, although observations on blow- 

 flies ^^ indicate the enzyme is an «-galactosidase. This sugar is digested by 

 some molluscs. 



Table 29 shows that the sugar-digesting enzymes of various animals are not 

 the same and that, except for maltose and sucrose (probably all a-glucosides), 

 no group of sugars is used by all animals. Present limited data indicate that 

 disaccharidases occur by stereochemical classes and that the utilization of 

 sugars can be accounted for by relatively few enzymes in any one group of 

 animals. For example, the utilization of eight out of twelve sugars by blowfly 

 adults can be accounted for by postulating an a-glucosidase and an a-galactosi- 

 dase but no /?-glucosidase or yS-galactosidase. ^^ In mammals only an a-glucosi- 

 dase and a yS-galactosidase are used. 



Sugar digestion may change with age. Honeybee larvae can use lactose,-** 

 adults cannot. ^•^■'' Sucrase first appears in the pharyngeal gland of the worker 

 bee when it starts foraging, and it then increases very greatly. •'"* Larvae of 

 numerous moths and butterflies ha\'e sucrase in addition to a lipase and pro- 

 tease in the mid-gut, whereas those adults which suck nectar have salivary and 

 mid-gut sucrase but no other digestive enzymes, while nonfeeding moths have 

 no digestive enzymes whatever. ^•''^- ''^•' Sugar digestion is weak in carnivor- 

 ous animals. For example, the blood-sucking tsetse-fly Chrysops has no salivary 

 carbohydrases and has amylase and sucrase only in the stomach (posterior mid- 

 gut), whereas the closely related non-blood-sucking CaUiphora has amylase in 

 the salivary glands and active amylase, maltase, and sucrase in both anterior 

 and posterior mid-gut. ^^^ 



Carbohydrates are absorbed as monosaccharides. Part of the sugars in mam- 

 mals, at least, must be phosphorylated in the intestinal wall before they are 

 transferred across the mucosa and into capillaries. Sugars then pass to the liver, 

 where they may be converted to glycogen for storage. 



DIGESTION OF FATS 



Fats are hydrolyzed by the action of lipases to fatty acids and alcohols. 

 Neutral fats consist of higher fatty acids linked to the trihydric alcohol 

 glycerol : 



O H 



I 



C-H H 



I 

 lipase O HO-C-H 



R-C-O 



O 



II 

 R_C-0-C-H + 3H.O-»3R-C-OH + HO-C-H 



O I HO-C-H 



R_C-0-C-H H 



I 

 H 



where R refers to the chain of fatty acid such as palmitic, stearic, or oleic acid. 

 Neutral fat (triglyceride) can be absorbed without hydrolysis in some animals. 

 Lipases, at least the extracts of them which have been prepared, are not highly 

 specific as to substrate. They are generalized esterases and will attack most 



