Feedino and Dioestion 



169 



A classification of glycosides commonly used as substrates, that is. natural 

 disaccharidcs and glucosides with their monosaccharide components, follows: 

 a-glucosides: 



a-maltose'~^2 a-glucose 



sucrose'*' -^a-glucose -f- /^-fructose 



a-methyl glucoside-*o-glucose + methyl alcohol 



/if -glucosides: 



a-galactosides: 



cellobiose^/y-glucose + /3-glucose 

 salicin->^-glucose -f saligenin 

 /:^-gentiobiose-^2 /y-glucose 

 /i-methyl glucoside-^/:;-glucose + methyl alcohol 



t- 



melibiase 



*a-galactose-|-sucrose 



/S-galactosides: 



rafhi 



'a-melibiose -)- fructose 

 a-melibiose^galactose -|- glucose 

 a-methyl galactoside-^a-galactose + methyl alcohol 



/J-lactose— ^y^-galactose+a-glucose 

 ^-methyl galactoside-^^tf-galactose + methyl alcohol 

 /J-fructosides (fructofuranosides) : 



sucrose '^■^/3-fructose + a-glucose 

 raffinosef-^/y-fructose-f-a-melibiose 



Polysaccharides are highly polymerized carbohydrates which occur in great- 

 est variety in plants as stored food and as structural elements. The most im- 

 portant storage carbohydrate is starch and its animal homologue glycogen. 

 Starch and glycogen are complex molecules built of a-glucoside units together 

 with small amounts of other compounds. Starch grains are digested by animal 

 amylase (diastase) only if boiled or broken mechanically, or if the coating 

 amylopectin of the starch grains is broken. In the course of digestion a series 

 of dextrins are formed, with maltose split ofiF at each step. Glycogen can be 

 hydrolyzed by the same amylase that splits starch; reports of a separate glyco- 

 genase are questionable. 



The most important structural polysaccharide of plants is cellulose, which 

 makes up the bulk of plant cell walls. This is built of chains of /S-glucoside 

 units, is much less soluble than starch, and is resistant to boiling and mild acid 

 treatment. Many microorganisms and a few metazoa have enzymes which can 

 attack cellulose. Perhaps the reason most animals are unable to digest cellulose 

 is that their glucosidase and amylase act only on a-compounds. A few animals 



*Sucrose can be attacked from the /J-fructose or the al-glucose end. The best known 

 sucrase is from yeast and is a fructofuranosidase, whereas animal sucrase is probably an 

 a-glucosidase. 



tRaffinose is a trisaccharide which can go to galactose plus sucrose by the enzyme 

 melibiase or to melibiose + fructose by yeast saccharase. There is lack of agreement as 

 to whether animals attack raffinose with an a-galactosidase or with a /J-fructofuranosidase. 



