DIGESTION 



319 



(hydrolyze nucleic acids to polynucleotides and nucleotides) , nucleotidases 

 (hydrolyze nucleotides to the corresponding nucleoside and phosphate, 

 e.g., adenylic acid + HoO -^ adenosine + phosphoric acid) , nucleosidases 

 (split nucleosides into purine or pyrimidines and pentose, e.g., adenosine + 

 H20-> adenine + ribose), phosphatases (hydrolyze phosphate esters into 

 the corresponding alcohol and phosphoric acid, e.g., 



O 



!l 

 HO-POCH2— CHNHo— COOH + H,0 



xjQ ^*\phosphatase 



Serine phosphate 



HOCHo— CHNH2-COOH + H3PO4 



Serine 



sucrase (hydrolyzes sucrose to glucose and fructose), maltase (hydrolyzes 

 maltose to two molecules of glucose), and lactase (lactose + HoO— > 

 glucose + galactose). JNIucin is secreted by epithelial cells of the small 

 and large intestines; it lubricates the movement of material through this 

 portion of the intestinal tract. The flow of intestinal juice is markedly 

 stimulated by the application of mechanical pressure to the intestinal 

 wall; therefore, the mere physical presence of food will stimulate secre- 

 tion. 



Schematic representation of enzymatic degradation of major foodstuffs 



a amj'lases 

 starch or fi amylases maltases , 



glycogen ^ "^^^^^^^ *■ g^"^°^® 



sucrase r , . • 



sucrose *• fructose + glucose 



, . pepsin, trypsin polypeptides + carboxypeptidase amino 



chymotrypsin, etc. ammo aClUS aminopeptidase aCldS 



I dipeptidase, etc. ^ 



fat — ]P^:^ ^ fatty acids + glycerol 



ABSORPTION FROM THE GASTROINTESTINAL TRACT 



Fairly large quantities of ethanol, methanol, and water are absorbed 

 in the stomach, and hydrocyanic acid is rapidly taken up at this site 

 in fatal amounts. The mucosa of the small intestines, however, is the 

 most important location for the absorption of foodstuffs. The intestinal 

 wall is covered with a large number of microscopic, protruding processes 

 known as villi. Each of the villi contains a small blood vessel and a lymph 

 vessel (lacteal). The villi are the principal absorbing units of the small 

 intestines. The materials that can be absorbed are transported across 

 the membranes which separate the intestinal content from the blood and 



