Chapter 12 



DIGESTION 



by G. W. E. PLAUT 



Assistant Professor, Institute for Enzyme Research, 

 University of Wisconsin 



Most foods have to be converted to the proper physical and chemical 

 state before they can be utilized by the body. Digestion is the series of 

 mechanical and chemical processes which accomplishes this result. 



SALIVARY DIGESTION 



Composition of saliva 



Food particles are reduced to smaller size by the mechanical action of 

 the teeth. While in the mouth they are moistened and mixed with saliva, 

 the secretion of the submaxillary, sublingual, and the parotid glands. 

 The sublingual glands secrete a thick fluid which is rich in the glycoprotein, 

 mucin. When mucin is hydrolyzed it yields, in addition to protein, sul- 

 furic acid, acetic acid, glucuronic acid, and glucosamine. Mucin serves 

 to lubricate the food for its subsequent passage through the esophagus 

 to the stomach. A thin watery fluid, low in organic matter (serous 

 secretion) , is produced by the parotid gland. The submaxillary gland 

 contributes a mixture of the two types of secretion. 



The saliva contains inorganic constituents found in blood. A small 

 amount of thiocyanate is also present. Some organic compounds char- 

 acteristic of blood such as uric acid, urea, and creatinine are also present. 

 An a-amylase called ptyalin is present in saliva. It catalyzes the hy- 

 drolysis of starch and glycogen to maltose and polysaccharides of lower 

 molecular weight. The action of the amylase on starch persists on the 

 way from the mouth to the stomach. The activity stops in the stomach 

 when the acidity becomes too unfavorable. This enzyme has been crys- 

 tallized from human saliva. It is inactivated upon dialysis against dis- 

 tilled water, but the activity can be restored by the addition of chloride 

 ions. Human saliva has a neutral reaction (about pH 6-8). 



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