CHAPTER XXVII 

 THE CHEMISTRY OF DIGESTION 



The Object of Digestion is twofold ; to prepare the various foods 

 for absorption by the lining of the digestive tract, which means 

 that they must be made soluble if not already so ; and to convert 

 them into forms in which the Body can make use of them after 

 they have been absorbed. Digestion is confined to the nutrients; 

 the inorganic salts of the food are soluble, and are used by the 

 Body in essentially the same form as eaten, they therefore need no 

 digestion. The accessories either perform their function in con- 

 nection with the process of digestion itself, or are absorbed and 

 used by the Body in the form in which they are taken. 



Nature of the Digestive Process. Although the foods requiring 

 digestion are of very different sorts chemically, the method of 

 digestion is at bottom the same for all of them. It consists of the 

 process known in chemistry as hydrolysis. Hydrolysis is a chemi- 

 cal reaction in which one molecule of the substance involved com- 

 bines with one molecule of water and the resulting compound splits 

 into two or more simpler molecules. By repeated hydrolyses very 

 complex substances may be split into comparatively simple 

 ones. 



Hydrolysis is a common reaction of organic chemistry. It is 

 probable that it is the most frequently occurring reaction of the 

 living Body. Not only the digestive processes, but many of the ac- 

 tivities of living cells are of this nature. The digestive hydrolyses 

 are all carried on through the agency of enzyms. There is a special 

 and specific enzym for each particular reaction; the enzym that 

 splits starch is without effect on protein or fat. These enzym 

 reactions are all simple chemical reactions; they are carried on in 

 the alimentary tract as in a chemical laboratory, and will go on 

 just as well in test-tubes kept at Body temperature as in the Body 

 itself. They are not therefore " vital " processes in the sense that 

 they cannot occur except in the presence of living cells. 



Digestion Products. Before beginning a detailed description 



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