ALIMENTATION 105 



the number of cleavages involved depending upon the composi- 

 tion of the food. What happens in the case of complex sub- 

 stances such as proteins, is a continuous separation of soluble 

 fractions from a residue that becomes more and more soluble as 

 its chemical composition is simplified, until the original complex 

 molecule is converted entirely into simpler and more soluble 

 molecules. 



Enzymes. — The hydrolytic cleavages of foodstuffs in the 

 alimentary canal are brought about by organic catalytic agents 

 known as enzymes. A catalytic agent is a substance whose 

 presence hastens chemical reaction. It does not initiate the 

 reaction but greatly accelerates the rate of reaction. Thus, to 

 use a common example, hydrogen and oxygen which do not 

 combine in any appreciable degree at ordinary temperatures to 

 form water, may be made to do so if exposed to spongy platinum. 

 Since the catalytic agent, platinum, does not appear in the 

 product of the reaction, a very small amount is capable of pro- 

 ducing an infinite change. Similarly, digestive enzymes bring 

 about changes in food without becoming a part of the product 

 of digestion. 



Enzymes are organic catalytic agents produced by cells from 

 which they may be extracted by water, salt solutions, or by 

 glycerin. From such an extract the enzyme may be obtained 

 in a fairly pure state by precipitation with an excess of alcohol. 

 The action of enzymes is destroyed by temperatures between 

 60 and 100°C. and greatly retarded by temperatures near 0°C. 

 Enzymes are specific in their reaction; thus proteolytic enzymes 

 act only on proteins, fat-splitting enzymes only on fat, etc. 

 Enzymes are reversible in their reaction, the direction of the 

 reaction being determined by whether or not the products of the 

 reaction are removed. If the products are removed as rapidly 

 as formed, the reaction may go to completion in one direction. 

 This is what happens in all probability in the alimentary canal, 

 where the products may be absorbed as soon as formed. If the 

 products are allowed to accumulate, the reaction comes to 

 equilibrium, under which conditions there is as much tendency 

 for the reaction to go in one direction as the other, and thus, 

 for all practical purposes, to come to a halt. 



It has been shown that an enzyme may be secreted by the cell 

 in an inactive form, to which the general term zymogen is applied. 



