66 STORAGE AND DIGESTION OF FOODS 



Digestion. — The removal of solid food from the place 

 of temporary or permanent storage, for example, the re- 

 moval of accumulated starch from a potato leaf to a 

 potato tuber, or from the tuber to the growhig sprout, 

 implies the conversion of starch, which is insoluble in 

 water, into some other substance which, dissolving in 

 water, thereby becomes movable. Such chemical change 

 from insoluble to soluble material is known as digestion. 

 In some cases digestion merely involves the conversion 

 of one soluble substance into another. 



The Means of Digestion. — Starch may be converted 

 into sugar in the laboratory by treatment with dilute 

 acid, dilute alkali, and enzyms. But the action of dilute 

 acids and dilute alkalis, if it is to be rapid, must take 

 place at temperatures higher than those which prevail 

 in the living body. The process of digestion, therefore, 

 must be carried on more rapidly than dilute acids and 

 alkalis, acting at body temperatures, make possible. The 

 means employed by animals and plants in digesting their 

 food are certain substances known as enzyms. These are 

 complex compounds having various common characters 

 the most significant of which is their capacity to change 

 the rate of chemical reactions which might otherwise go 

 on without them. They are therefore catalyzers. 



Catalyzers. — Charcoal, platinum sponge, and various 

 other substances have the remarkable quality of chang- 

 ing the rate of chemical reactions taking place in their 

 presence. Some catalyzers are employed in mdustrial 

 processes, for example, the use of platinum sponge in the 

 treatment of ammonia in munition manufacture, and of 

 finely divided nickel in converting oils into solid fats. 

 But the catalyzers of most importance to man are those 

 enzyms concerned in the physiological processes in living 

 organisms. Catalyzers prorhice effects out of proportion 

 to their own quantities. Often a very small quantity of 

 a suitable enzym, diastase, for example, will convert a 

 very large amount of starch into sugar if the temperature 



