CHAPTER XVIII 

 STARCH-MAKING AND DIGESTION 



111. Digestion. We have learned that the simplest food 

 resulting from photosynthesis is probably sugar. Experiments 

 have shown that in many plants only sugar is formed. Most of 

 our common plants, however, contain starch. In our own ex- 

 periments we found starch in the leaves that had been exposed 

 to the light, and none in the leaves that had been kept in the 

 dark. Now, what became of the starch that must have been 

 present in the leaves before we began our experiments ? 



The study of osmosis shows that starch, like many other 

 substances, cannot diffuse through a cell wall. Such substances 

 are called colloids (meaning " like glue "), to distinguish them 

 from sugars and salts and other substances (called crystalloids) 

 that diffuse through membranes more or less readily. Experi- 

 ments show us that these colloids are changed into crystalloids 

 and then pass through cell walls by osmosis. The process is 

 called digestion and can be easily demonstrated. 



In the grains and in other seeds containing starch the absorption 

 of water leads to the development of a substance called diastase, which 

 is capable of converting starch into sugar in the presence of plenty 

 of water. Diastase has been extracted from malted barley (that is, 

 barley that has been kept moist until the grains sprouted), from rice, 

 and from many other seeds. It can be bought in the stores. A sub- 

 stance that behaves in many ways like diastase is found in human 

 saliva and in the digestive juices of many other animals. 



The change from starch to sugar makes it possible for 

 carbohydrates to pass through cell walls by osmosis. 



112. Ferments. Substances like diastase and the active 

 part of the saliva are called ferments, or ensyms, and many 



78 



