HOW FOOD IS TAKEN IN 



129 



106. Digestion universal. Digestion seems to go on in nearly 

 all living things. In the ameba, which consists of a single cell 

 (see section 47), a solid particle of food can be swallowed by the 

 naked protoplasm and then digested inside the cell. Among the 

 bacteria, which are the smallest living things known, each indi- 

 vidual is a single cell consisting of protoplasm and cell wall. 

 These tiny plants can get food only in a liquid state, yet many of 

 them live on solid food that is not soluble in water. Under suit- 

 able external conditions each cell throws out through the cell 



Fig. 73. Digestion by bacteria 



The organism, a, lying on a solid, b, which may serve as food, secretes an enzym, or 

 ferment, which passes out of the cell, c, and changes the material to a liquid, I. This 



is absorbed into the cell by osmosis, / 



wall, by osmosis, a liquid containing a ferment capable of di- 

 gesting the solid, or insoluble, food material. The liquid re- 

 sulting from the digestion is then absorbed by osmosis. This 

 is why meat or cheese becomes fluid when it rots. The rotting 

 in such cases is the work of the ferments contained in the diges- 

 tive juices given off or secreted by the bacteria (Fig. 73). In 

 higher animals like ourselves a similar process of digestion 

 takes place; but instead of every cell's pouring out digestive 

 juices into its immediate neighborhood, only certain portions of 

 the body produce and throw out such juices. 



HOW FOOD IS TAKEN IN 

 I. Diffusion 



Materials capable of diifusion 



Gases ; liquids ; solids in solution 



Conditions in which diffusion can take place 

 Free contact of diffusible substances 

 Certain kinds of membranes (semipermeable) 



