204 NUTRITION AND METABOLISM 



depend upon soluble food since they have no means of incorporating 

 insoluble compounds into their protoplasm. The protoplasm, however, 

 must be considered the center of metabolism, and the digestion of food 

 and the formation of energy must take place in the protoplasm if the 

 cell is to profit by it. Since the food cannot diffuse into the cell, and 

 the protoplasm does not diffuse out, the food must be dissolved. This 

 is accomplished by the cell itself by secreting certain agents with 

 peculiar qualities. These agents, the so-called enzymes, act upon the 

 insoluble foods, changing them into soluble compounds which then can 

 diffuse into the cell where they are digested or fermented. The final 

 digestion or fermentation of the food must take place within the cell. 

 Energy production outside the cell serves the same purpose as a stove 

 outside the house. The dissolution of insoluble compounds by cell 

 secretions must be considered a preparatory process which has no direct 

 relation to intra-cellular food digestion or fermentation. Enzymes are 

 not produced by microbial cells exclusively. All living cells produce 

 enzymes. They were known before the science of microbiology had 

 been established. In fact, microbial activity was considered for a 

 long time as an enzymic chemical process. Enzymes in the animal 

 and plant body serve largely the purpose of metabolic changes. In 

 the animal body, many enzymes help to dissolve the insoluble food 

 which cannot pass from the alimentary canal into the body except by 

 diffusion through the mucous membrane. There is diastase in the 

 saliva which acts upon starch, there is pepsin in the stomach and 

 trypsin in the intestine, both dissolving protein bodies; there is ereptase 

 for the peptones, lipase for the fat, invertase for the saccharose, and 

 many other enzymes. The object of all these enzymes is apparently 

 to prepare the food for passing through the membrane into the proto- 

 plasm of the cells, where the final changes which liberate energy take 

 place. The same processes occur with microorganisms but in a more 

 simple manner. Surrounded by a liquid medium, they secrete enzymes; 

 these dissolve certain insoluble foods, which then diffuse through the 

 cell wall to be decomposed further. 



The food-preparing processes are all supposed to be simple hydrolytic 

 processes. For some of these changes the chemical equations are well 

 known. The hydrolyzation of starch to maltose by means of diastase is 

 represented by the equation 



2(C 6 H 10 5 ) n + nH 2 = nCi 2 H M Oii. 



