MECHANISM OF METABOLISM. 131 



soluble food since they have no means of incorporating insoluble com- 

 pounds into their protoplasm. The protoplasm, however, must be con- 

 sidered the center of metabolism, and the digestion of food and the forma- 

 tion 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, which secretes certain agents having 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 

 may 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 pro- 

 tein 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 mem- 

 brane into the protoplasm of the cells, where the final changes which 

 liberate energy take place. The same processes occur with micro- 

 organisms but in a more simple manner. Surrounded by a liquid 

 medium, they secrete specific substances (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 = 



