PHYSIOLOGY OF CELLS 55 



up protoplasm, must first be brought into solution. Proteins and fats 

 cannot be used while they remain proteins and fats for neither can diffuse 

 through the colloids of the cell. In the animal cell the same state of 

 affairs exists. Foods are rendered soluble and diffusible during the proc- 

 ess of digestion. According to an accepted definition digestion is the 

 process of bringing foods into solution and into a state of diffusibility 

 by means of a chemical agent. The agents employed by plant and 

 animal cells during the process of digestion are solutions elaborated 

 by the protoplasm. The power of producing digestive solutions is 

 possessed by all living cells. Digestive solutions are either alkaline or 

 acid in reaction, and in addition to the acid or alkali present in the 

 solution there are always one or more very important substances to which 

 has been given the name of enzyme. 



Enzymes. — Enzymes are substances whose exact chemical nature 

 is yet imknown. Extracted enzymes may respond to protein tests, but 

 this seems to be due to proteins associated with the enzymes, from which 

 the latter have only occasionally been separated. Enzymes act like 

 certain substances which have the property of hastening chemical reac- 

 tions. These accelerating substances are called catalyzers or catalysts 

 or catalytic agents, and the effect which they have in the reaction is called 

 catalysis. Among inorganic substances which act as catalyzers may be 

 mentioned finely divided platinum. So far as can be determined cataly- 

 zers hasten reactions by entering into the formation of intermediate 

 substances from which they are promptly released. They are thus 

 present undiminished at the end of the reaction. Enzymes also act 

 somewhat in this fashion. Unlike the inorganic catalyzers, enzymes are 

 affected by extremes of heat or cold. Most of them operate best at a 

 temperature of 30-45°C. but are destroyed by temperatures of 60-75°C. 

 Certain of them are rendered active by small quantities of free acid, 

 others by alkali. The ions of certain metals inhibit their action. En- 

 zymes may be extracted from plant or animal bodies, precipitated from 

 their solutions, purified, and again brought into solution without loss 

 of activity. A given enzyme may take part in only a single kind of 

 reaction. This reaction, however, is reversible, that is, the enzyme may 

 assist in the splitting of a substance into two others or it may serve as the 

 agent for causing the combination of the two simple substances into the 

 one. The limitation of an enzyme to a certain specific action is visualized 

 by Fischer's famous "lock and key" hypothesis which assumes that the 

 chemical configuration of the enzyme corresponds very closely to that of 

 the substance on which it acts, and that the two are thus fitted to each 

 other as a kej^ is fitted to a lock. From the fact that many enzymes may 

 take part in but one kind of reaction it maj^ be inferred that to accomplish 

 the multitude of chemical reactions that are going on continuously or 

 periodically in the animal or plant a number of enzymes must be present 



