ENZYMES— MODE OF ACTION 57 



resulting from enzyme-action are removed by the plant as soon 

 as formed. 



Enzymes frequently act only in the presence of another 

 substance, an activator, which is commonly a salt, acid, or alkali, 

 although sometimes more complex. These activators differ from 

 the enzjanes in being able to diffuse through a parchmerrt mem- 

 brane, and can consequently be separated from the latter by 

 dialysis. 1 Examples are afforded by the small quantitj- of acid 

 requisite for the action of pepsin, and the necessity of the pres- 

 ence of certain complex phosphatic compounds for the action of 

 the zymase of Yeast. 



Just as these substances have the effect of accelerating enzyme 

 action, so, too, there are others which exert a retarding 

 influence, and there is every reason to believe that either the one 

 or the other can be produced as may be required by the living 

 cell. Moreover, the rate of the reaction depends on the amount 

 of the enzyme present, and this latter is regulated bj^ the cell's 

 activity. It is clear, then, that the character and vigour of 

 catalytic activity is subject to considerable modification, and 

 is indeed intimately related to the momentary requirements of 

 the organism. 



1 That is to sa)', placing the mixture of enzyme and activator in a 

 parchment tray floating on a large bulk of water. 



