184 CHEMISTRY OF PLANT LIFE 



hydrolyses, changes in oxygen content, or other simple decompo- 

 sitions or condensations, which will otherwise proceed so slowly 

 as to be practically negligible, it is customary to speak of the 

 enzyme as " acting upon " the material in question. It should be 

 understood, however, that this is a misstatement, as the enzyme 

 cannot actually initiate a reaction, or " act upon " any sub- 

 stance; it only acts as a catalyzer to accelerate the action of 

 water, oxygen, etc., upon the material in question. 



Generally speaking, most enzymes are colloidal in form and, 

 hence, do not diffuse through membranes such as the cell- walls. 

 Some of them perform their characteristic functions only within 

 the cell, or organ, which elaborates them, and can be obtained 

 outside these tissues for purposes of study only by first rupturing 

 the cell-wall or other membrane with which they are surrounded. 

 Such enzymes are known as " intracellular." Others are regu- 

 larly secreted by glands which discharge them into other organs, 

 as the stomach or intestines of animals, where they perform their 

 useful functions; or, as in the case of germinating seeds, they 

 move to other parts of the organ, and can be extracted from the 

 tissue by simple treatment jvith water. These are known as the 

 " extracellular " enzymes. 



Enzymes are specific in their action. Any given enzyme 

 affects only a single reaction; or at most acts only upon a single 

 group of compounds which have similar molecular configuration. 

 Usually it is only a single compound whose decomposition is accel- 

 erated by the action of a particular enzyme; but there are a few 

 enzymes, such as maltase (which acts on all a-glucosides) and 

 emulsin (which acts on all 0-glucosides) which act catalytically 

 upon groups of considerable numbers of similar compounds. 



Enzymes, like all other catalysts, act more energetically at 

 increased temperatures; but for each particular enzyme there is an 

 " optimum temperature," (usually between 40 and 65) above 

 which the destructive effect of the temperature upon the enzyme 

 itself more than offsets the accelerating influence of the increased 

 temperature. At still higher temperatures (usually 80 to 100) the 

 enzymes are " killed," i.e., rendered permanently inactive. All 

 enzymes are " killed " by boiling the solutions in which they are 

 contained. Dry preparations of enzyme material can withstand 

 somewhat higher temperatures, for somewhat longer periods of 

 time, than can the same enzyme in moist condition or in solution. 



