200 



PLANT PHYSIOLOGY 



for every rise of 10° C. according to Van't HofTs general statement 

 for chemical reactions, but the deleterious effect of the heat upon 

 the enzyme at the higher temperatures tends to diminish the effect 

 after the optimum temperature is reached. At that point, the 

 harmful effect upon the enzyme is greater than the beneficial 

 effect upon the reaction. 



Light also favors the destruction of the enzyme. Ultra-violet 

 light and the shorter wave lengths are especially deleterious, 

 while the harmful effect of ordinary sunlight is much increased 

 by the presence of oxygen and depends largely upon it. 



4. They are very sensitive to acids, alkalies, salts, etc. Each 

 enzyme works best in a medium of definite pH value and any 

 change from this optimum is seen in the action of the enzyme. 

 Thus pepsin works best in the acid medium of the stomach, while 

 trypsin, in the intestine, requires an alkaline medium. A very 

 small amount of excess acid or alkali inactivates an enzyme. This 

 also may be explained on the basis of the colloidal nature of the 

 enzymes. Colloids, as has been noted, are especially sensitive to 

 carriers of electrical charges such as acids, bases, and salts. En- 

 zymes, however, are less affected by antiseptics, such as toluene, 

 which injure protoplasm. 



5. With few exceptions they are very specific in their action. 

 Only a limited number of substances can be acted upon by any 



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La 



A B C D E 



Fig. 8. — Illustrating the lock and key hypothesis of enzyme specificity. A 

 key the shape of A, B, or C would fit the lock and might unlock it, while a 

 key the shape of D or E could not be made to enter the lock. 



given enzyme, and in many cases only one. Thus invertase acts 

 only upon cane sugar and lactase upon milk sugar; but maltase 

 is able to hydrolyze both maltose and alpha methyl glucoside; 

 and emulsin, one of the glucosidases, can decompose milk sugar, 

 amygdalin, arbutin, salicin, coniferin, and still other substances. 

 It is probable, however, that emulsin is really a mixture of enzymes. 



