182 CHEMISTRY OF PLANT LIFE 



organism ("in vivo "), is due to the universal presence in the 

 living protoplasm of certain organic catalysts, known as enzymes. 



ENZYMES AS CATALYSTS 



The phenomenon known as " catalysis " is of common occur- 

 rence in both inorganic and organic chemistry. The effect of a 

 small amount of manganese dioxide in aiding in the liberation of 

 oxygen from potassium chlorate is an example which is familiar 

 to all students of elementary chemistry. Similarly, spongy 

 platinum accelerates the oxidation of sulfur dioxide to sulfur 

 trioxide, in the commercial manufacture of sulfuric acid. Again, 

 the hydrolysis of sucrose into fructose and glucose proceeds very 

 slowly in the presence of water alone, but if a little hydrochloric 

 acid or sulfuric acid be added to the solution, the velocity of the 

 hydrolysis is enormously accelerated. Many other examples of 

 the accelerating effect of various chemicals upon reactions into 

 which they do not themselves enter, might be cited. 



The essential features of all such catalytic actions are: (1) 

 the velocity of the reaction is greatly altered, usually accelerated; 

 (2) the catalytic agent does not appear as one of the initial sub- 

 stances, or end-products, of the reaction, and is not itself altered 

 by the chemical change which is taking place; (3) the accelerating 

 effect is directly proportional to the amount of the catalyst which 

 is present; (4) relatively small amounts of the catalyst produce 

 very large results in the reacting mixture; and (5) the catalysts 

 cannot themselves initiate reactions, but only influence the velocity 

 of reactions which would otherwise take place at a different rate 

 (usually much more slowly) in the absence of any catalytic 

 agent. 



Enzymes conform to all of these properties of catalysts, and 

 are commonly defined as the " catalysts of living matter." They 

 are almost universally present in living organs of every kind, and 

 perform exceedingly important functions, both in the building-up 

 of synthetic materials and in the rendering soluble of the food of 

 both plants and animals, so that it can be translocated from place 

 to place through the tissues of the organism. 



Enzymes differ from inorganic catalysts in being destroyed by 

 heat, in not always carrying the reaction to the same stage as does 

 the inorganic catalyst which may accelerate the same reaction, and 



