CHAPT^E III 



PLANT ENZYMES 



Some indication has been given in the previous chapter of the large 

 number of complex processes which take place in the plant, and it has 

 been mentioned that many of these are controlled by enzymes. 



The most remarkable feature in connexion with the chemical pro- 

 cesses of plant metabolism is the ease and rapidity with which, at ordi- 

 nary temperatures, chemical reactions take place, when under artificial 

 conditions they need a much longer time and higher temperatures. 



It has been found that many of the chemical reactions in the plant 

 can be brought about in vitro on addition of certain substances which 

 can be extracted from the plant. These substances are known as enzymes. 

 It is the property of enzymes that they are able to accelerate reactions 

 which, in their absence, would only take place very slowly. The enzyme 

 cannot initiate these reactions and does not form part of their final 

 products. 



Some inorganic substances have the same property of accelerating 

 reactions, and such substances are termed catalysts. For example, when 

 water is added to ethyl acetate, the latter begins to be decomposed slowly 

 into ethyl alcohol and acetic acid : 



ethyl acetate + water — >- ethyl alcohol -|- acetic acid, 



but if, in addition, some hydrochloric acid is added, hydrolysis takes place 

 with much greater rapidity, and at the end of the reaction the hydro- 

 chloric acid is found unchanged. Hence in this case hydrochloric acid 

 is an inorganic catalyst. Many other similar instances are known as, for 

 example, the catalyzing effect of a small quantity of manganese dioxide 

 which brings about the liberation of oxygen from potassium chlorate at 

 a much lower temperature than by heat alone. 



By analogy, therefore, an enzyme may be defined as an organic 

 catalyst produced by the plant. 



Another point in connexion with the above-mentioned reaction of 

 water with ethyl acetate, is the fact of its being representative of the 

 type known as reversible. After a certain amount of acetic acid and 

 ethyl alcohol has been formed, these recombine to form ethyl acetate 

 until in time a certain point of equilibrium is reached. Since the same 



