ORGANIC SUBSTANCES AS PLANT FOOD 213 



their action. It has been shown that starch is converted into sugar 

 only by diastase, and cellulose only by cytase, and that the hydroly- 

 sis of two disaccharides, maltose and sucrose, is brought about, 

 respectively, by maltase and invertase. The classification and 

 nomenclature of enzymes are based on this selective action. Enzymes 

 which act upon carbohydrates, fats, and proteins, are usually 

 divided into three large groups; carbohydrases, which hydrolyze 

 carbohydrates; lipases, which split fats and complex esters in 

 general; and proteases, which attack the proteins. The name of 

 an enzyme is usually composed of the name of the substance acted 

 upon plus the suffix ase. Only such enzymes that have been known 

 for a long time: diastase, invertase, pepsin, etc., retain their his- 

 torical names. 



The specificity of enzymatic action should not be understood 

 in the sense that each disaccharide or polysaccharide, each protein, 

 each fat, etc., can be hydrolyzed only by a special enzyme. Quite 

 the contrary, it is established beyond doubt that there are fewer 

 enzymes than are often described, and that various proteins, for 

 example, are acted upon by one and the same enzyme. Their 

 specific action consists largely in the fact that each enzyme pos- 

 sesses the ability to break down any bond of a certain character in 

 the complex molecule of organic compounds, and that in the pres- 

 ence of two optical isomers, each enzyme possesses the ability to 

 act only upon one of them. 



The question of reversibility of enzymatic action is of great 

 importance and interest. According to some general laws of 

 physical chemistry, catalyzers, accelerating the course of a reac- 

 tion, do not, as a rule, change the points of equilibrium between 

 the initial and the end products. But the reactions of hydrolysis 

 are, generally speaking, reversible, that is in the case of a large 

 number of products, accompanying the decomposition and release 

 of free water there is also observed the reverse process of condensa- 

 tion. Theoretically, at least, it should be expected that the same 

 enzymes will accelerate both the decomposition and the synthesis 

 of a complex reserve substance in the organism. In the majority 

 of enzymatic reactions, however, as for instance in the decompo- 

 sition of starch by diastase, this theoretical requirement is not 

 satisfied. And in those cases in which by the aid of an enzyme it 

 has been possible to obtain synthesis, the product, at best, was 

 found to be only an isomer of the initial substance. From a con- 



