20 NATURE OF ENZYMES 



cannot use them as economically as the plant does. In the 

 wheat, for example, the proteid material is largely confined to 

 the outer portion of the grain (Fig. 12). This layer is of a 

 darker color and in the "best flour" it is excluded in the milling 

 process, leaving the flour very white. So the most nutritious 

 part of the wheat does not appear in white bread, but it should be 

 remembered that white bread is better than the much exploited 

 "whole wheat" products because the proteid material is digest- 

 ible to only a slight degree. These reserve foods are made 

 available to the plant in the same way as to the animal. They 

 are first put into solutions by ferments or enzymes. Ptyalin in 

 the saliva of animals and diastase in the plant are common ex- 

 amples of enzymes which change starch into sugar. So there are 

 enzymes which transform each reserve food and render it ca- 

 pable of transport and incorporation into the substance of the 

 plant body. These enzymes are formed by the living substance 

 of certain cells and resemble in many ways the living matter 

 itself. They are most extraordinary in their action and behavior. 

 The decomposition of the substances upon which they react is 

 effected by the mere presence of the ferment — at least there is 

 no permanent union between ferment and the substance. Conse- 

 quently a very small amount of a ferment may effect a decom- 

 position of an almost unlimited amount of a given substance. 

 More commonly water enters into the composition of the sub- 

 stance as a result of the presence of the enzyme. As a result its 

 stability is upset and it breaks down into simpler compounds. 

 This decomposition due to the incorporation of water is termed 

 for this reason hydrolysis. We say that diastase hydrolyzes 

 starch into simpler compounds, as maltose and dextrines. 

 Some enzymes appear to effect decompositions without the co- 

 operation of other substances. Still more remarkable is the 

 action of these ferments in that some of them appear to have a 

 reverse action, i. e., the power of building up again complex 

 material from the parts into which they have separated it. In 

 many ways enzymes behave like living matter, being quickened 

 in their activities by suitable temperatures, the concentration 

 of the fluid or by the presence of certain acids and alkalies; or 



