ENZYMES— MODE OF ACTION 55 



is to say, the compound is broken down with the addition of 

 water, thus : 



Ci.H,,On + H,0 = CeHi.Oe + C,}1,,0, 1 

 cane-sugar water glucose fructose 



(CeHioOa)^ + "H.O = nCoH,,Oe 

 inuHn water fructose. 



In other cases, however, the action appears to be different, as, 

 for instance, that of the fermenting enzymes (cf. p. 256) and of 

 the oxidising enzymes or oxidases which are in part responsible 

 for the change in colour of the cut surface of an Apple, and of 

 many Fungi, when exposed to air. Evidence is accumulating in 

 support of the view that all the chemical processes depending on 

 enzyme-action are reversible, taking place in one direction or the 

 other according to the prevailing conditions. Thus, for example, 

 it is beheved that the building up of starch from sugar and the 

 reverse process are both dependent on the same set of enz}anes. 

 Tittle is known as to the chemical nature of enzymes, but all 

 of them are colloids, and as a consequence the rate of diffusion 

 through parchment and similar membranes is either very slow 

 or practically nil. 



Our knowledge of this class of substances is almost entirely 

 confined to their mode of action. The reactions influenced by 

 enzjmies are all such as require an appreciable interval of time 

 for their completion, so that it is possible to measure their rate 

 under anv given set of conditions. One of the most important 

 aspects of enzMne-activit}' is the small quantity of the enzj'me 

 necessary to bring about a pronounced chemical change ; thus 

 invertase is stated to invert 200, coo times its weight of cane- 

 sugar. Moreover, at the end of the reaction the enzjTne appears 

 to be unaltered both as regards amount and characteristics. 

 In both these respects enzymes resemble the so-called catalytic 

 agents employed in certain chemical processes. As examples 

 we mS-Y mention the use of small quantities of manganese dioxide 

 to accelerate the liberation of oxygen from chlorate of potash, 



1 Although the formulae for glucose and fructose are the same, these 

 two compounds differ in the arrangement of the atoms -within their 

 molecules. 



