78 ELEMENTS OF BIOLOGY 



specifically mean the breakdown of sugar by yeasts. The reaction 

 by which the yeast breaks down grape sugar is usually written: 



5ugar alcohol carbon dioxide 



CeHisOe = ^lCsHbOH + 1CO2 . 



The particular substance which effects this splitting of the sugar 

 molecule is known as an enzyme, this particular enzyme being 

 named zymase. Yeasts may ferment starches and other carbohy- 

 drates, but the reaction in these cases takes place in several steps, a 

 diflferent enzyme effecting each step, finally converting the carbohy- 

 drate into the form of grape sugar and then zymase acts to produce 

 alcohol and carbon dioxide. Hence there may be a chain of enzyme 

 reactions involved in a living process. 



AGENT 



SUBSTANCE A ^-s-^ SUBSTANCE B 



PRODUCT 

 Fig. 25. — Scheme representin.fj the acti\ation of a chemical reaction by a colloidal 

 catalytic agent. It is assumed that substance A and substance B, because of their 

 electrical characteristics, do not ordinarily react with each other. Both are taken 

 up (adsorbed) by the agent, and their contact with the agent so alters their 

 electrical properties that they react and produce a third substance which is dif- 

 ferent from either. 



Catalysis. In the non-living world a class of substances are 

 known which also effect reactions without being used up in the 

 process. These are termed catalysts (Fig. 25). The phenomenon 

 of catalysis may be illustrated by the common device known as a 

 platinum sponge. When held over escaping illuminating gas it ap- 

 pears to accumulate the molecules of the oxygen in the air and the 

 gas molecules in such fashion that they unite and a flame results. 



