132 THE COLOURLESS PLANT CELL. THE YEAST PLANT 



Alcoholic Fermentation. The yeast plant is of 

 special interest both biologically and practically, because 

 it is able for a time, in a suitable nutritive medium 

 containing sugar, to cany on a process which we must 

 regard as a modified kind of respiration, in place of 

 ordinary respiration. The yeast cells produce an 

 enzyme (zymase), by the catalytic activity of which 

 the molecule of sugar (glucose) is split into two mole- 

 cules of ethyl alcohol and two of carbon dioxide : 



2C0 2 



glucose ethyl alcohol carbon 

 dioxide 



thus liberating energy from the sugar molecule 

 without using free oxygen. This process in the living 

 cell is called anaerobic respiration, as opposed to the 

 ordinary aerobic x respiration t in which free oxygen is 

 used. Of the sugar actually absorbed by the yeast 

 cells some is split up in this way and the energy is 

 used in growth, while some is doubtless used, together 

 with nitrogenous substances, for the formation of new 

 protoplasm. None of the zymase formed by the 

 cell passes out into the surrounding sugar solution, 

 but quantities of sugar pass into the yeast cells, and 

 are split up into alcohol and carbon dioxide. The 

 carbon dioxide forms bubbles in the liquid, which froths, 

 while the alcohol accumulates, and the energy set 

 free raises the temperature of the liquid. Meantime 

 the yeast cells grow and bud with great rapidity, enor- 

 mously increasing their number. This is the process 

 known as alcoholic fermentation, and is the foundation 

 of the making of beer and wine. 



Brewing. The liquid fermented in brewing beer is 

 called wort, and is a hot water infusion of malt, i.e. sprout- 

 * " Living in air " from Greek d^p, air, and fiiog, life. 



