392 MICROORGANISMS 



Under certain conditions, and probably in all cases in which 

 yeasts are able to grow and multiply, they digest, absorb, 

 assimilate, and respire sugar, essentially as other fungi do or 

 as animals do. But under other conditions, especially in the 

 absence of oxygen, and to some extent at all times when 

 they are supplied with plenty of sugar, they simply digest 

 and absorb the sugar into their bodies where by the action 

 of an ENZYME, the sugar molecules are each split up into alcohol 

 and carbon dioxide. The alcohol and the carbon dioxide then 

 escape from the yeast cell into the culture. 



444. How Yeast may Derive Benefit from Changing Sugar 

 into Alcohol and Carbon Dioxide. Sugar is made up of the 

 elements, carbon, hydrogen, and oxygen; likewise alcohol is 

 made up of carbon, hydrogen, and oxygen while carbon dioxide 

 is composed of carbon and oxygen. The chemist is able to 

 show, however, that all of these elements which were origi- 

 nally in the sugar reappear in the alcohol and carbon dioxide 

 produced. None of the material is lost or disappears. Conse- 

 quently, it is evident that, in this process, the yeast plant does 

 not use any of the sugar for building up its own body. It is 

 true, however, that the alcohol and carbon dioxide which result 

 from this breaking up of a molecule of sugar do not together 

 contain quite so much energy as did the sugar molecule. 

 Therefore, in the process, a little energy must be liberated 

 within the yeast plant and the plant is probably able in some 

 unknown way to utilize this energy. If this latter thing is 

 true, it may explain the role that this process plays in the life 

 of the yeast. At the best, however, the yeast gets very little 

 energy out of the process, for the alcohol that results contains 

 very nearly as much energy as does the sugar from which it 

 comes. 



It has been suggested that another possible good that the 

 yeasts derive from this process lies in the fact that the accu- 

 mulation of alcohol in a yeast culture hinders the growth of 

 other organisms in the culture, and thus gives the yeasts a 

 better chance to multiply. It is also a fact, however, that 



