310 FOUNDATIONS OF BIOLOGY 



proteins for nitrogen but obtain it in less complex forms. 

 But the essential fact of interest at present is the chemical 

 changes associated with Yeast metabolism the transforma- 

 tion of a large proportion of the sugar content of the medium 

 in which they live into alcohol and carbon dioxide. This 

 process of alcoholic fermentation may be approximately 

 expressed by the formula : 



C 6 H 12 O 6 (sugar)+yeast = 2 C 2 H 5 OH (alcohol) +2 CO 2 

 The explanation is not far to seek. Deprived of an adequate 

 supply of air, Yeasts resort to the energy released when, with 

 the decomposition of the sugar, the carbon and oxygen unite 



FIG. 159. Yeast cells, very highly magnified. A, cell showing granular 

 cytoplasm and a large vacuole; B, showing nucleus; C, cell budding; 

 D, mother cell and bud after division is completed. 



as CO 2 . The formation of alcohol by the remnants of the 

 sugar molecules is, from the standpoint of the Yeasts, a mere 

 incidental factor which is, so to speak, unavoidable. On the 

 other hand, from the broad viewpoint, the waste products 

 of the action of the Yeast plants' enzymes represent an impor- 

 tant phase in the general simplification of organic compounds 

 in nature. And Man turns to account in numerous ways both 

 products of the Yeasts' destructive powers the alcohol 

 and the carbon dioxide. 



Thus the Yeasts are practically independent of free oxygen 

 and in this they agree with many kinds of Bacteria as well as 

 some animals, chiefly parasitic worms, which are able to 

 secure the necessary oxygen by the rearrangement of the 

 atoms within a molecule or the disruption of the molecule 



