FOOD OF MICROORGANISMS 227 



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phates to hydrogen sulphide, and litmus or methylene blue to the 

 colorless leuco-compounds. Such removal of oxygen from a molecule 

 requires energy, and is possible only when the bacterium by using the 

 oxygen for oxidation of organic matter can obtain a larger amount 

 of energy. The following example shows such a possibility: 



2 KNO 3 + 36.6 Cal. = 2KNO 2 + O 2 

 C 2 H 5 OH + O a = CH 3 CO 2 H + H 2 O + 115 Cal. 



This process leaves an energy balance of 115 36.6 = 78.4 Cal. for 

 the needs of the bacterium. 



Such decompositions are sometimes referred to as "reducing fermen- 

 tations" but this term is not correct, as the reduction must always be 

 accompanied by a simultaneous oxidation process. 



The amount of energy liberated by a fermentation without oxygen 

 is much smaller than that furnished by complete oxidation; the intra- 

 molecular change always leaves organic compounds which contain a 

 considerable amount of the total energy. Yeast, in presence of very 

 much oxygen, oxidizes sugar completely to water and carbon dioxide. 



C 6 H 12 O 6 + 120 = 6CO 2 + 6H 2 O + 674 Cal. 



while in the absence of oxygen it will change the sugar to alcohol and 

 carbon dioxide. 



C 6 H 12 O 6 = 2C 2 H 5 OH + 2 CO 2 + 22 Cal. 



The energy gained in the first process is about thirty times as large 

 as that gained in the second process. This was demonstrated as early 

 as 1861 by Pasteur. He grew yeast in sugar solutions, varying only 

 the amount of oxygen in contact with the medium. At the end of 

 the experiment, the weight of the dry yeast and the decomposed sugar 

 was determined, and the amount of sugar necessary to produce one 

 part of yeast was computed. He found: 



In a closed flask, without any air i part yeast required 1 76 parts sugar. 



In a closed flask, with large air space i part yeast required 23 parts sugar. 



In a thin layer, a few mm. thick i part yeast required 8 parts sugar. 



In a very thin layer, in 24 hours i part yeast required 4 parts sugar. 



This experience led Pasteur to the conclusion that fermentation 

 corresponded to the respiration process of animals, that fermentation 

 was respiration without oxygen. 



It is quite evident that since the utilization of the food in the 



