ANAEROBIC RESPIRATION AND FERMENTATION 311 



has a total heat of combustion of 652 kg. calories, so that, of the 



possible 709 calories which can be obtained by complete oxidation 



under aerobic conditions, only 709-652 or 57 calories are obtained 



under the conditions of fermentation. Anaerobic respiration is 



57 

 thus seen to be only — or less than 9% as efficient as the aerobic 



652 



method. To supply the same amount of energy the plant must 

 then respire nearly twelve times as much material. The actual 

 difference is even more than that, and all kinds of fermentation 

 require much greater amounts of material than the corresponding 

 complete aerobic oxidations. 



It might be expected that the plant would use oxygen if it were 

 available, but such seems not to be the case. In the presence 

 of abundant oxygen, the plant grows rapidly, but the amount of 

 fermentation is not very much increased. Due to lack of oxidases 

 to combine with atmospheric oxygen, the yeast plant seems unable 

 to use much oxygen even when present, although under aerobic 

 conditions growth and reproduction proceed much more rapidly 

 than in the absence of oxygen. According to the results of Pasteur, 

 the age of the cell plays an important role in this connection; young 

 cells are more independent of oxygen than old ones. In the brew- 

 ing and yeast-growing industries, cultivation is carried on in the 

 presence of oxygen because of its influence on growth and repro- 

 duction. The oxygen thus exerts an indirect influence upon fer- 

 mentation. 



In the absence of nutrient materials (food) the production of 

 carbon dioxide and alcohol continues for some time. Unlike the 

 ordinary respiration, this aufofermentation is accompanied by a 

 loss in dry weight of the yeast plant, since it is now fermenting 

 the food stored up within itself and lacks the opportunity to re- 

 place this matter by food from the outside. A similar phenom- 

 enon is seen when one compares the changes in dry weight of a 

 plant grown in light, where it can photosynthesize, with those in 

 one grown in the dark, where no food can be made. 



The yeasts are of chief importance in the brewing and baking 

 industries. Wild yeasts are found free in nature blowing about 

 in the dust of the air, and it is a matter of common knowledge 

 (these days) that almost any kind of fruit juice which is rich in 

 sugar, will ferment without the special addition of yeasts if merely 

 exposed to the air. If the fermentation is produced by molds, 



