538 ANAEROBIC RESPIRATION 



they produce no amylase. This is the reason that germinated barley (malt) 

 is used rather than the ungerminated grains in the brewing industry, since 

 the sugar content of the grains increases greatly during germination. 



Alcoholic fermentation is an anaerobic process, occurring without any 

 utilization of atmospheric oxygen. Oxidation is accomplished by intermolecu- 

 lar atomic shifts which take place in such a manner that the sum total of the 

 energj^ remaining in the resulting compounds is less than that present in the 

 original substrate, the excess energy being released. Alcoholic fermentation 

 results in only an incomplete oxidation of hexose molecules, hence the quan- 

 tity of energy released — about 25 kg.-cal. per mol of glucose — is much less 

 than in aerobic respiration, in which oxidation of one mol of glucose sets free 

 673 kg.-cal. In spite of its relative inefficiency the process of fermentation 

 is the method by which yeast plants obtain necessary energ3^ This is the 

 fundamental biological significance of the process. 



It might be supposed that eificient aeration of a sugar solution containing 

 yeast plants would result in complete oxidation of the sugars present by 

 a process of aerobic respiration. Such, however, is not the case; ethyl alcohol 

 and carbon dioxide are the principal end products whether the reaction 

 occurs in the presence or absence of oxygen. Some aerobic respiration, (as 

 much as one-third of the total, according to some investigators) does occur 

 when oxygen has access to the yeast cells. The predominance of anaerobic 

 respiration even in the presence of oxygen is generally ascribed to the possession 

 by yeast cells of a relatively ineffective oxidizing enzyme mechanism, as com- 

 pared to a highly active zymase system. In the presence of oxygen multiplica- 

 tion of yeast cells occurs at a more rapid rate than in its absence. This is 

 probably due to the much greater energy production resulting from the occur- 

 rence of some aerobic respiration when oxygen is available. 



When sugar in solution is fermented by yeast one of the end products 

 — ethyl alcohol — accumulates in the solution, while the other — carbon dioxide 

 ■ — escapes as a gas. However, there is a definite limit to the accumulation of 

 alcohol. When the proportion of alcohol in the liquid reaches about 15 per 

 cent the yeast cells are poisoned and the fermentation process stops. 



Anaerobic Respiration. — Any higher animal which is deprived of oxygen 

 will die within a very few minutes. Higher plants, however, will continue 

 to live in an atmosphere devoid of oxygen. Some plants or plant organs can 

 survive under such conditions for a long period, others succumb within a 

 day or two. Corn (maize) seedlings, for example, will not remain alive 

 much more than a day in an oxygen-free atmosphere. On the other hand, 

 apple and pear fruits can survive storage in an atmosphere of pure hydrogen 

 or pure nitrogen for months without injur}^ Even under such conditions 



