METABOLISM 131 



In its essential characteristics — intake of oxygen, liberation of 

 energy and raising of temperature — the aerobic respiration of 

 plants is exactly comparable to the respiration of animals, a fact 

 which the complexities of plant metabolism sometimes obscure. 



Anaerobic Respiration {Fermentation). — Anaerobic respiration, 

 in which the breaking down of chemical compounds and the 

 consequent liberation of energy is not accompanied by an intake 

 of free oxygen, is characteristic of certain lowly plants and some- 

 times of higher ones when temporarily deprived of oxygen. The 

 best known examples of this process are alcoholic fermentations 

 and alhed phenomena. These were named "fermentations" 

 from the fact that the activity of ferments (now more commonly 

 called enzymes) was here first clearly shown. There is conse- 

 quently a confusion in the application of the term fermentation, 

 the commonest usage regarding it as practically synonymous 

 with anaerobic respiration, the other expanding it to cover all 

 activity brought about by the agency of enzymes. 



The most important characteristic of anaerobic respiration 

 is the fact that it does not lead to a complete breaking down of the 

 organic substance or food but only to its partial decomposition, 

 with the result that a quantity of more or less complex by- 

 products is formed which still contain a considerable amount of 

 potential energy. The accumulation of these by-products often 

 stops the process itself. 



The fermentation of sugar by yeast is the classic example of 

 anaerobic respiration. The yeast plants — minute, single-celled 

 organisms — thrive in rather weak solutions of sugar. Those 

 which have easy access to the air usually respire aerobically, but 

 if the supply of free oxygen is limited (as is the case anywhere 

 below the surface of the liquid) or if it becomes exhausted, the 

 yeast respires anaerobically. The cells now obtain their energy 

 through a partial decomposition of the sugar, with the formation 

 of carbon dioxide and a complex by-product, ethyl alcohol, thus: 



CeHioOe = 2CO2 + 2C2H5OH (alcohol) 



This change is effected by the activity of an enzyme, zymase, 

 which is secreted by the yeast cells. It is evident that only a 

 portion of the potential energy in the sugar has been liberated, 

 for the resulting alcohol may be absorbed by another organism, 

 or may be burned, and will then yield a considerable amount of 

 additional energy. When the concentration of alcohol has reached 



