96 PLANT PHYSIOLOGY 



Within a fairly wide range, the amount of oxygen in the air 

 has no perceptible influence on the intensity of respiration. 

 Only when the amount of oxygen has decreased to about 8 per 

 cent does the respiratory quotient begin to increase perceptibly. 

 Alcohol then accumulates in the tissues of the plants as a result 

 of anaerobic respiration, i.e., respiration without oxygen, which 

 replaces normal respiration in conditions of a deficiency of oxygen. 



23. Anaerobic Respiration and Alcoholic Fermentation.— 

 Normal respiration represents an oxidation process that continues 

 during the uninterrupted absorption of oxygen of the air. When 

 a plant is transferred into an oxygen-free atmosphere, respiration 

 must inevitably cease. Together with it are discontinued the 

 processes directly connected with the liberation of energy 

 obtained through respiration, e.g., growth and protoplasmic as 

 well as organic movements. When the plant is permanently 

 deprived of oxygen, it will perish, since for its support, life 

 requires a continuous supply of energy. 



In the absence of free oxygen, death of a plant does not occur 

 immediately, since life may be supported in the cells for some 

 time by anaerobic respiration. As, in both normal and anaerobic ' 

 respiration, carbon dioxide is eliminated, oxidation processes 

 take place in both cases. In the absence of air, however, the 

 necessary oxygen is taken from various compounds, e.g., from 

 water or the hydroxyl groups of sugar molecules; hence, oxidation 

 of one organic substance or group during anaerobic respiration 

 is accompanied by reduction of another organic substance or 

 group. 



Carbon dioxide is the most oxidized product of anaerobic 

 respiration, while alcohol is a highly reduced product. The 

 general reaction may be represented by the following equation: 



CeHisOe -> 2CO2 + 2C2H5OH + 25 cal. 



When compared with the amount of free energy released by 

 complete oxidation of a molecule of glucose, 674 cal., this quan- 

 tity is very small. It is but natural, therefore, that the energy 

 of anaerobic respiration is not sufficient to maintain all the 

 life functions that usually are supported through ordinary 

 respiration. 



Besides the low energy value, anaerobic respiration has the 

 other disadvantage of accumulating alcohol, a substance known 



