258 TEXTBOOK OF PLANT PHYSIOLOGY 



understood. They appear to belong to a group of facultative 

 saprophytes, that is, they can develop on organic substances. 

 The oxidation of methane and hydrogen is of great advantage from 

 a kinetic point of view, since in the combustion of hydrogen a 

 great quantity of energy is liberated. These bacteria are able to 

 utilize this energy for the decomposition of CO2, but only in the 

 presence of a considerable amount of ferrous salts. 



80. Anaerobic Respiration and Alcoholic Fermentation. — Nor- 

 mal respiration represents an oxidation process which 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, as, for instance, 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 either case. In the absence of air, however, the necessary 

 oxygen is taken from various compounds, as, for instance, water 

 and the hydroxide groups of oxidized sugar molecules; hence oxida- 

 tion during anaerobic respiration is accompanied by reduction. 



Carbon dioxide is the most oxidized product of anaerobic res- 

 piration, while alcohol is a highly reduced product. The general 

 reaction may be represented by the following equation: 



C0H12O0 = 2C0 2 + 2C2H5OH + 25 Cal. 



When compared with the amount of free energy released by com- 

 plete oxidation of a molecule of glucose, viz., 674 Cal., this quantity 

 is very small. It is bat natural, therefore, that the energy of 

 anaerobic respiration is not sufficient to maintain all the processes 

 which usually are supported through ordinary respiration. 



Besides the low energy value, anaerobic respiration has the 

 other disadvantage of accumulating alcohol, a substance known to 

 possess poisonous properties. Consequently, in the absence of 

 oxygen, a plant perishes not merely from an excessively low supply 



