548 RESPIRATION AND FERMENTATION 



in the presence of oxygen for the same production of alcohol. On the 

 other hand, free oxygen depresses the metabolic activity of obligate 

 anaerobes, and the character of the food exercises an important influence 

 upon the result produced, as when Saccharomyces oxidizes alcohol in the 

 absence of sugar, and Bacterium aceti vinegar in the absence of alcohol. 

 In certain cases the maximal fermentative activity is attained in the 

 presence of a trace of oxygen, and many plants in which aerobic and 

 anaerobic respiration can proceed simultaneously, may find optimal condi- 

 tions for growth and development only when both are possible. 



It is not the case that in the absence of free oxygen aerobic respiration 

 continues by means of oxygen drawn from reducible substances present 

 in the cell, but rather that, as in normal aerobic metabolism, changes and 

 decompositions are induced in one or more substances, which in general 

 lead to a liberation of energy and to a simultaneous production both of 

 reduced and of oxidized products. Carbon dioxide is the most commonly 

 excreted oxidized product, while among the products of reduction are 

 hydrogen, sulphuretted hydrogen, alcohol, &c. The heat of combustion of 

 alcohol is greater than that of sugar for the same number of carbon atoms, 

 or for the same weight. During the anaerobic metabolism of carbon- 

 compounds, not only may various carbon affinities be satisfied, and com- 

 pounds of carbon and hydrogen and carbon and carbon decomposed, but also 

 combinations and dissociations may occur similar to those involved in aerobic 

 metabolism. In neither case is the formation of carbon dioxide an absolute 

 necessity (Sect. 98), and it is not impossible that certain anaerobic organisms 

 may be able to obtain all the energy they require from chemical actions 

 between inorganic compounds, such as sulphur and potassium nitrate, for 

 example. It depends entirely upon the specific character of the organism 

 itself whether it has or has not the power of utilizing to a sufficient extent 

 the energy latent in a given substance, and hence it is always possible that 

 organisms may be discovered which obtain energy from totally different 

 sources to those with which we are at present familiar l . Energy can be 

 obtained from a greater variety of substances by aerobic than by anaerobic 

 respiration, and hence usually only one or a few substances are suitable for 

 the fermentative activity or respiratory metabolism of an anaerobe. 



The products of anaerobic respiration may induce further changes ; 

 thus sulphuretted hydrogen is a powerful reducing agent, as is also nascent 

 hydrogen. Hence it is often difficult or impossible to say whether a given 

 substance is an actual product of respiratory metabolism, or results from 



1 Fermentative changes need not necessarily always follow precisely the plan given by Hoppe- 

 Seyler, Pfliiger's Archiv f. Physiol., 1875, Bd. XII, p. i ; Physiol. Chem., 1877, p. 116. See also 

 Ad. Mayer, Gahrungschemie, 1895, 4. Aufl., p. 200, and Frankland, Centralbl. f. Bact., 1894, Bd. XV, 

 p. 103. 



