ENERGY SUPPLY OF THE CELL 63 



nitrogenous material of his medium. The aspartic 

 acid acted as hydrogen acceptor and was reduced to 

 succinic acid and ammonia. 



COOH-CH2-CH(NH2)-COOH + H2 = 



COOH-CH2-CH2-COOH + NH3 



By adding aspartic acid to the medium, still more 

 hydrogen could be '^deviated" and the ratio of pro- 

 pionic to acetic acid fell to 1.6:1. 



We might conclude from this that the equation for the fermenta- 

 tion of lactic acid is correct, and that the aspartic acid interfered. 

 Such a statement, however, might be thought to prescribe a definite 

 course to a biological reaction, and to establish standards rather than 

 to give an unbiased interpretation of the facts. The unprejudiced 

 description of the process would give each of the partial reactions 

 (as mentioned on p. 47) and add the above reduction of aspartic 

 acid. This would leave to the bacterium the ''choice" of the 

 hydrogen acceptor, instead of introducing the human element of 

 ''correct equations" and "deviations." 



This is probably the best known, quantitatively 

 studied example, but many others are known, in which 

 the protein food acts either as hydrogen donator or as 

 hydrogen acceptor. 



Another very commonly found hydrogen acceptor is 

 the elementary oxygen. In the common types of alco- 

 holic fermentations (beer, wine, distilled alcohol), the 

 influence of oxygen is negligible because the continuously 

 formed CO2 prevents appreciable amounts of oxygen 

 from being taken up by the yeast. In the aerated 

 tanks used for the production XDf bakers yeast, the ratio 

 of alcohol to CO2 is changed considerably (see p. 176). 



A further good example of oxygen as a hydrogen 

 acceptor is its influence upon the gas ratio of Bad. coli, 

 i.e., the ratio C02:H2. Keyes and Gillespie (1912, 



