102 BACTERIOLOGICAL CHEMISTRY 



it follows from this consideration that the less com- 

 plete the oxidation the more of a given substance must 

 be broken down to supply the needs of the organism. 

 The incomplete oxidation of non-nitrogenous compounds 

 is what we usually call fermentation, and the incomplete 

 oxidation of nitrogenous compounds is usually referred to 

 as putrefaction. 



Pasteur considered that the main factor controlling 

 fermentation was the oxygen supply. He showed that 

 under anaerobic conditions the growth of yeast cells is 

 much restricted but that fermentation is very active, 

 whilst under aerobic conditions with a good oxygen supply 

 the growth of the cells is rapid but fermentation is 

 repressed. The aerobic breakdown of 7 grams of sugar 

 is associated with the formation of 1 gram of yeast cells, 

 whilst anaerobically the production of 1 gram of cells 

 needs 70 grams of sugar, a striking illustration that 

 complete oxidation is much more economical than 

 incomplete oxidation. 



Various theories have been propounded to explain the 

 fact that such compounds as carbohydrates, amino-acids 

 and fatty acids are readily oxidised in part or completely 

 by cells at ordinary temperatures, whilst in the laboratory 

 they are only oxidised by much more drastic means. 

 Usually these theories involve the activation of either the 

 substrate or the hydrogen acceptor (whether it be oxygen 

 or some other compound) or sometimes both of them. 



Wieland has put forward the idea that all oxidations 

 are due to a hydrogen transfer. He came to this con- 

 clusion as a result of experiments in which he found that 

 many substances could be anaerobically oxidised, in 

 presence of such metallic catalysts as spongy palladium 

 or platinum, by loss of hydrogen which is taken up by 

 the catalyst. Examples are the oxidation of hydro - 

 quinone to quinone : — 



