ENERGY SUPPLY OF THE CELL 73 



Table 9 shows that H2O and CO2 and sulfur could 

 not be considered as hydrogen acceptors, and that 

 H2SO4 is a very poor acceptor, while nitrate and nitrite 

 are excellent. Better than all of these (except H2O2) 

 is oxygen gas which requires no energy whatever and, 

 therefore, gives the highest energy yields. 



The computation of the energy balance shows us which 

 reactions are possible, and which reactions can take 

 place more easily than others. But it is not sufficient 

 that a reaction is thermodynamically possible. It is 

 the type of catalyst which decides whether the possible 

 processes actually happen. Very few organisms can 

 reduce nitrates to nitrogen gas, though this seems 

 the most economical type of reduction from the energy 

 viewpoint. No organisms are known to reduce carbon 

 monoxide, though it would be quite profitable with the 

 proper mechanism. 



Eventually, the catalysts for such oxido-reductions 

 are already present in the culture medium. Demeter 

 (1930) has shown that sterile broth, asparagin solution, 

 etc. will reduce nitrates to nitrites at 45°C. 



The importance of chemical structure in addition to energy con- 

 siderations is most striking in the anaerobic species. Though their 

 energy supply depends upon oxido-reductions hke that of all other 

 organisms, they cannot utilize the best of all hydrogen acceptors, 

 i.e. oxygen. This is not merely due to their sensitivity to oxygen 

 because some species which are rather indifferent to it show the same 

 lack of oxygen utihzation. The author cultivated Strept. cremoris at 

 30°C. in a flask half full of milk and closed by a rubber stopper with 

 a large inverted U-tube whose other -end opened into a glass of water. 

 While with Bad. coli and B. cereus in peptone solution, the water 

 was always drawn into the tube as a result of consumed oxygen, 

 the level of water never changed with milk cultures of Strept. cremoris, 

 not even when a tube of soda-Ume was suspended in the flask to 

 absorb eventual CO 2 formed. Addition of methylene blue made no 



