ENERGY SUPPLY OF THE CELL 77 



that by hydrolysis, an acid and an alkali are formed. 

 We can write the urea fermentation in two steps : 



CO(NH2)2 + H2O = CO2 + 2NH3 



2NH3 + CO2 + H2O = (NH4)2C03 



The energy yield of the second process has been deter- 

 mined, and Landolt-Bornstein mentions two values, 

 10.7 and 15.9 cal. per gram molecule of ammonium car- 

 bonate. The energy yield of the first process, the real 

 hydrolysis, can not been measured separately. Com- 

 bustion heats of urea are given only with nitrogen gas as 

 final product. By combining the values for heats of 

 formation of urea, ammonia and water, the computed 

 energy yield of the hydrolysis proper is negative, —1.5 

 cal. This value is quite uncertain, representing the 

 difference of various, none too exact, heats of formation; 

 but it is probably near zero. The only energy available 

 to the cells of urea bacteria is that of the second process; 

 namely, the heat of neutralization. This does appear a 

 strange source of energy. 



The same principle probably applies to the organic acid amids 

 which are contained in the protein hnkages. The heat of neutrahza- 

 tion of acetic acid by ammonia approximates 12 calories per gm. 

 molecule. These values are quite considerable if we realize that the 

 energy yield obtained from the alcoholic fermentation of sugar is 

 only about 26 cal. per gm. molecule. The heat of neutralization 

 also must enter into some of the inorganic fermentations (formation 

 of nitrous acid from ammonia, of sulfuric acid from hydrogen sulfide 

 or sulfur, and in denitrification and sulfate reduction).* 



This discussion of the role of oxygen for the energy 

 requirements leads us to wonder to what extent our 

 distinction between aerobic, facultative and anaerobic 

 bacteria is justified. Since we judge this property 

 primarily by the growth of bacteria, it really should 



* Another peculiar type of oxygen-free fermentation is the spUtting 

 of fatty acids into CH4 and CO2 (Thayer, 1931). 



