PASTEUR EFFECT 51 



oxidized at the expense of the other part, which accordingly is re- 

 duced. Energetically probably the most eflBcient reaction is one such 

 as the propionic acid fermentation: 3 CgHigOg = 4 CoHjCOOH + 

 2 CH3COOH + 2 CO2 + 2 H2O. The energetic yield is 61 kg.-cal. 

 of heat per mole of glucose. According to Burk (2), the change in 

 free energy is approximately 18 kg.-cal. higher per mole of glucose 

 than the heat exchange. The probable maximum for a fermentative 

 breakdown of carbohydrate thus amounts to 79 kg.-cal. per mole. 

 This is 11.5 per cent of the 686 kg.-cal. to be obtained by respira- 

 tory breakdown. The more common lactic acid and alcoholic fer- 

 mentations do not reach this maximum but yield only 54 kg.-cal. 

 (36 kg.-cal. plus 18 kg.-cal. entropy change), or 7.9 per cent of the 

 heat of combustion. 



These values represent the theoretical maximum for fermentation 

 and respiration. To compare the eflBciencies of the two reactions in 

 the cell we must know how much of the energy of each reaction is 

 actually available to the cell. In a recent paper (3) the author has 

 pointed out that from 40 to 70 per cent of the theoretical fermenta- 

 tion energy is utilizable. This is deduced from the fact that in the 

 muscle up to 40 of the 54 kg.-cal. derived from glycolysis can be 

 stored as four energy-rich phosphate bonds in phosphagen, the 

 energy of which is utilizable for muscular work and other purposes. 



Until fairly recently the view was favored that respiration energy 

 was much less utilizable than fermentation energy— in other words, 

 that fermentation energy was relatively more valuable than would 

 be indicated by a comparison of theoretical caloric yields. Recent 

 results, however, for the conversion of oxidation into phosphate 

 bond-energy strongly indicate that such is not the case. With 

 oxidation of pyruvic acid in the brain, Ochoa (4) found that for each 

 molecule of oxygen consumed four energy-rich phosphate bonds 

 were generated. With carbohydrate oxidation in heart muscle, ac- 

 cording to Belitzer and Tzibakova (5) as many as seven energy-rich 

 phosphate bonds (for nomenclature cf. ref. 3) might be formed per 

 molecule of oxygen. One energy-rich phosphate bond represents 

 from 10 to 12 kg.-cal. of utilizable energy. The six moles of oxy- 

 gen oxidizing one mole of carbohydrate could therefore generate 

 from 11 X (4 to 7) X 6 = 260 to 460 kg.-cal. of utilizable energy, 

 or 40 to 68 per cent of the theoretical. 



The unexpectedly high yield obtained in these recent experiments 

 shows that there is probably no great diflFerence in utilizability be- 

 tween fermentation and respiration. Therefore it seems permissible 



