26 PHYSIOLOGY OF BACTERIA 



while those with a large energy yield are endo-enzymes. 

 Lactase with twenty-three calories is sometimes soluble; 

 sometimes, it is an endo-enzyme. Miquel's claim that 

 urease is a soluble enzyme will be discussed on p. 36. 



Attention should be called to the fact that the entire 

 amount of liberated energy does not always become 

 available for growth, for thermodynamic reasons. These 

 will be discussed in the chapter on Growth. Ordinarily, 

 in organic fermentations, the difference between avail- 

 able and total liberated energy is small; furthermore, 

 only a small fraction of the available energy is used for 

 growth, even under ideal conditions. 



(c) METHODS OF MEASURING ENERGY YIELDS 



We have various means of measuring the amount of 

 energy liberated in a fermentation. It is possible to com- 

 pute it as the difference of the heats of combustion of the 

 fermented material and of the products. This compu- 

 tation is not very accurate since the error in combustion 

 heat determinations becomes greatly increased if we have 

 to consider the difference of two combustion heats of 

 nearly the same magnitude. The inaccuracy is increased 

 by the necessary corrections for the heats of solutions of 

 products and for gases escaping. A good compilation 

 of combustion heats is given by Kharasch (1929). 



This method may be illustrated by the following example: 

 The combustion heat of glucose is 3,739 calories per gram; if a mold 

 in a very well ventilated culture oxidizes glucose directly to carbon 

 dioxide and water, the total amount of energy Uberated amounts 

 to 677.2 cal. per gram molecule. If oxygen is not available, yeast 

 could change the sugar to alcohol +CO2 according to the equation 



CeHuOe = 2C2H5OH + 2CO2 



There is a considerable amount of potential energy left in the two 

 molecules of alcohol formed, and the yeast can liberate only the 



