174 PHYSIOLOGY 



expenditure or evolution of energy. This is evident if we examine the 

 heat evolved by the total combustion of one gramme molecule of the 

 initial and final substances in a number of typical reactions. In the 

 following Table these are given for the substances involved in typical 

 instances of the three classes of chemical change that we have just 



been considering 



(1) HYDROLYSIS 



Initial Heat of com- 



bustion per 

 SUbstancc gram molecule 



Maltose . 1350 



Glucose .... 677 

 Hippuric acid . . . 1013 



Heat 



Final of 



substance combustion 



2 Glucose . . . 1354 

 2 Lactic acid 659 



(Glycine . 235 \ 



^Benzoic acid . 773 J 



1008 



(2) DEAMINATION 



Initial Heat of 



substance combustion 



Alanine .... 389-2 



Leucine .... 855 



Aspartic acid . . . 386 



Final Heat of 



substance combustion 



Lactic acid . . 329-5 



Caproic acid . . 837 

 Succinic acid 354 



(3) DECARBOXYLATION 



Initial Heat of 



substance combustion 



Alanine .... 389 

 Leucine 855 



Final Heat of 



substance combustion 



Ethylamine. . . 409 



Amylamine. . . 867 



(4) OXIDATION AND REDUCTION. The fourth class of chemical 

 reactions differs from those just described in being attended 

 with a very considerable energy change. This class involves the 

 processes of oxidation and reduction. In almost every living cell 

 by far the largest amount of the energy available for the discharge 

 of the functions of the cell is derived from the oxidation of the 

 food-stuffs, and even in the plant the energy is obtained from the 

 oxidation of the food-stuffs, built up in the first instance at the 

 cost of the energy of the sun's rays. If we take the final changes 

 in the food-stuffs, the very large evolution of energy attending 

 their oxidation is at once apparent. Thus in the v conversion 

 of glucose into C0 2 and water there is an evolution for each 

 gramme molecule of 677 calories. In the combustion of glycerin 397 

 calories are evolved. In the oxidation of a fat such as trimyristin 

 there are 6650 calories evolved. The change does not, in the living cell, 

 occur all at once, but the molecule is oxidised step by step. In each step 

 the heat change will, however, be probably greater than the heat changes 

 in the other types of chemical change which we have been considering. 



Since the mechanism of oxidation in the animal body will have to 

 be discussed at length in a subsequent part of this work, we may at 

 present confine our attention to the other types of chemical change. 



