156 PHYSIOLOGY 



(3) DECABBOXYLATION 



Initial Heat of 



substance combustion 



Alanine .... 389 

 Leucine 855 



Final Heat of 



substance combustion 



Ethylamine . . 409 



Isoamylamine . . 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 oxida- 

 tion is at once apparent. Thus in the 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. Of these, 

 all which involve a splitting of a large molecule into smaller ones with the 

 taking up of one or more molecules of water, as well as, in all probability, 

 those in which the reverse change of dehydration and synthesis occur, are 

 effected in the body by meajis of ferments. To the same agency are also 

 ascribed the process of deamination which takes place in many organs of 

 the body and, though with less certainty, the processes which involve 

 decarboxylation. 



FERMENTS 



Under the name ferments we include a number of substances of indefinite 

 composition whose existence is chiefly known to us by their action on other 

 substances. A ferment has been defined as a body which on addition to a 

 chemical system is able to effect changes in this system without supplying 

 any energy to the reaction, without being used up, and without taking 

 any part in the formation of the end products. It differs therefore from 

 the reacting substances in the absence of any strict quantitative relation- 

 ships between it and the substances included in the system in which its 

 effects are produced. Minimal quantities of ferment can induce chemical 

 changes involving almost indefinite quantities of other substances, the only 

 result of increasing the quantity of ferment being to quicken the rate of 

 the change. Since they are effective in minimal doses they occur in living 

 tissues in minute quantities, and it is partly due to this fact that it has 



