METABOLISM 9 



which contain amongst them all of these elements. These must 

 also be eliminated from the body, and this process of elimination 

 of waste products constitutes the function of excretion. This 

 function may be performed in a variety of ways and by a variety 

 of organs. In so far as the carbon dioxide is concerned it is, as 

 we have already seen, an essential part of the function of 

 respiration. Urea, on the other hand, a nitrogenous substance 

 which is perhaps the most characteristic waste product in the 

 higher animals, is eliminated by special excretory organs, such 

 as the kidneys. In the case of the higher plants the waste 

 products are for the most part stored up in the leaves and 

 got rid of when these are shed. 



We have thus seen that the supply of energy to a living 



Living Protoplasm 



Food Material. / \Waste Products. 



FIG. 1. Diagram of Metabolism. 



organism, and therefore also its life, depends upon a series 

 of complex chemical processes which take place within the body. 

 All these processes collectively are spoken of as metabolism, and 

 we may distinguish between two sets of metabolic changes : 

 those which are constructive and lead to the building up of 

 new living protoplasm out of food material, and those which 

 are destructive and lead to the decomposition of the body 

 substance, the liberation of energy and the formation of waste 

 products. The former are termed anabolic and the latter 

 katabolic. 



We may roughly illustrate these elementary conceptions of 

 the chemical processes which take place in the living body by 

 the accompanying diagram, in which a mass of living protoplasm 

 is represented as balanced in a very unstable position on the 

 apex of a triangle. It is constantly undergoing destruction, 



