90 



GENERAL BIOLOGY 



transformation into gases and ash liberates energy for its 

 work. It is unlike the engine in that, far frombeing a mere 

 contrivance of chambers in which transformations and reac- 

 tions may occur, it is itself changed constantly, formed and 

 reformed, regularly gathered from and returned to the 

 stream. 



The dissimilation process (katabolism), whereby the 

 complex organic compounds are broken up into simpler 

 ones, with the liberation of their energy for use, has not 

 hitherto been traced step by step in detail: indeed, it is 

 even less understood than the assimilative. Its results are 

 well enough known: the end products are simple com- 

 pounds, CO 2, H^O, and nitrogen compounds not wholly 

 reduced to the grade of composition they had when first 

 taken up from the water (and therefore, a little energy that 

 they still retain is lost to the body) . Their energy has re- 

 appeared in various forms, mechanical movement, bodily 

 heat, luminescence, etc. 



From the chemical side it therefore appears that assimila- 

 tion (anabolism) is the process of separating chemical 

 affinities and of storing up chemical energy in complex 

 compounds, and that dissimilation (katabolism) is the pro- 

 cess of reuniting affinities in stable compounds with the 

 liberation of energy for use. 



Plant and animal differ typically in the nature of their 

 intake and output of matter and energy, and the main 

 features of that difference are expressed graphically in the 

 diagram at the top of the following page. 



In this table the facts are of necessity stated broadly. 

 For example oxygen is given off by the green plant only 

 in the light, and among animal foods organic and inor- 

 ganic materials are set down together. The latter consti- 

 tutes a very small part of animal food, never- the-less the 

 diagram should aid in forming a definite conception of the 

 fundamental nutritive relations of plants and animals. 



