260 



NUTRITION AND METABOLISM 



reserve substances, largely fat and protein. If the animal dies, a 

 decomposition process will take place, which breaks down the organic 

 compounds to simpler products and finally the carbon will be com- 

 pletely oxidized to carbon dioxide. Even the marsh gas which might 

 be liberated in this process will find organisms that oxidize it to carbon 

 dioxide and water. Every product will find an organism to break it 

 up further until it is completely disorganized and the carbon atoms can 

 start the same circulation anew. Undoubtedly as long as organic 

 life has existed on earth, microorganisms have been present, in order 

 to render the dead organic matter again available for plant and animal 

 life. Fig. 114 gives a schematic illustration of the carbon cycle; the 

 microbial activity is marked by heavy lines. 



Mtrates 



FIG. 115. Nitrogen cycle. 



NITROGEN CYCLE. Nitrogen shows the same continuous change 

 as carbon. Plants take up nitrogen in mineral form usually as nitrates. 

 The plants change this mineral nitrogen to the most complex bodies, 

 proteins, where it is combined with the other elements of organic nature. 

 The plants may be eaten by animals; part of the protein is then digested 

 to urea or hippuric or uric acid, which in turn are readily decomposed 

 by microorganisms to ammonia (Fig. 115). Part of the protein will be 

 stored in the growing animals, and if the animal dies, the body will 

 decay or putrefy, and the nitrogenous compounds of that body will 

 pass through the various stages of decomposition to the final product, 



