126 



NUTRITION AND METABOLISM. 



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. Figure 49 gives a schematic illustration of the carbon cycle; 

 the microbial activity is marked by heavy lines. 



NITROGEN CYCLE. Nitrogen shows the same continuous change as 

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



Dead 

 Organisms 



//2t rates 



Pro tein 



FIG. 50. Nitrogen cycle. 



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. 50). 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, am- 

 monia. Ammonia is then oxidized to nitrites and nitrates, when the nit- 

 rogen cycle is completed. 



There is, however, one discrepancy in this cycle. It has been men- 

 tioned already that some organisms are able to reduce nitrates to nitrogen 

 gas. This is one of the "leaks" in the rotation of elements which would 



