44 



ELEMENTS OF BIOLOGY 



tassium nitrite, KNOj, by some types of nitrogen-fixing bacteria. 

 The cells of green plants can oxidize this nitrite to the nitrate, 

 KNO3, and then employ this salt in the series of reactions that 

 eventually forms proteins. For the most part, however, other soil 

 bacteria convert the nitrite to the nitrate and thus accomplish an- 

 other step in the process before the salt is absorbed by the green 



ANIMAL METABOLISM 



PLANT OR ANIMAL 

 PROTEIN 



PLANT 

 PROTEIN 



ANIMAL PROTEIN 



UREA 



\ 



GREEN PLANT 

 METABOLISM 



INTERMEDIATE 

 PRODUCTS 



Q 



PLANT AMINO-ACIDS 



BACTERIA METABOLISM 



O 



AMMONIA 



© 



SOIL 

 @ I AIR 



NITRITES 



\y NITRATES 



Fig. 9. — The nitrogen cycle. A scheme to illustrate the sequence in the utiliza- 

 tion of nitrogen to form proteins and its return to the soil. The scheme also 

 illustrates the nature of the changes which occur during the digestion and assimila- 

 tion of protein by animals. 



plant. Still other soil bacteria, termed denitrifying bacteria, may 

 undo this synthesis and rob the soil of its fixed nitrogen by breaking 

 down these molecules and liberating free nitrogen gas. In the 

 scheme showing the cycle of nitrogen the dependence of Man and 

 all other animals on green plants and on the metabolic activity of 

 the obscure nitrogen-fixing bacteria appears very clearly. From the 

 relations that it represents one may also conclude that soil fertility is 



