THE ANIMAL AND ITS ENVIRONMENT 



93 



drates to obtain energy. It must be pointed 

 out, however, that plants also produce 

 carbon dioxide during the daytime, but 

 because it is utilized immediately in the 

 process of photosynthesis, its release is ob- 

 scured. The burning of organic matter and 

 decaying of dead plants and animals also 

 release carbon dioxide into the air. Such is 

 the extent of the carbon cycle. 



I I I 

 ENERGY 



i I 

 SUNLIOH 



The nitrogen cycle (Fig. 5-5) is some- 

 what more complicated than the carbon 

 cycle, primarily because plants cannot uti- 

 lize atmospheric nitrogen. Nitrogen in the 

 air must be converted first to nitrites 

 (NO:-) and then to nitrates (NO3) before 

 the plants can make use of it in producing 

 proteins. This conversion is brought about 

 by N-fixing bacteria in the nodules which 



I J 



cjlucosa 

 COi+ HiO-^C6Hii06 



GLUCOSE H^O. STARCH 



PROTEINS 





,;P" 



:i-«! 



,>#.^-» 

 ,„„.»»'"" 



= ,,/////^^ 



PATS 



i^l MI III ' ^^^^ JJJJJJ J^^p ^ 



PATS ^- H^O fo+fy ocids-^r^Mf ATS 

 storogg I Q„j gycarol 



glycogen :: g*O^GLUcoS£ .^»<>starch 

 "^^ / itiltrogcn 



ominoocids .^t!!iP proteins 



growth dnd 

 rgpoir 



Fig. 5-6. Solar energy is incorporated into the foods manufactured by plants. This energy is released from the foods 



by animals for their own use. 



