92 



THE RISE OF ANIMAL LIFE 



Fig. 5-4. The carbon cycle. 



which — the carbon and nitrogen cycles — 



will be discussed briefly. 



Carbon, being the core element of proto- 

 plasm, is conspicuously present in all living 

 things and, like all elements, follows a cyclic 

 pattern ( Fig. 5-4 ) . Plants utilize the carbon 

 in carbon dioxide to manufacture fats, car- 

 bohydrates, and proteins, as well as many 



other essential food products. These foods 

 are eaten, digested, and absorbed by ani- 

 mals, and the carbon becomes a part of 

 the body of the animal. During destructive 

 metabolism carbohydrates are burned, re- 

 leasing carbon dioxide into the air again. 

 Similarly, carbon dioxide is released at 

 night by plants as they oxidize carbohy- 



niTroqen in th^ air 



,-MOj used by 

 plant to make 

 protein 



nitroqcD 



fixinq 

 boCTeria 



Fig. 5-5. The nitrogen cycle. 



