PRODUCTS OF METABOLISM. 125 



one dependent upon the products of the other; animals, however, could 

 be excluded from the circle without interfering with a continuation of 

 life on earth. 



CARBON CYCLE .Carbon is the main element in organic nature, and 

 so dominant that the term carbon compounds is practically identical with 

 organic compounds. 



Carbon is contained in the carbon dioxide of the air which is usually 

 considered a mineral or inorganic compound. It is absorbed in this 

 condition by the green plants, and is changed by the chlorophyl gran- 



Oranisms 



, Protein 



FIG. 49. Carbon cycle. (Original.) 



ules of the leaves to organic compounds of various types, either to car- 

 bohydrates (cellulose, starch, sugars) or to fats, or to protein substances, 

 occasionally to organic acids or other compounds. The plants will either 

 die and decay, or will be eaten by animals. In the first case, the decay 

 will be caused exclusively by microorganisms; if the plants are eaten, 

 they will be digested; part may be used to build up the animal body or 

 stored as 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 

 completely oxidized to carbon dioxide. Even the marsh-gas which 

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



