184 ' NUTRITION AND METABOLISM 



organic compounds of various 13^)68, either to carbohydrates (cellulose, 

 starch, sugars) or to fats, or to protein substances, occasionally to 

 organic acids or other compounds. The plants will €ither 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 com- 



JPeac^ Onanisms 



rroCe/n 



\Ca,r6on<{ioxiak 



CarSoAucirates 

 ^t, fy-otein 



, Fig. 102. — Carbon cycle. 



pletely oxidizea to carbon dioxide. Even the marsh gas which might 

 be liberated in this process will find organisms that oxidize it to 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. Fig. 102 gives a schematic illustration of the carbon cycle; the 

 microbial activity is marked by heavy lines. 



