376 



How Liv'mg Things Affect One Another unit vii 



;reen 



P^a/3, 



CO2 



(Carbon dioxide) 



Fig. 327 This diagraiu shows how 

 hydrogeji, oxygen, and carbon are 

 kept m circnlatioji by living things. 

 Note that the carbon and hydrogen 

 occur as compounds, not as ele- 

 7nents. What two processes keep 

 these ele?)ients in circulation? The 

 amounts of oxygen, carbon dioxide, 

 and water on the earth are affected 

 by many other processes, in which 

 living things do no-" play any part. 

 Can you find out what they are? 



0:?CJDATlO>i 



the animal produces carbon dioxide. 

 Thus carbon dioxide is returned to the 

 air which surrounds us. 



Most plants, however, are not eaten by 

 animals, and animals do not oxidize all 

 the carbon compounds that they eat. 

 This carbon does not remain "frozen" in 

 the plants or animals. It, too, gets back 

 into circulation when the organisms die. 

 The organisms then decay. Various fungi 

 and bacteria cause this decay. They oxi- 

 dize the carbon to carbon dioxide. 



Because of the organisms that cause 

 decay, our planet is not piled deep with 

 the dead bodies of plants and animals in 

 which carbon is locked. And because of 

 these bacteria, the same carbon atoms 

 can be used again and again, thus never 

 becoming exhausted. Try Exercise 5. 



Of course, some carbon atoms are 

 taken out of circulation for a fairly long 

 period of time. For example, coal is car- 

 bon that was made from the trunks, 

 leaves, and roots of plants many millions 



of years ago. By burning coal we put 

 this carbon back into circulation. 



The circulation of nitrogen. The cir- 

 culation of nitrogen is somewhat more 

 complicated. Green plants get nitrogen 

 from simple compounds in the soil that 

 contain nitrogen. Most plants can use 

 only nitrates as a source of nitrogen. 

 Others can also use ammonium com- 

 pounds. Both types of compounds con- 

 tain nitrogen. In the plant the nitrogen 

 becomes a part of proteins. If the plant 

 is eaten by an animal, some of the plant 

 protein is changed into animal protein. 

 Some is oxidized and nitrogenous mate- 

 rial is returned to the soil in urea and 

 other wastes eliminated by the animal. 

 These wastes contain nitrogen in a form 

 not usable by plants. Only a small pro- 

 portion, however, of plant proteins are 

 ever eaten by animals; they and the ni- 

 trogen they contain remain locked up in 

 plants, sometimes for a long time. Thus 

 nitrogen is continually being taken from 



