CHAPTER IV. 



RESPIRATION AND OTHER FORMS OF METABOLISM. 



33. Nature of Metabolism. By various processes, of which 

 photosynthesis is an important example, a large number of 

 complex substances are formed in the plant. The synthesis of 

 complex compounds from those of simpler composition is 

 termed constructive metabolism. In this process a portion of the 

 oxygen in the simple compounds is liberated. Thus in photo- 

 synthesis water and carbon dioxide, both containing oxygen, 

 are combined and a portion of the oxygen is set free. This 

 liberation of oxygen makes constructive metabolism what is 

 known in chemistry as a reducing process. On the other hand, 

 when the complex foods thus formed are used by the plant, 

 oxygen is taken up and the complex substances are resolved 

 into others of simpler composition. This is known as destruc- 

 tive metabolism. Since oxygen is absorbed in destructive meta- 

 bolism, it is essentially an oxidizing process. One of its most 

 important forms is respiration. 



34. Respiration. In respiration, which is directly opposite 

 in character to photosynthesis, oxygen is absorbed, carbon 

 dioxide given off, and energy liberated in the forms of heat 

 and electricity. The oxygen needed is largely obtained from 

 the air, although in some instances it is derived from other 

 compounds in the plant which contain a large porportion of 

 it. The extraction of oxygen from one substance within the 



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