The Release of Energy 49 



atmosphere ; the roots obtain theirs from the air that is in the 

 soil. Wet soils are unsuited to the growth of many plants, not 

 because of the water present, but because of the lack of a suffi- 

 cient oxygen supply for the roots. Drainage is a valuable agri- 

 cultural practice, not only because it removes excess water, but 

 also because it draws air (oxygen) into the soil. When the 

 farmer breaks the crust on the surface, he is making it possible 

 for more oxygen to reach the roots of his crop. 



The plant and the process of respiration may be compared to 

 a manufacturing establishment and the work that goes on in it. 



The power stations are every Hving cell of root, stem, and leaf. 



The machinery is the protoplasm and enzymes. 



The fuel is foods, especially carbohydrates. 



The process is the combining of food and oxygen. 



The product is energy. 



The waste is carbon dioxide and water. 



The working hours are twenty-four hours a day. 



Respiration and photosynthesis contrasted. Respiration is the 

 reverse of photosynthesis. In photosynthesis, carbon dioxide 

 and water are combined, complex molecules of carbohydrates 

 are formed, and a large number of oxygen atoms are set free 

 in the process. In respiration, the complex carbohydrate mole- 

 cules are broken up, oxygen is again combined with them, 

 and simple molecules of carbon dioxide and water are formed. 

 In photosynthesis, the energy of the sunhght used in building up 

 the carbohydrates is stored in them. In respiration, this energy 

 is released when carbohydrates are oxidized and changed back to 

 the simple substances out of which they were made. 



When we wind up a clock spring, we put energy into the 

 tightened coil. When the springis allowed to uncoil, this energy 

 is released and turns the wheels of the clock. So in photo- 

 synthesis the energy is stored in the carbohydrates, and in the 

 process of respiration this energy is released and used in the life 

 processes of the cell. 



