Section II 



ANABOLISM 



CHAPTER VI 

 ACCUMULATION OF ENERGY 



A SUPPLY of available energy is necessary for both 

 plants and animals. As we shall see later a few organ- 

 isms can obtain energy by the oxidation of inorganic 

 substances, such as sulphur (see p. 211), but the main source 

 of energy is the radiant energy of the sun. All living cells 

 can synthesise but not all cells can store radiant energy, and 

 in this chapter we shall confine our attention to the storage 

 of radiant energy. 



The process of storing light energy as chemical energy is 

 called photosynthesis. 



Photosynthesis is most marked in cells which contain the 

 green colouring matter chlorophyll, and we shall apply the 

 chemical principles described in the preceding chapters to 

 the process of photosynthesis. 



PHOTOSYNTHESIS 



The fundamental points are as follows : (i) The plant 

 absorbs various inorganic salts and water by its roots. (2) 

 When exposed to light the green leaves absorb carbon dioxide 

 and set free oxygen. (3) A plant grown under proper condi- 

 tions can be shown to increase in weight, and the increase is 

 accompanied by an accumulation of carbon compounds. The 

 carbon is furnished by the carbon dioxide of the atmosphere 

 and a process of reduction occurs with the liberation of 

 oxygen. 



The first stage of the synthesis is the production of carbo- 

 hydrate. 



6CO 2 +6H 2 O *" C 6 H 12 6 +6O 2 



Carbon dioxide. Water. Sugar. Oxygen. 



These points can be proved experimentally, and we shall 

 examine them in this chapter, leaving the fate of the carbo- 

 hydrate to be discussed later. 



