CHAPTER III 

 PHOTOSYNTHESIS 



PHOTOSYNTHESIS is the process whereby chlorophyll-containing 

 plants, in the presence of sunlight, synthetize organic compounds 

 from water and carbon dioxide. The end-product of photosyn- 

 thesis is always a carbohydrate. Chemical compounds belonging 

 to other groups, mentioned in the preceding chapter, are synthe- 

 tized by plants from the carbohydrates and simple raw materials; 

 but in such cases the energy used is not solar energy and the process 

 is not photosynthesis. 



Under the ordinary conditions of temperature, moisture supply, 

 etc., necessary to plant growth, photosynthesis will take place if 

 the three essential factors, chlorophyll, light, and carbon dioxide 

 are available. 



PHYSIOLOGICAL STEPS IN PHOTOSYNTHESIS 



There are five successive and mutually dependent steps in the 

 process of photosynthesis, as follows: 



(1) There must be a gas exchange between the plant tissue 

 and the surrounding air, by means of which the carbon dioxide of 

 the air may reach the protoplasm of the chlorophyll-containing 

 cells. 



(2) Radiant energy must be absorbed, normally that of sun- 

 light, although photosynthesis can be brought about by the 

 energy from certain forms of artificial light. 



(3) Carbon dioxide and water must be decomposed by the 

 energy thus absorbed, and the nascent gases thus produced com- 

 bined into some synthetic organic compound, with a resultant 

 storage of potential energy. 



(4) This first organic synthate must be condensed into some 

 carbohydrate suitable for translocation and storage as reserve 

 food. , 



(5) The oxygen, which is a by-product from the decomposition 



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