CHAPTER IV 



CARBON ASSIMILATION 



THE fact has already been emphasized that the plant synthesizes all the 

 complex organic substances of which it is built from the simple com- 

 pounds, carbon dioxide, water and inorganic salts. The initial metabolic 

 process and the one from which all others have their starting-point is 

 that of a synthesis of a carbohydrate from carbon dioxide and water. 

 This synthesis can only be carried out in the light, and only in a green 

 plant, i.e. a plant containing chlorophyll. Chlorophyll may almost be con- 

 sidered the chemical substance of primary importance in the organic world, 

 for upon it depends the life of all plants and animals. Animals depend 

 for their existence on certain complex amino-acids, some of which they 

 are unable to synthesize for themselves, and which they derive from 

 plants. Plants in turn are unable to exist except by virtue of the pro- 

 perties of chlorophyll. 



The property of chlorophyll which is so important is the power it 

 possesses of absorbing the radiant energy of the sun's rays and converting 

 it into chemical energy by means of which a carbohydrate is synthesized. 

 This summarizes the whole process, which, however, can scarcely be very 

 simple, and probably consists of several reactions at present undifferen- 

 tiated. If the formula for carbonic acid is compared with that of a simple 

 carbohydrate such as a tetrose, pentose or hexose, the following relation- 

 ship is seen : 



H 2 CO 3 -^(H 2 CO)^ where x=4, 5 or 6, 



that is, in the synthesis of a carbohydrate a reducing reaction must take 

 place. 



Many hypotheses have been formulated as to the nature of these re- 

 actions. The one which has most frequently been advanced suggests 

 that formaldehyde is the first product of the synthesis from carbon 

 dioxide and water which takes place in the green plant ; that the re- 

 action involves reduction with elimination of oxygen : 



H 2 CO 3 =H 2 CO + O 2 , 

 and that this product is later condensed to form a hexose, 



