CHAPTER II 



THE PROTOTROPHIC ASSIMILATION 

 OF CARBON 



Since the energy transformations occurring in the meta- 

 boUsm of Hving organisms are chiefly brought about by 

 chemical changes in carbon compounds, the assimilation of 

 this element and the absorption of energy cannot be con- 

 sidered apart. Photosynthesis, the characteristic method of 

 carbon assimilation of the algae, occupies a central place in 

 the metabolism of these organisms and an account of the 

 process is an indispensable foundation for the consideration 

 of their other chemical activities. 



The process of photosynthesis may be represented by the 

 following over-all equation: 



C02+2H20->(CH20)+H20+02 . . (i) 



i.e. as an oxidation-reduction in which water acts as the 

 hydrogen donor and carbon dioxide as the hydrogen 

 acceptor. The oxygen evolved in photosynthesis has been 

 shown to come exclusively from the water used and there- 

 fore two water molecules are needed as hydrogen donors for 

 each one of carbon dioxide. The first stable product of the 

 reduction of carbon dioxide may for present purposes be 

 supposed to be a carbohydrate and is accordingly repre- 

 sented as (CHgO) in equation (i). The photosynthetic 

 quotient (Op =A02/-AC02) is then unity. Photosynthesis 

 is endergonic and proceeds with an accumulation of at least 

 112 k-cal. per mole of carbon dioxide reduced. This energy 

 is obtained as light of wavelengths between about 400 and 

 700 TnjLi, absorbed by the photosynthetic pigments. Chloro- 

 phyll acts as the photochemical sensitizer and photosynthesis 

 has never been found to occur in its absence. The minimum 

 number of light quanta required for the reduction of one 

 molecule of carbon dioxide to the reduction level of the 



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