Chapter 6 

 The Energy Relations in Photosynthesis 



1, The Energy Used in Photosynthesis 



When viewed in the broadest prospect, the most important feature 

 of the phenomenon of photosynthesis centers about the fact that the 

 radiant energy of the sun is transformed into potential energy of the 

 products synthesized. These are largely of the composition of carbo- 

 hydrates, although the plant elaborates, in a more or less indirect man- 

 ner, considerable amounts of fats and proteins as well. The significance 

 of this storing of the sun's energy for life on the earth has already been 

 touched upon, in general terms, in the first chapter. The need of more 

 specific knowledge regarding the manner of transformation of energy 

 in the photosynthetic process has been recognized for a long time. There 

 have been numerous speculations regarding the manner in which the 

 absorbed radiant energy is utilized in bringing about the chemical re- 

 actions constituting the reduction of carbon dioxide and water with the 

 synthesis of carbohydrates. There have, however, been few experimental 

 attacks on the problem. As a consequence our knowledge of the energy 

 relations of photosynthesis is exceedingly meagre. 



Undoubtedly the reason for this lack of more precise knowledge 

 of the energy relations in photosynthesis lies in the difficulties which are 

 encountered in endeavoring to subject the photosynthetic reaction to ex- 

 perimental conditions in which all the factors which influence the reaction 

 are carefully controlled and measured. The complex nature of photo- 

 synthesis has been repeatedly emphasized ; this arises from the fact that 

 there are so many factors which exert an influence on the process. It 

 is but necessary to recall that the factors of light intensity, temperature, 

 carbon dioxide-concentration, chlorophyll-content, water-content and cer- 

 tain yet vaguely recognized enzymatic factors control the rate of photo- 

 synthesis ; these are not independent of each other, but the intensity of 

 each must always be considered. Probably the most serious difficulty of 

 all arises from the fact that a photochemical reaction is involved, and 

 the laws governing the kinetics of photochemical reactions, even in the 

 simplest systems, are as yet but imperfectly understood. 



Our information regarding the energy used in ])hotosynthesis by the 

 leaves of land plants is largely confined to the results obtained from 

 the investigations of Brown and Escombe and of Puriewitsch. The 



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