Energy Relations in Carbon Assimilation. 131 

 Chapter VI. 



Energy Relations in Carbon Assimilation. 

 A. General Remarks. 



In the introductory chapter we have referred to the fundamental 

 1/ fact that radiant energy is utilised in carbon assimilation so that 

 compounds of higher energy content are produced from tiie simpler 

 ones of the surroundings. Beyond this the energy relations of the 

 green leaf are only very imperfectly known, although physical and 

 chemical methods for investigating such enei'gy relations have 

 reached a high degree of development. This aspect of carbon 

 assimilation exhibits perhaps more than any other an unfortunate 

 isolation of effort in research, the various workers on the subject 

 having generally neglected the results obtained by others, both 

 along their own and related lines of investigation. 



As in those aspects of the subject we have already dealt with, 

 so here also the complexity of the processes is again evident, and 

 it is diffieult to draw definite conclusions. It is possible to measure 

 quantitatively the radiant energy incident on the leaf, and also to 

 measure the amount transmitted. It is, however, by no means easy 

 to determine in what way the energy absorbed is utilised, because 

 we do not know with any approach to completeness how this 

 energy is expended into chemical or electrical energy or heat. 



It is generally assumed that the increase in the heat of 

 combustion of the leaf represents that part of the absorbed energy 

 transformed into chemical energy, but it should be pointed out that 

 in so doing, carbon assimilation is taken in its widest possible sense 

 (not carbohydrate assimilation merely) so as to include all the 

 substances formed in the leaf as a result of the photochemical and 

 possible contemporary processes. Thus it is by no means settled 

 as to what extent proteins are formed in the green leaf by photo- 

 chemical or other chemical actions. In any case an error is 

 introduced if proteins are formed, as their products of combustion 

 cannot be identical with the substances in the leaf from which they 

 are produced, so that their heat of combustion cannot have the same 

 value as the radiant energy used in the leaf in their formation. 



We shall first discuss the methods for estimating the amount 

 of material produced in assimilation and the conclusions which 

 have been drawn as to the energy relations of the green leaf from 

 such determinations, before passing on to a discussion of work in 

 which quantitative measurements of both radiant energy and heats 



