THE MECHANISM OF PHOTOSYNTHESIS 327 



The Mechanism of Photosynthesis. — Certain topics contributing to an 

 understanding of the chemical kinetics of photosynthesis have already been 

 considered: (i) the structure of the chloroplasts, (2) the general properties 

 of the chloroplast pigments, (3) the first sugar of photosynthesis, and (4) the 

 photosynthetic ratio. Other aspects of this problem will now be discussed 

 under appropriate sub-headings. 



I. The Role of the Chloroplast Pigments. — Although it has been known 

 for a long time that chlorophyll is essential for photosynthesis the mechanism 

 of its operation in the process is still largely a mystery. There have been 

 two general ideas regarding its role: (i) that chlorophyll combines chemically 

 with carbon dioxide or some other compound and takes part in the chemical 

 reactions involved in photosynthesis; and (2) that chlorophyll absorbs certain 

 wave lengths of radiant energy and either converts this energ}' into other 

 wave lengths that are utilized in photosynthesis, or else in some way transfers 

 the energ>^ absorbed directly to the compounds involved in the reaction. 



Determinations of the quantity of chlorophyll present in leaves before and 

 after active photosynthesis show that their chlorophyll content does not 

 diminish during the process. There is not only just as much chlorophyll 

 after active photosynthesis as before, but the proportion of chlorophyll a to 

 chlorophyll b is the same after a period of active photosynthesis as before. 

 These facts make improbable any theory that photosynthesis is associated with 

 the continuous destruction or transformation of chlorophyll. There is, how- 

 ever, some experimental evidence for the view that chlorophyll enters into a 

 temporary chemical union with carbon dioxide or carbonic acid and that it is 

 returned to its original condition when such groups are split off. Other in- 

 vestigators, however, doubt that even a temporary combination occurs between 

 carbon dioxide and chlorophyll during photosynthesis. The fact that carbonic 

 acid does not absorb any of the wave lengths of the visible spectrum has led 

 to the suggestion that chlorophyll may operate by converting visible radiations 

 into wave lengths which are effective in the reduction of carbonic acid. 



The universal presence of the carotinoid pigments in the chloroplasts sug- 

 gests that they may participate in the process of photosynthesis, but their role, 

 if any, is unknown. Chemical analyses of green leaves, made before and 

 after active photosynthesis, do show that both the absolute and the relative 

 amounts of the carotinoid pigments are changed during the process 

 (Table 30). The amount of carotene present decreases and the amount of 

 xanthophylls increases. However, since the conditions prevailing in such 

 experiments favor the oxidation of these pigments, it is doubtful if any con- 

 clusions regarding the role of the carotinoids in photosynthesis can be drawn 

 from them. 



