MECHANISM OF SENSITIZATION IN VIVO 



543 



tion from benzaldehyde was equal to about one-tenth the maximum rate 

 of normal photosynthesis. The "photosynthesis with substitute oxi- 

 dants" is thus about as efficient as the "photosynthesis with substitute 

 reductants" as carried out by hydrogen-adapted algae (c/. Chapter 6). 



C. Mechanism of Sensitization in vivo* 



We have now reviewed all photochemical reactions sensitized by 

 chlorophyll and bacteriochlorophyll in vivo (cf. Table 19.1). If we 



Table 19.1 



Reactions Sensitized by Chlorophyll in vivo 



consider photosynthesis as the "normal" photochemical reaction in 

 green plants, the other processes in the table can be attributed to the 

 replacement of one (or both) of the normal reaction components (CO2 

 and H2O) by "substitute oxidants" or "substitute reductants." 



Table 19.1 does not contain the combination "oxygen as oxidant 

 and water as reductant." Of course, the net chemical result of this 

 "autoxidation of water" would be zero; but it could perhaps be detected 

 by isotopic indicators. It would be interesting to find out whether this 

 reaction actually proceeds under certain conditions (e. g., in C02-starved 

 leaves) . Its yield may perhaps be much larger than that of the photoxi- 

 dation of organic hydrogen donors, which is responsible for the net 

 chemical change (consumption of oxygen). 



1. Hypothesis of a Common Primary Process in All 

 Chlorophyll-Sensitized Reactions in vivo 



The interpretation of photoreduction, photoxidation and nitrate 

 reduction as "photosynthesis with substitute oxidants or reductants" 

 leads to the working hypothesis that the primary photochemical process 

 is the same in all these reactions, and that their different final results are 

 due to secondary transformations of the same primary products. 



* Bibliography, page 560. 



