427 



Birgit Vennesland 



reaction. But it is impossible to inhibit the H^O^ formation by absorbing 

 O^ out of the gas phase In contact with the illuminated solution. The O^ Is 

 reduced to H^O_ before It gets into the gas phase. 



I refer you to the paper of Warburg and Krippahl for further details about 

 this interesting reaction. What I want to emphasize here is the fact that this 

 particular system provides a convincing demonstration that oxygen can be 

 formed and reutilized in the liquid phase without entering the gas phase 

 which is supposedly "in equilibrium" with the liquid. 



CO2 EFFECT ON H2O2 FORMATION 



The formation of large amounts of H^O^ by Illuminated Chlorella poi- 

 soned with HCN was first reported from Warburg's Institute in 1925 by Tana- 

 ka. The emphasis In this early paper was on the fact that H-O- is not 

 formed during dark respiration. Now we are toying with the idea that the 



light-Induced respiration — the back reaction of photosynthesis Is at least 



In part a peroxide respiration. 



H 0„ formation isn't limited to intact Chlorella. I have been able to 

 demonstrate H_0„ formation also by cyanide-poisoned Intact leaves. Light 

 is required, just as for Chlorella. With leaves, the first and most obvious 

 effect of light In the presence of HCN is a bleaching of the green pigment. 

 The leaf turns white in red light of an intensity quite Insufficient to do 

 damage in the absence of HCN. As the bleaching occurs, a small amount 

 of H^O^ (0.1 to 0.5 tjmole per 100 mg fresh weight spinach) appears in the 

 water in which the leaf is suspended. One is inclined to regard the bleach- 

 ing as the consequence of the Internally generated H-O^. The peroxide 

 that escapes into the medium is only the fraction that hasn't acted chemi- 

 cally to cause bleaching. 



Hydrogen peroxide formation by isolated chloroplasts, first reported by 

 Mehler, has been studied extensively and shown to be the consequence of 

 the reoxidation by 0„ of an autoxidizable Hill reagent. There are a va- 

 riety of naturally occurring substances which may function in this way, and 

 the specific identity of the responsible reagent has to be determined In each 

 case. 



The formation of H^O„ by intact Chlorella (measured in the presence of 

 0.01 M HCN) has been shown to be enhanced by CO (^^). H2O2 formation 



