Il8 PHOTOSYNTHESIS 



quinone. Thus in the presence of quinone Chlorella cells 

 resemble the non-living chloroplast preparations. In a dis- 

 cussion of the factors which affect the chloroplast reaction 

 we shall therefore include observations on whole cells to 

 which quinone has been added. 



(a) Inhibitors of oxygen production 



The chloroplast reaction is strongly inhibited by urethanes 

 and by o-phenanthroline in about the same concentration 

 range as is photosynthesis in the living cell. Azide and 

 hydroxylamine also inhibit the chloroplast reaction but there 

 is little agreement between diiferent investigators as to the 

 effective concentration range; this is partly due to the fact 

 that inhibition with these poisons is progressive. Cyanide 

 has no effect on the chloroplast reaction whereas, as we have 

 seen, it is a strong inhibitor of photosynthesis, probably 

 affecting the carboxylation reaction. Thus these results are 

 in agreement with the conclusion that the mechanism of 

 oxygen production in the chloroplast reaction is essentially 

 the same as that in photosynthesis. 



(b) Light intensity 



The chloroplast reaction shows saturation within the same 

 order of intensity as photosynthesis, suggesting that pos- 

 sibly there is a dark process in the chloroplast reaction which 

 becomes limiting at about the same intensity as in photo- 

 synthesis when the carbon dioxide concentration is high. 

 In the few experiments which have been reported there is 

 not an exact correspondence between the light curves of 

 photosynthesis and the chloroplast reaction. If the curves 

 were identical it would indicate that the quantum efficiency 

 of the two processes was the same, but up to the present, 

 slightly lower values have been obtained for the efficiency 

 of the chloroplast reaction. With Chlorella cells for low light 

 intensities of the same energy flux the oxygen production in 

 photosynthesis in alkaline media and in reduction of quinone 

 are of the same order. 



(c) Intermittent illumination 



The effect of intermittent illumination was investigated 



