414 



LIGHT AND LIFE 



PMS 



Relative 



Difference 



of Absorption 



Wave Length (m/i) 



Fig. 12. Changes of absorption induced in a suspension of spinach chloroplasis 

 by addition of either phenazine niethosulfate or potassium ferricyanide. The chemi- 

 cally induced differences at 700 m^ are of the same order of magnitude as the light- 

 induced changes shown in other figures. 



example, was ineffective. In a few preliminary measurements of this 

 type (using Anacystis preparations and ferrocyanide instead of PMS) , 

 it appeared that in mixtures of ferri/ferro-cyanide the 700 mfx. peak 

 is half maximal at a ratio Fe+ + + /Fe++ --3. Such data in- 

 dicate "P 700" to be a redox system with an E'„ value around +0.46 

 volt, the colored state being the reduced one. These latter results 

 are quite tentative, however. If confirmed, they might elucidate that 

 the fast oxidation of cytochrome / (£'„+ 0.36 volt) by light (Fig. 4) 

 is mediated by the light-induced oxidized form of "P 700." 



Conclusions 



The phenomena discussed in the last three sections can be satis- 

 factorily combined so as to expand the scheme of Fig. 7 into that 

 of Fig. 13, a working hypothesis for pigment interaction in photo- 

 synthesis. 



Each of the two light steps supjjosedly drives an oxidorcduction re- 

 action. In the step sensitized by chlorophyll a, the natural photo- 

 synthetic oxidant X becomes reduced and "P 700" is oxidized. By 

 means of the second, and more complex, light-induced step, "P 700" 

 is regenerated and oxygen evolved. Several types of evidence indicate 



