DANIEL I. ARNON 



541 





Chi :+,»^ 



LIGHT ~P 



H 

 t 



HgO 



I 



OH- 



-Cyt 



ADP 



-COH) >( 



-^(atpj 



Non- cyclic photophosphorylation (chloroplasts) 

 Fig. 19. Scheme for non-cyclic photophosphorylation in chloroplasts. 



Details in the text. 



Loo negative for cytochromes to serve here as electron carriers. This 

 leaves the electron transfer from water (hydroxyl ions) to chlorophyll 

 as the sequence of reactions in which cytochromes can serve as electron 

 carriers. 



The proposed mechanism, which assigns to cytochromes a role in 

 the electron transport from OH- ions (or water) to chlorophyll, 

 implies that the chlorophyll-cytochrome complex must generate a 

 sufficient oxidizing j^otential to drive the reaction 2 HoO — > Oo -f" 

 4H+ + 4e-, which at 25°C and at pH 7, has E', = +0.815 v. 



It might be argued that cytochrome /, the most oxidizing cytochrome 

 now known to occur in chloroplasts, has a redox potential lower than 

 oxygen (42) , i.e., E'„ rr: -|-0.365 v. However, it would be premature 

 to conclude that our knowledge of redox potentials of cytochromes in 

 chloroplasts is now complete. 



The proposed reactions leading to oxygen evolution appear to be 

 thermodynamically feasible. The energy contribution of one einstein 

 of red light, about 43 kcal, is equivalent to a potential of 1.9 v per 

 faraday, and is sufficiently large, after making allowances for TPN 

 reduction and ATP formation, to endow a chlorophyll-linked cyto- 

 chrome with a redox potential more oxidizing than 0.815 v, as is 

 needed for oxygen evolution. 



We wish to reiterate that, in our present state of knowledge, the 

 proposed mechanism for oxygen evolution must remain tentative. 

 The possibility is not excluded that the transfer of electrons from 

 OH~ to cytochromes may require an additional input of energy. 



