114 



Eugene Rabinowitch 



-0.4 

 V 



-0.1 



+ 0.2 



+ 0.5 



+08 



X/XH 



(O.OV)Cyt bg 



hi/1 



hi/2 



Z/HZ 



Cyt f (+Q37V) 

 System 2 



Fig. 2 



Interestingly enough, two other oxidation-reduction 

 systems have been identified recently in chloroplasts, one 

 ( plastoquinone) having a potential close to that of cyto- 

 chrome h^f the other ( plastocyanine ) one close to that of 

 cytochrome f. Perhaps, they function as regulating systems, 

 keeping the Fe3+/Fe''+ ratio in the cytochrome systems stable. 

 Another possible function of these quinones could be to pro- 

 vide transition from the "mono-valent" cytochromes to the 

 "di-valent" redox systems at the beginning and end of the 

 reaction sequence. 



The first oxidation-reduction step, which transfers the 

 hydrogen atom (or electron) from an oxidant, ZIT, with a 

 potential of about +0,8 V, to cytochrome _bg with a potential 

 close to zero, stores about 0,8 eV (or 18,5 kcal, per mole) 

 of the energy of the quantum, (The latter, in the case of 

 chlorophyll a, contains between 35 and 43 kcal, per mole). 

 The photochemically reduced cytochrome t^ then reduces, in a 



