PROCESSES IN THE PHOTOSENSITIVE COMPLEX 



1025 



This mechanism is represented in scheme 28. lA. The reason for assuming 

 the occurrence of the secondary back reaction (28.20d) will be discussed 

 later. If we assume, as suggested on page 1019, that /vo[A'H20] ^ Av[AC02], 

 the concentration of the oxidized form, [X.Chl.Z], can be neglected in 



cqa 



1 



XChlHZ 



+ A 



(^a) 



(28.20a) 



k' 

 (28.20a') 



ACO2 



L 



HX-ChIZ 



(28.20 b))(/(. 



r 



1 



AHCOz 



X-ChIZ 



(£"b) 



{28.20c) )(*«, 



r 



))$ 



X-Chl-HZ 



(28.20d 



I \ 



A +C02 



+ H2O 



r 



H20(orHR) 



+ A 



A'HaO (or A'HR) 



] 



AHO(orAR) 

 (fc) 



(fo) 



A + {CHgO} 



A' + 02 (orA+R) 



Scheme 28.IA. Photosynthesis according to equations (28.20a-d). 



green plants (though perhaps not in purple bacteria), compared to that of 

 the tautomeric form, [HX . Chi . Z ] ; the latter is then the only one the ac- 

 cumulation of which may affect the photosynthesis of green plants in 

 strong light. 



It may be argued that, if the reaction of Z with water is so much more 

 rapid than that of HX with carbon dioxide, the sequence of the secondary 

 reactions (28.20b and c) should be reversed, resulting in the following 

 mechanism (shown in scheme 28. IB): 



(28.21a and a') 



X- Chi -HZ 



k*I 



± HXChlZ 



k' 



