BESSEL KOK AND GEOIU'.E HOCH 115 



Light 

 Chlorophyll a and carotenoid 



X + p P + XH 



■^ * *^>-ed ■*— ^ : ox 



Spontaneous 



-— PMS — 



^l 



Chlorophyll a * 



Accessory pigment 



Light 



Fig. 13. Schematic summary of discussed observations. 



that the sensitization of this step, although mainly by accessory pig- 

 ment, is still mediated by a small fraction of the total chlorophyll a. 



As shown in the previous section, ferricyanide may spontaneously 

 carry out the first step, in other words, it can short-circuit the chloro- 

 phyll fl-sensitized light reaction (and the negative absorption change 

 induced by it) . But once all "P 700" is converted (and a correspond- 

 ingly small amount of Fe+++ is reduced to Fe++) , the reaction comes 

 to a halt. If now light of proper wavelength (accessory pigment 

 sensitization) is given, oxygen will be evolved, "P 700" is again re- 

 duced and will in turn reduce another batch of Fe+++, so that a con- 

 tinuous cycle occurs. This concept— or should we say oversimplifica- 

 tion — thus states that the Hill reaction with ferricyanide is only half 

 of the photosynthetic process. 



A similar reasoning can be based upon the second observation: 

 that PMS spontaneously converts P^^ -^ Pred- Obviously this entails a 

 short-circuit of the second (accessory-pigment-sensitized) light reac- 

 tion. Oxygen evolution is bypassed and, wuth all "P 700" in the re- 

 duced state, no positive light-induced absorption change will occur. 

 To drive a cyclic process in which ATP is generated, light is required, 

 but only the oxidative (chlorophyll-fl-sensitized) step suffices. This 

 prediction grossly fits the observed sensitization data of Fig. 10. No 

 guess can be made about the site in the above scheme where coupling 

 with phosphorylation (either in the Fe+++ or PMS system) is most 

 likely to be. An important question remains to be answered. In 



