THE CHEMIS'IRV OF PHOTOSYNTHESIS 131 



of light quanta, chlorophyll molecules are brought into the excited state and 

 expel electrons which are transported in a cyclic system as follows: 



light 

 2Chl — > 2Chl* + 2e 



oxidized vitamin K + 2e > reduced vitamin K 



reduced vitamin K + 2Fe^+-cvti > oxidized vitamin K + 2Fc''+-cyti 



2Fe2+-cyti + 2Chl* + ADP + ph > 2Fe3+-cyt, + 2Chl + ATP 



light 

 ADP + ph > ATP 



A modification of the vitamin K pathway is the FMN pathway occurring in 

 green plants. In this pathway the electrons also return to chlorophyll in a 

 cyclic system. However, this cycle includes TPN+ and a second cytochrome 

 cytn- Both pathways of cyclic electron transport as well as the non-cyclic 

 pathway for the generation of assimilatory power are shown schematically in 

 Figure 48. 



Arnon's general hypothesis of photosynthesis embracing green plants as 

 well as photosynthetic bacteria does not necessitate water photolysis. In 

 green plants the Oo production results from an "open" non-cyclic electron 

 transport. The electrons expelled from the excited chlorophyll molecules 

 do not return to chlorophyll, as in the cyclic pathways, but are removed by 

 TPN+, together with H ions originating from water in the presence of a 

 light-dependent reductase. 



TPN+ + 2e + H+ > TPNH 



The electrons removed must be continuously replenished. This is made 

 possible by the interaction of the remaining" OH ions from water and a cyto- 

 chrome (cytii) peculiar to the green plant and not found in photosynthetic 

 bacteria. Thus, the role of water is to produce H ions for TPNH formation 

 and OH ions to yield molecular Oo and donate electrons to a cytochrome 

 chain. According to this hypothesis water is still the source of O2 though the 

 process of photolysis is not involved : 



2H2O > O2 + 4H+ + 4e 



In bacterial photosynthesis external hydrogen donors are still needed. 

 However, their function is not the reduction of a precursor of O2 but merely 

 the production of H to reduce phosphopyridine nucleotides (PN), i.e. produce 

 one of the components of assimilatory power. The other component of as- 

 similatory power is ATP which photosynthetic bacteria are able to produce 

 with light energy by cyclic photophosphorylation. Figure 49 shows, ac- 

 cording to Arnon, the reaction schemes of photosynthesis in green plants and 

 photosynthetic bacteria. 



Warburg (59) discovered that CI ions are essential for photosynthesis in 

 green plants. Arnon's experiments confirm this finding, CI ions being re- 

 quired for reaction c as well as for the FMN pathway. They are not re- 



