THE CHEMISTRY OF PHOTOSYNTHESIS 



123 



water with ^'^O and normal ClOo, the Oo produced contains '-O. By con- 

 trast, when normal water and CIO2 labeled with ^^O are available to a plant 

 normal O2 should be produced. Thus, 



lighi 

 CO2 + 2 Ho'^O — '- — ^ (CH2O) + H2O + 



^O, 



C"«Oo + 2 H2O 



light 



■^ (CHsi^O) + Ho 1^0 + 0.> 



However, the experimental results show that neither reaction proceeds in 

 this theoretical way. Neither 100%i''O2 nor 100% Oo has ever been found. 

 A comparison of the work of various investigators revealed marked discrep- 

 ancies. It may be said that 15 to 40% of the O2 produced seems to originate 

 from GO2. This result is not surprising, if one bears in mind that it is not 

 known whether CO2 or H2CO3 is the reaction partner in photosynthesis. It 

 is very probable that H2CO3 is intermediately formed so that the following 

 equilibria have to be taken into consideration 



H.i^O + CO2 ^ H.>C'sO:i ^ H,0 + C'^Oo 



According" to Warburg (60, 61, 67), Brown and Frenkel (17) and to van Niel 

 himself (57), the experiments with '''O do not prove that the Oo produced 

 in photosynthesis originates from H2O. Similarly, they do not prove the 

 contrary view, so that they must be considered to be useless. 



LIGHT 



^r 



DARK 



~^ 



AHjO- 



4E CO2 



■4H >4E'H >(CH50) + H20 + E' 



► 40H 



4H2O + 2A + 4E" 

 BACTERIA 



2H,02 + 4E" 

 i 



2H2O + OJ 



GREEN PLANTS 



Fig. 47. Van NieFs scheme. 



In Figure 47 van Niel's views are depicted schematically. The symbols 

 E' and E" represent unknown factors which prevent the recombination of 

 H and OH produced by the photolysis of water. The factor E' combines 

 with H and transfers it to COo which is reduced to (CH2O). This is the 

 process occurring in green plants and in photosynthetic bacteria. The fac- 

 tor E" combines with OH and transfers it to the hydrogen donor H2A in 

 bacterial photosynthesis. In green plants OH is transferred to special 

 mechanisms which convert it to H2O and Oo. However, the intermediate for- 



