124 PROBLEMS OF PHOTOSYNTHESIS 



mation of H2O2, though accepted by other investigators, is still very doubtful. 

 According to this scheme, light should decompose H2O to H and OH. 

 Other investigators write water photolysis as follows 



light 

 H2O ^2[H] + [O] 



where [H] and [O] do not represent molecular hydrogen and oxygen, but re- 

 duced and oxidized hypothetical products. 



§ 49 Hill Reactions 



Hill (24) was the first to observe that isolated chloroplasts are able to re- 

 duce compounds other than GO2 upon illumination. He proposed the fol- 

 lowing general reaction equation 



Ught 

 2 A + 2 H.O ^ 2 HoA + O2 



where reducible substances serving as hydrogen acceptors are denoted by 

 A. Thus, the chloroplasts possess a mechanism by which O2 is developed 

 upon illumination and a hydrogen acceptor — but not CO2 — is reduced. 

 Hill replaced the qualitative bacteria method by a quantitative method meas- 

 uring the O2 production spectroscopically by means of the conversion of myo- 

 globin to oxymyoglobin. 



In his first experiments Hill (24, 25, 27) used ferric oxalate as a hydrogen 

 acceptor 



light 

 4 Fe»+ + 2 H2O — > 4 Fe-+ + 4 H+ + O2 



However, the ferrous oxalate produced is autoxidizable and as ferric oxalate 

 in light — without chloroplasts — is easily reduced and CO2 is produced, 

 photochemical experiments with this salt lead to complications which Hill 

 was able to avoid by adding potassium ferricyanide. Warburg and Krip- 

 pahl (62) used potassium ferricyanide in their experiments. This salt is 

 much more stable than ferric oxalate and its reduction product is not autoxi- 

 dizable. The correctness of the iron reduction equation has also been con- 

 firmed by Holt and French (28). 



The Hill reacdon can also be realized with quinone as a hydrogen ac- 

 ceptor. Warburg and Luttgens (59, 64) studied the action of quinone on cell- 

 free leaf extracts, intact and disintegrated chloroplasts and on living Chlorella 



light 

 2 quinone + 2 HoQ *■ 2 hydroquinone + O2 



As no CO2 occurs in the Hill reaction. Hill concluded that the primary photo- 

 chemical process must be the decomposition of water. However, this con- 

 clusion was over-hasty as nothing was definitely known about the mechanism 

 of this type of reaction. 



