THE CHEMISTRY OF PHOTOSYNTHESIS 183 



quinone + H COOH > hydroquinone + CO2 



2 quinone + 2H'20 > 2 hydroquinone + O2 



If we accept the hypothesis of water photolysis, a similar scheme can be out- 

 lined with the intermediate production of a phosphorylated HoOo: 



2 quinone + 2H.O + 2H:iP04 > 2 hydroquinone + 2H3PO5 



2H,P05 > 2H3P04'+ O2 



2 quinone + 2H2O > 2 hydroquinone + O2 



The formation of H4P2O9 instead of H3PO5 may be envisaged : 



2 quinone + H2O + 2H:iP04 > 2 hydroquinone + H4P2O9 



It does not seem very probable that such phosphorylated hydrogen peroxides 

 (peroxyphosphoric acid and peroxypyrophosphoric acid) play a biological 

 role. 



The investigations discussed in this Section are being continued in War- 

 burg's institute. This experimental work is difficult and painstaking but 

 the results already obtained show the prime importance of CO2. The many 

 interesting facts drawn from the experiments with fluoride and quinone have 

 enriched our knowledge of the very complicated process of photosynthesis. 



§ 70 Some Final Remarks 



In this chapter some basically important results of research in the chemis- 

 try of photosynthesis have been discussed. It must be admitted that, with 

 respect to the extreme complexity of the problem, these results are modest. 

 Most of them are far from being certain and are not universally accepted. 

 Generally, the work done consists of tentative approaches and there is a con- 

 siderable amount of disagreement between the investigators working in the 

 field. The main risk in all the schemes worked out in the last years to de- 

 scribe chemical pathways in photosynthesis lies in the fact that the student 

 may forget that most of it is pure "paper" chemistry. It is evident that such 

 schemes may be useful as working hypotheses but more or less definite con- 

 clusions should only be drawn when every intermediate reaction of a reaction 

 scheme is found to accord with experimental facts. It must be emphasized 

 again that at the present time the chemistry of photosynthesis is still in its 

 infancy. 



Three main directions of research can be distinguished to-day. They are 

 inspired by Arnon, Calvin and Warburg. Arnon's concept of light phos- 

 phorylation and Calvin's photosynthetic carbon cycle certainly show new and 

 important ways, though the general theories as advanced by these investi- 

 gators are difficult to accept. All theories based upon the photolysis of water 

 — a hypothesis for which not the slightest evidence can be advanced and which 

 was put forward about 20 years ago to explain the reactions discovered by 



