THE CHEMISTRY OF PHOTOSYNTHESIS 185 



Recently, the half-value pressures of COo in the quinone reactions have 

 been determined by Warburg (23) with the aid of a special manometric 

 arrangement allowing the measurement of very low COo pressures. The 

 values obtained are 7 to 8 mm HoO for Chlorella and only about 0.5 mm HgO 

 lor chloroplasts and grana. These results provide definite proof that CO2 is 

 indispensable for the Hill reactions. The hypothesis of water photolysis, 

 therefore, can no longer be accepted. The following original scheme of the 

 quinone reaction in Chlorella as well as in grana 



2 quinone + 2H2O > 2 hydroquinone + O2 



2 hydroquinone -|- CO 2 > 2 quinone + 2H2O + C 



CO2 ) C + O2 



has to be replaced by 



2 quinone + H4CO4 > 2 hydroquinone + 2O2 + C 



2 hydroquinone + Oo * 2 quinone + 2H2O 



CO2 ) C + O2 



According to Hill, Calvin and others, quinone oxidizes water and hydroquin- 

 one reduces COo in the first scheme. The second scheme shows how, accord- 

 ing to Warburg (23), quinone oxidizes CO2 (orthocarbonic acid: H4CO4) 

 and hydroquinone reduces Oo. 



With respect to COo, the difTerence between photosynthesis and the Hill 

 reactions lies in the fact that in photosynthesis the reduction product of 

 COo is only partially (about two-thirds) reoxidized to COo. However, in the 

 Hill reactions the reduction product of COo is completely reoxidized to CO2, 

 so that CO2 does not appear in the over-all reaction. 



In the early part of this expose a clear-cut distinction was made between 

 the energetics and the chemistry of photosynthesis. We considered them, for 

 the sake of simplicity, to be two distinct problems. However, the reader will 

 have noted that, in reality, there is no such line of demarcation. The more 

 recent findings in chemistry have been successively developed from the earlier 

 work on energetics. Warburg's discovery of the quantum requirement 3 

 led to the one-quantum theory advanced by Burk and Warburg. From this 

 fruitful concept started the work done on the oxygen capacity, the labile car- 

 bon dioxide and their relationships to chlorophyll which, in turn, led to War- 

 burg's equations of photosynthesis and the notion of the photolyte. This fur- 

 ther enabled Warburg to give a new explanation for the mechanism of the 

 Hill reactions. 



Warburg's important fundamental discov^eries are the result of clear-sighted 

 studies which are based not on simple theorizing but on painstaking measure- 

 ments — measurements of a high standard with irreproachable techniques 

 carried out along the lines of scientific thought and carefulness of the classical 

 old style and which can stand up to objective criticism. The many unjusti- 

 fied attacks of a polemical nature mostly hide the incompetence or lack of 

 experimental ability of those making the attacks and will not stop the ad- 



