THE CHEMISTRY OF PHOTOSYNTHESIS 181 



It should be borne in mind that quinone reactions involve heavy metal 

 catalysis. The forward reaction of O2 is catalyzed by the unknown heavy 

 metal which is inhibited by 0.0001 A' phenanthroline but not by 0.01 A^ HCN. 

 The enzymatic back reaction of Oo is catalyzed by the copper of the phenol- 

 oxidase. It is still doubtful whether a third heavy metal — the iron of the 

 very active catalase of green cells and grana — plays a role in the quinone reac- 

 tions. 



B 



Fig. 70. Manometer vessel for determining the CO., partial pressure 



We come now to the question how COo fixation is related to the experi- 

 mental results described. To study this, the partial pressure of CO2 must be 

 established. For this purpose, Warburg worked out a new type of manom- 

 eter vessel (Fig. 70). The main compartment contains the suspension of 

 grana or cells. The side-arm A connected with the main compartment 

 contains H2SO4. The trough contains KMn04 and the side-arm B connected 

 with the trough K4Fe(CN)e. When H2SO4 is tipped into the main compart- 

 ment CO2 evolves from any bicarbonate present in the suspension. When 

 K4Fe(CN)6 is tipped from the side-arm B to the trough containing KMn04 

 the contents of the trough become strongly alkaline and very rapidly absorb 

 all the CO2 present: 



KMn04 + 3K4Fe(CN)6 + 2H2O -> MnO. + 3K3Fe(CN)6 + 4KOH 



In this way it is possible to determine the CO2 partial pressures at times 

 to and t and to establish the exact change in COo partial pressure in the time 

 t — to. Any decrease in the CO2 partial pressure means CO2 fixation. In 

 spite of many variations in his experiments, Warburg always found that the 

 O2 evolutions in the presence of quinone and HCN were never accompanied 

 by changes in the CO., partial pressure. The explanation of these results lies 

 in the fact that there is no respiration. It was, indeed, found that no CO2 

 is fixed in grana as they do not respire. Similarly, Chlorella cells do not 

 respire in the presence of quinone and no CO2 fixation is ob.served. However, 

 when Chlorella cells are treated with 0.01 A' HCN in the dark, they show respi- 



