THE CHEMISTRY OF PHOTOSYNTHESIS 127 



these circumstances O2 respiration + "iron respiration'' produce the neces- 

 sary energy for reaction b. 



With isolated chloroplasts which do not respire, the end value of the O2 

 produced is equivalent only to the amount of ferric cyanide, no matter how 

 much CO2 the gas phase may contain. It is not the Hill reagent but the 

 O2 respiration which decides whether CO2 disappears or not in the light re- 

 action, i.e., whether sufficient energy is available for converting GO2 to CO2* 

 in reaction b. 



Of primary importance for the understanding of the Hill reactions is res- 

 piration — not normal respiration, but induced respiration. The Hill re- 

 agents do not replace CO2, but they do replace molecular O2. The Hill re- 

 actions must therefore be considered to be Oo-free respiration processes; this 

 type of respiration, like O2 respiration, produces the necessary energy for the 

 splitting of CO2 in the light (reactions b and c). These considerations lead 

 to the firm conclusion that the hypothesis of water photolysis has to be 

 abandoned. The explanation of the Hill reactions must be found in respi- 

 ration processes which are directly and inseparably connected with the pho- 

 tolysis of CO2. In photosynthesis and in the Hill reactions the light reaction is 

 the splitting of CO2. In photosynthesis CO2 decreases in the over-all re- 

 action, whereas in the Hill reactions CO2 remains constant and the Hill re- 

 agents decrease. 



In Section D Warburg's conclusive experiments dealing with the action of 

 quinone on living Chlorella, isolated chloroplasts and lyophilized cells will 

 be further discussed. These experiments provide definite evidence that the 

 hypothesis of water photolysis can no longer be maintained (63). This is 

 embarrassing insofar as many important contributions to the chemistry of 

 photosynthesis are based upon this hypothesis. 



Phenylurethane and ethylurethane inhibit true photosynthesis (65) as well 

 as the Hill reactions (27). This was considered by Hill and Scarisbrick 

 (26, 27) to be a very valuable argument in favor of the mechanism of O2 pro- 

 duction in vitro being identical with that in true photosynthesis. According 

 to Hill (24), the chloroplast reaction is not influenced by HCN. 



Phenanthroline inhibits the Hill reactions by about 60% (62). The addi- 

 tion of Zn ions, which are bound by phenanthroline, cancels this inhibitory 

 action (65). Schwartz (52) arrived at similar results with lyophilized cells. 

 According to Arnon and Whatley (9), other metal ions exert this eflfect. 

 Thus, phenanthroline (0.002 molar solution) specifically and reversibly in- 

 hibits photosynthesis, O2 production of isolated chloroplasts with quinone, the 

 ferric cyanide reaction with living Chlorella and the Oo capacity. Respiration 

 is not inhibited by phenanthroline (see Table 18). 



§ 50 Phosphorylations 



Ochoa and Vishniac (43, 44) found that, under appropriate conditions, 

 illuminated isolated chloroplasts can reduce DPN+ and TPN+. These re- 



