PHOTOSYNTHESIS 



permanent structural excess of electrons (?2 volume), permitting the 

 conduction of positive charges. The other layer would contain a 

 permanent structural deficiency of electrons {p volume), permit- 

 ting the conduction of electrons. With the creation of mobile 

 charges by light absorption there would follow a flow away from 

 the chlorophyll aggregate and across the semiconducting layers. 

 If the potential between these two layers could be measured, it 

 would be found equivalent to ^^42 kcal./mole minus the 5 

 kcal./mole loss postulated above, or 1.6 v. If the circuit between 

 these two "electrodes" is completed by chemical reactions, as 

 it is in photosynthesis, the potential at each electrode, relative 

 to the ground state of the system, will be simply that required 

 by the oxidants or reductants with which the electrons and 

 positive charges must react. Thus, in terms of our arbitrary 

 "ground," which is the standard hydrogen electrode potential, 

 the 1.6 V. potential across the "battery" will be distributed as 

 1.3 V. positive and 0.3 v. negative, since these are the potentials, 

 relative to our "ground," with which the electrodes must react. 

 From this point of view it appears that the recently developed 

 solar battery (22) may have been preceded by a similar but much 

 more efficient process in photosynthetic organisms ! 



The evolution of oxygen from hydrogen peroxide according 

 to the reaction 



H2O2 = V2 O2 + H2O 



proceeds with a positive potential of 0.37 v. for \0~^ M peroxide, 

 so that another 8.5 kcal./mole of excess energy is expended per 

 electron. There is some question as to whether this reaction 

 could be catalyzed by catalase (55), owing to the apparent lack 

 of inhibition of the Hill reaction by cyanide. However, this 

 reaction can be catalyzed by a number of simple inorganic 

 compounds, so perhaps this problem is not too serious. More- 

 over, inhibition of oxygen evolution in the Hill reaction by 

 orthophenanthroline has been observed (29), so some iron- 

 containing enzyme may be involved. Finally, the recent work 

 of Chance (21) has demonstrated the formation of catalase- 



57 



