y. A. BASSHAM AND M. CALVIN 



The decomposition of water will require by far the greater 

 portion of the energy available from the primary photochemical 

 reaction. From the half-reaction potential of the primary reduc- 

 ing agent, which in our scheme must be about 0.3 v., we can say 

 that the relative energy stored by the transfer of one electron to 

 the reducing agent is about 7 kcal./mole (taking the energy re- 

 quired to transfer an electron to 1 A^ H+ in contact with H2, g, 

 1 atm., as zero). If there is a loss of about 5 kcal. in the transfer 

 of the electron from the chlorophyll aggregate, there is left, from 

 a 42 kcal./mole quantum, about 30 kcal./mole to be stored in 

 each positive charge ("hole"). This would then be a poten- 

 tial of about 1.3 V. The potential required for the half reaction 



2H2O = H2O2 + 2H+ + 2 ^- 



is about 1.2 v. at/?H 7 and 10"^ Af H.2O2. Thus the reaction will 

 go as written, provided the very high activation energy required 

 for the removal of electrons from water-oxygen atoms, which 

 would result in formation of hydroxyl radicals in solution, can 

 be overcome. We may suppose that the hydration of a suitable 

 surface on the granar fragment, perhaps resulting in actual 

 hydrated compounds, results in an orientation of — OH groups 

 which permits the formation of O — O bonds concurrently with 

 the removal of the electrons from the water. 



The formation of the positive and negative potentials dis- 

 cussed above requires some mechanism for obtaining just the 

 right distribution of energy to achieve the necessary oxidation 

 and reduction. This can be accomplished by extending some- 

 what further the proposal for the separation of charges through 

 the agency of semiconductors. We may think of the subgranar 

 unit as a photoelectric battery. The driving force for this battery 

 is the light energy absorbed by the chlorophyll. The absorption 

 of light produces in the aggregate conduction electrons and their 

 corresponding positive "holes." This part of the structure can 

 be considered a conductor after light absorption. On either side 

 of the chlorophyll aggregate is a layer of semiconducting material. 

 This material may be lipid or lipoprotein. One layer contains a 



56 



