BENTLEY GLASS 863 



"Following ihc al)sc)ipii()n ol a (juaiiliiin in chlorophyll, il will mi- 

 gnite by resonance transler to a suitable site near the quinone, at 

 which jjoint electron transfer to the (|ninone will take place. The 

 resulting vacancy, or positive chlorophyll ion, can then migrate by 

 hole cliiriision, that is, electron transfer from normal chlorophyll, into 

 the vacant orbital of the neighboring chlorophyll positive ion. This 

 process is the one in the entire sequence which most nearly resembles 

 the properties of a semiconductor and permits the oxidizing point 

 (the chlorophyll positive ion) to separate from the reducing jjoint 

 (the electrons in the quinone orbitals) by a process which is very 

 nearly temperature-independent. The oxidizing point will make it- 

 self apparent as a chemical change, finally, wiien it captures an elec- 

 tron from a suitable reducing agent, in this case shown as a ferrocyto- 

 chrome, thus producing a ferricytochrome and regenerating chloro- 

 phyll." 



The virtue of this semiconductor model of the chloroplast lies, of 

 course, in the degree to which the proposed electron transfers can 

 lead to a rapid and relatively temperature-independent separation of 

 the electrons from the positive holes, and consequently a slowing 

 down of the back reaction. In this respect, the solid state lattice is 

 greatly superior to the solution. 



Barry Commoner, in his paper, takes issue with Calvin in respect 

 to the interpretation of the ESR signals. These may come from or- 

 ganic free radicals or semiconductors, but signals from triplets are 

 hard to detect. That ESR signals from systems at very low tempera- 

 ture exclude the occurrence of ordinary chemical reactions and favor 

 a semiconductor type of behavior in the chloroplast is denied. There 

 is evidence that at the temperatures of liquid nitrogen in glassy solu- 

 tions the absorption of light may eject an unpaired electron, and 

 leave the organic molecule in the form of a free radical so long as 

 the ejected electron remains trapped in the structure (Lewis and 

 Lipkin) . This situation restdts in an ESR signal. 



James Franck, in discussion following Calvin's paper, pointed out 

 a number of reasons for rejecting Calvin's "solar battery" hypothesis, 

 or at least for maintaining reservations in respect to it: (1) the quan- 

 tum yield of photoionization does not seem high enough to account 

 for the known high yield of photosynthesis; (2) the photoionization 

 hypothesis fails to explain the characteristic relationship between the 

 fluorescence intensity of chlorophyll and the utilization of light 



