PHOTOSYNTHESIS 



end, and a corresponding electron donor at the other end, to 

 drop electrons into the positive holes that have been photo- 

 created. In order to complete the separation of the electron and 

 the positive charge and make use of the electrical energy avail- 

 able, all that is needed is either a semiconductor which will 

 transmit only electrons or positive charges, or else an acceptor 

 molecule which can accept either an electron or a positive charge 

 (i.e., contribute an electron) or both, in such a way as to produce 

 an irreversible change. 



The possibility of a semiconductor is an interesting one in 

 that it provddes a possible function for the lipid constituents. One 

 might consider most of the lipid material as an insulator with 

 occasional conductor molecules to pass electrons through. 

 Such a function might be served by a conjugated molecule like 

 a carotenoid which could accept an electron at one end and give 

 an electron to a suitable acceptor at the other end. At the same 

 time, the positive charges left behind would be reacting with 

 water, probably through the agency of some metalloprotein, to 

 produce oxygen. The electron acceptor, in the case of the Hill 

 reaction, could be the supplied oxidant such as quinone, while 

 in photosynthesis the acceptor could be the primary carrier of 

 reducing power (thioctic acid). 



The conductor function for a carotenoid compound might 

 explain the stimulation of photosynthesis by catalytic amounts 

 of blue light observed by Warburg et al. (61). Warburg has 

 suggested the participation of carotenoid somewhere in the 

 transfer of electrons between the photochemical reaction and 

 the reduction of carbon dioxide as an explanation of the blue- 

 light effect. It should be noted, however, that Stanier (R. 

 Stanier, private communication) has studied mutants of Rhodo- 

 spirillum which contain no carotenoids but which nevertheless 

 are able to carry out reduction of carbon dioxide during photo- 

 synthesis. 



The advantage of the semiconductor arrangement would be 

 the physical separation of the points at which reducing agent 

 and oxidizing agents are formed. Also the nonspecificity of 



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