REACTION CENTERS IN PHOTOSYNTHESIS 391 



The transfer of energy to a reaction center could occur by migra- 

 tion of electric chargeorof excitation energy quanta. The first of these 

 possibilities corresponds to the view that the photosynthetic unit func- 

 tions as a semiconducting organic crystal, with light absorption leading 

 directly to local photoionization. There is abundant evidence (26, 39- 

 42) that photosynthetic tissues do behave like organic semiconductors. 

 They exhibit such properties as semi- conductivity, photoconductivity, 

 light-induced electric polarizability, thermo luminescence, and delayed 

 luminescence. These effects are indicative of photoionization accom- 

 panied by the trapping and untrapping of electrons. The quantum effi- 

 ciencies associated with these phenomena are extremely low or have 

 not been measured, whereas the transfer of excitation energy in 

 photosynthetic tissues (e.g., from accessory pigments to Chi) occurs 

 with efficiencies approaching lOO^c (24). Accordingly, it is generally 

 accepted that energy transfer in photosynthetic units proceeds through 

 the migration of energy quanta, and not of charge. 



Assuming that energy migrates in photosynthetic units as singlet 

 excitation quanta, one may ask whether the migration occurs by a slow 

 ("resonance transfer") or a fast ("delocalized exciton") transfer 

 mechanism. 8 "Slow" and "fast" correspond roughly to <10l2 and>10l3 

 transfers per sec, respectively. Slow transfer should be temperature- 

 dependent to the extent that temperature governs the amount of overlap 

 between the absorption and fluorescence bands of the participating 

 molecules. Fast transfer should be independent of temperature. 



Slow transfer is favored, in chloroplasts and algae, by theoretical 

 criteria involving the absorption spectra of Chi a in vivo and in vitro 

 (45). However, a slow transfer mechanism should barely allow a photo- 

 synthetic unit of 400 Chi molecules to function. During its lifetime 

 of 1,6 X 10"^ sec (46), a quantum of singlet excitation in Chi a will 

 visit at most about 1600 molecules, in a "random walk," by the slow 

 transfer mechanism. This difficulty suggests that the theoretical 

 criteria ought to be relaxed to allow a faster rate of transfer. 



In purple bacteria the photosynthetic unit is smaller and the fore- 

 going restrictions do not become serious. Furthermore, there is evi- 

 dence that a fast transfer mechanism prevails in chromatophores. The 

 efficiency of BChl-absorbed light in mediating the bleaching of P870 

 is nearly as great at 1°K as at 300°K (28). Thus the efficiency of en- 

 ergy transfer in the BChl system is virtually independent of temper- 

 ature; this result is probably incompatible with slow or resonance 

 transfer. It remains to be seen whether these extremes of temperature 

 have a significant effect on the amount of overlap between the absorp- 

 tion and fluorescence bands of BChl in vivo. 



A quantum of energy migrating in a photosynthetic unit will not be 

 available to a photochemical reaction center unless the center can 



8 Discussions of these mechanisms can be found in references 43 and 44. 



