118 



Eugene Uabinowitch 



Figure 3 represents a two-layer photochemical system. 

 V/ithin each layer, resonance transfer must take place both 

 between identical pigment molecules, (leading to energy 

 migration through the layer, at least within the confines 

 of a single photosynthet ic unit), and between pigments ab- 

 sorbing at the shorter wavelengths and those absorbing at 

 the longer wavelengths. The energy quanta absorbed in each 

 unit may be thus conveyed to a single enzymatic site. The 

 chlorophyll molecules immediately associated with this site 



may be different in the two layers; their absorp- 

 tion band may be located at 700-710 m^ in the hydrophobic 

 system 1, and nearer 670 mfi in the hydrophilic system 2. 

 The question arises: How is it possible for the photoxida- 

 tive process in one layer to be effectively coupled with a 

 photoreduct ive process in the opposing layer, preventing the 

 loss of energy by back reactions within the layer? Is the 

 existence of photosynthet ic units in both layers, with their 

 enzymatic centers in juxtaposition, sufficient to insure 

 effective correlation, evidenced by utilization of almost 

 all light quanta absorbed, (at least, in weak light)? 



In the light of Menke's results, one is tempted to equate 

 the photosynthet ic units in the hydrophilic layer with his 

 "crystallites," and consider the possibility of chlorophyll 

 molecules in the hydrophobic layer not being associated in 

 units at all. However, bringing quanta absorbed by such 

 scattered molecules into action at the proper reaction site, 

 would be very difficult. 



The effectiveness of coupling of events in two layers de- 

 pends on the time interval allowed between the two photo- 

 chemical reactions. The observations, by Myers and French, 

 of the Emerson enhancement in flashing light suggest that 

 the intermediates involved may be long-lived enough to per- 

 mit effective coordination of the two processes, even in 

 weak light. (Otherwise, the light curves, showing the rate 

 of photosynthesis as function of light intensity, would be 

 quadratic in weak light and only gradually approach linear- 



ity)! ,. 



Another question is that of the mechanism of function of 

 the sensitizing pigments. Do they themselves serve as re- 

 versible oxidation-reduction catalysts, (or more exactly, 

 "photocatalysts" ) in the electron (or hydrogen) transfer 

 chain? The search for evidence of reversible oxidation- 

 reduction reactions of chlorophyll in the sensitization of 



