120 



Eugene Rabinowitch 



fer chain. In this case, too, the possibility of this 

 change being in the nature of oxidation-reduction remains 

 uncertain, Witt considered certain difference bands he ob- 

 served in the short-wave region of the spectrum as indica- 

 tive of reversible oxidation and reduction of chlorophyll, 

 but this interpretation, too, remains hypothetical. 



The old problem of chemical (photocatalytical) versus 

 purely "physical" sensitizing action of chlorophyll in 

 photosynthesis, thus remains an elusive one. 



5. 



Two types of fluorescence measurements need to be inter- 

 preted for the two-layer pigment picture to be convincing. 

 One is the above-mentioned finding, by Franck and coworkers, 

 (as well as by Wassink and others), that the fluorescence 

 yield of photosynthesis doubles (from about 1.5^ to about 

 3^) when photosynthesis becomes light-saturated. 



In our picture, chlorophyll is supposed to fluoresce in 

 the hydrophilic layer, where it sensitizes the oxidation of 

 water, and to be non-fluorescent in the hydrophobic layer, 

 where it sensitizes the reduction of the organic substrate, 

 X. Additional ad_ hoc hypotheses are needed to explain in 

 this picture why the fluorescence yield should double at 

 light saturation of photosynthesis. (Pranck sees in this 

 finding the evidence that of the two photochemical steps, 

 one is brought about by fluorescent, and one by metastable 

 chlorophyll molecules). 



The other finding is due to S. Brody, who found in our 

 laboratory that the lifetime of chlorophyll fluorescence in 

 living cells is slightly less than one half of that in sol- 

 ution — while the steady fluorescence yield is at least 10 

 times smaller (3^ vs. 30^), Brody' s value was subsequently 

 confirmed by more precise measurements of Tomita and Murty, 

 They support the presence j_n vivo of two forms of chloro- 

 phyll, but suggest that the fluorescent form accounts for 

 only 20^ of the total, and the non-fluorescent for 80^, How- 

 ever, Brody' s results were obtained by means of very brief 

 flashes of illumination; the average light intensity was 

 very low, and the cells could be presumed to remain in the 

 dark-adapted state. Lifetime measurements should be repeat- 

 ed with cells illuminated with a background light, suffix 

 ciently strong to maintain steady photosynthesis. 



