105 



James Franck and J. L. Rosenberg 



in the reduced form. The rate woilLd then he so great that the 

 reversible bleaching becomes non- observable in difference 

 spectra experiments. Since photo-oxidation might attack any 

 exposed Chi at random, efficient statistical recovery requires 

 that practically all the Cyt molecules at the reaction centers 

 be in the reduced form during steady- state illumination. 



We nov face the problem: how can most of the Cyt be in the 

 reduced form? Simple kinetic analysis of our scheme shows that 

 if each photosynthetic unit were functionally independent, half 

 of all the Cyt should be in the reduced form and half oxidized 

 during steady light-limiting illumination. In fact in the intro- 

 duction we implied an independence of the imits. We now qualify 

 this statement by introducing an interaction at the level of the 

 exposed chlorophylls . We postiolate an energy migration of sing- 

 let excitation among the exposed chlorophylls in a larger super- 

 unit of the chloroplast. This proposal does not invalidate our 

 conclusion that the far-red absorbing modification in green 

 plants does not act as a trap for normal chlorophyll a singlet 

 excitation, as will be dlsc\issed in the section on the Emerson 

 effect. With this limited type of super-organization we gain 

 the advantage that the steady- state fraction of Cyt in the 

 oxidized condition is lowered to l/2 n, where n is the n\imber of 

 participating lonits in the exchange. We visualize n to be of 

 the order of magnitude of 100 or more. On this basis we can 

 •understand why good photosynthesis does not change the level of 

 oxidized cytochrome, as shown by difference spectra. 



That our scheme provides an automatic adjustment of equal 

 opportunity for photochemical use of singlet and metastable 

 excitations is obvious. 



Next we must mention several consequences of our scheme for 

 the fluorescence yield of exposed chlorophylls . 



a) Butler has shown that freshly formed chlorophyll in a 

 greening leaf has a fluorescence yield practically as high as 

 that of chlorophyll in organic solvents. W/ Absolute measure- 

 ments show that the fluorescence yield in vitro is about 25^ 

 while that of fully developed chloroplasts, in which only the 

 exposed chlorophyll a is responsible for fluorescence, is G°Jo in 

 the absence of photosynthesis. This difference may be ascribed 

 to the influence of the heavy atom, the iron of the coraplexed 

 Cyt, on the transition to the metastable state. 



