PRINCIPLES OF A THEORY OF ENERGY UTILIZATION IN PHOTOSYOTHESIS 

 James Franck and J. L. Rosenberg 



The present paper is an abstract insofar as it leaves out 

 many details for the sake of brevity. An extended discussion 

 will be published elsewhere. A short introduction which will be 

 neither historical nor comprehensive in literatiore citations, 

 will be presented first. 



Our theory developed slowly from early attempts to find a 

 general point of view for the meagre evidence available at that 

 time. They were mostly based on van Niel's principle of water 

 splitting, certain kinetic phenomena, and fluorescence observa- 

 tions . Along with proposals for which a sound experimental basis 

 existed were a number of guesses which, londer the influence of 

 subsequent experimentation, became obsolete. One of the deduc- 

 tions which we regarded, and still regard, as well founded is 

 that in green plants the excitation energy collected by the bulk 

 of the plant pigments is transferred always to one center of 

 photochemical activity, namely a chlorophyll a molecule in a 

 special position which enables it to use the energy for the 

 photochemical reactions of photosynthesis. Although this state- 

 ment referred originally only to photosynthetic ixnits in green 

 plants, we extended it later to those in all plant cells. The 

 energy- collecting part of the non-photochemically active regions 

 of the unit contains the "protected" pigments, so-called because 

 of the lack of contact between these dye molecules with water and 

 its solutes. Their natxore of the pigments may vary in different 

 classes of plants or even from unit to unit within a given plant 

 cell. In green plants, where all units contain chlorophyll a 

 chlorophyll b, and p-carotene, the energy transfer to the lowest 

 available excitation level is so quick, in accordance with well- 

 known principles applied to dense and partly ordered systems, 

 that the energy absorbed by all dyes reaches the exposed site as 

 chlorophyll a excitation. The excitation arriving at the reac- 

 tion center is usually that of the first excited singlet state, 

 with the consequence that fluorescence visible during photo- 

 synthesis at non-excessive light intensities is predominantly 

 the red fluorescence of chlorophyll a. 



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