1298 THE PIGMENT FACTOR CHAP. 32 



resonance stabilization, associated with a limited migration of excitation 

 energy. 



Returning now to the problems in connection with which the concept of 

 the "photosynthetic unit" was first introduced on p. 1280 — namely, the al- 

 ternative between a "preparatory" or a "terminal" process Hmiting the 

 maximum flash yield, we note that although the "unit" was first invented to 

 be able to attribute the flash yield limitation to the number of available re- 

 duction sites (i. e., of substrate molecules available for transformation in a 

 flash), it can be equally well combined with the assumption of a "terminal" 

 limitation — e. g., by the amount of a "stabilizing" enzyme. In this form, 

 the hypothesis does not contradict the observation that the "CO2 acceptor" 

 seems to be present, in photosynthesizing cells, in a concentration of the 

 same order of magnitude as that of chlorophyfl (and not <1% of it). 

 As to the effect of carbon dioxide on chlorophyll fluorescence in vivo, this 

 does not require, if energy migration occurs, the association of each chloro- 

 phyll molecule with a carbon dioxide molecule. (A case in point is pre- 

 sented by Scheibe's hnear poljoners, where, in a micelle containing up to 

 1000 dye molecules, the association of a single molecule with a quencher 

 effectively quenches the fluorescence of the whole micelle.) 



To sum up, the role of resonance energy migration between chlorophyll 

 molecules in the photosynthetic process is still uncertain. Theoretical es- 

 timates show that excitation energy should have a chance to migrate, 

 during an actual life-time of the order of lO-i*^ sec, over a small number 

 (of the order of 10 or 100) of chlorophyll molecules, if the pigment is dis- 

 tributed at random in the grana, and over a larger number (of the order of 

 1000) of molecules, if these are arranged in densely packed monomolecular 

 layers. 



Whether this migration actually takes place, we do not know. To prove 

 beyond doubt that energy migration does occur between chlorophyll mole- 

 cules in vivo, and to determine its reach, would be of great importance for 

 the understanding of the photochemical mechanism of photosynthesis; yet 

 even after this proof one could not be certain that energy migration is the 

 true cause of the kinetic phenomena (such as flash saturation) that had 

 first led to the concept of a "photosynthetic unit." The now most plausi- 

 ble interpretation of these phenomena is in terms of an enzymatic compo- 

 nent ordinarily present in a concentration of between 1/300 and l/2400th 

 of that of chlorophyll. Effective cooperation of this "hmiting enzyme" 

 with all chlorophyll molecules could be achieved in three ways — by migra- 



