390 LIGHT AND LIFE 



connection with this {liscussion, will have in it the last third of the talk 

 which this audience had no opportunity to hear. Perhaps the readers will 

 thus be better equipped to judge the importance and the value of the notion 

 that a charge migration through a pi-system of electrons involving several 

 if not many molecules is the essential feature required for the separation 

 of the high-energy products of the primary quantum conversion act in 

 photosynthesis. In this sense, and in this sense only, do we invoke some 

 of the notions of solid state electronic (hole) conductivity, a phenomenon 

 which is now well estal)lished for the type of organic molecules available in 

 the chloroplasts. 



Dr. Rabinowitch: Dr. Calvin is right when he objects to the treatment 

 of photosynthesis as a homogeneous reaction; the structure is important as 

 he and many others have pointed out for a long time. (I have called photo- 

 synthesis a "topochemical reaction" since 1940.) I think that one should be 

 careful in going to the other extreme and saying that since photosynthesis is 

 not a reaction in a homogeneous solution, it must be a reaction in a solid- 

 meaning a crystalline solid, because the only really solid systems are crystals. 

 One can, of course, interpret any intermediate situation by starting from 

 the one or from the other end; and it is useful to explore how far one can 

 get in a case like that of photosynthesis, by starting from the phenomena in 

 the crystalline state. However, I think that if one looks at what we know 

 about the real structure of chloroplasts and the distribution of chlorophyll 

 in it, one is inclined to see it as rather far from a true crystal. The mono- 

 molecular layers of chlorophyll supposed to exist in the chloroplasts, are- 

 as witnessed by their absorption spectra— not two-dimensional crystals. In 

 two-dimensional crystalline chlorophyll monolayers, the absorption band is 

 shifted to 740 ni/x as against 675-680 m/x in the chloroplasts. (It lies at 

 670 m/t in non-crystalline colloids.) If a solid state were realized, and 

 excitation were to throw electrons out of a molecule cither into the crystal- 

 line interspace, or outside, where they can join some electron acceptor, the 

 absorption spectrum of chlorophyll in vivo would differ from that of chloro- 

 phyll in molecular dispersion much more strongly than it actually does. 

 The band of chlorophyll in the living state is rather similar to the absorption 

 band of chlorophyll in molecular solution. The arrangement of chlorophyll 

 molecules in the chloroplasts is dense enough to permit migration of energy 

 because this migration requires closeness only on the scale of the wavelength 

 of visible light-which is large compared to the scale of molecular diameters. 

 It is not close enough, I believe, to permit free movement of electrons 

 because this requires overlapping of eigenfunctions. Howe\er interesting 

 it may be to explore the solid state picture I think one has no reason to 

 be too optimistic about explaining with its iulp what is going on in the 

 chloroplasts. 



Dr. Kasha: May I make one comment between these three comments of 

 Calvin and Rabinowitch. .Mthough it is true that there may Ix; electron 

 transport between the pigments, electronic overlap would be required. On 



