864 LIGHT AND LIFE 



energy for photosynthesis; and (3) the hypothesis leaves unexplained 

 the Emerson effect, whereby light of a shorter wavelength, if sup- 

 plied together with the wavelength characteristically absorbed by 

 chlorophyll a, produces a synergistic effect. According to Franck, it 

 is more probable that the primary effect of the absorption of a light 

 quantum is to produce a photochemical intermediate stable enough 

 to persist through an intervening dark period and to undergo a 

 further photochemical change w^hen exposed to light of a shorter 

 wavelength. Also, why does the oxygen present in aerobic cells not 

 interfere with photosynthesis? Franck concludes that the photosyn- 

 thetic oxidants are present in substrate quantities, not merely in 

 catalytic amounts. He therefore favors the idea of a double keto-enol 

 transition occmring in both phosphoglyceric acid and chlorophyll at 

 the expense of energy stored temporarily in a metastable state of the 

 excited chlorophyll a. The double transition would store about 20 

 kilocalories of energy. In this scheme, a second step is then initiated 

 in the form of a singlet excitation of the complex. This energy, plus 

 the stored 20 kcal, is enough to split the water molecule and reduce 

 phosphoglyceric acid. The OH group is taken up by a cytochrome. 

 Two of the radicals formed by addition of one H atom to a phospho- 

 glyceric acid molecule might dismutate to yield phosphotriose and 

 phosphoglyceric acid, ^vith a release of 40 kcal of energy, available to 

 form ATP. 



Calvin pointed out that the quantum yield of photoionization in 

 his model system was one, and that chlorophyll molecules in super- 

 abundance and not in contact with quinone molecules might trans- 

 fer their energy by resonance transfer. He emphasized that the essen- 

 tial feature of his hypothesis was that of a charge migration through 

 a TT-system of electrons involving at least several molecules, and 

 that it is only in this sense that the theory resembles "solid state 

 electronic (hole) conductivity." Rabinowitch objected that if the 

 theory were true, the absorption spectrum of chlorophyll in vivo 

 should differ far more than it does from that of chlorophyll in molecu- 

 lar dispersion. Hut Kasha pointed out that electron overlap was the 

 prime parameter in electronic transport of this type, and that lack 

 of shift of the absorption spectrum is no sign that there is not ade- 

 quate electronic overlajx li might be the orientation of adjacent 

 chlorophyll molecules rather than the distance between them that 

 is responsible for the smallness of the shift in the spectrum. 



Connnoner's paper points out that with a refinement in the sensi- 



