PREFACE IX 



is close to 8, the second one suggested that it may be 6.5 or 7.0 in strong 

 light, and as low as about 4 in weak light (as suggested earlier by Kok). 



Significant new observations of the reversible changes in absorption svec- 

 trum and fluorescence of chlorophi/Il in vivo have been described by Witt,^^ 

 as well as by Chance and co-workers, '^ by Strehler and Lynch,!^ ^^^ by 

 Coleman, Holt, and Rabinowitch.'" The latter suggested that in Chlorella, 

 chlorophyll undergoes, in light, a reversible change (probably reduction), 

 leading to a bleaching in the red and the appearance of a band at 525 mn. 

 However, discrepancies between the observations of Coleman et al, on the 

 one hand, and of Duysens (and Strehler and Lynch), on the other, call for 

 further study, especially in the red region of the spectrum. 



The chemical reduction -path of carbon dioxide, elaborated by Calvin, 

 Benson, and co-workers on the basis of C(14) studies (c/., Chapter 36) has 

 received a suggestive confirmation in the demonstration by Racker'^ that 

 reduced pyridine nucleotide, adenosine triphosphate, and bicarbonate will 

 in fact produce sugar if provided with the eleven enzymes postulated in 

 Calvin and Benson's cycle. 



Franck'6 made the interesting suggestion that the transfer of hydrogen 

 from hydrated chlorophyll to an acceptor, such as the carboxyl group in 

 PGA, is achieved by the intervention of an excited triplet state of the pig- 

 ment, formed by cooperation of two photons— one producing the meta- 

 stable triplet state, and the other (supplied by resonance transfer from 

 near-by chlorophyll molecules) transferring the metastable molecule into 

 the excited state. Obviously, this mechanism requires a minimum of 8 

 photons per reduced COo molecule. The existence of a triplet metastable 

 state of chlorophyll b has been confirmed, and its energy found equal to 

 about 33 Kcal./mole, by phosphorescence studies of Becker and Kasha." 



This is only a short (and arbitrary) selection of important new facts and 

 speculations added to the field under review within a few months after this 

 monograph was "completed!" 



The author's thanks are due to Dr. Robert Emerson, the Photosynthesis 



"Witt, M. T., Natunvissenschaften, 42, 72 (195,5); Z. physik. Chem., 4, 1920 

 (1955); Z. Elektrochem., 59, 981 (1955); Gatlinhurg Conference on Photosynthesis of 

 the NAS, Oct. 1955 (in press). 



12 Chance, B., Gatlinburg Conference on Photosynthesis of the NAS, Oct. 1955 (in 

 press). 



'3 Strehler, B. L., and Lynch, V. M., Science, 123, 462 (1956). 



" Coleman, J., Holt, A. S., and Rabinowitch, E., Second GatHnburg Conference 

 on Photosynthesis of the NRC, Oct. 1955 (to be pubUshed); Science (in press). 



15 Racker, E., Nature, 175, 249 (1955). 



16 Franck, J., Daedalus, 86, 17 (1955) (Rumford Medal Lecture, American Academy 

 of Arts and Sciences in Boston). 



" Becker, R. S., and Kasha, M., in The Luminescence of Biological Systems, Prince- 

 ton Univ. Press, Washington, 1955, p. 25. 



