372 J. A. BASSHAM AND K. SHIBATA 



In the original interpretation of the Emerson-Arnold shoit-flash exporiments, 

 two constants had been derived. One was a concentration, 5-10~* X n(Chl): 

 it was supposed to be either the concentration of "pliotosynthotic units," or that 

 of a "finishing" enzyme (enzyme B in the terminology of Franck and Hertzfeld), 

 n being the numl^er of times the enzyme has to act for one molecule of oxygen to 

 be liberated. 



On the other hand, from the duration of the dark period needed to saturate the 

 Hash yield, a rate cons^tant (50 second"' at room temperature) was derived, which 

 was interpreted as the action constant of the same enzyme. We note that these two 

 constants were derived quite independently — one from the maximal fla.sh yield, 

 the other from the length of the dark period needed to attain this maximum. If 

 Kok's mechanism is substituted, the first constant retains its significance as the 

 concentration of "units" (U), but the second "constant" is replaced by a function 

 of the rate constants of the two reaction steps. Therefore, the decay function 

 ceases to be representable by a simple exponential curve. However, with brief 

 flashes, the second of Kok's two steps dominates the kinetics, and the decay is 

 approximatelj^ exponential, with a rate constant {ca. 0.01 second) similar to that 

 derived from Emerson and Arnold's experiments. However, the concentration 

 constant and the rate constant now apply to different catalytic components. The 

 picture has thus become less simple than before, but it seems to be able to account 

 for results obtained with both long and short flashes. 



I do not see, however, the amended theory as providing new arguments for the 

 existence of photosj'nthetic units as physical entities. I believe Kok could rewrite 

 his equations in terms of two successive independent enzymes, Ei and E2, instead 

 of a "unit," U, and an enzyme, E. Instead of postulating a unit which is excited 

 as a whole — a unit containing 2000 /n chlorophyll molecules — one can assume that 

 one molecule of an enzyme is present per 2000/71 molecules of chlorophyll, and 

 photoproducts formed by all of the latter must be worked up by the one enzyme 

 molecule. The only new element in Kok's picture is that, instead of enzyme Ei 

 transforming the primary photoproduet directly into the final product, it now 

 hands it over — after some sort of change — to a second enzyme, E2, which trans- 

 forms it into the final product. 



Whittingham : That is exactly the way Briggs had formulated the problem. 



References 



1. Gaffron, H., and Wohl, K., Naturwiss., U, 81, 103 (1936). 



2. Wohl, K., Z. physik. Chem., B37, 105, 122, 160, 186, 209 (1937); Neiv PhytoJo- 



gist, 39, 33 (1940); 40, 34 (1941). 



3. Tamiya, H., and Chiba, Y., Studies Tokugawa Inst., 6, No. 4 (1949). 



4. Bassham, J. A., Shibata, K., and Calvin, M., Biochim. et Biophys. Acta, 17, 332 



(1955). 



5. Strehler, B., Arch. Biochem. and Biophys., 34, 239 (1951); Strehler, B., and 



Arnold, W., J. Gen. Physiol, 34, 809 (1951); Arnold, W., and Davidson, 

 J. B., /. Gen. Physiol., 37, 677 (1954); Strehler, B., reported at Gatlinburg 

 Meeting on Photosynthesis, 1955; Arnold, W., reported at Gatlinburg Meet- 

 ing on Photosynthesis, 1955. 



6. Briggs, G. C, Proc. Roy. Soc. (London), BlSO, 24 (1941). 



