?,r>2 B. KOK 



olementary light reactions. The rate of each will be proportional to light intensity. 

 Tn the final step, two Oil-groups will have to conihiiio ai a rate proportional to the 

 square of their concentrntions. I think one could get Kok's quadratic term in this 

 way. 



Kok: Photooxidation and photoinhibition are not the same thing. As to the 

 mechanism which results in a "two-quanta process," 1 am open-minded; but 

 probably it has to occur within a single pigment unit. 



Wassink: \ye decided formerly upon a kind of ))hotosynthetic unit, similar to 

 Wohl's "kinetic model" (Enzymologia, 10, 365-372 (1942)). I think this theory 

 is consistent with what Kok foimd, or at least it can be made consistent with his 

 finding. It simpl>' implies that a ciuantum, or whatever it is, caimot travel thiough 

 tlie whole amount of chlorojjhyll present, but only over a certain fraction of it, 

 which may consist of, let's say, 400 or 600 chlorophyll molecules — or whatever 

 number you may derive from the experiment. In this average area "it" is captured 

 by an acceptor molecule (IT). As soon as you knock out, say, four out of five, or 

 two out of five, of the acceptor molecules (U), this mechanism does not work any 

 more, or works less adequately. I think that is a connecting link between your 

 scheme and Rabinowitch's kinetic suggestion. 



Kok : The first intermediate, in which the excitation energy, absorbed by chloro- 

 phyll, is stabilized can be conceived either as a "floating" enzyme, or as a "reser- 

 voir" structurally and functionally tied up with its chlorophyll molecules in 

 some kind of a "unit." 



The observation that this "reservoir" is loaded by a first-order process in re- 

 spect to light intensity indicates that a one-quantum process is involved. There- 

 fore, rather than to accept that all the 4, 8, or 10 quanta, ultimately required for 

 the evolution of one oxygen molecule, are successively stored in one and the same 

 primary stabilizing molecule, it is simpler to assume — and more likely — that 

 each center stores and transfers the energy of one quantum at a time. This, then, 

 decreases the required size of the "unit" by a factor of 4, 8, or 10 — say, from about 

 2000 to approximately 400 chlorophyll molecules per stabilization center. 



In the alternative concept, the stabilization centers can be conceived as enzyme 

 molecules floating about at random between, say, 400 times more numerous, 

 unorganized pigment molecules. Then, in order to retain a high efficiency of 

 light utilization in weak light, we have to ascribe a sufficient lifetime to the ex- 

 cited chlorophyll complex, long enough to allow for the diffusion of the enzyme 

 to it (or the photoproduct must live long enough to diffuse to the enzyme and react 

 there). 



In the absence of functional or structural correlation between C and U (i.e., 

 if virtually each chlorophyll molecule could react with each U molecule available), 

 a decrease in the number of U molecules (which can be computed from the maxi- 

 mum flash yield) would only very gradually impair the efficiency in weak light. 

 As long as excitations occur sufficiently infrequently, a few U molecules would 

 suffice for the stabilization of all photoproducts. On the other hand, since U is 

 involved in a rate-limiting dark reaction (as flashing light data show), one would 

 expect the saturation rate to be far more strongly dependent upon the concentra- 

 tion of U. 



Photoinhibition now provides a means for decreasing (U): the maximum flash 



