RECENT RESULTS AT WAGENINGEN 303 



yield decays immediately upon exposure to excessive light. However, it is found 

 that the quantum efficiency also decaj^s immediately (but the saturation rate 

 only after some delajO- 



This indicates that after the inactivation of a molecule U, a large number of C 

 molecules are deprived of their outlet for excitation energy; this is our main argu- 

 ment in favor of assuming a functional cooperation between U and an assembly of 

 C's- — at least, of a regular spatial distribution of the two components. 



But even with the "statistical" interpretation of the "unit," we still have a 

 choice between two possibilities. 



(a) The primary photoproduct (C*) could diffuse, as a chemical entity, to U 

 (or conversely V could diffuse to C*). Then we have to accept that the lifetime t 

 of C* is long enough to allow the enzyme U to "scan" 400 pigment molecules. 

 In this model, the size of a "unit" could be determined solely by the value of t 

 and the diffusion velocity of U. But we can repeat here the arguments used 

 above: If t were much longer than the time required bj- U to reach C*, a de- 

 crease of (U) by photoinhibition would only gradually become manifest as a de- 

 crease in maximum quantum yield (since U molecules in this model are able 

 to invade each other's areas). If t did not, or did only barely, fulfill the require- 

 ments set by the diffusion velocity of U, severe light losses and low quantum 

 yields would be the consequence. 



To prevent such losses by accepting a safe (sufficienth' long) value for i, we 

 have to return to the concept of a restricted "working area" assigned to each indi- 

 vidual U molecule. 



(b) The other alternative is energy migration through a series of pigment mole- 

 cules by way of inductive resonance. Such migration has been shown actually to 

 occiu" extensivelj' in the chloroplast. This "physical" diffusion of excitation energy 

 toward a final trap yields a satisfactory interpretation of the unit. Our relatively 

 simple kinetic scheme satisfies most observations made so far; the assumptions of 

 a relatively long lifetime of C* would require an additional reservoir between light 

 and r. 



Rabinowitch : I have pointed out repeatedly in the past that all experiments in 

 which the same inhibition (for example, by ultraviolet light, or by excess light, as 

 found by Kok, or by narcotics) is observed in strong and in weak light, require a 

 correlation between saturation rate and maximum quantum yield. If, for ex- 

 ample, you have a surface covered with chlorophyll, a molecule of the limiting 

 enzyme must be assigned to a certain surface area and permitted to serve only 

 the molecules of chlorophyll contained in this area. Thus, knocking out a molecule 

 of the enzyme will "inactivate" all chlorophyll molecules in the whole service area. 



I am not against a "physical" unit; I am onlj' saying that the only reason the 

 unit theory fits the kinetic results is because it permits the energj^ quantum, ab- 

 sorbed in one point, to end up at another point where it is needed, e.g., because 

 this is where an enzyme molecule is available. However, any chemical product, 

 formed at the original place of absorption, can similarly diffuse to the enzyme 

 that has to service it (or this enzyme itself can go arountl and gather up the prod- 

 uct). All that is needed is a division of spheres of influence between the intlividual 

 chemical "messengcis," or enzyme molecules, eacli l)cing as.signed exclusively, 

 or at least preferentially, to a certain group of i)igjnciit molecules. I don't see 



