1040 THE LIGHT FACTOR. I. INTENSITY CHAP. 28 



It is, of course, possible to combine the above derivations with the as- 

 sumption of slow carbon dioxide supply (in consequence of slow diffusion, 

 slow carboxylation or low content of the enzyme Ea), in other words, to 

 drop the assumption that equation (28.41e) is an instantaneous reaction. 

 In this case, one would have to take into consideration the accumulation 

 of the chlorophyll complex also in the "carbon dioxide-denuded" form, 

 A-Chl-A'HoO. Such derivations were actually carried out by Franck and 

 Hcrzfeld, using their eight-step mechanism. 



It may be noted that in the case of mechanism (28.41), as in that of the 

 previously considered mechanisms, the result would be formally the same 

 if the limiting finishing catalyst Eb would be assigned to the stabilization 

 of the oxidation product {HO} instead of the reduction product, JHCO2I, 

 i. e., if this catalyst would be needed to make possible reaction (28.41d), 

 rather than reaction (28.4 le). 



The reason Franck and Herzfeld had to assume that the catalyst En works on 

 all seven mtermediate products (and does not merely create a single "bottleneck," 

 e. g., after the fourth, or seventh photochemical step) is as follows: If one assumes eight 

 photochemical steps and only one bottleneck, then, in strong light, the intermediate 

 just before the bottleneck will accumulate at the expense of all the others. If now the 

 light intensity is suddenly reduced, a certain time must elapse until the distribution of 

 intermediates can become uniform. Uniform distribution is, however, necessary to ob- 

 tain the maximum quantum yield (since for this, all eight photochemical steps must occur 

 with equal frequencies). It follows that sudden reduction of light intensity from the 

 saturation range to the linear range should cause photosynthesis to drop "too low," and 

 then gradually recover to the normal value. Franck and Herzfeld took it for granted that 

 no such effect exists, and this caused them to assume equal proportions of back reactions 

 (i. e., equal role of the catalyst Eb) for all eight intermediates. 



However, Steemann-Nielsen (1942, 1949) has described experiments in which "in- 

 duction losses" following a transition from stronger to weaker light have in fact been ob- 

 served in the algae, Fucus serratus (1942), and Cladophora insignis (1949). (These ob- 

 servations will be described in chapter 33, dealing with induction phenomena.) It is, 

 however, not at all certain that the induction effects observed by Steem.ann-Nielsen are 

 actually caused by unequal distribution of intermediates in strong light; the observed 

 duration of the induction period (up to 30 niin.) seems much too long for this origin. It 

 seems more likely that these induction losses are related to "narcotization," photoxida- 

 tion, or some other type of inhibition or ]iartial destruction of chlorophyll in strong light. 

 This loss of active chlorophyll is not noticeable when the over-all rate is limited by a 

 chlorophyll-independent catalyst, such as Eb; consequently, in strong light, even if 

 only a fraction of chlorophyll is active, its activity may suffice to keep all Eb occupied. 

 In the light-limited state, on the other hand, absorption of light by "narcotized" (or 

 otherwise inhibited) chlorophyll must reveal itself in a proportionally decreased rate of 

 photosynthesis. 



Obviously, questions about the distribution of intermediates become unnecessary if 

 one postulates only one photochemical step followed by dark dismutations or coupled 

 oxidoreductions. 



