PROCESSES IN THE PHOTOSENSITIVE COMPLEX 1031 



approximating rectangular hyperbolae will permit a correct estimation of 

 the third parameter from the experimental values of the two other para- 

 meters. 



We will find later (c/. chapter 32) evidence that the principal rate- 

 limiting reaction in photosynthesis is a dark reaction catalyzed by a cata- 

 lyst (Eb) which is present in the cells in a concentration only 0.05% of 

 that of chlorophyll. We will also see (page 1038) that this type of limita- 

 tion leads to light curves which are hyperbolae, but not rectangular hyper- 

 bolae. It remains to be seen whether light curves can be measured pre- 

 cisely enough to exclude one of the two mechanisms. 



Is it possible for two or more bottlenecks to exist in photosynthesis, 

 each allowing the passage of about the same amount of reactants — the 

 maximum rate of passage through one bottleneck being determined by the 

 product of the concentration Chlo, the (approximately equal) concentra- 

 tion, Ao, the (bimolecular) constant, K, of reaction (28.21c) and the quan- 

 tum yield n (O.OS^ x 5 X 0.1 = 1.25 X 10"^); and the other, by the 

 product of the enzyme concentration Eb and its (monomolecular) rate 

 constant (2.5 X 10"^ X 50 = 1.25 X 10^^). Such a coincidence seems 

 not implausible; it could even be considered as admirable economy in 

 the allotment of catalysts to the cell. (Why have more of a certain 

 catalyst than can be utilized because there is not enough of another one?) 

 However, certain experimental results are not consistent with the as- 

 sumption that restoration of chlorophyll is the bottleneck which limits 

 (or "co-limits") the maximum rate of photosynthesis; these data indicate 

 that a chlorophyll molecule which has taken part in the primary photo- 

 chemical process needs much less than 4Qn (—4) sec. to return to the photo- 

 sensitive form. We mean here the observations of Willstatter and Stoll 

 (c/. Table 28. V; see also chapter 32, fig. 32.2), that aurea leaves have 

 P'^^''- values only slightly lower than those of ordinary green leaves, 

 although they contain only one third (or less) of the normal amount of 

 chlorophyll. This is obviously inconsistent with equation (28.35), and indi- 

 cates that P"^^"^- is determined not by the rate of restoration of the photo- 

 chemically tautomerized chlorophyll complex (rate constant kr in equation 

 28.35), but by the rate of transformation of a substrate by a catalyst (such 

 as Eb) that is kinetically independent of chlorophyll. The above-esti- 

 mated value of kr (— 5 (sec. mole)~i) is therefore merely a lower limit; in 

 fact, quantitative observations which aurea leaves indicate that the true 

 value of this constant (which determines how often a given chlorophyll 

 mole('uIe is available for the primary photochemical reaction) is at least 

 ten times higher. This means h >50 sec.-^ (for [ACO2] = 0.05 mole/ 

 liter), assuming that the primary photochemical reaction is (28.21c). 

 Because of fundamental significance of these conclusions, a reinvestigation 

 of the kinetics of photosynthesis in aurea leaves seems desirable. 



