934 CONCENTRATION FACTORS CHAP. 27 



of the oxidation of water, such as A'HO, or with some other celkilar oxi- 

 dants) to form ACO2 (or, according to a hypothesis of Franck, A + CO2). 

 The latter possibihties will be discussed in section le of chapter 28. 



We will briefly consider here the effects of the "acceptor blockade." The conserva- 

 tion equation (27.4) must in this case be replaced by: 



(27.71) Ao = [A] + [ACO2] + [AHCO2] 

 Assuming that the restoration of the acceptor is a monomolecular process 



k' 



(27.72) AHCO2 '- — > A + HCO2 



we can write the equations for the stationary concentration of ACO2 and (assuming the 

 validity of 27.6) also for the rate of photosynthesis. Taking the simplest case — that 

 discussed in section a (carboxylation equilibrium undisturbed by slow diffusion or slow 

 carboxylation) — we obtain: 



(27.73) P= nA.K. [CO.W. 



l+TValCOo] 



(27.74) 



(27.75) 



The carbon dioxide curves (27.73) are thus hyperbolae the half saturation of which is 

 shifted, with increasing light intensity, toward the lower concentrations of carbon dioxide 

 (a shift opposite to that caused by slow diffusion or slow carboxylation; and apparently 

 not encountered in experimental curves). Because of the occurrence in the denomina- 

 tor of (27.73) of the product A;*[C02], the rate cannot exceed the "absolute maximum": 



(27.76) PZ^. = n/cj'Ao 



i. e., n times the maximum rate of restoration of the acceptor according to (27.72). 



(g) Calculation of Carhoxijlation Constant from Carbon Dioxide Curves 



If one wants to use the carbon dioxide curves for the determination of 

 the carboxylation constant K^^, as was done in chapter 8 (Vol. I), one should 

 avoid the region of "absolute saturation," and determine systematically, 

 and with a high degree of precision, the carbon dioxide curves in the region 

 where the yield is still proportional to light intensity. In this way, one 

 could perhaps determine which of the several above discussed mecha- 

 nisms of carbon dioxide supply provides the best interpretation of the facts. 

 The experimental data available at present are neither exact nor systematic 

 enough for such an analysis. 



