936 



CONCENTRATION FACTOHS 



CHAP. 27 



However, the linear extrapolation procedure is not always reliable. 

 While slow diffusion and slow carboxylation tend to shift the half-saturat- 

 ing concentration upward with increasing light intensity, other influences, 

 such as the "acceptor blockade" (equation 27.75), or the deficiency of 

 catalysts, may shift it downwards. Equation (27.43) shows, for example, 

 that a limited amount of the carboxylase Ea may cause 1/JCO2] either to 

 increase or to decrease with increasing light intensity, depending on whether 

 Kf, is smaller or larger than K^. In this case, extrapolation of 1/2 [CO2] to 

 zero light intensity should still give the correct value of K^, but systematic 

 data will be needed to make the required nonlinear extrapolation possible. 



< 



llJ 

 o 



z _ 



O V. 



O a, 



O E 



O 



< 



< 

 CD 



< 



X 



Doto of Hoover et al 

 I r I I I 



2 4 6 8 10 12 14 16 18 20 22 24 

 LIGHT INTENSITY, klux 



Fig. 27.9. Extrapolation of 1/JCO2] to / = 0. 



Finally, under certain conditions the value of i/^ [CO2] may have no rela- 

 tion to the carboxylation constant at all. This is obviously true if the 

 ACO2 complex is practically undissociable. Equation (27.49), for ex- 

 ample, contains only the kinetic constants k^, and 14 ■ Similarly, in section 

 e, where we postulated that the carbon dioxide saturation is caused by 

 "detautomerization" of the photocomplex HX-Chl-Z, equation (27.68) 

 showed 1/2 [CO2] to be a function of the kinetic constant k^ and kr only. 



To sum up, the constant 1/Ks, may be ^ 1/JCO2] or <C 1/2 [CO2], 

 and the carbon dioxide cui-ves may nevertheless be hyi^erbolae. The 

 ol)servations of the "pick up" and of the uptake of C*02 in the dark speak 

 ill fa^'()r of a reasonably stable ACO2 complex, and thus against the second 

 possibility. Consequently, the above calculated value of iva ('^ 5 X 10 ~^ 

 niole/1.) and of AF( — 7.9 kcal/mole) are to be considered as upper, 

 rather than lower, limits; in other words, the ACO2 complex is either as 



