932 CONCENTRATION FACTORS CHAP. 27 



limited quantities and carboxylated by a slow dark reaction; or whether 

 one could perhaps explain all carbon dioxide saturation phenomena by ref- 

 erence to limited supply of light. The occurrence of phenomena such as 

 carbon dioxide "pick-up" after intense photosynthesis (Vol. I, page 206) 

 provides, however, direct evidence that a dark carboxylation reaction actu- 

 ally does occur, and that it has an effect on the rate of the over-all reaction 

 of photosynthesis. On the other hand, it is undoubtedly true that some, at 

 least, of the carbon dioxide curves, particularly those measured in weak 

 light, owe their hyperbolic shape entirely or preponderantly to the limited 

 rate of supply of light quanta. 



Reaction mechanism (28. lA, eqs. 28.20), which we have used, provides 

 that the reaction of the primary photoproduct, HX-Chl-Z, with the oxi- 

 dant, ACO2 (rate constant, A,), competes with the "deactivating" reaction 

 that converts HX • Chi • Z back to X • Chi • HZ (rate constant k'). An alter- 

 native mechanism (28. IB, eqs. 28.21) also is discussed in chapter 28, in 

 which the "primary" back reaction of the photoproduct, HX-Chl-Z, is 

 ehminated by immediate reaction of HX • Chi • Z with (free or bound) water, 

 converting it to HX-Chl-HZ. This simplified mechanism will be used in 

 chapter 28 to analyze another possible kinetic effect within the photosensi- 

 tive complex — the accumulation, during strong photosynthesis, of the 

 chlorophyll complex in the changed, photoinsensitive form (HX • Chi • HZ in 

 the mechanism used). In deriving equation (28.14), the simplification 

 [HX • Chi • Z ] «; [X • Chi • HZ ] ^ Chlo was made ; we do not make a similar 

 assumption in respect to HX-Chl-HZ, but postulate that the "reduced" 

 form accumulates and brings about saturation of light curves. (With in- 

 creasing light intensity more and more chlorophyll complexes will be pres- 

 ent, in the steady state, in the photoinsensitive form, HX-Chl-HZ.) This 

 assumption leads to equation (28.27) for P as function of / and [ACO2], and 

 to eciuation (28.28), if the equilil)rium value (27.3) is substituted for [ACO2] 

 in (28.27). Considering (28.28) as equation of carbon dioxide curves (/ = 

 constant), we obtain the following expressions for these curves: 



P — P k*I 



'^'•*'' "p.„. " f/(l + K.[C0,1 ) + k,K,A,lCO,] 



("•™' V.ICOJ - J; (s;j^,) 



Equation (27.70) shows that the half-saturating carbon dioxide concentra- 

 tion rises with increasing light intensity, and that the equilibrium constant 

 Kg^ can be obtained by an extrapolation of 1/JCO2] to high light intensities 

 (linear extrapolation with 1/7 as abscissa) . It will be recalled that in the 

 case of the carbon dioxide supply limitation (c/. equation 27.22 or 27.33) 

 we had to obtain the same value by extrapolation to low light intensities 



