1028 THE LIGHT FACTOR. I. INTENSITY CHAP. 28 



duced if one uses for [ACO2] one of the "kinetic" expressions, taking into 

 account the hmited rates of supply processes (carbon dioxide diffusion and 

 carboxylation) . Absolute saturation, Pmll'. will result if it is assumed that 

 a maximum rate of supply exists which is independent of [CO2], e. g., as a 

 consequence of a limited amount of the carboxylating catalyst, Ea- 



We will now consider the second phenomenon, which is without parallel 

 in "ordinary" photochemistry, and arises from the assumed longevity of 

 the "activated state" of the photosensitive complex: the accumulation of 

 these complexes in a changed form, and the consequent lack of propor- 

 tionality between the incident light intensity, /, and the photochemically 

 significant light absorption, I^. For this purpose, we use the alternative 

 mechanism (28.21), since the postulated practical instantaneousness of 

 reaction (28.21b) gives us some right to neglect in this case the primary back 

 reaction, (28.21a) {i. e., to assume /co[A'H20] ^ A;') The steady state 

 concentration of the reduced form, [HX.Chl.HZ], is, under these condi- 

 tions : 



(28.26) [HX-Chl-HZ] = k*I Chlo/(A-*/ + MACO,]) 



(implying that, in the absence of ACO2, all chlorophyll complexes would 

 go over, in light, into the reduced form, HX-C1il-HZ). The rate of photo- 

 synthesis is: 



(28.27) P = /(/,v[HX-Chl-HZ][AC02] = nAvChloA-*/[ACO.>]/(A-*/ + AvlACO,]) 



Assuming further that the complex ACO2 is in equilibrium with free carbon 

 dioxide, and no diffusion gradient exists, so that [CO2] — [CO^la we obtain: 



(28.28) P = nkrChU*IAoKAC0.2]/(k*I + A\[COo]^"*/ + A-.AoKalCOa]) 



an equation for the rate of photosynthesis when neither the preparatory 

 nor the finishing dark reactions, neither on the "reduction side" nor on the 

 "oxidation side," have a rate-limiting influence. Light saturation is in 

 this case due entirely to the limited amount of the acceptor, Ao; and car- 

 bon dioxide saturation, to the limited amount of chlorophyll, Chlo. 

 The light curves (28.28) are hyperbolae: 



(28.29) P/(P'°^^- - P) = A-*/(l + A'alCO.,])/Av.-lo/va[CO,I 

 The half-saturating light intensity is: 



(28.30) ,/./ = AvAoA\[CO,l/(A-* + A*A'JCO,]) 



Extrapolating to [CO2J = <», we obtain: 



(28.31) y/c = AvAo/A-* 



If the carboxyln tion equilibrium is not maintained in light, we have, more 



generally, instead of (28.30) ; 



(28.31a) y/ = kr[AC02]/k* 



