PROCESSES IN THE PHOTOSENSITIVE COMPLEX 1027 



In discussing the consequences of the primary back reaction, on the basis 

 of mechanism (28.20), we will neglect the second anticipated phenomenon — • 

 the accumulation of tautomerized chlorophyll complexes in strong Ught. 

 (We thus introduce a new cause for carbon dioxide saturation, but no new 

 cause for light saturation.) Later, we will use mechanism (28.21) to con- 

 sider the second phenomenon (accumulation of HX-Chl-HZ in strong 

 Hght, and consequent light saturation), while in turn neglecting the pri- 

 mary back reaction. 



By assuming [HX • Chi • Z ] «: [X . Chi . HZ ], conditions become formally 

 analogous to those prevailing in "ordinary" photochemistry in vitro, as dis- 

 cussed above. From the reaction sequence (28.20a-c) we obtain, for the 

 photostationary concentration [HX-Chl-Z], the Stern-Volmer type equa- 

 tion: 



(28.22) [HX-Chl-Z] = A;*/[X-Chl-HZ]/(A;' + kAkCO-A) ^ 



k*ICh.Wik' + kr[A.CO,]) 



and hence: 



(28.23) P = nkrk*I [AC02]Ch\o/(k' + ^^[ACOa]) 



(in expected formal analogy to equation 28.1). Comparison with equation 

 (27.6) shows that A-*, while proportional to /, is now in fact also a function 

 of [ACO2]: 



(28.24) k* = krk*I Ch\o/{k' + A-JACO2]) 



If [ACO2] increases indefinitely, P approaches the maximum rate: 



(28.25) Pmax. = nA:*7Chlo = nZ aChlo 



which corresponds to the maximum quantum yield, n. Similarly to the 

 yield (28.18), n is independent of Hght intensity. 



Equation (28.23) shows that back reactions in the photosensitive com- 

 plex can explain the increase in yield with increasing [CO2] and the final 

 [CO2] saturation, even if the acceptor A is available in unlimited quantities 

 (e. g., if [ACO2] stands for dissolved carbon dioxide, the quantity of which 

 can be increased practically indefinitely by raising the partial pressure of 

 carbon dioxide over the system). In other words, equation (28.23) indi- 

 cates how a limited supply of light quanta can account for hyperbolic car- 

 bon dioxide curves, without the assumption of a limited amount of a [CO2] 

 acceptor, or slow diffusion, or slow carboxylation. 



The effects of carbon dioxide supply can, of course, be superimposed 

 upon those of limited light supply, by introducing the corresponding ex- 

 pressions for [ACO2] into (28.23). Using for this purpose "static" equa- 

 tion (27.3), i. e., taking into consideration only the limited amount of the 

 [CO2] acceptor, will not lead to light saturation; the latter will be intro- 



