1024 THE LIGHT FACTOR. I. INTENSITY CHAP. 28 



encountering a molecule with which it has to react (be it ACO2 or A'H20) 

 creates a new source of dependence of the rate on the concentration [ACO2] 

 (or [A'H20]). 



We now consider the second anticipated phenomenon — the breakdown, 

 in strong light, of proportionality between irradiation and absorption. 

 This complication does not occur significantly in "ordinary" photochemis- 

 try, where the rate constant of the process by which the excited molecule 

 returns to the normal state is at least a million times higher than the rate 

 constant of light absorption, even in the strongest available Ught (order of 

 magnitude of the maximum frequency of absorptions: 10 sec.~\ of. page 

 838; order of magnitude of the rate constant of deactivation, by fluores- 

 cence or energy dissipation : > 10^ sec. "0 • In strongest available light, the 

 photostationary concentration of activated molecules ceases to be negligible 

 compared to that of the normal molecules if the life-time of the activated 

 form exceeds 10"- sec. (light absorption: once every 0.1 sec; lifetime 

 of the activated state: 0.01 sec; therefore, stationary concentration of 

 activated molecules: 10%). If we assume such a longevity for the ac- 

 tivated state of the chlorophyll complex, a sizable proportion of this com- 

 plex will be present in the changed state during strong photosynthesis, and 

 this will lead to a lack of proportionality between the "useful" light absorp- 

 tion, Ta, and the incident light intensity, I ("useful" meaning the absorp- 

 tion of light by chlorophyll complexes in the unchanged form — assuming 

 that, if the changed form does absorb visible light at all, this absorption 

 is photochemically "useless"). A new reason is thus added for the de- 

 pendence of the yield on light intensity. 



We will analyze these two phenomena — (1) the competition of the "de- 

 tautomerizing" back reaction in the photosensitive complex with the photo- 

 chemical forward reaction; and {2) the depletion of the normal form 

 of this complex during intense photosynthesis — by using two simple 

 mechanisms in which the photochemical forward reaction is assumed to 

 involve the tautomerized chlorophyll complex, HX.Chl.Z, and either the 

 carbon dioxide acceptor compound, ACO2 (mechanism 28.20), or the hydro- 

 gen donor, A'HR, where HR may stand for water, or for a "substitute" re- 

 ductant (mechanism 28.21) : 



k*I 



(28.20a ami a') X-Chl-HZ , HX-Chl-Z 



K 

 (28.201)) TlXChIZ + ACOo > X-Chl-Z + AHCO, 



(28.20e) X-Chl-Z + A'HR ^ X -Chi -HZ + A'R 



(28.20(1) AHCO2 + A'R > ACO2 + A'HR 



