1972 KINETICS OF PHOTOSYNTHESIS CHAP. 37D 



single quantum is likely to move only one electron, these primary photo- 

 chemical products in all probabiHty will be radicals (the product of attach- 

 ment of free radicals to valence-saturated "carriers" also are free radicals). 

 Unless these radicals are given sufficient energy in the act of their formation, 

 to break out of the "cage" (in the terminology of Franck and Rabinowitch), 

 before they are stopped, they will be in danger of "primary recombination." 



The amount of energy lost in the conversion of electronic to chemical 

 energy will depend on whether the sensitized reaction is brought about by 

 the unstable singlet, or the metastable triplet state. In the first case, the 

 residual vibrational energy must be equivalent to roughly the distance 

 between the peaks of the absorption and the fluorescence bands (in the case 

 of chlorophyll in vivo, about 1 kcal/mole). If the triplet state participates 

 in sensitization, the vibrational energy residue will be larger (because the 

 change in stable nuclear configiu'ation must be greater for the triplet- 

 singlet than for the singlet-singlet transition) . 



Franck's tentative estimate of total energy losses in the photochemical 

 steps is 7 kcal pei quantum, leaving 35 kcal if the singlet state is involved, 

 and ^ 30 kcal if the triplet state is responsible for sensitization. 



After the two free radicals, formed m the primarily photochemical step, 

 have separated (the sensitizer itself may be one of them) the chemical 

 stage of photosynthesis begins. We do not know its details, except for 

 the conclusion, derived from radiocarbon studies, that phosphogly eerie 

 acid (PGA) is the first identifiable organic product. It is presumed to be 

 formed by earboxylation of a C2 phosphate (or of a C5 diphosphate, the 

 product splitting into two PGA molecules). Probably, this earboxylation 

 is followed by the reduction of PGA to a triose, as first step in a cycle in 

 which one part of the reduction products is stabilized as hexose sugars, and 

 another is converted back into the C02-acceptor, to take up more carbon 

 dioxide. 



Whether in this or in another specific way, the chemical stage of photo- 

 synthesis undoubtedly consists of a complex series of reactions, mostly or 

 entirely enzymatic. It probably includes, in addition to oxidation-reduc- 

 tions, also phosphorylations, condensations and carboxylations. Consider- 

 ing the low concentration of the intermediates (particularly radicals) which 

 can be present in the steady state, each step can proceed rapidly and com- 

 pletely in the desired direction only if it is at least slightly exothermal; 

 and this means that a certain amount of energy must be converted into heat 

 in each step. The more steps are involved in a suggested mechanism, the 

 larger are the expected energy losses. (Warburg and Burk's "new scheme" 

 of photosynthesis, in which at least three times more oxygen molecules are 

 produced as intermediates than are left in the net result, would, for this 

 reason, entail considerably greater losses than the earlier, simpler schemes.) 



