1974 KINETICS OF PHOTOSYNTHESIS CHAP. 37D 



mechanism of oxygen liberation can be made. One is that mechanisms of 

 oxygen production are conceivable which would not involve a peroxide at 

 all, or involve a peroxide of lower energy, capable of dismutation with a 

 relatively small loss of energy (or a peroxide with such high energy that it is 

 capable of straight decomposition, R1OOR2 -> RiRo + O2). In other 

 words, one is not under obligation to postulate an energy-wasting "catalatic" 

 mechanism of oxygen liberation. The second objection is that the energy 

 of decomposition of the peroxide could be somehow fed back into the re- 

 action cycle (e. g., by coupling the peroxide composition with the formation 

 of high energy phosphate esters, and utilizing the latter in reactions such as 

 the reduction of PGA to glyceraldehyde) . 



To the first suggestion — which was discussed in some detail in chapter 

 11 — one can say that no mechanism of oxygen evolution not involving 

 peroxide as intermediate has yet come to light, and that it is unhkely 

 that the 0=0 double bond can be formed in one act without prehminary 

 formation of an O — O single bond. However, the possibility of a peroxide 

 thermochemically radically different from H2O2 (or from typical organic 

 peroxides) cannot be denied a priori (cf. chapter 11, p. 293); the 45 kca] 

 item in the energy balance may therefore be too large. 



As to the possibility of feeding the peroxide dismutation energy back 

 into the reaction cycle, two aiguments against this hypothesis come to 

 mind. No such coupHng has yet been demonstrated in respiratory proc- 

 esses; and the yield determinations on purple bacteria and adapted algae 

 (where no oxygen is evolved, and correspondingly more energj'^ is available 

 for "ploughing back" into photosynthesis), indicate no energy saving com- 

 pared to true photosynthesis. Admittedly, both arguments are suggestive 

 rather than conclusive. 



To sum up, the conclusions that can be derived from these estimates 

 (differing in some numerical detail from those of Franck, but in general 

 agreement with the latter) are as follows : 



{1) If 3 quanta (or less) would be proved to suffice for steady photo- 

 synthesis, this would mean that plants have found a way to get around all 

 general rules of reaction kinetics and photochemistry, except the law of 

 conservation of energy. 



(2) A quantum requirement of 4 is thermochemically only possible — 

 and then unlikely — if plants have found a way to liberate oxygen without a 

 peroxide as an intermediate, or to re-utilize the peroxide decomposition 

 energy. 



(3) If a peroxide with even one half of the dismutation energy of 

 H2O2 is involved in photosynthesis, and its dismutation energy is dissipated, 

 the minimum plausible quantum requirement is 6, with 7 or 8 the more 

 likely alternatives. 



