THERMOCHEMICAL CONSIDERATIONS 1973 



Franck counted "on the reduction side" of photosynthesis a minimum loss 

 of about G kcal in the enzymatic dismutation of four photochemically 

 formed radicals (XH in chapter 7) into two molecules of a saturated re- 

 ductant, and one of 13 kcal in the formation of PGA by reduction, phos- 

 phorylation and carboxylation of the C2 "acceptor." To this, one would 

 have to add the losses in the second stage of the reduction process — prob- 

 ably the conversion of PGA into a triose phosphate (followed by poly- 

 merization of the latter to hexose phosphate, and dephosphorylation). 

 Even without these additional items, we have by now accumulated losses 

 of at least 7 kcal per quantum in the primary photochemical process, and at 

 least 19 kcal per carbon dioxide molecule in the carboxylation and reduc- 

 tion reactions. For a four quanta mechanism, this amounts to a total 

 loss of 7 X 4 + 19 = 47 kcal per reduced CO2 molecule; for a three quanta 

 process, the minimum loss woukl be 7 X 3 + 19 = 40 kcal. These amounts 

 must be added to the free energy of formation of a mole of CH2O groups. 



As Franck pointed out, free energy rather than total energy of photo- 

 synthesis must be used in such calculations. This amounts to 120 kcal/ 

 mole in the free atmosphere, and to 116-117 kcal/mole in C02-enriched 

 media (cf. chapter 1). The total free energy requirement, estabhshed so far, 

 is therefore 163-167 kcal/mole for a four quanta process, and 156-160 

 kcal/mole for a three quanta mechanism. Four quanta of red light (168 

 kcal/einstein) are barely enough to cover these requirements; four meta- 

 stable quanta (148 kcal/einstein) are too little; three quanta are insuf- 

 ficient in both cases. 



The calculation so far has neglected all losses on the "oxidation side" of 

 photosynthesis, i. e., in the conversion of the primary photochemical oxi- 

 dation product into molecular oxygen. This conversion probably involves 

 at least two steps {cf. chapter 11). The first one transforms the primarily 

 produced free radicals (called Z, or {OH}, in chapter 7) into peroxides 

 ("photoperoxides"), while the second one dismutes the peroxide into oxy- 

 gen and an oxide. The latter process is known to be very wasteful of energy. 

 Whether the peroxide is hydrogen peroxide (which is unlikely), or an organic 

 peroxide, its dismutation into oxygen and an oxide liberates, according to 

 the general experience in peroxide chemistry, an energy amount of the order 

 of 45 kcal (per mole of hberated oxygen). Adding this huge loss to the 

 previously estimated losses would make photosynthesis unachievable even 

 by 5 quanta of red light (requirement: 7X5+ 19 + 45 -f- 116 or 120 = 

 215 or 219 kcal/mole), and make six quanta the smallest plausible number. 

 (If the reaction occurs via the metastable state, even six quanta would be 

 hardly enough : available energy < 6 X 37 = 222 kcal; required energy, 

 7 X 6 + 19 + 45 + 116 or 120 = 222 or 226 kcal/mole.) 



Two criticisms of the assumption of a 45 kcal loss in the "photoperoxide" 



