1674 CHEMICAL PATH OF CARBON DIOXIDE REDUCTION CHAP. 36 



2C=0 > C— O + O— C=0 



should have a AH of —29 kcal. Empirically, the energies of this type of 

 reactions ("Cannizzaro reaction") are not quite as large, but they are 

 negative ; a coupling with dismutation could therefore reduce the carboxyla- 

 tion energy markedly — e. g., bring it from the standard value of close to 

 zero (table 8. VII) to as low as —20 kcal. This should shift the free energy 

 of carboxylation (which, in table 8. VIII, differs by about 15 kcal from the 

 total energy) to about —5 kcal, or close to the above estimate. 



This estimate of AH and AF is very crude, but it points to one impor- 

 tant relationship. 



It has been repeatedly emphasized in this book that photosynthesis in- 

 volves two reduction steps of different types: carboxyl to carbonyl, and 

 carbonyl to hydroxyl. Because of the stabilization of carbon-oxygen bonds 

 by accumulation at a single carbon atom, the first step requires consider- 

 ably more energy than the second one. (According to table 9. IV, the nor- 

 mal potentials of transitions of the first type are about -|-0.5 volt, those of 

 the second type, about -fO.2 volt; the difference of 0.3 volt corresponds to 

 a free energy of dismutation of about —2 X 0.3 X 23 = —14 kcal.) In 

 schemes 36.IIIB and 36. V, light energy is used in one step only (conversion 

 of PGA to TP) which is the more difficult of the two required reduction 

 steps. By converting six carboxyl groups into six carbonyl groups — twice 

 as many as are needed for the final carbohydrate synthesis — we acquire 

 the possibility to dismute the three extra carbonyls, and to use the energy 

 of their dismutation to drive forward an otherwise difficult reaction — fixa- 

 tion of CO2 in a carboxyl. This is achieved in the coupled reaction (36.8) 

 which can thus be considered as a chemical mechanism for indirect use of 

 stored light energy to facilitate carboxylation. A further "assist" may be 

 provided (as first suggested by Ruben, cf. p. 201) by a "high energy phos- 

 phate" ; specifically, such a phosphate may be involved in the introduction 

 of a second phosphate residue in ribulose diphosphate. However, the 

 amount of energy stored in this way is likely to be considerably below the 

 10-12 kcal available in carboxyl phosphates. 



The assumption that sedoheptulose and ribulose phosphates are not 

 involved in the synthesis of hexoses in photosynthesis is supported by the 

 observation {cf. table 36. VII) that the distribution of tracer carbon in them 

 bears no similarity to that in hexoses. (One does not see how a hexose 

 with preferential C labelling in position 3,4 could be derived from a ribu- 

 lose with preferential labelling in C atom 3, or a sedoheptulose with equal 

 labelling of atoms 3, 4, 5.) If it were not for this argument, one would be 

 tempted to consider the formation of the C5 and C7 sugars as related to the 

 "alternative path" of respiration, first suggested by Warburg, and more 

 recently established by Horecker {cf. Horecker 1951 and later), in which 

 glucose phosphate is oxidized to phosphogluconate, decarboxylated in 

 ribose, and the latter split into a triose and a C2 compound. 



