1990 EPILOGUE CHAP. 38 



from two molecules of water), the hypothe.sis of H-transfer as primary 

 photochemical process naturally suggests that the quantum requirement 

 of photosynthesis (in higher plants as well as in bacteria) should be at 

 least 4. We say "at least," because one quantum is likely to give the 

 H-atom an insufficient reductive potential to enable it to reduce carbon 

 dioxide to the carbohydrate level {e.g., the hydrogen in reduced pyridine 

 nucleotide cannot be transferred to a carboxyl group without an additional 

 supply of energy). A "booster" effect could be derived from the reversal 

 of a part of the primary photochemical processes, and storage of the 

 chemical energy liberated in the back reactions ("energy dismutation" 

 hypothesis, cj. p. 164). For example, back reaction could be made to 

 create high-energy phosphate esters (ATP), which, we know, can assist 

 reduced pyridine nucleotides in reducing phosphoglycerate to phospho- 

 glyceraldehyde. "Energy dismutation," if it does occur, should increase 

 the minimum quantum requirement above 4 — how far above, depends on 

 the effectiveness with which the energy of the back reactions is utilized for 

 the forward reaction. Calvin suggested recently that the additional c^uan- 

 tum requirement is close to 3, leading to a total quantum yield of about ] /7. 



The hypothesis, according to which the primary chemical process of 

 photosynthesis is the transfer, by one quantum, of one hj^drogen atom 

 (directly or indirectly) from water to pyridine nucleotide; and that re- 

 oxidation of a fraction of the reduced pyridine nucleotide molecules pro- 

 duces high-energy phosphates, permitting other molecules of the reduced 

 pyridine nucleotide to reduce phosphoglyceric acid to triose phosphate, 

 seems, at this writing, the most popular (but by no means finally estab- 

 lished) mechanism of the primary photochemical process in photosynthesis. 

 It is supported — as mentioned above — by the observations of the accumu- 

 lation of adenosine triphosphate in illuminated cells {cf. p. 1702). The 

 "quantum yield controversy" (cf. Chapters 29 and 37D), remains unset- 

 tled, but it seems that its most likely final outcome will be the experimental 

 establishment of a minimum ((uantum recjuirement of between 6 and 8 

 quanta per oxygen molecule*^ — ^a value compatible with the above described 

 theoretical picture. On the other hand, it is not yet impossible that the 

 actual minimum quantum requirement will turn out to be 8, thus support- 

 ing the "eight (juantum hypothesis" {cf. Chapter 8), which suggests that 

 two quanta are used for the transfer of each hydrogen atom from water 

 to its ultimate desitination. 



Continued assertions that the minimum quantum requirement is less 

 than 4 are, on the other hand, highly implausible; this applies in particular 

 to the suggestion that the true minimum requirement is as low as '2.83, 



* New results, falling into this range, were published by Yuan, Evans and Daniels 

 {Biochivi. et Biophys. Acta, 17, 185, 1955) and Bassham, Shibata and Calvin (ibid., 17, 

 332, 1955). 



