H. GAFFRON 



tive side reaction, he has provided the first plausible model for a con- 

 tinuous chenioreduction of carbon dioxide. 



As an experimental approach to tie up the phosphorylation 

 reaction and photosynthesis, it is obviously insufficient to demonstrate 

 that phosphorylations take part in the metabolism of plant cells and 

 that the photosynthetic production of carbohydrates changes the dark 

 equilibrium of various phosphate esters. It w^ould be strange if it 

 were not so. In a respiring or fermenting cell, the model for the chemi- 

 cal reduction of carbon dioxide as presented by Lipmann should work 

 with any available metabolic hydrogen, particularly with free hydrogen 

 activated by hydrogenase. Since, under the latter circumstances, 

 the over-all energy requirements are very small, there is no reason 

 why fermenting algae should not exhibit a clear-cut reduction of car- 

 bon dioxide in the dark. This is not the case, at least not at a 

 noticeable rate. Apparently some extra energy for activation is 

 necessary. If so, we have still to explain why, as found by Rieke in 

 unpublished experiments, the number of quanta for the reduction of 

 one carbon dioxide molecule remains ten when the algae begin to 

 consume hydrogen instead of liberating oxygen. Energetically, one 

 or two quanta would now suffice. The simplest explanation would 

 involve the assumption of the existence of a rigid coupling of the re- 

 duction of carbon dioxide with the utilization of water as hydrogen 

 donor. This would mean the exclusion of energetically "cheaper" 

 hydrogen from a direct participation in the photochemical reaction 

 during anaerobic photoreductions in algae and, by analogy, also in 

 the photosynthetic purple bacteria. We cannot present in detail the 

 points for and against this explanation. It is also not necessary, for 

 in van Niel's comprehensive reviews (20) and Rabinowitch's book 

 (14) the reader will find much stimulating discussion. 



Interrelation of Carbon Assimilation with Oxidation 

 Reactions in Plants and Purple Bacteria 



We have seen above that there is a possibility that substrates, 

 intermediates, and final products in photosynthesis may undergo re- 

 duction and polymerization as phosphorylated compounds. Experi- 

 ments supporting Ruben's or Lipmann's hypotheses (11,15) are still 

 missing, and Ochoa (12) has just reported the formation of oxalo- 



42 



