876 LIGHT AND LIFE 



produced a masterly synthesis, major features of which must be ac- 

 cepted. Some aspects remain in dispute and some are still hotly 

 contested; others are at the moment more speculative than solidly 

 supported by evidence. Be that as it may, the review is certain to 

 be highly stimulating and should provoke a great deal of experi- 

 mentation. To summarize a 78-page review of such scope is indeed 

 not easy. Fortunately, Arnon has materially lightened the task by 

 providing a number of diagrams of his conceptual schemes. 



The great problem in photosynthesis remains that of the exact 

 role of light. Originally, of course, light was supposed to be the 

 direct mediator of CO2 fixation in the form of carbohydrate. The 

 discovery in the 1880's of the chemosynthetic bacteria by \Vinograd- 

 sky, and of the purple photosynthetic bacteria by Englemann, showed 

 otherwise. But some forty years were required before it was clear 

 that all organisms can assimilate COo although they differ as to the 

 energy sources they can apply to the process. Similarly, a great lag 

 in the undertanding of photosynthesis occurred because the evolu- 

 tion of oxygen was thought to be essential to the process; and not 

 until the classic work of van Niel was it established that bacterial 

 photosynthesis is quite similar to that in green plants except for the 

 nature of the hydrogen donor. 



Arnon states that the energy requirements for CO. assimilation are 

 satisfied byTPNH and ATP in combination. The work with 

 isolated chloroplasts provided no evidence for photochemical forma- 

 tion of any special reductant (H) capable of directly reducing COo 

 to carbohydrate by some special photosynthetic reaction, as van 

 Niel had postulated. Instead, the investigation of CO.j assimilation 

 by isolated chloroplasts has "completed the chain of evidence for 

 the view that carbohydrate synthesis is removed from the domain of 

 photosynthetic reactions proper in which light is converted into 

 chemical energy. In green plants the first stable, chemically de- 

 fined products of this energy conversion are not intermediate of 

 CO2 assimilation but are the two comjjonents of assimilatory power, 

 TPNH. and ATP." 



This view of the problem may be rather less than certain on ac- 

 count of the recent discovery of Warburg, confirmation of which is 

 presented by Vennesland in this Symposium, that carbon dioxide 

 functions catalytically in the Hill reaction. Warburg expects that 

 in the whole cell this function of carbon dioxide, intimately related 

 to the light reaction and pr()l)ably to chloro|)hyll itself, will be the 

 starting point for carbon assimilation in ])hotosynthesis. Until the 



