H. GAFFRON 



In addition to what has been said in the preceding paragraphs 

 about the probable evolution of the photosynthetic system and about 

 the three main types of carbon dioxide reduction, the scheme tries to 

 coordinate the following fairly well-established facts (13): 



(7) Chlorophyll is necessary even in those cases in which the 

 light is effectively absorbed by other plant pigments such as xantho- 

 phyll and phycocyanin. 



(2) The oxygen liberated does not originate from carbon 

 dioxide but from molecules of water entering somewhere as ultimate 

 hydrogen donors into the reaction. 



(J) Carbon dioxide is fixed initially by way of a reaction which 

 is reversible and nonphotochemical, probably in the form of a car- 

 boxy 1 group. 



{4) There are several intermediate steps between the photo- 

 chemical reaction proper and the appearance of free oxygen. The 

 proof for this is found in experiments with specific inhibitors and in 

 the fact that the reversible switching from oxygen evolution to an 

 equivalent consumption of hydrogen ("photoreduction") does not 

 change the quantum yield. 



(5) The chemoreduction in the dark has many traits in com- 

 mon with photoreduction. Yet we should expect differences between 

 some of the intermediates produced and utilized in the dark and those 

 made under the impact of a light quantum with an energy content of 

 40,000 calories. 



Since neither the reactions of chlorophyll or of any other catalyst 

 involved nor the nature of any one of the latter has been clearly estab- 

 lished, the scheme will convince the reader that there is a host of 

 problems to be solved. The interested investigator is likely to look 

 upon that partial problem as the most exciting and, hence, important 

 one to whose solution he believes he can contribute something sig- 

 nificant. Since each of the partial reactions of which the whole of 

 photosynthesis consists is indispensable, truly none can be more im- 

 portant than the other. However, the utilization of radiant energy 

 and the evolution of free oxygen sets photosynthesis apart from other 

 better analyzed metabolic reactions. It is here, therefore, where we 

 must expect the greatest deviations from the common types of cellular 

 catalyses and where we may find the unprcdicted. 



Though the picture given in the scheme could be seen to 



