146 PHOTOSYNTHESIS 



corresponds almost exactly with the mean energy of the 

 quanta of light of the fluorescence spectrum of chlorophyll a 

 in the plant. If the light energy is used efficiently it might 

 be possible to observe under some condition the production 

 of O2 with a quantum efficiency of one. This is actually what 

 Warburg has claimed from the results obtained during 

 the alternate light and dark intervals with Chlorella. But 

 any theoretical approach here needs to be guided by more 

 experimental results. 



Interpretation of quantum efficiency rneasurements 



There does not seem to be any evidence from physico- 

 chemical knowledge that more than one electron could be 

 transferred for each quantum of light against an energy 

 gradient. The transfer of one hydrogen equivalent could 

 provide a very strong reducing agent which could theoreti- 

 cally reverse any of the known enzyme systems concerned 

 with the fixation and reduction of COg to the level of carbo- 

 hydrate. But the presence of the cytochrome components 

 may indicate that some process involving a stepwise transfer 

 of electrons occurs during photosynthesis. The transfer in 

 this second hypothetical case would be in the direction of 

 a more oxidizing potential as it is in respiration. Only if it 

 is assumed that the characteristic cytochrome components 

 in the chloroplasts are inactive, can the simplest formulation 

 of photosynthesis, as given by van Niel (with « = 2 in 

 equations a and /5), represent the process in biochemical 

 terms. 



This discussion was given in order to show that at the 

 present moment little is known concerning the actual 

 mechanisms operating in the light dependent reactions. If 

 it is finally found that n in the equations a and /? turns out 

 to be more than 2, then, from a theoretical standpoint, the 

 term quantum requirement of photosynthesis has little 

 meaning. On the other hand from the physiological point of 

 view the term may be used in a perfectly definite sense. The 

 evidence that the plant in general requires from 8 to 12 

 quanta of light per COg molecule for the overall process is 

 overwhelming: the question of minimum requirement is 



