J. A. BaSSHAM and M. CALVIN 



nounced at longer periods of time than those required for the 

 early steps in the reduction of carbon described earlier. It 

 seems likely that such effects are due to changes in the relative 

 rates of transformation for various photosynthetic intermediates 

 into other substances. These changes in rate of specific reactions 

 are probably photocatalytic and the light energy is used only in 

 the activation or deactivation of an enzyme. 



Another photoactivation has been reported by Warburg 

 et al. recently (61). In this case it was found that the mano- 

 metrically measured quatum yield with either red or green light 

 was greatly affected by catalytic amounts of added blue light. 

 Some rather special conditions for the culturing of the algae used 

 in the measurements appear to be necessary for tliis effect to be 

 seen, since in other experiments reported by Warburg, high 

 quantum efficiencies were obtained with red light only, so that 

 the role of the blue light again appears to be in the activation of 

 an enzyme, but in this case, one which affects the efficiency of 

 the energy conversion path. Another possible interpretation of 

 this effect will be presented in the section on the quantum 

 requirement. 



The step in photosynthesis which is perhaps most character- 

 istic is the efficient conversion of energy of an excited state of 

 chlorophyll a to the stored energy of new chemical bonds. The 

 first point to consider is the quantity of energy actually available 

 for transformation. It has been frequently proposed that the 

 primary excited state of chlorophyll a has such a short half life 

 (10~^^ sec.) that direct conversion of the electronic energy of 

 this state to some chemical reaction or the transfer to some other 

 pigment might not take place before the energy was lost by 

 fluorescence. Consequently, it was believed that transition to a 

 metastable triplet state with concurrent loss of some of the 

 electronic energy must first occur, followed by conversion or 

 transfer of the energy of the triplet state. However, according 

 to Scheibe (52), transfer of electronic energy in a condensed 

 pigment system can occur in 10~^* sec. It seems possible, 

 therefore, that in the aggregated chlorophyll-lipid-protein 



46 



