MECHANISM OF PHOTOSYNTHESIS I29 



Now when two pigments are present in a solution it is 

 sometimes found that the Hght absorbed by one pigment 

 will excite the fluorescence of the other. Chlorophyll a and h 

 show this phenomenon of sensitized fluorescence. In a 

 mixed solution the fluorescence is nearly all that of chloro- 

 phyll a. This has been studied in relation to concentration 

 by Duysens (1952). The results were in agreement with the 

 calculations of Forster and showed that with concentrations 

 of pigment approaching those in the chloroplast the eflici- 

 ency of energy transfer from chlorophyll a to chlorophyll h 

 could indeed be 100%. In Chlorella ahhough chlorophyll h 

 is present it does not show fluorescence in the cell and 

 Duysens concludes that the transfer of energy from chloro- 

 phyll b is nearly 100% within the cell. We may represent 

 this transfer as 



Chi 6*+ Chi a - Chi 6+ Chi a* 



The transfer of energy by inductive resonance depends on 

 there being an overlap in the wavelengths of the fluorescence 

 spectrum of the first pigment (Chi h) with the absorption 

 spectrum of the second pigment (Chi a). While this over- 

 lap is essential for inductive energy transfer, the actual 

 presence of an overlap does not tell us whether a given pair 

 of pigments will show sensitized fluorescence. When there 

 is no observed overlap in the spectra it has been shown in 

 certain cases that energy may be transferred provided the 

 molecules are in contact, as in a molecular chemical com- 

 pound. (For example, the haemprotein CO compounds are 

 dissociated by light not only by wavelengths absorbed by 

 the pigment, but also by wavelengths absorbed by the 

 protein.) The distinguishing feature of the inductive trans- 

 fer is that it can operate over an intermolecular distance. 



Energy transfer in the chloroplasts 



The quantum efficiency of photosynthesis was deter- 

 mined by Emerson and Lewis for Chlorella and shown to be 

 nearly constant over a range of the spectrum which includes 

 the absorption bands of both chlorophyll a and b. This must 

 imply that both the chlorophylls are equally effective in 



