130 PHOTOSYNTHESIS 



photosynthesis. Further as we have seen in Chapter 4 in the 

 brown algae, the blue-green algae and the red algae the 

 light absorbed by pigments other than chlorophyll is also 

 efficiently used in photosynthesis. Thus light absorbed by 

 the carotenoid fucoxanthol was found to be as efficient in 

 photosynthesis as the light absorbed by chlorophyll a\ also 

 in the living cell the absorbed light is equally efficient in 

 sensitizing the fluorescence of the chlorophyll. In the blue- 

 green and the red algae similar relationships are found 

 between phycoerythrin and phycocyanin and chlorophyll a, 

 but in these algae experimental data have not been obtained 

 to show that energy transfer occurs only by inductive 

 resonance. 



In summary, it has been found experimentally that there 

 is a transfer of energy in a physical sense between the pig- 

 ment molecules in the chloroplast. This fact leads to the 

 important question: with what substance, a pigment or 

 otherwise, does the 'physics' end and the 'chemistry* begin. 



The photochemical cycle and the first chemical step 



Chlorophyll a is the one photosynthetic pigment which 

 is common to all plants including the various classes of 

 Algae. The possibility therefore exists that chlorophyll a or 

 some fraction of it can be regarded as the essential photo- 

 chemical catalyst. Moreover French and Young (1953) have 

 pointed out that it is chlorophyll alone which shows charac- 

 teristic induction effects in intensity of fluorescence (with 

 constant illumination) in the living cell; in the red algae for 

 example the phycoerythrin and phycocyanin show no fluctua- 

 tions in the intensity of their fluorescence. This appears to 

 place chlorophyll a in a unique position with regard to the 

 other pigments. Thus it may be said that with chlorophyll a 

 we transform light energy from a physical point of view. 

 Perhaps, then, this one pigment may pass through a cycle 

 of chemical change. 



Physico-chemical theories of the transformation of light energy 



The function of theory developed on these lines is to 

 direct and guide experiments concerned with the discovery 



