62 



LIGHT ABSORPTION EFFECTS 



1.75 - 



0.25 



o o 



400 450 500 550 600 650 700 750 



Fig. 26. The effectiveness of various wavelengths of light in producing photo- 

 synthetic carbon dioxide fixation. Two clear peaks are in evidence. The fact 

 that the effectiveness does not go down to zero between the peaks indicates the 

 existence of at least one more peak in the wavelengths between the two. 



we could plot, as in Fig. 26, the relative effectiveness of the various 

 wavelengths. Of course, to obtain these data we need to measure the 

 incident light intensity and the fraction of the light absorbed by the 

 system. This implies that we must use a solvent medium (photosynthetic 

 algae would be swimming in some specially designed medium) which is 

 entirely nonabsorbing in the spectral region studied, or that the fraction 

 of the light absorbed by the medium must be separately measured and 

 corrected for. 



The shape of the effectiveness spectrum or, as it is usually called, the 

 action spectrum, is then studied to see whether it resembles any known 

 pigment or collection of pigments. In the figure above, we see basically 

 three peaks. By comparing this spectrum with those in Fig. 27, we see 

 that two of the peaks are similar to those of chlorophyll, the third to that 

 of carotenoids. Thus we deduce that both pigments are involved. 



To know that carotenoids are transferring their absorbed photon 

 energy to chlorophyll requires additional experimentation involving 

 fluorescence measurements. When a wavelength absorbed only by chloro- 

 phyll is used to illuminate the organisms, the typical red chlorophyll 

 fluorescence is found. When organisms are irradiated with wavelengths 

 absorbed only by carotenoids, no typical carotenoid fluorescence results; 

 only the chlorophyll fluorescence is observed. Thus we may deduce that 

 the energy transfer is only from carotenoids to chlorophyll. Since the 

 efficiency of production of photosynthesis by carotenoid absorption is 

 nearly as high as that of photons absorbed by chlorophyll, we conclude 



