LIGHT AND THE PIGMENT 



101 



effectiveness of various wavelengths in the process at hand, providing 

 a so-called action spectrum for that process. If only the effective 

 wavelengths are absorbed by the pigment, as seems fairly logical from 

 the first law of photochemistry, then the color properties of the 

 pigment can be described. 



--I20 ^ 



100 



wave length in millimicrons 



Figure 7-1 

 The action spectrum for photosynthesis in Ulva taeniata, a green alga, 

 compared with the absorption spectra of the alga, chlorophyll-a, and 

 beta-carotene. The action spectrum most closely matches the absorption 

 spectrum of the alga, except for a shoulder between 460 and 500m/x 

 which is accounted for by the photosynthetically inactive carotene. 

 The absorption spectrum for chlorophyll probably accounts for most of 

 the absorption spectrum of the alga and the action spectrum, but in ether 

 solution the peaks are very sharp and shifted to the left compared to the 

 spectrum produced by chlorophyll in water or the cell. Data for the iso- 

 lated pigments from various plant physiology texts ; for the algae from 

 F. T. Haxo and L. R. Blinks, 1950, /. Gen. Physiol. 330, 389-422. 



The classical example is that of photosynthesis, and the action 

 spectrum for the process in a green alga is compared in Fig. 7-1 to 

 the absorption spectra of chlorophyll, carotene, and the plant as a 

 whole. Obviously the match between absorption of chlorophyll in 

 ether solution and the action spectrum for photosynthesis is far from 



