ACTION SPECTRA AND OPTICAL PROPERTIES 

 OF CELLULAR PIGMENTS 



C. S. FRENCH 

 Department of Plant Biology, Carnegie Institution of Washington, 



Stanford, California 



Any effect which is caused by light must be due to the participation of 

 some pigment which absorbs that hght. The major reason for wanting 

 to know the action spectra of photochemical and photobiological 

 processes is to identify this photosenshive pigment. The active pigment 

 is the first chemical substance involved in the process caused by the 

 light. The action spectrum for a photochemical reaction is the plot of 

 the effectiveness of different wavelengths in causing the reaction to 

 take place, and under certain favorable conditions it may be an exact 

 replica of the absorption spectrum of the active pigment. In some 

 cases there may be a difference between the actually measured action 

 spectrum and the true absorption spectrum of the pigment itself. An- 

 other practical value in knowing the action spectrum, of course, is 

 that it tells us the most suitable color of the light to use for producing 

 a desired effect. 



It will clarify the problems of measuring and understanding action 

 spectra to consider first some of the peculiar optical properties of liv- 

 ing cells. For pigment identification it is necessary to be aware of the 

 difference between the absorption spectra of pigments in living cells 

 and in pure solutions. These differences are equally relevant to the 

 consideration of action spectra since action and in vivo absorption 

 spectra are usually plotted together to substantiate the identification of 

 the active pigment. 



ABSORPTION OF LIGHT BY LIVING CELLS 



Since this conference represents both zoologists and botanists, I have 

 chosen Euglena to illustrate our first topic. Figure 1 compares the 



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