ACTION SPECTRA 



63 



O.D. 



400 440 480 520 560 600 640 680 

 Wavelength in m^ 



Fig. 27. The absorption spectra of chlorophyll and carotene. The chloro- 

 phyll peaks at about 430 and 665 m/x and the carotene peaks near 450 and 

 490 mfx are identified as being responsible for the action spectrum peaks in the 

 preceding figure. 



that virtually all of the energy absorbed by carotenoids is transferred to 

 chlorophyll. 



Action spectra then may be used to establish the absorption spectra 

 of compounds involved directly (or indirectly, by energy transfer) in 

 any given light-mediated process. Indeed, in many important investiga- 

 tions the action spectrum has given the first information about the 

 nature of the molecular species involved. The student should recall that 

 the comparison is between the action spectrum and the plot of the 

 extinction coefficient, E, against wavelength. Some additional examples 

 of action spectra utilization include the fitting of the action spectrum for 

 human vision to the absorption spectrum of rhodopsin, the action spec- 

 trum of phototaxis with the absorption spectrum of carotenoids, and the 

 action spectrum for flowering of certain plants with the absorption 

 spectrum for a molecule of what is now known as phytochrome. 



Up to this point we have considered situations in which the absorbed 

 light promotes a given effect. There is a second type of action spectrum 

 in which the absorbed light inhibits or inactivates some process. A trivial 

 example is that of flowering inhibition by far-red light which reverses 

 the action of the phytochrome — this action spectrum has turned out to 

 be that of an excited state of the phytochrome molecule itself. 



The more general case of inhibition results from the use of light of 

 such great energy as to damage the pigment. Ultraviolet light is the 

 usual agent of this kind of experimentation, since its photons are suffi- 

 ciently energetic to break chemical bonds, thereby altering the chemical 

 structure of the absorbing structures. Viruses, bacteria, and many kinds 



