. Elementary biophysics: selected topics . 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. Ind
. Elementary biophysics: selected topics . 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 s
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