XIII. ACTION SPECTRA AND ABSORPTION SPECTRA 



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and 9). I'hiis one cannot say definitely on the basis of action spectra 

 alone that the universally destructive effects of wavelengths shorter 

 than 320 m/x are due to the action of these- wavelengths on proteins 

 and/or nucleic acids, although this is probably true in some cases. 



Mutations are also produced by the same wavelengths, and there 

 is reason to believe that the light absorber in this case is nucleoprotein, 

 as action spectra suggest (19), because of the close association of these 

 substances with the genetic mechanism of the cell. 



When photosensory responses are concerned, whether phototropic 

 bending of plants or visual mechanisms in animals, one may on various 



< 



O 



1.0 



Sso.eh 



t 0.6k 



< 



o 



< 0.41- 



a. 

 cc 

 o 



< 



0.2 



200 



350 



250 300 



WAVELENGTH, m^ 



Fig. 9. ARi and AR-z are action spectra for antirachitic action in the rat (de- 

 terminations of two different sets of investigators). E is the absorption spectrum 

 of 7-dehydrocholesterol in ethyl alcohol. Ordinates (units arbitrarily chosen to 

 bring maxima in agreement) for action spectra are 1 //(, and for absorption spectra, 

 log (I /la). Oiiginal references are cited in {12a). 



grounds, including evolutionary reasoning, suspect carotinoid pig- 

 ments {26) . Tetrapyrrole ring compounds, including chlorophjdl and 

 porphyrins, are widely distributed in living organisms and likely to 

 be photoactive, but are by no means universally so, nor are the por- 

 phyrins the principal offenders in human photopathology they were 

 once thought to be {2, Ch. 19). 



Examples could be multiplied, Init it is best perhaps to close in- 

 stead with a brief word of eauti(jn — that in this field, as elsewhere in 

 biophysics, exact physical measurements are apt to be of limited 

 usefulness unless linked with sound biological interpretation. 



