ACTION SPECTRA 61 



simple molecules, the information may be enough to permit a fairly good 

 preliminary model of the three-dimensional structure. 



Finally, it should be noted that when light is absorbed by dichroic 

 structures, the re-emitted light (the so-called fluorescence) will be polar- 

 ized, provided only that the structures (or their constituent parts) do not 

 rotate appreciably during the time (about 10 -8 sec) required for the re- 

 emission of the light. The study of the fluorescent light then provides 

 additional information about the structures involved. 



4. Action spectra 



For light to produce an effect it must be absorbed, and for absorption 

 to take place there must be energy level differences equal to the energy 

 of the incident photon. Once the photon is absorbed, the pigment (the 

 substance absorbing the light) may itself produce the effect being ob- 

 served or it may pass the light energy on to another substance by one of 

 several mechanisms. For example, it may happen that a pair of energy 

 levels of the pigment chance to coincide with a pair of levels of an 

 adjacent molecule. If so, the energy may be directly transferred, with the 

 result that the electron of the pigment molecule ends up in its lowest 

 energy state, while an electron of the adjacent pigment is raised to the 

 excited energy state. If this adjacent molecule can produce the effect 

 being studied, the energy transfer will permit the utilization of light not 

 directly absorbed by the effective molecule, thereby expanding the effi- 

 ciency of the system. 



As an example, if light is incident on chlorophyll, the wavelengths ab- 

 sorbed are found to be in the red and blue regions of the spectrum, so 

 that the chlorophyll color is that of the unabsorbed green light; the result 

 of the absorption is the production of photosynthetic activity. Carot- 

 enoids absorb in the blue-green, and it is found that such light is 

 effective in photosynthesis, thereby suggesting that the carotenoids and 

 chlorophyll are energetically linked. To prove the linkage, we would 

 have to show that the carotenoids themselves effect no photosynthesis. 

 This will be shown later. 



To obtain this information, we measure the effectiveness of light of 

 various wavelengths in producing the effect (in our example of photo- 

 synthesis, the effect could be oxygen production, C0 2 fixation, etc.). 

 This is done by irradiating an organism at such an intensity that the 

 photosynthetic activity is not saturated, which means that we should 

 choose an intensity such that the amount of photosynthesis would double 

 if the intensity were doubled. Thereby we obtain the amount of photo- 

 synthesis per incident photon of the given wavelength. Then the entire 

 experiment is repeated with light of various wavelengths, so that finally 



