MECHANISM OF PHOTOSYNTHESIS I27 



development of knowledge concerning all the light absorb- 

 ing pigments is an essential step in the study of photo- 

 synthesis. 



When light is absorbed by the pigment system in a plant, 

 theory would suggest a number of alternative mechanisms 

 by which the energy of the quantum of light is transformed. 

 In general, with light absorbing substances, the energy is 

 degraded into heat. With a 'black' substance this may be 

 virtually complete for radiation of all wavelengths as with 

 the thermopile surface (used for measurements of radiant 

 energy). There is however little opportunity for obser\ang 

 what is going on in the absorption of light by a black body. 

 Fortunately, however, many of the pigments associated with 

 photosynthetic mechanisms show, both in the living cell and 

 in organic solution, the property of fluorescence. 



Fluorescence of pigments 



The fluorescence of chlorophyll shows a spectrum which 

 is independent of the wavelength of the light absorbed. 

 This means that there is no characteristic fluorescence 

 associated with the higher energy transition which would 

 result from the absorption of blue or violet light. The 

 necessary condition that the wavelength of the exciting 

 light must be shorter than the wavelength of the emitted 

 light is shown by the maximum of the fluorescent spectrum 

 being of a longer wavelength than the maximum of the 

 absorption band of chlorophyll in the red. With chloro- 

 phyll a, for example, we may write 



Chl+Avi - Chi*; Chi* -- Chl+Avj 



where v^ is the frequency absorbed and -i-., the frequency 

 corresponding to the fluorescence spectrum and Chi* repre- 

 sents the 'fluorescent state' of the molecule which reverts 

 to the normal state with the emission of light. 



The energy corresponding to the fluorescent state 

 remains within the pigment molecule for a time which is 

 long compared with the frequency of the exciting light; the 

 half life of the fluorescent state may be measured in the 

 following way. If the pigment is dissolved in a viscous 



