PHOTOSYNTHESIS 



to the longer wavelengths with increasing crystal size observed. 

 A maximum shift of the red peak to about 7450 A was found, 

 beyond which there was no further shift with larger crystals. 

 This shift is ascribed to resonance interaction between identical 

 chromophores and to migration of the resonance energy through 

 the array. 



The phenomenon of energy migration through the pigment 

 aggregate brings us to a consideration of the light absorption 

 process. According to present views (24) it appears that most 

 of the energy absorbed by plant pigments for subsequent con- 

 version to chemical energy is either absorbed directly by chloro- 

 phyll a or transferred to chlorophyll a. The excited state is 

 presumed to differ in energy from the ground state by only about 

 42 kcal./mole, corresponding to 6800 A light, the longest wave- 

 length light that brings about photosynthesis with high efficiency. 

 It is postulated that the extra energy that is absorbed at shorter 

 wavelengths, either by chlorophyll a or other pigments, is con- 

 verted to vibrational energy and eventually lost as heat. Thus 

 the course of energy transfer from chlorophyll on would be 

 unaff"ected by the wavelength of the light absorbed. It has long 

 been known, in fact, that the yield of oxygen evolved per quan- 

 tum of light absorbed is as high for red light as at any other 

 wavelength. 



On the other hand, light absorbed at wavelengths around 

 4800 A produces a relatively lower quantum yield, indicating 

 that pigments which absorb in that range may transfer their 

 energy to chlorophyll inefficiently or may transfer some of their 

 energy to other chemical reactions inefficiently. In the latter 

 case, the course of subsequent steps in photosynthesis should be 

 to some extent affected by the light energy converted to chemical 

 energy without passing through the excited chlorophyll a stage. 

 Such an effect has recently been reported by Voskrenskaya (59), 

 who has studied the products of carbon reduction, using G'^ as 

 a function of the wavelength of the incident light. She reports 

 an enhanced ratio of protein to carbohydrate in blue light as 

 compared with red light. This effect seems to be more pro- 



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