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radicals, this is one of the minority class called stable free radicals, which 

 can be maintained for a comparatively long time. (Most free radicals, 

 for example OH, are highly reactive and therefore cannot be kept as 

 such except for very short periods.) 



By repeating the magnetic measurements at 25°C and — 150°C, it is 

 possible to show that the free radical builds up in a fraction of a second 

 at both temperatures. This suggests that the free-radical production 

 does not involve a separate chemical reaction. The unpaired electrons 

 disappear much more rapidly at the higher temperature. This indi- 

 cates that their disappearance is associated with a chemical reaction. 



The available evidence on the nature of the light reaction, then, may 

 be summarized as follows. Photons may be absorbed by any of a 

 number of pigment molecules, raising these to an excited level. All 

 the different types of pigments are somehow coupled together so that 

 energy absorbed by any one of them may be transferred to any other 

 one. (This is indicated by the appearance of the chlorophyll a fluores- 

 cence spectrum in intact chloroplasts. The quantum mechanical basis 

 for this process is under study.) In some fashion, the energy is con- 

 verted into a charge separation, and the charges so separated form 

 comparatively stable free radicals. (Calvin describes this by saying 

 that the free electrons fall into "traps.") These free radicals then react 

 to form chemical free radicals which are able to move about and lead 

 to the ultimate reactions necessary for photosynthesis. Various models 

 of the grana of chloroplasts have been built and are constantly being 

 revised in order to try to make this picture of the light reaction seem 

 reasonable in terms of molecular form and arrangement. It appears 

 that the lamellar structure of the granum may be intimately associated 

 with the over- all process of energy conversion. 



One of the reasons for supposing that the energy of excitation appears 

 directly as a charge separation is that the over-all efficiency is very high. 

 Another reason is that the resonant structure of single and double bonds 

 in both the chlorophyll and also the carotenoid pigments may make 

 both suitable for short semiconductors. (Alternate double and single 

 bonds are discussed in Chapter 27.) To justify the statement about the 

 efficiency of photosynthesis, one needs to measure this efficiency. These 

 measurements give variable results depending on how they are carried 

 out; at one time, these efficiencies were the source of an active contro- 

 versy between research workers in the field of photosynthesis. 



The efficiency of photosynthesis has been of interest for a number of 

 reasons. Before photophosphorylation and the carbon cycle were 

 understood, attempts were made to find the minimum number of steps 

 in photosynthesis. It was assumed (albeit incorrectly) that each photon 

 necessary catalyzed a different step and schemes were built for photo- 



