334 RADIATION BIOLOGY 



bii\i that for the purple bacteria is a variety of more reduced substances 

 (Sects. 1 and 2). Evidence has accumulated in favor of the view that the 

 photosynthetic chain may be divided into three well-distinguishable parts, 

 each of which may consist of a number of elementary steps. The three 

 parts are (1) the dehydrogenation of a hydrogen donor, (2) the "photo- 

 chemical process" proper, and (3) the reduction of carbon dioxide (Sect. 2.) 

 The light energy enters directly only into link 2 ; the other processes con- 

 sist of only dark reactions. The first noticeable chemical act of the light 

 is the production of a "reducing agent." The evidence for this view has 

 been collected from various independent lines of research in relation to 

 photosynthesis, e.g., studies on comparative biochemistry, studies of 

 chlorophyll fluorescence along with photosynthesis, studies using tracer 

 elements (e.g., C^^), in vitro studies of partial reactions, studies on redox 

 potentials, and studies on phosphate conversions. A detailed discussion 

 of the evidence from these various fields is given in Sect. 4. 



Some remarks on the structure of the photosynthetic apparatus are 

 given in Sect. 3; some remarks on possible modes of energy transfer inside 

 the chloroplasts or photosynthetic bacterial cells are given in Sect. 5. 



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