310 RADIATION BIOLOGY 



are active in photosynthesis in the green alga Ulva. Red algae, which, 

 owing to environmental conditions, had developed little phycobilin, 

 showed higher photosynthetic efficiency of chlorophyll a. 



Some further discussion on energy transfer will be found in Sect. 5. 



4. EVIDENCE FOR THE MECHANISM OF PHOTOSYNTHESIS 

 AS OUTLINED IN SECT. 2 



4-1. OBSERVATIONS AND CONSIDERATIONS 

 OF A COMPARATIVE BIOCHEMICAL NATURE 



Van Niel (1940) points out that certain purple bacteria can act on the 

 same hydrogen donors in two different ways, namely, in the dark with 

 oxygen as the ultimate hydrogen acceptor and in the light with carbon 

 dioxide as the acceptor. He also points out that both in light and in 

 darkness various hydrogen donors may be consumed simultaneously and 

 apparently independently, so that the total rate of conversion is the sum 

 of the rates obtained for each hydrogen donor. These observations make 

 it seem unlikely that radiant energy brings about a special activation of 

 the hydrogen donors in photosynthesis. 



It is well known, moreover, that many organisms can reduce carbon 

 dioxide in the dark, so that a special photoactivation is doubtful for 

 carbon dioxide. Van Niel states: •'. . . . The probability has to be 

 seriously considered that the reduction of this substance takes place only 

 after its incorporation into some organic molecule, and as a result of 

 reducing systems active in the dark but generated in the light." This 

 conclusion is in accord with conclusions previously reached by Ornstein 

 et al. (1938) and by Wassink and Katz (1939) from comparative studies 

 of chlorophyll fluorescence and photosynthesis, and with more recent 

 results by Calvin and Benson (1948). 



These facts give support to the postulation of the partial processes 

 (1) and (3), as formulated in Sect. 2, and especially to their separation 

 from the photochemical reaction (2). 



4-2. THE COMBINED STUDY OF PHOTOSYNTHESIS 

 AND CHLOROPHYLL FLUORESCENCE 



Light absorption in the pigment-protein complex, as was mentioned in 

 Sect. 3, leads to a certain concentration of excited chlorophyll molecules, 

 made evident by fluorescence, which is controlled by the input of light 

 quanta and the transfer of excitation energy. Other molecules may use 

 this transferred energy in chemical reactions or convert it into heat. 

 Since the chief conversions of light energy into chemical energy occur in 

 the process of photosynthesis, it is plausible to assume an intimate con- 

 nection between changes in the rate of photosynthesis and changes in the 

 amount of chlorophyll fluorescence. The suggestion has been made 



