THE MECHANISM OF PHOTOSYNTHESIS 299 



From this discussion it is clear that, in purple bacteria, independent 

 variation of the hydrogen donor and of carbon dioxide and allied com- 

 pounds is possible, which suggests that these two "poles" of the photo- 

 synthetic chain are independent to a high degree. 



Various facts to be discussed later on strengthen this view. It appears 

 that the photosynthetic mechanism can be rendered essentially in three 

 reaction complexes (Dorrestein et at., 1942; Wassink, 1947), namely: 



(1) A dark-reaction complex in which the hydrogen donor is dehydro- 



genated. 



(2) A photochemical-reaction complex in which light energy is fixed. 



(3) A dark-reaction complex in which carbon dioxide is converted into 



carbohydrate. 



The arguments for this view are derived chiefly from the following 

 types of studies: (1) observations of a comparative biochemical nature, 

 (2) combined study of chlorophyll fluorescence and photosynthesis, (3) 

 tracer work, (4) in vitro studies of partial reactions, (5) combined study 

 of photosynthesis and redox potentials, and (6) studies on the relation of 

 phosphate conversion and photosynthesis. In the following sections are 

 discussed the chief observations of these types. This discussion will be 

 preceded by some remarks on photosynthetically active pigment systems 



(Sect. 3). 



In concluding this general survey, one final remark should not be 

 omitted. The presentation given here and later on has a shghtly sub- 

 jective character. This seems both inevitable and indicated, because a 

 discussion covering every viewpoint and every type of observation would 

 largely exceed the space available for this chapter. The author thinks 

 his presentation to be in accordance with the observations and views of 

 Van Niel, Calvin, and others. For a discussion of certain points in a 

 somewhat different manner, the reader might consult Franck's survey 

 articles (Franck, 1949, 1951^). 



3. THE PHOTOSYNTHETIC APPARATUS AND PRELIMINARY REMARKS 



ON ENERGY TRANSFER 



Chloroplasts were first recognized long ago. Leeuwenhoek, according 

 to Weier (1938), may have been the first to observe and to describe them. 

 More regular observations began about the end of the eighteenth century, 

 simultaneously with the start of the study of photosynthesis. Figure 

 5-1, reproduced here from Schleiden's textbook of 1845, indicates that 

 careful observations were already being made. Chloroplasts are also 

 pictured and described in Jussieu's textbook of about the same time. 

 The view was held by then that chlorophyll in the cell is deposited upon 



2 The author wishes to thank Professor Franck for his kindness in showing him this 

 discussion in manuscript. 



