CHAPTER 5 



The Mechanism of Photosynthesis 



E. C. Wassink 



Laboratory for Plant Physiological Research, Agricultural University, 



Wageningen, Netherlands 



Introduction. General characterization of the photosynthetic process. The photo- 

 synthetic apparatus and preliminary remarks on energy transfer. Evidence for the 

 mechanism of photosynthesis as outlined in Sect. 2: Observations and considerations of a 

 comparative biochemical nature — The combined study of photosynthesis and chlorophyll 

 fluorescence — Tracer experiments — Experiments on isolated chloroplasts — The connection 

 between redox potentials and photosynthesis — On the connection between phosphate metab- 

 olism and photosynthesis. Some remarks on energy transfer in photosynthesis. Siim- 

 mary. References. Addendum. 



1. INTRODUCTION 



In older literature the process now called "photosynthesis" was often 

 simply denoted as "assimilation" or, more precisely, as "carbon dioxide 

 assimilation." The inadequacy of these older terms has gradually been 

 recognized. "Assimilation" should mean all kinds of conversion of sub- 

 strates into cell constituents, including reserve substances; "carbon diox- 

 ide assimilation" should indicate all cases in which carbon dioxide is 

 incorporated into these compounds. In this sense both terms cover a 

 range of phenomena wider than photosynthesis. The term "photosyn- 

 thesis" is now restricted to the conversion of carbon dioxide into cell 

 constituents, chiefly carbohydrates, by the interaction of radiant energy. 

 The discrimination between carbon dioxide assimilation and photosyn- 

 thesis became desirable with the recognition — about 1935 — that various 

 types of heterotrophic cells more or less continually incorporate carbon 

 dioxide into metabolic products (cf. Kluyver, 1939). After 1940 this view 

 was corroborated by tracer studies (Van Niel et al., 1942). Moreover, 

 the chemosynthetic carbon dioxide assimilation of certain bacteria capa- 

 ble of oxidizing inorganic substrates, yielding, for example, nitrates and 

 sulfates, has been known as a counterpart of photosynthesis for more 

 than sixty years. 



These considerations suggest that the assimilation of carbon dioxide 

 by way of photosytithesis is not completely isolated in a biochemical 

 sense and that light is not strictly necessary for the incorporation of 



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