56 OVER-ALL REACTION OF PHOTOSYNTHESIS CHAP. 3 



one assumes the intermediate formation of molecular hydrogen, while the 

 newer one postulates a transfer of hydrogen atoms from water to carbon 

 dioxide (either directly, or through the intermediary of catalysts). We 

 prefer the second theory because the intermediate formation of molecular 

 hydrogen in photosynthesis could hardly have remained unnoticed. 



After having characterized in general terms, the chemical nature of 

 photosynthesis, it seems appropriate to add a similar general description 

 of its physical nature, by classifying photosynthesis as a sensitized photo- 

 chemical reaction. It must be sensitized by a pigment, because the reac- 

 tion substrate (CO2 + H2O) does not absorb visible light. The concept 

 of sensitization is familiar from the photographic plate, from so-called 

 " photodynamic effects" in biology, and from many photochemical reac- 

 tions in vitro. In the exact sense of the term, sensitization means a 

 photochemical reaction induced by a light-absorbing substance which is 

 not itself permanently affected by the reaction. True sensitizers are thus 

 substances which act as catalysts in light. However, substances are often 

 called "sensitizers" even if they take an active part in the photochemical re- 

 action (as this is probably the case in most photodynamic effects). We 

 cannot entirely avoid using "sensitization" and "sensitizer" in the usual 

 loose manner, and shall therefore speak of " photocatalysts " when desir- 

 ing to emphasize that we are dealing with a case of "true" sensitization. 



Chlorophyll is a photocatalyst, since no decrease in the concentration 

 of chlorophyll in leaves has been observed after intense photosynthesis 

 (c/. Chapter 19, page 549). Therefore, only truly photocatalytic reac- 

 tions can be adduced as imitations of photosynthesis in vitro. This has 

 often been neglected by investigators who have attempted to reproduce 

 photosynthesis outside the living cell, as will be demonstrated by many 

 examples in chapter 4. 



Bibliography to Chapter 3 

 The Over-All Reaction and the Products of Photosynthesis 



A. The Quantitative Balance of Photosynthesis 



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