PHOTOSYNTHESIS AS A SENSITIZED OXIDATION-REDUCTION 51 



An even more striking characteristic of photosynthesis has been claimed by Spessard 

 (1940), who asserted that photosynthesis results in "conversion of hght into matter." 

 His experiments purported to show that a sealed vessel containing photosynthesizing 

 plants increases in weight with the progress of photosynthesis, and certainly will receive 

 a less spectacular explanation. 



D. Photosynthesis as a Sensitized 

 Oxidation-Reduction * 



After having described the over-all chemical reaction of normal 

 photosynthesis by equations (3.6) and (3.7), we will now assign to this 

 reaction its proper place in the general classification of chemical reac- 

 tions, by identifying it as a sensitized photochemical oxidation-reduction. 



When the first light was thrown on the chemistry of photosynthesis 

 by the investigations of Ingen-Housz and Senebier, it appeared as "de- 

 composition of fixed air" {i. e., carbon dioxide) with the oxygen escaping 

 into the air, and carbon retained by the plants. Even when de Saussure 

 in 1804 added water to the reaction components, he did not doubt that 

 all oxygen liberated in photosynthesis was the product of decomposition 

 of carbon dioxide, while the role of water was vaguely described as "con- 

 tributing its elements" to the formation of organic matter. Later, the 

 "decomposition" of carbon dioxide was generally recognized as a re- 

 duction of this compound, and different paths of reduction were devised, 

 e. g., by Liebig (1843) and Baeyer (1870). According to Liebig, the plant 

 acids— oxalic, malic, succinic, tartaric — are the main intermediates in 

 the reduction of carbon dioxide to carbohydrates; while according to 

 Baeyer, formic acid and formaldehyde are the two main stepping stones 

 in this reduction. The question of the way in which water participates 

 in the reduction was left aside by both authors. 



However, some chemists have looked on photosynthesis from a dif- 

 ferent angle. As early as 1864, Berthelot suggested that water is decom- 

 posed by photosynthesis into hydrogen and oxygen, while carbon dioxide 

 is dissociated into carbon monoxide and oxygen, after which the two 

 products unite to form a carbohydrate, 

 (3.8a) CO + H2 > { CH2O ) 



Although this theory was vague, it clearly made both carbon dioxide 

 and water subjects of primary transformations in photosynthesis. Fifty 

 years later, Bredig (1914) and Hofmann and Schumpelt (1916) turned 

 the spotlights entirely on the transformation of water. They suggested 

 that the primary effect of light in photosynthesis is the decomposition of 

 water into oxygen and hydrogen. The former escapes into the atmosphere, 

 while the latter reduces carbon dioxide to the carbohydrate level (by a 

 secondary process, not specifically defined). 



* Bibliography, page 60. 



