52 OVER-ALL REACTION OF PHOTOSYNTHESIS CHAP. 3 



Willstatter and Stoll (1918) were opposed to concepts of this kind. 

 They thought that the exact equivalence between the consumption of 

 carbon dioxide and the evolution of oxygen can be understood only if 

 one assumes that oxygen originates in the decomposition of carbon dioxide, 

 or, since water has to be given a place in the scheme, in the decomposition 

 of carbonic acid: 



light 



(3.9) H2CO3 > O2 + { CH2O j 



plant 



These three schemes of photosynthesis: (a) decomposition of carbon 

 dioxide (with a subsequent reaction of one of the products with water) ; 

 (6) decomposition of water (with a secondary reaction between one of the 

 products and carbon dioxide); and (c) decomposition of carbonic acid 

 (after a preliminary combination of CO2 and H2O to H2CO3), have been 

 widely used in the literature; but it was some time before it became 

 clear that all three of them implied, without telling it in so many words, 

 that photosynthesis is an oxidation-reduction reaction between carbon 

 dioxide and water. That photosynthesis is a reduction of carbon dioxide, 

 was generally acknowledged; but that reduction presupposes a reductant 

 and that in photosynthesis the only possible reductant is water (which 

 is oxidized to oxygen) was ignored. It seemed strange to call "oxidation " 

 a process in which free oxygen is produced; but the removal of hydrogen 

 from the water molecule is oxidation by any general definition of this 

 term. In the above-mentioned scheme (a), carbon dioxide is reduced to 

 carbon, and the latter "hydrated" by water, a process which seems to 

 imply no oxidation at all. In scheme (c), of Willstatter and Stoll, neither 

 the hydration of carbon dioxide to carbonic acid, nor the decomposition 

 of carbonic acid into formaldehyde and oxygen, seems to bear the charac- 

 ter of oxidation-reduction. However, both the "hydration" of C to 

 H2CO and the "decomposition" of H2CO3 into H2CO and O2, involve 

 transfers of hydrogen atoms from oxygen to carbon, and this is the mark of 

 an oxidation-reduction, even if the transfer occurs intramolecularly, i. e., 

 between two atoms belonging to the same molecule, and not inter- 

 molecularly, as in typical oxidation-reduction reactions. To say that 

 photosynthesis is an oxidation-reduction reaction between water and 

 carbon dioxide, is not to suggest an hypothesis, but to make a statement 

 of fact. 



In recent years, the mechanisms of many biological oxidation-reduc- 

 tions have been elucidated, and the transfer of hydrogen atoms (or elec- 

 trons, cf. page 219) from molecule to molecule, has emerged as the most 

 important elementary act in these processes. Thus Wieland (1913, 1914) 

 explained respiration as the transfer of hydrogen atoms from a substrate 

 (glucose, for instance) to oxygen; and Kluyver and Donker (1926) and 



