1916 KINETICS OF PHOTOSYNTHESIS CHAP. 37D 



pretation of photosynthesis as an oxidation-reduction reaction. Experi- 

 mental tests of this assumption, reported in chapter 3, were of two kinds: 

 Measurements in which 0(18)-enriched water (or carbon dioxide) were 

 used as substrates of photosynthesis (Ruben, Randall, Kamen and Hyde, 

 1941, c/. table 3.VI); and measurements with normal, isotopically equili- 

 brated H2O and CO2, depending on the small difference in 0(18)-content of 

 these two compounds in isotopic eciuilibrium (Vinogradov and Teiss, 1941, 

 1947). Both types of measurements lead to the conclusion that the iso- 

 topic composition of the oxygen evolved in photosynthesis corresponds to 

 that of water taking part in the reaction and is independent of that of car- 

 bon dioxide. 



Kamen and Barker (1945, cf. also Kamen 1946) pointed out that meas- 

 urements of the first type — with 0(18) enriched water — are not entirely 

 reliable. The rate of isotopic equilibration between water and carbon di- 

 oxide, which occurs via reversible hydration, e. g.: 



(37.4A) C02<i«' + H20('«' , HaCOs^i^'i*) . COj^^"''*' + HjO^i^'i" 



depends on acidity {cf. chapter 8, p. 176). In the interpretation of table 

 3. VI, the rate of equilibration was assumed to be that prevailing in the 

 alkaline buffer medium, neglecting the possibility of a much faster equilibra- 

 tion inside the cells, where the reaction may be neutral or even slightly 

 acid. The more convincing evidence of the origin of photosynthetic oxy- 

 gen is therefore that provided by experiments with isotopically equilibrated 

 reactants. 



Two additional investigations of the latter type have been made since 

 the appearance of Vol. I; and their results were contradictory. Yosida, 

 Morita, Tamiya, Nakayama and Huzisige (1942) found that water pre- 

 pared from photosynthetic oxygen (produced by Elodea densa) was 3.3 

 7/ml. lighter than water prepared from atmospheric oxygen; from the 

 known isotopic composition of atmospheric oxygen, and of oxygen present 

 in natural water, it follows that photosynthetic water was about 3.5 7/ml. 

 heavier than natural (fresh) water. The Japanese authors interpreted 

 this result as indicating that about one-third of the photosynthetically 

 produced oxygen must have originated in carbon dioxide, and not in water — 

 a result which they interpreted by the following equations (6 is used to 

 designate oxygen originally present in carbon dioxide) : 



(37D.4B) C62 + 2 H2O ^ [H2C (Vs 6 + Vs O)] + (Vs 6 + Vs 0)2 + 



H2(V3 6 + V. O) 



(37D.4B) H2C (73 6 + V3 O) + Ve H2O > H2C (V2 6 + V2 ) + Vs H26 



Equation (37D.4B) describes photosynthesis from one molecule CO2 and 

 two molecules H2O, leading to oxygen gas with Vs of its oxygen atoms 



