THE MECHANISM OF PHOTOSYNTHESIS 297 



of the process. Each of these reactions may act as the master reaction 

 governing the total rate of the process; the reaction that predominates 

 depends on which environmental condition is the limiting factor. In this 

 way properties of both reactions have been established, e.g., the quantum 

 yield of the photochemical reaction and its independence of temperature, 

 and the cyanide and temperature sensitivity of the dark process (cf. 

 Warburg, 1925; Warburg and Negelein, 1923; Wassink et al, 1942). It 

 was found, furthermore, that what has been denoted as "dark reaction" 

 consists of several processes, each of which may act as the master reac- 

 tion, depending on external limitations. 



It follows from Eq. (5-1) that the ratio of CO2 absorbed to O2 evolved 

 theoretically equals unity. In many cases this quotient does not deviate 

 much from 1. This suggests that the oxygen evolved originated from 

 carbon dioxide, a conclusion later found to be unjustified. Arguments 

 were collected for the view that carbon dioxide is reduced by hydrogen 

 derived from the water from which the oxygen is evolved. The oxygen 

 of carbon dioxide enters partly into the organic matter formed, partly 

 into water. Equation (5-2) does more justice to this viewpoint than 

 Eq. (5-1): 



CO2 + 2H2O -^ (CH2O) -1- O2 + H2O. (5-2) 



The correctness of this view was proved by investigations that made 

 use of the isotope O^^. Ruben et al. (1941) used either water or carbon 

 dioxide artificially enriched in O^^ in photosynthesis experiments. The 

 0'* content of the evolved oxygen was always close to that of the water 

 and was distinctly different from that of the carbon dioxide used in the 

 experiments. It has been argued that this result probably was not fully 

 conclusive, since, with a reasonable rate of exchange in the cells or in the 

 chloroplasts, the excess of water would easily give rise to the observed 

 effect. This criticism, however, does not apply to similar experiments 

 by Dole and Jenks (1944), who made use of the natural small differences 

 in 0'* between water and carbon dioxide with essentially the same result 

 as that of Ruben et al. The restriction should be made that, strictly 

 speaking, the result is valid only for the objects investigated, i.e., a green 

 alga and a few land plants. There is, however, no reason to suppose 

 that other plants will behave differently. 



One type of facts fails to fit into this scheme. It is assumed that the 

 oxygen of the air has originated chiefly from the photosynthetic process, 

 or at least that the oxygen now present has passed through the metabolic 

 cycle many times. From the above statements it would be assumed that 

 the O^^ content of the atmospheric oxygen would closely resemble that 

 of water and not that of carbon dioxide (or of carbonate rocks). Actu- 

 ally the reverse is true (Kamen and Barker, 1945). Notwithstanding 

 the fact that this discrepancy has still to be clarified, it is not considered 



