ROLE OF BICARBONATE IONS IN PHOTOSYNTHESIS 



195 



carbonates, these cations could account for the absorption of 8.5 X 10~* 

 mole, or 19 ml. carbon dioxide. This is a little more than the observed 

 effect; but not all alkahne earths need to be present as carbonates. 

 Insoluble phosphates, as well as manganese (found in the ash), also can 



0.2 0.4 0.6 0.8 



Carbon dioxide pressure, atmospheres 



1.0 



Fig. 19. — The bicarbonate-ion concentration determined from e.m.f. measurements 

 (A) compared with the total combined carbon dioxide obtained by gas-analytical 

 methods (o) and that calculated from the buffer action of the phosphates (D) in the 

 sunflower-leaf sap (after J. H. C. Smith 1940). 



contribute to the absorption of carbon dioxide by the water-insoluble 

 fraction. 



3. Role of Bicarbonate Ions in Photosynthesis 



The preceding section showed that plant cells (at least those of the 

 higher plants, since no data are available on algae) usually contain, in 

 equilibrium with the atmosphere, considerably more bicarbonate ions 

 than carbon dioxide molecules. The role of these ions in photosynthesis 

 has been much discussed in the literature, but most arguments used in 

 this discussion are now obsolete; they were based on the effect of the 

 presence of bicarbonate ions in the environment on the photosynthesis of 

 aquatic plants. 



When Draper (1844) discovered that plants can live in bicarbonate solutions without 

 a carbon dioxide supply, he concluded that bicarbonate ions can be used as such in 

 photosynthesis. Later, it was reaUzed that all bicarbonate solutions contain carbon 

 dioxide molecules; but it was thought that quantitative determinations of the rate of 

 photosynthesis in relation to the concentrations, [CO2] and [HCOj"], can reveal 

 whether the bicarbonate ions participate directly in photosynthesis or not. 



Natanson (1907, 1910) postulated that CO2 molecules are the only form in which 

 carbonic acid is utilized in photosynthesis, while Angelstein (1911), who had observed 



