196 FIXATION OF CARBON DIOXIDE CHAP. 8 



that at a constant value of [CO2], the rate of photosynthesis is improved by the addition 

 of bicarbonate, beheved in the availabiUty of the latter for the photosynthetic process. 

 Wilmott (1921) found, on the other hand, that the rate of oxygen production by Elodea 

 is the same in acid carbon dioxide solutions and in alkaline bicarbonate solutions with 

 the same concentration of CO2 molecules. Romell (1927) attributed Angelstein's 

 results to the capacity of bicarbonates to renew the supply of carbon dioxide (cf. page 

 177), an interpretation confirmed by James (1928), who noticed that the improve- 

 ment of photosynthesis, caused by the addition of bicarbonate ions, disappeared with 

 an increase in the rate of circulation of the medium — thus indicating that it was predi- 

 cated upon a local exhaustion of carbon dioxide. 



Because of the permeability of cell membranes to carbon dioxide 

 molecules, changes in the external concentration of this molecular species 

 produce shifts in the carbonate concentration inside the cell (accom- 

 panied by changes in the acidity of the aqueous cell phases). Whether 

 variations in the external concentration of bicarbonate ions, for which 

 the cell membrane is almost impermeable, also affect the composition 

 of the carbonic acid system in the cell, is a complicated problem of mem- 

 brane equilibrium, and as long as we do not know the answer, experi- 

 ments with varying concentrations of carbonate ions in the external 

 medium do not tell us anything definite about the part which these ions 

 play inside the cell. 



One thing, however, can be stated with certainty. If the presence of 

 carbonate anions in the medium does have an influence on the composi- 

 tion of the carbonic acid system within the cell (in the equilibrium state 

 or in the steady state of illumination), this influence is at least several 

 orders of magnitude smaller than that of free carbon dioxide molecules. 

 Warburg's buffers contain tens of thousands of HCOs" and CO3 ions 

 for each CO2 molecule (cf. Table 8.V). Nevertheless, the curves show- 

 ing the yield of photosynthesis in relation to the concentration of the 

 species CO2 in these mixtures have approximately the same shape as 

 those obtained in experiments with land plants supplied with free CO2 

 molecules only. For instance, according to chapter 27 (Vol. II), the sat- 

 uration of the photosynthetic apparatus of Chlorella occurs, in carbonate- 

 bicarbonate buffers, at [CO2] = 5 X IQ-^ mole/1., with 7.5 X IQ-^ 

 mole/1. HCOs" and 2.5 X 10"^ mole/1. CO3 ions also present in solution; 

 while the photosynthetic apparatus of wheat is saturated when the 

 concentration of carbon dioxide is of the order of 2 to 4 X 10~^ mole/1. 

 In other words, the presence of an enormous excess of HCOs" and CO3 

 ions does not essentially affect the carbon dioxide saturation, which 

 remains determined, in the first approximation, by the concentration of 

 the carbon dioxide molecules alone. 



The assertion that carbonate ions cannot penetrate into the cells as 

 rapidly as do the carbon dioxide molecules is based not only on the gen- 

 eral experience that ions, with their clusters of water molecules, are much 



