CONVERSION OF CARBON DIOXIDE INTO BICARBONATE 189 



1. Solubility of Carbon Dioxide in Plant Sap 



In all measurements of the carbon dioxide absorption by plant 

 tissues, the solubility in cell water has to be corrected for in order to 

 determine the extent of "chemical" binding. This component is small 

 at the low carbon dioxide concentrations, but grows with increasing 

 pressure, when the chemical absorbers become saturated. 



According to table 8.1, the cell water, if it wxre pure, would con- 

 tain, in contact with the free atmosphere at 25° C, about 9 X 10"^ mole 

 per liter of CO2 molecules, and in contact with an atmosphere of pure 

 carbon dioxide, approximately 3 X IQ-^ mole per liter. Since an aver- 

 age leaf is about 80% water, the first concentration corresponds to about 

 2 X 10-''%, and the second to 0.6% dissolved CO2, relative to the dry 

 weight of the leaves. 



A correction is needed in exact calculations for the effect of salts and 

 nonelectrolytes on the solubility of carbon dioxide. However, this 

 effect cannot exceed a few per cent (c/. page 179). Smith (1940) found 

 that partial drying of leaves affects the absorption of carbon dioxide 

 somewhat more than can be accounted for by the amount of evaporated 

 water, and ascribed this effect to the solubility-depressing influence of 

 sugars and salts. He noticed also that expressed and acidified cell sap 

 absorbs about 10% less carbon dioxide than the same volume of pure 

 water. Leaves of Sedum prealtum, whose sap has a strongly acid reaction 

 (pH 4.08) and in which no bicarbonates can be formed, also absorb less 

 carbon dioxide than calculated from the solubility of this gas in pure 

 water. 



The possible contribution of lipoids to the reversible absorption of 

 carbon dioxide by plants (c/. page 179) has never yet been taken into 

 consideration. The small volume of the lipoid phase compared with the 

 hydrophilic phases (cytoplasm and cell sap) perhaps makes the omission 

 permissible. 



2. Conversion of Carbon Dioxide into Bicarbonate in Plants 



If the cell water were unbuffered, about 15% of dissolved carbon 

 dioxide would be in the form of bicarbonate ions in ordinary air, and a 

 smaller proportion in an atmosphere enriched in carbon dioxide. The 

 sap, under these conditions, would be acid. The absorption of carbon 

 dioxide in excess of normal solubility, actually observed with almost all 

 investigated plants, must be attributed to a conversion of carbon dioxide 

 into bicarbonate by alkahzing agents. The most important of these are 

 soHd alkaline earth carbonates and dissolved primary phosphates. 



The presence in plants of solid carbonates was discovered by Berthelot 

 and Andre in 1887. They removed "free" carbon dioxide (that is. 



