CONVERSION OF CARBON DIOXIDE INTO BICARBONATE 193 



The total quantity of chemically bound carbon dioxide in Helianthus 

 leaves, equilibrated with an atmosphere of pure carbon dioxide, is 17-19 

 ml. per 10 g. fresh leaves, corresponding to an average CO2 concentration 

 of 0.1 mole per liter, or 2% CO2 relative to the dry weight of the leaves. 

 This absorption equilibrium can have nothing to do with chlorophyll, 

 whose average concentration in the leaves is only of the order of 2 X 10"' 

 mole per liter. 



This conclusion is borne out by the observations that yellow leaves 

 absorb the same quantities of carbon dioxide as green leaves (Willstatter 

 and Stoll), that white leaves also yield carbon dioxide in vacuo (Schafer), 

 and that stalks, roots, and petals show^ the same reversible carbon dioxide 

 absorption as leaves (Smith). Schafer found that the quantity of 

 dissociable carbon dioxide increases in light; but this can scarcely be 

 taken as an indication of a direct relationship between the agent absorbing 

 carbon dioxide and the photochemical apparatus of the leaves. 



Not all figures in table 8. XI can easily be interpreted. The properties 

 of fractions 3, 4, 5 and 6 are understandable, but the carbon dioxide up- 

 take of whole leaves (rows 1 and 2) is considerabl}^ larger than the sum 

 of the volumes taken up by fractions 3 and 4, and its distribution between 

 "reversible" and "irreversible" CO2 is remarkably different for the living 

 and the frozen leaves. 



Fraction 3 behaves as a buffered solution which takes up carbon dioxide under 

 pressure (in excess of the solubiUty of this gas in pure water) by conversion into bicar- 

 bonate, but releases all of it upon evacuation. It will be shown below (cf. Fig. 19) that 

 this uptake can be attributed practically entirely to the presence of a phosphate buffer. 



The behavior of fraction 4 is that of an insoluble carbonate, which absorbs reversibly 

 an equivalent quantity of carbon dioxide by conversion into bicarbonate (c/. p. 179), 

 and thus contains, upon saturation, equal amounts of "reversible" and "irreversible" 

 carbon dioxide. This interpretation is confirmed by the properties of fractions 5 and 6, 

 since they show that the carbon dioxide-absorbing component of fraction 4 is completely 

 soluble in carbonated water. 



Fractions 3 to 6 were prepared from 10 g. of frozen leaves. An ahquot portion of 

 whole frozen leaves (No. 2) took up the expected quantity of "reversible "carbon 

 dioxide (roughly the sum of those absorbed by fractions 3 and 4), but proved to contain 

 considerably more "irreversible" carbon dioxide than did these two fractions together. 

 Fresh leaves showed an even stronger deviation from additive behavior: the amount 

 of "reversible" CO2 was only one-third of that of fractions 3 and 4, while that of "irre- 

 versible" carbon dioxide was four times larger. 



Since carbonates yield equal quantities of "reversible" and "irreversible" carbon 

 dioxide, while phosphates take up only "reversible" carbon dioxide, the combined 

 action of these two agents should lead to the uptake of more "reversible" than "irre- 

 versible" CO2 — while fresh leaves in table 8.XI show the reverse relation. This can 

 only be explained by assuming the presence of carbonates in such a state or location 

 that they are unable to take part in the absorption of gaseous carbon dioxide, but can 

 be decomposed by acid. 



Smith suggested that the difference between Uving and frozen leaves can be ex- 

 plained by the rapid carbon dioxide production by respiration in the former ones — a 



