198 FIXATION OF CARBON DIOXIDE CHAP, 8 



of photosynthesis at constant [002] may thus have been caused by the 

 alkahnity of the medium rather than by the carbonate ions themselves. 

 It has, for example, been reported by van der Honert (1930) and van der 

 Paauw (1932), that Hormidium does not thrive in carbonate buffers at 

 all; Chlorella cells are more resistant but they too are affected by the 

 more alkaline of Warburg's buffers (Emerson 1936; Emerson and Green 

 1938). 



To sum up : all experiments described so far do not teach us anything 

 about the true role of carbonate ions in photosynthesis. The occasional 

 improvement in rate caused by an addition of these ions at a constant 

 concentration of carbon dioxide can be attributed to the removal of 

 COa-depletion effects, while the occasional decrease in rate caused by the 

 same treatment may be due to changes in pH. The meaning of Arens' 

 observations on the direct transfer of ions across the leaves cannot be 

 assessed without new quantitative experiments. 



While we find no basis for the claim that carbonate ions play a direct 

 part in photosynthesis, neither can we assert that they play no such 

 part at all. Even if these ions do not penetrate into the cell from the 

 outside, they are produced inside by the interaction of carbon dioxide 

 molecules with alkahzing buffers (HPO4 — , CaCOs, etc.). In equilib- 

 rium with the atmosphere, the concentration of HCOa" ions in the cell 

 usually is many times larger than that of free CO2 molecules. We 

 have assumed that the immediate substrate of reduction is a complex, 

 {CO2}. The participation of HCOs" (and CO3 — ) ions in the formation 

 of this complex may be twofold. If this complex can be used for photo- 

 synthesis both as a carboxylic acid RCOOH and as its anion, RCOO~, 

 then the presence of bicarbonate ions can increase its equilibrium concen- 

 tratiov. If, however, only the neutral complex can take part in photo- 

 synthesis, then the presence of anions, although it cannot affect the 

 equihbrium concentration of the reduction substrate, may accelerate the 

 rate of its formation. 



One argument has been presented in favor of the assumption that the 

 main reaction sequence of photosynthesis does not include the inter- 

 mediate formation of bicarbonate ions. It was based on an estimate of 

 the rate of hydration of carbon dioxide in the cell, a rate which appears to 

 be too slow to allow all the reduced carbon dioxide molecules to pass 

 through the hydration stage. It was mentioned on page 175 that the 

 (noncatalyzed) hydration of carbon dioxide requires about one minute 

 at room temperature. Consequently, only about 5 X 10~^ X p moles 

 of carbon dioxide (p = carbon dioxide pressure in mm.) can be hydrated 

 (noncatalytically) each second in one liter of cells. On the other hand, 

 at p = 1 mm. (a pressure sufficient for carbon dioxide saturation of pho- 

 tosynthesis in most plants), as much as 10~^ mole carbon dioxide can 



