888 CONCENTRATION FACTORR CHAP. 27 



HCO3- > CO2 + OH- 



or partially, according to the equation : 



2 HCO3- > CO32- + H2O + CO2 



In Volume I (page 157), we said that Arens' results are in need of ex- 

 perimental verification, and that, if they prove to be correct, they may be 

 explained (a) by the diffusion of ions through the leaf without penetration 

 into the interior of cells, and (6) by cell wall penetration l:»y neutral salt 

 molecules, such as KHCO3. In connection with the latter possibility, it 

 would be important to obtain quantitative information on the rate of pene- 

 tration of bicarbonate as compared to that of free carbon dioxide ; conceiv- 

 ably, the observations of Arens could be explained even if the first rate is 

 only one hundredth or one thousandth of the second one, and thus negligible 

 from the point of view of the kinetics of photosynthesis. 



By considering the penetration problem as a quantitative rather than 

 qualitative one, we can anticipate that the influence of the external con- 

 centrations [HCOa"] and [COs^"] on the carbonic acid system inside the 

 cell will depend on whether we deal with an approximate equilibrium (z. e., 

 work in the dark, or in low light), or with a photostationary state in which 

 carbonic acid is rapidly consumed by photosynthesis. In the first case, 

 even a very slow penetration of salt molecules may result in considerable 

 changes in the composition of the cell fluids, while, in the second case, the 

 effect of such slow penetration may be completely negligible in comparison 

 with that of the much more rapid flow of CO2 molecules. 



More recently, Steemann-Nielsen (1946) revived Angelstein's (1911) 

 argument. He studied the rate of photosynthesis as a function of the con- 

 centration [CO2] in the medium, using the two aquatic plants Myriophyl- 

 lum spicatum and Fontinalis antipyretica. In Fontinalis, the rates of oxygen 

 liberation found in alkaline solutions (pH 8.3, containing from 0.5 X 10-^ 

 to 5 X 10-3 mole HCO3- per liter, together with from 0.5 X 10"^ to 5 X 

 10-^ mole CO2 per liter) were hardly different from those observed in acid 

 solutions with the same amount of CO2, but practically no HCO3- ions. 

 A significantly different result was obtained with MyriopMjllum: In this 

 plant, the yield of photosynthesis in alkaline bicarbonate solutions was 

 ten times higher than in acid solutions with the same content of CO2 mole- 

 cules! In some alkaline solutions the rate of oxygen liberation by Myrio- 

 phyllum was as much as one third of the rate found in acid solutions with 

 the same total concentration ([CO2] + [HCO3-]). This was interpreted 

 by Steemann-Nielsen as indication that Myriophyllum uses IICO3- ions 

 directly with about one third the efficiency with which it uses neutral CO? 

 molecules. The difference between the two species, Fontinalis and Myri- 

 ophyllum, was tentatively related by Steemann-Nielsen to the fact that 



