CARBON DIOXIDE MOLECULES AND CARBONIC ACID IONS 887 



were carried out with land plants, in an atmosphere containing gaseous 

 carbon dioxide, or with aquatic plants, in either acid or alkaline solutions, 

 despite the fact that in the last-named case carbonic acid was present 

 mainly in the form of carbonate and bicarbonate ions. This presentation 

 is chosen because the neutral molecular species CO2 easily enters and leaves 

 the cell, while ions appear to encounter a much greater difficulty in diffusing 

 through the cell membrane. Consequently, the intracellular concentra- 

 tions of all molecular species of carbonic acid, including the ions HCOs" 

 and CO3"", probably are determined mainly by the extracellular concentra- 

 tion of the species CO2 and largely independent of the concentration of the 

 carbonate ions (and thus also of the pH of the medium, since at a given 

 value of [CO2], variations of [HCO3-] and [COs^-] are uniquely correlated 

 with changes in the pH). 



This simplification is convenient, but is likely to prove an oversimplifi- 

 cation. In chapter 8 (Vol. I, page 195), we described the controversy be- 

 tween Natansohn (1907, 1910), Wilmott (1921), Romell (1927) and James 

 (1928), on the one hand, and Angelstein (1911) and Arens (1930, 1933, 

 1936), on the other. Natansohn believed that only the concentration of 

 neutral carbon dioxide molecules in the medium is of importance for the 

 photosynthesis of aquatic plants ; the occasionally observed rate-enhancing 

 effect of bicarbonate ions (quoted by Angelstein as proof of their availabil- 

 ity for photosynthesis) was interpreted by Wilmott and Romell as a buf- 

 fering effect. (The dissociation of HCO3- ions into OH" and CO2 provides 

 ample replacement for the carbon dioxide molecules used up by photosyn- 

 thesis.) James found, in fact, that, if care is taken to avoid exhaustion of 

 carbon dioxide by strong circulation, the influence of bicarbonate ions on 

 the rate of photosynthesis tends to disappear. In many plants, excess 

 carbonates may even produce an inhibition attributable either to alkaline 

 reaction (c/. Vol. I, page 339) or to the damaging effect of the cations (as 

 indicated by the different influences of the bicarbonates of sodium and 

 potassium; cf. Vol. I, page 340, and p. 835). Experimental support of 

 Natansohn's concept was provided by the experiments of Osterhout and 

 Dorcas (1926), in which the rate of penetration of carbonic acid into the 

 interior of giant Valonia cells was proved to be proportional to the external 

 concentration of the carbon dioxide molecules and independent of the 

 concentration of the anions of carbonic acid. 



However, Arens (1930, 1933, 1936) observed that, in light, HCO3- ions 

 were taken up by the lower surface of leaves of aquatic plants (such as Elo- 

 dea), while COs^- or OH - ions were set free at the upper surface. He inter- 

 preted this observation as proof that HCOs" ions actually penetrate into 

 cells and are used there for photosynthesis, either completely, according to 

 the equation: 



