ROLE OF BICARBONATE IONS IN PHOTOSYNTHESIS 197 



less capable of penetrating through cell membranes than neutral, par- 

 ticularly lipophilic molecules, but also on direct experiments of Osterhout 

 and Dorcas (1926), who found that the rate of penetration of carbonic 

 acid into the interior of the unicellular alga, Valonia, is proportional to 

 the external concentration of carbon dioxide and unaffected by the addi- 

 tion of a large quantity of carbonate and bicarbonate ions. 



At first sight, certain results of Arens (1930, 1933, 19361-2) seem to contradict the 

 conclusions of Osterhout and Dorcas. He investigated the well-known fact that 

 aquatic plants, Elodea or Potamogeton, for instance, while carrying out photosynthesis 

 in natural waters, often become covered by a precipitate of calcium carbonate; at the 

 same time, the water in the neighborhood of the leaves becomes alkahne. Both observa- 

 tions are easily explained by shifts in the equihbrium (8.9), caused by the elimination 

 of carbon dioxide by photosynthesis. The interesting aspect of the phenomenon is 

 that the deposition of calcium carbonate often takes place on the upper surface only. 

 This would be natural if the consumption of HCO3" ions also took place only there. 

 Arens found, however (by experiments in which leaves were used as membranes between 

 two water-filled cells), that the bicarbonate— Ca(HC03)2 or KHCO3— is consumed on 

 the lower surface of the leaf, while an equivalent quantity of Ca++ (or K+) ions emerges 

 at the upper surface, accompanied either by CO3 ions (in the case of potassium 

 bicarbonate), or by OH" ions (in the case of calcium bicarbonate). This directed 

 transfer of ions through the leaves takes place only in light and thus appears to be 

 related to photosynthesis. Although the results of Arens indicate a penetration of ions 

 across the leaf, they do not necessarily clash with the conclusions of Osterhout and 

 Dorcas. According to the latter, the flow of carbonic acid into the cells is maintained 

 practically exclusively by the molecules of CO2, even when the medium contains a 

 large excess of carbonate or bicarbonate ions. This makes it probable that the ions, 

 HCO3- and CO3 — , cannot penetrate through the membranes at all. However, carbon- 

 ate solutions contain a small proportion of undissociated salt molecules (KHCO3, 

 K2CO3 etc.). It seems plausible that salt molecules can pass through the membranes 

 as easily as acid molecules. If this is so, the results of Arens could be attributed to 

 the penetration of the cell by these molecules, rather than by free ions. A salt, e. g. 

 KHCO3, would enter the cell in the form of neutral molecules, dissociate there into 

 ions, have a part or all of its HC03~ ions consumed by photosynthesis, and escape on 

 the opposite side of the cell in the form of other neutral molecules, e. g., K2CO3 or KOH. 

 Simultaneously with this comparatively slow flow of carbonates and bicarbonates across 

 the cell, a much larger quantity of free carbon dioxide — unobserved in Arens technique — 

 enters the cell (as shown by Osterhout and Dorcas), to be completely consumed there 

 by photosynthesis. 



It must be added that the correctness of the results of Arens is not beyond doubt. 

 Gessner (1937) found, for instance, by experiments with vaseline-covered leaves, that 

 both surfaces of Elodea leaves are equally active in supplying carbon dioxide for 

 photosynthesis. 



A complete interpretation of the transport of ions across the leaves must also take 

 into consideration the possibility of diffusion through the cell walls without actual 

 entrance into the membrane-shielded interior of the cells. 



It was repeatedly stated that the ratio [HCOs-J/ECOa] in the me- 

 dium cannot be changed without simultaneous change in acidity. The 

 occasionally observed depressing influence of carbonates on the rate 



