THE CARBON DIOXIDE FACTOR 1891 



Steemann-Nielsen 1952) : new measurements of the bicarbonate transport 

 through leaves of Potomageton lucens showed the rate of this transportation 

 to be high enough to play an important role in photosynthesis. 



In contrast to Arens, Steemann-Nielsen observed that bicarbonate was taken up 

 through the upper as well as through the lower leaf surface; the liberation of CO3 

 ions (or OH" ions), on the other hand, occurred only on the uppei- surface. For ex- 

 ample, after 180 min. illumination (24 klux) of a leaf placed between a 2.7 X 10 "^ N 

 HCO3- solution (below) and distilled water (above), the upper solution was 25 X 10 "^ 

 A^ in OH- 37 X IQ-^ N in CO3— and 10 X 10"^ N in HCOr (joH 10.6, as against pH 

 7.9 before illumination). If the solutions in the two compartments were exchanged, 

 no alkali was released into the lower compartment. 



In the experiment with distilled water in the upper compartment, the amount of 

 carbon transferred across the leaf was of the same order of magnitude as that used in 

 photosynthesis. The results could be quantitatively accounted for by assuming that 

 bicarbonate entered the leaf from below and suffered the following fate: about 15% dif- 

 fused without change into the upper compartment; about 50% was dismuted (2HCO3- 

 -* CO3— + CO2 + HoO) and about 35% was split (HCO3- -* CO2 + 0H-). The 

 carbon dioxide produced in the leaf by these two processes was used for photosynthesis, 

 while the OH" and CO3 — ions diffused out into the solution above. The result was the 

 same, whether the light fell on the upper or on the lower surface of the leaf. No such 

 directed transfer of ions could be observed with fronds of Ulva lactuca. 



Steemann-Nielsen 's experiments on Potomageton showed, in contrast to those of 

 Arens, no significant difference between potassium bicarbonate and calcium bicarbonate 

 in the ratio of CO3"" and OH" ions liberated on the upper side of the leaf. 



From these experiments, Steemann-Nielsen (1951, 1952) concluded that the en- 

 trance of bicarbonate ions into Potomageton cells occurs by "passive" diffusion, at a rate 

 proportional to the concentration gradient, while the excretion of OH" ions is a directed 

 "active" diffusion, the rate of which is independent of the concentration gradients. 



The experiments of Osterlind, Gaffron and Steemann-Nielsen provide 

 strong evidence for the penetration of bicarbonate ions into the cells of 

 certain species (or strains) of higher aquatic plants and algae. Other 

 plants — not only terrestrial, but also acjuatic — seem to be impermeable to 

 bicarbonate. In the case of higher aquatics, the capacity to take in bicar- 

 bonate seems to be, to a certain extent, a matter of adaptation to the 

 natural habitat — some of those living in alkaline waters, poor in free carbon 

 dioxide, have acquired it, while those living in neutral or acid waters have 

 not. However, not all aquatic plants appear capable of such adaptation; 

 Steemann-Nielsen found Fontinalis picked in alkaline lakes (pH up to 8.2) 

 to be as indifferent to bicarbonate as were plants of the same species col- 

 lected in acid waters (pH down to 5.5), or as plants of Fontinalis dalecarlica, 

 a species that grows only at pH 4.2-6.6. Gaffron's experiments seem to 

 indicate that adaptation to bicarbonate can be very rapid in unicellular 

 algae. In every case, adaptation is probably due to a change in the per- 

 meability of the cell membrane, rather than to an alteration in the chemical 

 mechanism of carboxylation in photosynthesis. Changes in the amount of 



