THE CARBON DIOXIDE FACTOR 



1893 



The possible participation of HCOs" ions in the flow of carbon into the 

 cell further complicates the situation, particularly since the relative role 

 of CO2 molecules and HCO3- ions appears to depend on the species and the 

 strain used, and may be subject to change by individual adaptation. 

 From this point of view, the only strictly comparable carbon dioxide curves 

 are those obtained in gaseous carbon dioxide atmosphere, or in acid CO2 

 solutions, such as the curves reproduced in figures 27. 2B and 27.3. In 

 plotting the results obtained in bicarbonate solutions, or carbonate-bicar- 

 bonate buffers, one faces the uncertainty whether one should use as abscissa 

 [CO2], or [HCO3-] or, more generally, [CO2] + a;[HC03-], with the coeffi- 

 cient X differing from species to species, strain to strain, and perhaps even 

 changing, in a given specimen, from one measurement to another (c/. fig. 

 37D.2). 



I ^J-- -^^" ' 



Hormidium flaccidum 

 Trilicum salivum 



Myriophyllum spicauUun 



U2 

 free C02,volume percent 



L 

 0.t 



0.6 



0.8 



1.0 



U 



Fig. 37D.3. Three types of carlwn dioxide curves, P = /[COj] (at two light 

 intensities) (Steemann-Nielsen 1952). 



On p. 888 it was suggested that, when HCOa" ions contribute to the carbonic acid 

 flow into the cell, they may traverse the cell membrane as neutral salt molecules, such 

 as KHCO3 and dissociate again in the cell sap. This remains a possible but unproved 

 speculation. 



Steemann-Nielsen (1952) suggested that the plants in which carbon 

 dioxide has only a short liquid diffusion path (of the order of 1 m) to reach 

 the chloroplasts — such as the higher terrestrial plants and terrestrial algae 

 (e. g., Hormidium), have no great difficulty in securing adequate supply of 

 carbon dioxide, even when its concentration in the medium is low (free air, 

 [CO2] = 0.03%, or 1 X 10-^ M). Their carbon dioxide saturation is 

 reached at concentrations such as [CO2] = 7 X 10"^ mole/1. {Triticiwi, 

 fig. 27.3), or lower— e. g., 2 X 10"^ mole/1, in Hormidium (fig. 27.2B). 

 Such plants do not need — and did not develop — a mechanism permitting 



