THE CONDUCTANCE OF THE PLASMALEMMA FOR CO2

Permeability coefficients (PS values) for CO2 of the plasma membrane (PM) of the unicellular green algae Eremosphaera viridis, Dunaliella parva, and Dunaliella acidophila, and of mesophyll protoplasts isolated from Valerianella locusta were determined from 14CO2 uptake experiments using the rapid separation of cells by the silicone oil layer centrifugation technique. The experimental PS values were compared with calculated numbers obtained by interpolation of Collander plots, which are based on lipid solubility and molecular size, for D. parva cells, mesophyll protoplasts isolated from Spinacia oleracea, mesophyll cells and guard cells of Valerianella, and guard cell protoplasts isolated from Vicia faba.The conductivity of algal plasma membranes for CO2 varies between 0.1 and 9 × 10-6 m s-1, whereas for the plasmalemma of cells and protoplasts isolated from leaves of higher plants values between 0.3 and 11 × 10-6 m s-1 were measured. By assuming that these measurements are representative for plants and algae in general, it is concluded that the CO2 conductivity of algal PM is of the same order of magnitude as that of the higher plant cell PM. Ps values of plasma membranes for CO2 are lower than those for SO2, but are in the same order of magnitude as those measured for H2O. On the basis of these results it is concluded that theoretical values of about 3000 × 10-6 m s-1 believed to be representative for higher plant cells (Nobel, 1983) and which are frequently used for computer-based models of photosynthesis, lack experimental confirmation and represent considerable overestimations. However, with several systems, including higher plant cells, the conductance of the PM for CO2 was significantly higher in light than in darkness. This suggests that in light, additional mechanisms for CO2 uptake such as facilitated diffusion or active uptake may operate in parallel with diffusional uptake. © 1990 Oxford University Press.