| Abstract: | Permeability coefficients (PS values) for CO2 of the plasmamembrane (PM) of the unicellular green algae Eremosphaera viridis,Dunaliella parva, and Dunaliella acidophila, and of mesophyllprotoplasts isolated from Valerianella locusta were determinedfrom 14CO2 uptake experiments using the rapid separation ofcells by the silicone oil layer centrifugation technique. Theexperimental PS values were compared with calculated numbersobtained by interpolation of Collander plots, which are basedon lipid solubility and molecular size, for D. parva cells,mesophyll protoplasts isolated from Spinacia oleracea, mesophyllcells and guard cells of Valerianella, and guard cell protoplastsisolated from Vicia faba. The conductivity of algal plasma membranes for CO2 varies between0.1 and 9 ? 10–6 m s–1, whereas for the plasmalemmaof cells and protoplasts isolated from leaves of higher plantsvalues between 0.3 and 11 ? 10–6 m s–1 were measured.By assuming that these measurements are representative for plantsand algae in general, it is concluded that the CO2 conductivityof algal PM is of the same order of magnitude as that of thehigher plant cell PM. Ps values of plasma membranes for CO2are lower than those for SO2, but are in the same order of magnitudeas those measured for H2O. On the basis of these results itis concluded that theoretical values of about 3000 ? 10–6m s–1 believed to be representative for higher plant cells(Nobel, 1983) and which are frequently used for computer-basedmodels of photosynthesis, lack experimental confirmation andrepresent considerable overestimations. However, with severalsystems, including higher plant cells, the conductance of thePM for CO2 was significantly higher in light than in darkness.This suggests that in light, additional mechanisms for CO2 uptakesuch as facilitated diffusion or active uptake may operate inparallel with diffusional uptake. Key words: Conductivity, CO2, permeability coefficient, photosynthesis, plasmalemma |
