Analysis of Aquaporin Facilitated CO2 Diffusion Across Biological and Artificial Membranes

The contribution of membrane proteins to membrane diffusion of CO2 in biological systems has been questioned for two major reasons: the violation against the Meyer-Overton rule and the disregarding of unstirred layer effects. According to Meyer-Overtons rule membrane diffusion of gasses is not limited by the membrane itself nor can it be improved by membrane proteins. Due to theoretical considerations and experimental evidence it was concluded that the solubility diffusion model has to be applied for CO2 membrane diffusion.We studied membrane CO2 flux (JM,CO2) into Arabidopsis mesophyll cells using a scanning pH microelectrode, which has been shown to be sensitive enough to detect possible limitations by unstirred layers. Arabidopsis thaliana mesophyll cells were exposed to saturating light intensities to trigger photosynthesis and induce cellular CO2 uptake. The data indicate that under these conditions JCO2 of mesophyll cells depends on the expression of the aquaporin AtPIP1;2. Inhibition of a different aquaporin (AtPIP2;3) did not modify JM,CO2. It can be concluded that unstirred layers are not rate limiting for cellular CO2 uptake of Arabidopsisthaliana mesophyll cells, but the expression of the integral membrane protein AtPIP1;2. The results provide new arguments to the ongoing debate about the validity of the lipid bilayer model system and the Meyer-Overton rule for cellular gas transport and suggest a different view on molecular gas transport mechanisms of living cells.