A refined model of water and CO₂ membrane diffusion: effects and contribution of sterols and proteins.

Black lipid bilayers, a general model system for biomembranes were studied for diffusion rates of small molecules such as water or CO2 using advanced analysis techniques and cell free synthesized proteins. We provide evidence that by simple insertion of proteins or sterols the diffusion rates of water or those of CO2 decrease. Insertion of cell free synthesized water permeable aquaporins restored water diffusion rates as well as insertion of CO2-facilitating aquaporins the CO2 diffusion. Insertion of water or CO2 impermeable proteins decreased the respective diffusion rates. Therefore, for normal high cellular CO2 diffusion rates specific aquaporins are mandatory.