10 Membrane permeability
Diffusion across lipid bilayers
Lipid bilayers are freely permeable to small unchanged molecules (e.g. O2, CO2, urea) and hydrophobic molecules (e.g. steroids) but are impermeable to ions and large polar molecules (e.g. glucose, Na+). Passive transport is linearly related to concentration.
Surprisingly, membranes are relatively permeable to water, which diffuses passively up the concentration gradient of a solute, the osmotic gradient. In some cells (e.g. kidney proximal tubule), the movement of water may be facilitated by specific water channels, called aquaporins.
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