Selective permeability means that a cell membrane permits different substances to pass at different rates under comparable conditions. Both the lipid bilayer and the available membrane proteins contribute to this selection.
Artificial boundaries can also have selective permeability. For osmosis, specify which solutes cross and over what observation interval: a boundary that retains one solute need not retain another. A semipermeable membrane is the ideal solvent-only boundary used in the osmotic pressure calculation.
membrane permeability describes passage of a specified substance through a membrane under stated conditions. It is distinct from lateral diffusion within the membrane and from the mechanical force needed to rupture the membrane.
A semipermeable membrane ideally passes the solvent while excluding the specified solutes. In the aqueous osmosis model below, water can cross but the counted dissolved particles cannot. Real selective permeability is often a matter of different rates, so this idealization needs checking over the measurement interval. Neither an impermeable sheet nor a freely mixing opening realizes this model.[ref]
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