| semipermeable | Freely permeable to water (or other solvent) but relatively impermeable to solutes. Depending on the context, it has been used to imply impermeability to all solutes except very small uncharged molecules (e.g., a cell membrane), or merely impermeability to very large molecules such as proteins (e.g., a capillary membrane). (05 Mar 2000) |
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| semipermeable membrane | <biology, physiology> A membrane that is selectively permeable to only one (or a few) solutes. The potential developed across a membrane permeable to only one ionic species is given by the Nernst equation for the species: this is the basis for the operation of ion selective electrodes. (18 Nov 1997) |