any of a group of organic compounds that facilitate the transport of ions across the cell membrane.

(ion-carrier) - a lipophilic charged or neutral organic molecule, which has an affinity for a particular ionic species in solution (i.e. forms selectively reversible complexes with a particular ionic species). For ion-selective electrodes, there are introduced into a polymeric membrane, which forms a barrier between the test solution and the electrode. The detected ion is passed from one ionophore molecule to the next by diffusion under a concentration gradient and hence is selectively transferred across the membrane and causes a build up of electrical charge on the inside of the membrane.

An ionophore is a lipid-soluble molecule usually synthesized by microorganisms to transport ions across the lipid bilayer of the cell membrane. There are two broad classifications of ionophores.