A proposed mechanism of membrane transport. A transmembrane protein, the activated porter, exposes its binding site for the transported molecule to one membrane face and then to the other. In the case of a uniport porter, activation does not depend upon the solute, so transport can be effected in either direction; a symport porter is activated only when both (or neither) of the two solutes are bound; an antiport porter is activated when either, but not both, of the two solutes is bound. This proposal is often coupled with the concept of solvation substitution, the identification of solute binding as the replacement of bound water by specific liganding groups of the porter, e.g. chelation of ions, to help explain the specificity of the porter and how it maintains a barrier against transport of water across the membrane.