Scheme of an epithelial cell performing transcellular transport of a solute. The solute enters the cell at the apical membrane (ap) via active transport at rate and exits at the basolateral membrane (bl) to an adjacent unstirred layer (ul), from which it is extracted at rate . One of the simplest situations is represented, where the cell interior is assumed to be a homogeneous compartment characterized by a single value of solute concentration (). The extracellular unstirred layer is also characterized by a single value of the solute concentration ().
