Different models of transcellular solute transport across an epithelial cell (cf. Figure 1). The dotted lines denote limits of unstirred layers (cf. Figure 1), the rest of the symbols as in Figure 1. The model in (a) (Model I) corresponds to the situation depicted in Figure 1. (b) shows a more complex model (Model II), where the solute distributes into two distinctive intracellular compartments (at concentrations and ). In the general scheme of (c), the solute may distribute along a larger number of intracellular compartments (at concentrations and to ). In particular, this work considers models having from two (Model III) to four (Model V) intermediate states “.” (d) corresponds to a simple case of bidimensional intracellular distribution of the solute (Model VI).