In a stirred container, the solute concentration is equalized in the bulk of the liquid. However, close to the membrane surface, molecules only move by diffusion rather than by convection. As a solute flows through the membrane, a concentration gradient builds up in close proximity to the membrane, weakening the driving force. According to equation 1, so-called measurable apparent permeability (papp) is composed of contributions: pUWL, fneutral and pM.