Since the physical interpretation of practical Kedem-Katchalsky equations is not clear, we consider an alternative, mechanistic approach to membrane transport generated by osmotic and hydraulic pressure. We study a porous membrane with randomly distributed pore sizes (radii). We postulate that the reflection coefficient (σp) of a single porę may equal 1 or 0 only. From this postulate we derive new (mechanistic) transport equations. Their advantage is in clear physical interpretation.
The classical version of the Kedem-Katchalsky equations is suitable for describing substance transport in membrane systems with well-stirred bathing solutions. However, when dealing with biological reality we are faced with a more complicated situation. For instance, in the living cell one can distinguish the central bulk area where the cytoplasm is well stirred due to its natural streaming and a certain relatively thin layer adjacent to the plasma membrane where there is no stirring. In such a situation, the passive transport of substance can be well described using the Kedem-Katchalsky equations in their more general form [1], The equations can, however, be applied provided the membrane boundary unstirred layers of cytoplasm are treated as diffusion layers. This has been shown in detail in the present work.