The flow of solute and solvent across a two-membrane system
Abstract
Equations are derived, utilizing the methods of irreversible thermodynamics, for the steady state flow of solvent and a single, neutral solute across a simple, non-homogeneous membrane system. The model system analyzed is (solution, homogeneous membrane 1, solution, homogeneous membrane 2, solution), where the two homogeneous membranes have different parameters. An active transport mechanism for solute is also included as a part of membrane 1. A variety of qualitative properties of the system are derived for two special cases. In case I (pure active transport) the concentrations in the outer solutions are the same and the solute pump is functioning. In case II (pure osmotic flow) the concentrations in the outer solutions are different and the solute pump is inactive. For case I, one of the results is that there will be solution flow across the system, which may be hypertonic, isotonic, or hypotonic. It is also possible to have solvent flow with no net solute flow. For case II, a result is that the solvent flow is not linearly related to the concentration difference between the outer solutions. Furthermore, the system will exhibit rectification of solute and solvent flow. Possible applications of this model to biological systems are discussed.
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