Ideal gas models for equilibrium osmotic pressure and for nonequilibrium flow of solvent through an ideal semi-permeable membrane are presented. These models can easily be introduced in general physics courses for students of the life sciences. The models are justified in Appendices, which include a summary of the irreversible thermodynamics of flow through membranes. Some physiological examples are presented, including the Nernst equation and Gibbs-Donnan equilibrium.

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