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Osmotic Pressure. The force exerted by a dissolved substance on a semipermeable membrane that separates it from the pure solvent (q.v.) and through which it cannot penetrate. Yeast cell (q.v.) walls represent typical semipermeable membranes and the cells begin to show the inhibiting effect of osmotic pressure on their activity when they lose moisture as they are exposed to a medium containing 5% or higher concentrations of sugar (q.v.).