Water permeability of phospholipid vesicles.

The water permeability of phospholipid vesicles 0.5 to 10 μ in diameter bounded by one or by several lipid bilayers was measured by following the change in turbidity of a suspension after mixing in a stopped flow apparatus. A semi-empirical formulation for evaluating volume changes of vesicles with a broad size range by measurement of turbidity is developed. The rate of flow is analyzed in terms of reaction rate theory. The water permeability coefficients for phosphatidylcholine vesicles were approximately 44 μ/sec at 25°C and 70 μ/sec at 37°C. The activation energy for water transport was 8.25 kcal/mole. The results were consistent with the view that water permeates by dissolution and diffusion in the membrane.