Diffusion through a selectively-permeable membrane: practical

Particles of some substances are small enough to diffuse through holes in a selectively-permeable membrane. Particles of these substances move through the selectively-permeable membrane in both directions. Predict the direction of net movement of particles through a selectively-permeable membrane. Observe the direction of net movement of a substance through a selectively-permeable membrane (e.g. Visking tubing). Explain observations of the diffusion of a substance through a selectively-permeable membrane.