Key learning points

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

Keywords

• Selectively-permeable membrane - A membrane is selectively-permeable if some substances can move through it but others cannot.

• Particles - All substances are made up of particles that are too small for us to see.

• Diffusion - Diffusion is the net movement of particles of a substance down a concentration gradient.

• Concentration gradient - A concentration gradient is a difference in the concentration of particles from one area to another.

• Net movement - Net movement is the overall movement of particles from one area to another.

Common misconception