Osmosis is the diffusion of water molecules from a dilute solution (high concentration of water) to a more concentrated solution (low concentration of water) across a selectively permeable membrane.

By doing this, water moves down the concentration gradient.

A selectively permeable membrane allows some small substances to pass through but prevents larger substances doing the same.

The cell wall is totally permeable and has no role in controlling what enters and leaves a cell.

It is a rigid structure that provides a plant cell with support and keeps non-woody plants upright.

Diagram of a selectively permeable membrane. Large solute molecules (coloured blue) are unable to pass through the membrane. Small water molecules (coloured red) are able to pass through the membrane.