Learning outcomes:
understand definitions of diffusion, osmosis and active transport
understand that movement of substances into and out of cells can be by diffusion, osmosis and active transport - b - c - d
understand the importance in plants of turgid cells as a means of support
understand the factors that affect the rate of movement of substances into and out of cells, to include the effects of surface area to volume ratio, temperature and concentration gradient - b - c - d
describe experiments to investigate diffusion and osmosis using living and non-living systems.

Diffusion – the movement of molecules from high concentration to low concentration, down a concentration gradient.

Osmosis – the movement of water molecules from high concentration to low concentration through a partially permeable membrane
Active Transport – the movement of molecules from low concentration to high concentration against the concentration gradient. Energy is required for movement to occur. As is a protein carrier.


Diffusion and osmosis occur because molecules have kinetic energy. The molecules constantly bounce off each other all the time, gradually spreading out. Eventually there will be an even mixture of molecules, which is called an equilibrium. Diffusion can be affected by;

- temperature (increases Kinetic energy)
- stirring (increases Kinetic energy)
- surface area for diffusion
- thickness / distance molecules have to diffuse
- the size of the concentration gradient
- the surface area to volume ratio

Plant cells are normally turgid (swollen full of water). This is important because it provides strength to plants. Plant cells have a cell wall to stop them bursting when turgid. When plant cells start to lose water they become flaccid. Flaccid plants lose their strength and start to wilt. Eventually, flaccid cells become plasmolysed as the cell membrane begins to peel away from the cell wall. This kills the cell.

You need to give examples of diffusion and osmosis living and nonliving situations. Good examples of diffusion are ink chromatography, or the diffusion of KMnO4 crystals (purple) into water. Diffusion of gases in the lung or leaf are also good examples.
Osmosis can be shown artificially using visking tubing, or potato chips in salt solutions of different concentrations.