Active Transport – the movement of molecules from low concentration to high concentration against the concentration gradient. Energy is required for movement to occur.
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
Diagram: The Difference Between Osmosis and Diffusion
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
In I/GCSE Biology, you need to give examples of diffusion and osmosis in living and non-living situations - The circulatory system and Heart. 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.
Effect of Osmosis on plant cells
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.
That's the end of the topic!
Drafted by Joey (Biology)