Cells need to take in nutrients and remove waste. Two passive processes — diffusion and osmosis — move substances across cell membranes without using energy.
Diffusion
Diffusion is the movement of molecules from a region of higher concentration to a region of lower concentration, until they are evenly spread. It is a passive process (requires no energy).
Examples of diffusion:
- The smell of perfume spreading across a room.
- Oxygen diffusing from the lungs into the blood.
- Carbon dioxide diffusing out of a leaf during respiration.
Osmosis
Osmosis is the movement of water molecules from a region of higher water concentration (a dilute solution) to a region of lower water concentration (a concentrated solution) through a selectively permeable membrane.
| Feature | Diffusion | Osmosis |
|---|---|---|
| Substance moved | Any molecule (gas or solute) | Water only |
| Membrane required? | No | Yes — selectively permeable membrane |
| Direction | High → low concentration | High → low water concentration |
| Energy needed? | No (passive) | No (passive) |
Effects of Osmosis on Cells
- In a dilute (hypotonic) solution: water enters the cell by osmosis. Animal cells may swell and burst (haemolysis); plant cells become turgid (firm) because the cell wall prevents bursting.
- In a concentrated (hypertonic) solution: water leaves the cell. Animal cells shrink (crenation); plant cells become flaccid and the cell membrane pulls away from the wall — called plasmolysis.
- In an isotonic solution: no net movement of water; the cell stays the same.
Summary
Diffusion moves any molecule from high to low concentration without energy. Osmosis is the special case of diffusion where water moves through a selectively permeable membrane. Turgidity, flaccidity and plasmolysis describe the effects of osmosis on plant cells.
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