Transport: Diffusion, Osmosis and Active Transport
Diffusion
Net movement of particles from a region of high to low concentration (down the gradient). Passive — no energy needed.
- Faster with a bigger concentration difference, higher temperature, and larger surface area.
- Examples: O₂ into cells, CO₂ out, urea into blood.
Osmosis
Diffusion of water across a partially permeable membrane, from a dilute solution (high water) to a concentrated solution (low water).
- Plant cells in water → turgid; in concentrated solution → plasmolysed (shrink).
Active transport
Moves particles against the gradient (low → high), so it requires energy from respiration.
- Examples: root hair cells absorbing minerals; the gut absorbing glucose into the blood.
Comparison
| Direction | Energy? | Moves | |
|---|---|---|---|
| Diffusion | High → low | No | any particle |
| Osmosis | High → low water | No | water only |
| Active transport | Low → high | Yes | any particle |
Surface area : volume ratio
Smaller organisms / specialised exchange surfaces (lungs, villi, gills) have a large SA:V, speeding up exchange.
Exam tip
Only active transport uses energy. Osmosis is specifically about water — never use it for other substances.