Transport: Diffusion, Osmosis and Active Transport
Moving substances in and out of cells
Cells constantly exchange substances with their surroundings — taking in what they need (oxygen, glucose, water) and removing waste. There are three transport processes: diffusion, osmosis and active transport.
Diffusion
Diffusion is the net movement of particles from a high concentration to a low concentration (down a concentration gradient). It is passive — it needs no energy.
- Examples: oxygen and carbon dioxide moving in/out of cells; the smell of food spreading through a room.
- Rate of diffusion increases with:
- a steeper concentration gradient (bigger difference);
- higher temperature (particles move faster);
- larger surface area of the membrane.
Osmosis
Osmosis is the movement of water molecules from a dilute solution (high water concentration) to a more concentrated solution (low water concentration) through a partially permeable membrane. It is also passive.
- Water moves into a cell when the outside is more dilute; the cell swells.
- Water moves out of a cell when the outside is more concentrated; the cell shrinks.
- In plants: enough water makes the cell turgid (firm, supportive); too little makes it flaccid, then plasmolysed.
Active transport
Active transport moves particles from a low concentration to a high concentration — against the concentration gradient. This requires energy from respiration (so needs mitochondria).
- Example: root hair cells absorbing mineral ions from very dilute soil water, where there's already more inside the cell.
- Example: absorbing glucose from the gut into the blood when gut glucose is low.
Comparing the three
| Direction | Energy needed? | Moves what | |
|---|---|---|---|
| Diffusion | High → low | No (passive) | Any small particles/gases |
| Osmosis | High → low water | No (passive) | Water only |
| Active transport | Low → high | Yes (from respiration) | Ions/molecules against the gradient |
Surface area to volume ratio
Small or flat objects have a large surface area to volume ratio, so exchange is efficient. Larger organisms need specialised exchange surfaces (lungs, gills, small intestine) — often with a large surface area, thin walls and a good blood supply.
Worked example
Why do root hair cells use active transport to absorb minerals?
- Soil water is very dilute, so there are fewer mineral ions outside than inside the cell. Moving them in goes against the gradient, which needs energy — active transport. ✓
Common mistakes
- Saying osmosis moves "particles" — it moves water specifically.
- Forgetting active transport needs energy (from respiration) and goes against the gradient.
- Confusing the direction of osmosis — water moves to the more concentrated solution.
Exam tips
- Learn the three factors that speed up diffusion.
- Always state active transport is against the gradient and uses energy from respiration.
- For exchange-surface questions, mention large surface area, thin membrane, good blood supply.
Key facts to remember
- Diffusion: high→low, passive, any particles. Osmosis: water, high→low water, through a partially permeable membrane, passive.
- Active transport: low→high (against gradient), needs energy from respiration.
- Efficient exchange needs a large surface area, thin walls and good transport/blood supply.