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Osmosis vs diffusion vs active transport
Three ways substances cross cell membranes, and how to tell them apart in an exam.
Diffusion is the net movement of particles from high to low concentration. Osmosis is the diffusion of water across a partially permeable membrane. Both are passive. Active transport uses energy to move substances against a concentration or electrochemical gradient. ATP can power it directly or maintain an ion gradient that drives co-transport.
Covers GCSE, AP and A-level. Exam boards differ slightly, so check your course specification.
Side by side
| Feature | Diffusion | Osmosis | Active transport |
|---|---|---|---|
| What moves | Particles, such as oxygen or a dissolved solute | Water | Ions or molecules |
| Direction | High to low concentration | From a dilute solution to a more concentrated one | Low to high concentration (against the gradient) |
| Energy needed? | No metabolic energy input | No metabolic energy input | Yes, directly from ATP or indirectly from an ion gradient |
| Membrane needed? | No | Yes, partially permeable | Yes, with carrier proteins |
| Example | Oxygen moving from the air in the alveoli into the blood | Water entering root hair cells | Mineral ion uptake by roots, glucose absorption in the gut |
Try it: change the solution outside the cell
Drag the slider to make the solution outside more dilute or more concentrated than the cell's contents, for an animal cell or a plant cell.
What affects the rate of diffusion
Diffusion is faster when:
- the concentration gradient is steeper
- the surface area is larger (many alveoli provide a large total area, and villi increase the gut's surface area)
- the distance is shorter (thin exchange surfaces)
- the temperature is higher (particles move faster)
At A-level this is summarised in Fick's law: the rate of diffusion is proportional to surface area × concentration difference ÷ thickness of the exchange surface.
Key terms
- Concentration gradient
- A difference in concentration between two regions.
- Partially permeable membrane
- A membrane that lets some substances through, such as water, but not others.
- Water potential
- A measure of water's potential energy relative to pure water. Net water movement is from higher to lower water potential. Dissolved solutes lower it; pressure also affects it.
- Hypotonic
- A solution with a lower concentration of effectively non-penetrating solutes than the cell, so water enters.
- Hypertonic
- A solution with a higher concentration of effectively non-penetrating solutes than the cell, so water leaves.
- Turgid / plasmolysed
- A plant cell full of water and firm (turgid), or one that has lost water so the membrane pulls away from the wall (plasmolysed).
Common mistakes
Osmosis is only about water. Don't say solute particles move by osmosis.
Water moves down a water-potential gradient. At equal pressure, water moves from dilute to more concentrated solutions. In plant cells, pressure can balance the effect of solutes and stop net movement.
Only active transport needs energy. Diffusion and osmosis are passive. Don't say they use ATP.
Plant cells don't burst in water. The cell wall stops them. They become turgid instead.
A worked example
This is the kind of answer SnapBio gives you when you snap a question: the answer first, then the steps.
A potato cylinder gets heavier in pure water. Explain why.
How to get there
- 1
Pure water is more dilute than the cell contents, so it has higher water potential.
- 2
Water moves into the cells by osmosis across the partially permeable membrane.
- 3
The cells gain water, so the cylinder's mass increases.
Check yourself
Which process needs energy from respiration?
Active transport moves substances against a concentration gradient, which needs energy. Diffusion and osmosis are passive.
What happens to an animal cell placed in pure water?
Water enters by osmosis. An animal cell has no cell wall, so it can swell until it bursts.
In which direction does water move by osmosis?
Water moves from a dilute solution (high water potential) to a more concentrated one (low water potential) through a partially permeable membrane.
FAQ
What is the difference between diffusion and osmosis?
Diffusion is the net movement of any particles from high to low concentration. Osmosis is a special case: the movement of water across a partially permeable membrane.
What is a hypotonic solution?
A solution with a lower solute concentration than the cell. Water moves into the cell by osmosis.
Why don't plant cells burst in pure water?
The cell wall is rigid. As water enters, the cell becomes turgid (firm) and the wall pushes back, so it doesn't burst.
What is an example of active transport?
Root hair cells take in mineral ions from soil where their concentration is lower than in the cell, and the gut absorbs glucose against its gradient. Both use energy from respiration. See cellular respiration.
Sources and further reading
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