Diffusion is the net movement of particles from where they are more concentrated to where they are less concentrated. Osmosis is the net movement of water molecules from a region of higher water potential (a dilute solution) to a region of lower water potential (a concentrated solution) through a partially permeable membrane. Both are passive and need no energy from respiration. The exam trap is forgetting that osmosis is only about water, and only across a membrane.
Why these two matter
Diffusion and osmosis come up all over the 0610 course: gas exchange in the lungs, absorption in the small intestine, water uptake in root hairs and support in plants. Examiners want precise wording, so it is worth learning the definitions as the syllabus words them.
Diffusion
The 0610 syllabus describes diffusion as the net movement of particles from a region of their higher concentration to a region of their lower concentration (down a concentration gradient), as a result of their random movement.
Points that earn marks:
- The word net matters. Particles move in both directions, but more move from high to low concentration overall.
- The energy comes from the kinetic energy of the random movement of molecules and ions, not from respiration.
- It continues until the particles are evenly spread.
- Factors that affect the rate include surface area, temperature, concentration gradient and distance.
An everyday example is food colouring spreading through water. In the body, oxygen diffuses from the air in the alveoli into the blood, and carbon dioxide diffuses the other way.
Osmosis
The syllabus describes osmosis as the net movement of water molecules from a region of higher water potential (dilute solution) to a region of lower water potential (concentrated solution), through a partially permeable membrane.
What is different from diffusion:
- Only water molecules are involved.
- A partially permeable membrane is needed. It lets water pass but holds back larger solute molecules.
- Direction is from dilute to concentrated solution, which is the same as from higher to lower water potential.
Water potential is a measure of how freely water molecules can move. Pure water has the highest water potential, and adding solute lowers it.
In the syllabus the full definition above sits in the Supplement (Extended) content. The Core content asks you to state that water diffuses through partially permeable membranes by osmosis, and to know that water moves into and out of cells by osmosis. If you are on the Core entry, learn the shorter version well and check your syllabus for the exact points.
Osmosis and diffusion side by side
| Feature | Diffusion | Osmosis |
|---|---|---|
| What moves | Any particles | Water molecules only |
| Membrane needed | No | Yes, partially permeable |
| Direction | High to low concentration of that substance | High to low water potential |
| Energy from respiration | Not needed | Not needed |
| Example | Oxygen into the blood at the alveoli | Water into a root hair cell |
Cells in different solutions
Plant and animal cells behave differently because plant cells have a cell wall.
Plant cells
- In pure water or a dilute solution, water enters by osmosis. The cell swells and pushes against its wall, so it becomes turgid. The wall stops the cell bursting, and the pressure of water inside the cells supports the plant.
- In a concentrated solution, water leaves by osmosis. The cell becomes flaccid. If enough water leaves, the cell surface membrane pulls away from the wall, which is plasmolysis.
Animal cells (no cell wall)
- In pure water, water enters and the cell can swell and burst.
- In a concentrated solution, water leaves and the cell shrinks.
That is why the water potential of blood plasma is carefully controlled: red blood cells would otherwise swell or shrink.
A potato practical, with numbers
A standard investigation places pieces of potato in solutions of different concentration and measures the change in mass. Here is a made-up set of results to show the method.
Three potato cylinders each start at 5.0 g and are left in solution for a set time:
- Distilled water: the water potential outside is higher than inside the cells, so water enters. The mass rises to 5.6 g.
- Strong sugar solution: the water potential outside is lower, so water leaves. The mass falls to 4.4 g.
- A solution with the same water potential as the cells: no net movement, so the mass stays at about 5.0 g.
To compare fairly, calculate percentage change in mass: change divided by the starting mass, times 100.
- Distilled water: \frac{5.6 - 5.0}{5.0} \times 100 = +12\%
- Strong sugar solution: \frac{4.4 - 5.0}{5.0} \times 100 = -12\%
If you plot percentage change against concentration, the line crosses zero where there is no net water movement. That point tells you the concentration of solution with the same water potential as the potato cells. When an exam asks you to explain a change in mass, name osmosis, give the direction of water movement and link it to the water potential difference.
Where active transport fits
Diffusion and osmosis are passive. Sometimes a cell has to move particles against the concentration gradient, from low to high concentration. The syllabus describes active transport as the movement of particles through a cell membrane from a region of lower concentration to a region of higher concentration, using energy from respiration. A standard example is ion uptake by root hairs. At Extended level you also state that protein carriers move the molecules or ions across the membrane.
Revision checklist
- Diffusion: any particles, net movement down the gradient, passive.
- Osmosis: water only, partially permeable membrane, higher to lower water potential, passive.
- Plant cells: turgid, flaccid, plasmolysis. Animal cells: can burst or shrink.
- Active transport: against the gradient, uses energy from respiration.
- Percentage change in mass: change divided by the original mass, times 100.
Learn the exact definitions, and several marks become straightforward. The longer-answer technique in how to answer 6-mark questions in IGCSE Biology shows how to turn these definitions into separate marking points.
Using this with past papers
ExamPal does not cover Biology yet, so you cannot have Biology answers marked there at the moment. Use official past papers and mark schemes from your school or from Cambridge and check your definitions word by word against the mark scheme. If you are studying another subject too, the past papers hub shows which subjects can be marked now, and how to check your answer against a mark scheme works for any subject.
Frequently asked questions
- What is the difference between osmosis and diffusion?
- Diffusion is the net movement of particles from a region of higher concentration to a region of lower concentration. Osmosis is the net movement of water molecules from a region of higher water potential to a region of lower water potential through a partially permeable membrane. Osmosis is the diffusion of water across a membrane.
- Is osmosis active or passive?
- Osmosis is passive, so it does not need energy from respiration. Water moves down its water potential gradient by itself. Active transport is different: it moves particles against a concentration gradient using energy from respiration.
- Why does a plant cell not burst in pure water?
- Water enters the cell by osmosis and it becomes turgid, but the cell wall resists further expansion, so the cell does not burst. An animal cell has no cell wall, so in pure water it can swell and burst.
- Do I need to know the osmosis definition for Core or only for Extended?
- The full definition using water potential is listed under the supplement (Extended) content in the 0610 syllabus. Core candidates need to know that water moves through partially permeable membranes by osmosis. Check which content applies to your entry.