Diffusion is the net movement of particles from a region of higher concentration to a region of lower concentration, down a concentration gradient. It is a passive process requiring no energy, and it is how oxygen enters cells and carbon dioxide leaves them. Diffusion is a core concept in the KS3 national curriculum for biology.
What is a concentration gradient?
A concentration gradient exists when there is a difference in the number of particles of a substance between two regions. Particles in a gas or liquid move randomly in all directions all the time. Because there are more particles on the high-concentration side, more random collisions push particles towards the low-concentration side than in the other direction. The result is a net movement from high to low concentration — this is diffusion.
The steeper the gradient (the bigger the difference in concentration), the faster the rate of diffusion. When concentrations equalise on both sides, there is no longer a net movement — the system is in dynamic equilibrium, meaning particles still move randomly but with no overall drift in one direction.
What factors affect the rate of diffusion?
Three main factors are assessed at KS3 and GCSE:
1. Concentration gradient A larger difference in concentration means faster diffusion. If there is almost no difference in concentration between two regions, diffusion is very slow.
2. Temperature Higher temperature gives particles more kinetic energy, so they move faster and collide more often. This increases the rate of diffusion.
3. Surface area More surface area (for example, a larger cell membrane or longer exchange surface) means more opportunities for particles to cross, increasing the rate of diffusion. This is why the small intestine has villi and microvilli — enormously increasing surface area for absorbing digested food.
A fourth factor sometimes tested is distance (the thickness of the membrane or the distance particles must travel): a shorter distance means faster diffusion.
A worked example: oxygen in the lungs
The lungs are one of the best examples of diffusion in a living system.
When you breathe in, the concentration of oxygen in the air sacs (alveoli) is high — roughly 21% of air at the alveolar surface. The blood arriving at the alveoli from the right side of the heart has been used up around the body and has a low oxygen concentration.
Because there is a concentration gradient from the alveoli (high) to the blood (low), oxygen diffuses across the thin alveolar membrane into the blood. The reverse is true for carbon dioxide: blood arriving at the alveoli has a high concentration of carbon dioxide (produced by respiration), while the alveoli have a low concentration because you have just breathed in. Carbon dioxide therefore diffuses from the blood into the alveoli and is breathed out.
The alveoli are adapted to maximise the rate of diffusion:
- Very large total surface area (roughly 70 m² in an adult human — about the size of a tennis court, according to British Lung Foundation estimates)
- Very thin walls (just one cell thick) to minimise diffusion distance
- Rich blood supply to constantly remove oxygen from the far side of the membrane, maintaining the gradient
Diffusion in plant cells
Leaves carry out photosynthesis and need a constant supply of carbon dioxide. Carbon dioxide diffuses from the air through tiny pores called stomata on the underside of leaves. Inside the leaf, cells are actively using carbon dioxide for photosynthesis, keeping the concentration low. The air outside has a higher concentration (about 0.04% or 420 parts per million as of 2024), so carbon dioxide diffuses inward down the gradient.
At the same time, oxygen produced by photosynthesis builds up inside leaf cells. Because the air outside has a lower oxygen concentration than the inside of the leaf (during active photosynthesis), oxygen diffuses out through the stomata.
Diffusion versus osmosis versus active transport
These three types of transport are commonly confused at KS3. Here is a direct comparison:
| Feature | Diffusion | Osmosis | Active transport |
|---|---|---|---|
| Substance moved | Any dissolved or gaseous substance | Water only | Any substance (against gradient) |
| Direction | High → low concentration | High → low water concentration | Low → high concentration |
| Membrane needed? | Not always | Yes (partially permeable) | Yes |
| Energy needed? | No (passive) | No (passive) | Yes (ATP) |
The key distinction: both diffusion and osmosis are passive and move particles down a gradient; active transport moves them against the gradient and requires energy.
How does diffusion relate to cell size?
This is an important KS3 biology concept. As a cell grows larger, its volume increases faster than its surface area. A large cell has a smaller surface area to volume ratio, meaning fewer particles can cross the membrane relative to the cell's needs. This is one reason why cells stay small: a small cell can supply all its needs by diffusion alone. Larger organisms use specialised exchange organs (lungs, intestinal villi, gills in fish) to overcome this limitation.
You can model this with cubes. A 1 cm cube has a surface area of 6 cm² and a volume of 1 cm³ (ratio 6:1). A 3 cm cube has a surface area of 54 cm² and a volume of 27 cm³ (ratio 2:1). The larger cube has a much less favourable ratio for diffusion.
What does the KS3 curriculum say about diffusion?
The Department for Education's Science Programmes of Study for Key Stage 3 specifies that pupils should understand "the role of diffusion in the movement of materials in and between cells." BBC Bitesize KS3 biology content, aligned with this curriculum, describes diffusion as movement of particles from high to low concentration without energy use — the same definition used by all major UK exam boards including AQA, Edexcel, and OCR.
Frequently asked questions
What is diffusion in simple terms for KS3?
Diffusion is when particles spread out from where they are crowded to where they are less crowded. They move down the concentration gradient — from high concentration to low concentration — without needing energy.
Does diffusion require a membrane?
No. Diffusion can occur in open spaces — for example, the smell of cooking spreading through a room. However, when diffusion occurs across a cell membrane, the membrane may slow or limit what can pass through. Osmosis is a special case of diffusion that specifically requires a partially permeable membrane.
How do you increase the rate of diffusion?
To increase the rate of diffusion, you can steepen the concentration gradient (increase the difference between the two sides), raise the temperature (particles move faster), increase the surface area available for crossing, or reduce the distance the particles must travel.
What is the difference between diffusion and active transport?
Diffusion is passive — particles move from high to low concentration and no energy is needed. Active transport moves particles against the gradient (from low to high concentration) and requires energy from the cell in the form of ATP.
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