Topic 3.1 · Core
Factors Affecting Diffusion
Diffusion itself never changes — it is always a net movement down a concentration gradient, driven by random particle motion. What does change is how fast that net movement happens, and Cambridge limits the required explanation to exactly four factors: surface area, temperature, concentration gradient and distance.
Surface area
A larger surface area gives more particles the chance to cross at the same time, so:
larger surface area → faster diffusion
This is the reasoning behind why exchange surfaces in living organisms — alveoli in the lungs, the lining of the small intestine, root hairs in a plant — so often have features that increase the area available for exchange. The important idea to hold onto for an exam answer is that it is specifically the area available for exchange that matters, not simply that a structure happens to be physically large. A structure can be large overall and still have a small exchange area if its shape does not maximise the surface actually in contact with the substance diffusing across it.
Temperature
Raising the temperature gives particles more kinetic energy. They move faster and change direction more often, so they spread through a substance more quickly:
higher temperature → faster diffusion
Lower the temperature and the opposite happens — particles move more slowly, so diffusion slows down with them. This is one of the more intuitive factors, but it is worth stating the mechanism rather than just the outcome: an answer that says “heat makes diffusion faster” without connecting that to kinetic energy and particle movement is stating the pattern without explaining it, and explaining it is usually where the mark actually sits.
Concentration gradient
A bigger difference in concentration between two regions produces a steeper concentration gradient. When the gradient is steeper, the imbalance between the number of particles moving in each direction is greater, so the net movement is faster:
steeper concentration gradient → faster diffusion
As diffusion proceeds, it reduces the very difference in concentration that was driving it, so the gradient becomes less steep and the rate of net movement falls as a result. This is worth remembering specifically for graph-based questions: a rate-against-time graph for diffusion typically curves, fast at first and slowing as the gradient shrinks, rather than staying a straight line.
Distance
Particles diffuse more quickly across a short distance than a long one, so:
shorter diffusion distance → faster diffusion
This is exactly why a thin exchange surface is more effective for rapid diffusion than a thick one — a thinner barrier is a shorter distance for particles to cross, nothing more mysterious than that. When you see a structure described as “thin” or “one cell thick” anywhere else in this syllabus, that phrasing is almost always pointing at this exact factor.
Investigating factors that affect diffusion
A diffusion investigation should change one factor at a time, while keeping the other conditions that could also affect the result as similar as possible. For example, an investigation comparing diffusion at different temperatures should try to keep the concentration gradient, the surface area available, and the diffusion distance the same across every repeat — otherwise you cannot be confident that temperature was the reason for any difference you observe.
A suitable measurement for this kind of investigation might be:
- the time taken for diffusion to reach a fixed distance,
- the distance travelled in a fixed time, or
- another measurable change that represents the progress of diffusion.
The same logic applies whether you are investigating surface area, concentration gradient or distance instead of temperature. A valid comparison needs:
- a clearly identified independent variable,
- a measurable dependent variable,
- the important control variables kept constant, and
- repeated measurements where possible.
When you are asked to explain a result rather than just describe it, connect the observation back to the mechanism rather than restating the pattern. A higher temperature increases the kinetic energy and movement of the particles; a steeper concentration gradient increases the imbalance between particles moving in each direction and so increases net movement from the high-concentration region towards the low-concentration region. That causal chain — condition changes, particle behaviour changes, net movement changes — is what turns a description into an explanation.
For the underlying definition of diffusion itself, and why it needs no energy from respiration, see diffusion.