Edexcel IGCSE Biology · Spec 2.16
Factors Affecting Diffusion and Movement Rate
Covers recall the four factors affecting movement rate, Explain each factor using particles and Apply the factors to biological exchange surfaces.
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Factors Affecting Diffusion and Movement Rate
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Explained
The four things that change how fast substances move
Pearson asks for four factors, and only four: surface area to volume ratio, distance, temperature and concentration gradient. Learn them as a set. Questions on this topic almost always give you a situation and expect you to pick out which of the four is doing the work.
What each factor does, and why
Every one of them comes back to particles moving randomly and spreading out from where there are more of them to where there are fewer.
- A larger surface area relative to volume gives more membrane for particles to cross, so more can cross at once.
- A shorter distance means less to travel, so particles arrive sooner.
- A higher temperature gives particles more kinetic energy, so they move faster and spread more quickly.
- A steeper concentration gradient means a bigger difference between the two sides, so the net movement in one direction is greater.
Notice that none of these makes an individual particle choose a direction. Diffusion is random movement. What changes is how many particles cross, and how quickly.
Why surface area to volume ratio, not just surface area
This is the factor students most often state incompletely. A whale has an enormous surface area and still cannot rely on diffusion alone, because its volume is larger still. What matters is the surface available compared with the amount of tissue that needs supplying.
As anything gets bigger, volume grows faster than surface area. That single fact explains why large organisms need lungs, gills and a circulatory system, while a single celled organism manages with its own outer membrane.
What examiners say about this topic
Principal examiner reports for Edexcel International GCSE Biology note that most learners can state what diffusion means, so the marks are not there. They are in the explanation.
On the classic ammonia and litmus paper experiment, reports describe the best responses as those that said the litmus changed colour more quickly because the molecules diffused faster, and linked that to a higher concentration gradient. Saying only that the paper changed colour sooner describes the result. The mark is for the reason.
Reports also show these questions asking learners to calculate a mean rate of diffusion, and to modify a method to investigate temperature. It is worth being able to do both, not just describe the factors.
Applying the factors to a real exchange surface
Take an alveolus. It is one cell thick, which shortens the distance. There are millions of them, which raises the surface area to volume ratio. Blood flowing past constantly removes oxygen, which keeps the concentration gradient steep. Body temperature keeps the particles moving.
All four factors, in one structure. A root hair cell, a villus and a gill filament each do the same job with the same four ideas, which is why examiners can ask about any of them and expect the same reasoning.
A common trap in the wording
If a question says a surface is "well ventilated" or "well supplied with blood", it is telling you about the concentration gradient, not about surface area. Ventilation keeps replacing the air, so the difference across the surface stays large. Answers that mention only the size of the surface miss the point of the phrase.
Spec 2.16
What you need to know
- Recall the four factors affecting movement rate
- Explain each factor using particles
- Apply the factors to biological exchange surfaces
Active recall
Quick check
Answer each question before opening the answer.
Name the four factors affecting movement rate
Surface area-to-volume ratio, distance, temperature and concentration gradient
Why do one-cell-thick walls increase diffusion rate?
They create a short diffusion distance
How does ventilation maintain rapid oxygen diffusion?
It replaces oxygen-depleted air and maintains a steep oxygen concentration gradient
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