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Diffusion is the net movement of particles from high to low concentration. Free GCSE AQA Biology notes with key terms and 14 questions answered.
What is diffusion and what affects its rate? is part of Transport in cells in GCSE AQA Higher Biology.
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Diffusion is the net movement of particles from high to low concentration. Free GCSE AQA Biology notes with key terms and 14 questions answered.
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Use this lesson to stay inside Transport in cells while connecting the detail back to the wider Cell biology topic.
Read the core explanations from What is diffusion and what affects its rate? before testing yourself from memory.
Picture dropping a sugar cube into a cup of tea. Without stirring, the sweetness spreads throughout the liquid. This spreading happens because particles are constantly moving and bumping into each other - that's diffusion in action.
Here's the key insight: particles don't 'know' where to go. They move randomly in all directions. But when there are more particles in one area, more will randomly move away from that crowded area than move into it. This creates net movement from high to low concentration.
Trap: Many students think particles only move from high to low concentration. Actually, particles move in all directions randomly - it is that more move away from crowded areas, creating the net movement we observe.
Your cells rely on diffusion every second you're alive. Substances move into and out of cells across cell membranes via diffusion - no energy required from the cell.
Three crucial examples you need to know:
Oxygen and carbon dioxide in gas exchange: In your lungs, oxygen diffuses from air (high concentration) into your blood (lower concentration). Carbon dioxide does the reverse - from blood into air to be breathed out.
Urea removal: This waste product diffuses from cells (where it's made) into blood plasma (lower concentration). Your kidneys then filter it out for excretion.
Nutrients entering cells: Glucose and other nutrients diffuse from blood into cells where they're needed for respiration.
Three factors control how fast diffusion happens. Understanding these helps explain why your lungs have millions of tiny air sacs, not one big balloon.
Factor 1: Concentration gradient - Think of it like a hill. The steeper the hill, the faster things roll down. A bigger difference in concentration means faster diffusion.
Factor 2: Temperature - Higher temperature gives particles more kinetic energy. They move faster and collide more often, speeding up diffusion.
Factor 3: Surface area of the membrane - More area means more space for particles to cross. It's like having more checkout lanes in a supermarket - more customers can be served at once.
You won't get complex calculations in GCSE, but you need to predict and compare diffusion rates. Here's how to think through these problems systematically.
Worked example: Comparing Diffusion Rates
Two identical cells are placed in glucose solutions. Cell A is in 10% glucose, Cell B is in 5% glucose. Both cells initially contain 2% glucose. Which cell will have glucose entering faster?
Answer: Cell A will have glucose entering faster due to the steeper concentration gradient (8% vs 3%)
Trap: Students often just look at the outside concentration and think 10% > 5% so A is faster. You must calculate the actual gradient - the difference between outside and inside concentrations.
Diffusion questions appear in every GCSE biology paper. Here's how to tackle them and avoid the traps that catch most students.
Exam depth extension: In what is diffusion and what affects its rate?, connect what is diffusion? to diffusion in living cells using clear cause-and-effect statements. When you explain biological processes, include what changes, why it changes, and the consequence for cells, organisms, or populations. If calculations appear, show each step, include units, and end with a sentence that interprets the result in biological context.
Command word focus: 'Explain' questions want you to give reasons. Use 'because' to link your points. 'Describe' questions want you to state what happens, not why.
Common exam mistake: Writing 'particles move from high to low concentration' without mentioning it's NET movement. Examiners want to see you understand particles move randomly in all directions.
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Move to the related lessons or back to the topic page when you need the wider sequence before exam-style practice.
Diffusion is the net movement of particles from high to low concentration. Particles move randomly in all directions, but when there are more particles in one area, more move away from that crowded area than move into it, creating the net movement.
Three factors affect the rate of diffusion: the concentration gradient, temperature and the surface area of the membrane. A bigger difference in concentration, a higher temperature and more surface area all make diffusion faster.
The steeper the concentration gradient, meaning the bigger the difference in concentration between two areas, the faster the rate of diffusion. It is like a hill: the steeper the hill, the faster things roll down.
Diffusion happens because particles move randomly in all directions, but when there are more particles in one area, more will randomly move away from that crowded area than move into it. This creates a net movement from high to low concentration.
Higher temperature speeds up diffusion because it gives particles more kinetic energy. The particles move faster and collide more often, speeding up diffusion from high to low concentration.
A larger membrane surface area speeds up diffusion because there is more space for particles to cross at once, like having more checkout lanes in a supermarket. This is why lungs have millions of tiny air sacs rather than one big balloon.
Examples of diffusion in living cells include gas exchange in the lungs, where oxygen diffuses from air into blood and carbon dioxide from blood into air. Urea also diffuses from cells into blood plasma, and glucose diffuses from blood into cells for respiration.
Urea, a waste product, diffuses from cells where it is made into the blood plasma, where its concentration is lower. The kidneys then filter it out of the blood for excretion.
In the lungs, oxygen diffuses from the air, where it is at high concentration, into the blood, where its concentration is lower. Carbon dioxide does the reverse, diffusing from the blood into the air to be breathed out.
Diffusion rate is affected by three factors: the concentration gradient, temperature and the surface area of the membrane. A steeper concentration gradient, a higher temperature and a larger surface area all make diffusion faster.
Net movement in diffusion means the overall movement of particles from higher to lower concentration, even though each particle moves randomly in all directions. Writing that particles move from high to low concentration without mentioning net movement is a common exam mistake.
To work out which cell has faster diffusion, calculate each concentration gradient as the difference between outside and inside concentrations. A cell in 10% glucose containing 2% has a gradient of 8%, while one in 5% glucose has 3%, so glucose enters the first cell faster.
A common mistake when comparing diffusion rates is looking only at the outside concentration, for example thinking 10% is faster than 5%. The actual gradient, the difference between outside and inside concentrations, must be calculated, because a steeper gradient means faster diffusion.
Diffusion does not need energy from the cell. Substances move into and out of cells across cell membranes by diffusion because particles are constantly moving randomly, which creates net movement from high to low concentration.