What is diffusion and what affects its rate? Revision Notes | GCSE AQA Higher Biology

1. What is Diffusion?

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.

2. Diffusion in Living Cells

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.

3. What Affects the Rate of Diffusion?

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.

4. Calculating Diffusion Effects

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?

1. Cell A: Outside 10%, Inside 2% Cell B: Outside 5%, Inside 2% — Always write down what you know first
2. Cell A gradient: 10% - 2% = 8% Cell B gradient: 5% - 2% = 3% — Gradient = difference between outside and inside
3. Cell A has gradient of 8% Cell B has gradient of 3% 8% > 3% — Steeper gradient means faster diffusion
4. Cell A will have glucose entering faster — Higher concentration gradient in Cell A drives faster diffusion

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.

5. Exam Technique and Common Mistakes

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.