Factors Affecting Diffusion

IGCSE Edexcel Biology
2.16 Factors Affecting Movement of Substances
Key Concept: Diffusion rate depends on surface area, distance, temperature, and concentration gradient.

Section A: Factors

1. State four factors that affect the rate of diffusion. [4]
2. Explain why a large surface area to volume ratio speeds up diffusion. [3]

Section B: Calculation

3. A cube has side length 2 cm. Calculate its surface area to volume ratio. [3]

Section C: Application

4. Predict what happens to diffusion rate if temperature increases. [2]
5. Describe how a steeper concentration gradient affects diffusion rate. [2]
Total marks: 14

Mark Scheme

1. Surface area, distance/thickness, temperature, concentration gradient [4]
2. More surface for exchange and shorter diffusion distances per unit volume, so particles move in and out faster [3]
3. Surface area $= 6 \times 2^2 = 24\,cm^2$; volume $= 2^3 = 8\,cm^3$; ratio $= 24:8 = 3:1$ [3]
4. Rate increases because particles have more kinetic energy and collide more often [2]
5. A steeper gradient increases the rate because the difference in concentration is greater [2]