Diffusion

This lesson covers diffusion as required by the Edexcel GCSE Combined Science specification (1SC0). You need to define diffusion, explain the factors that affect the rate of diffusion, give biological examples and understand the importance of surface area to volume ratio.

---

What Is Diffusion?

Diffusion is the net movement of particles from an area of higher concentration to an area of lower concentration, down a concentration gradient.

Key points:

• Diffusion is a passive process — it does not require energy from respiration.
• It occurs because particles are in constant random motion (kinetic energy).
• Diffusion continues until particles are evenly distributed (equilibrium is reached), although random movement still occurs.
• Diffusion can occur in gases and liquids (solutions), and across partially permeable membranes.

Exam Tip: The specification requires you to say "net movement". Individual particles move randomly in all directions, but overall the net movement is from high concentration to low concentration.

---

Factors Affecting the Rate of Diffusion

Several factors affect how quickly diffusion occurs: