What is diffusion and what affects its rate? Revision Notes | GCSE OCR Foundation Biology

What this lesson covers

Learn about the process of diffusion and the key factors that control how fast it happens, including concentration gradients, temperature, and surface a...

• Define diffusion as the net movement of particles from high to low concentration
• Explain how concentration gradient, temperature and surface area affect diffusion rate
• Apply diffusion principles to biological examples like gas exchange and waste removal
• Calculate and compare diffusion rates under different conditions

Key ideas to keep in view

Use this lesson to stay inside B2.1 Supplying the cell while connecting the detail back to the wider B2: Scaling up topic.

• B2: Scaling up
• B2.1 Supplying the cell
• GCSE OCR Foundation Biology

Lesson notes preview

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.
• The spreading out of the particles of any substance in solution, or particles of a gas, resulting in a net movement from an area of higher concentration to an area of lower concentration.
• 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.
• 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.
• The difference in concentrations between two areas. The steeper the gradient (bigger difference), the faster the rate of diffusion.

Continue through the topic

Move to the related lessons or back to the topic page when you need the wider sequence before exam-style practice.