Organisms: Breathing and Digestion – Lesson 2
Gas Exchange and Diffusion (PPT + Worksheet)

This Year 9 lesson builds on students’ understanding of the respiratory system by exploring the science behind gas exchange. Pupils define diffusion and the role of concentration gradients, identify the adaptations of the alveoli that make gas exchange efficient, and explain how oxygen and carbon dioxide move between the lungs and the bloodstream. Students then analyse real‑world data on gas concentrations in the circulatory system to deepen their scientific reasoning.

• Define diffusion and the role of concentration gradients — understanding particle movement from high to low concentration.
• Identify the adaptations of the alveoli for efficient gas exchange — large surface area, thin walls, moist lining and rich blood supply.
• Explain how oxygen and carbon dioxide move between the lungs and blood — diffusion across the alveolar–capillary membrane.
• Analyse data regarding gas concentrations in the circulatory system — interpreting tables, graphs and patterns in oxygen and carbon dioxide levels.

• KS3 Biology: Gas exchange, diffusion, and organ systems
• Working Scientifically: Data interpretation, scientific explanation, and use of models