Gas exchange and respiration covers how air reaches the lungs, how oxygen and carbon dioxide cross the alveolar wall, and how cells use that oxygen to release energy. It links to transport, exercise, fermentation and many practical data questions.
For the exact wording of the content you are examined on, check the subject page on the Cambridge website for your exam year. This page gives the learning route, and the lessons give the detail.
What should you know before you start?
You should understand diffusion and concentration gradients, so movement across membranes is a useful base. It also helps to know how blood carries substances, which is covered in circulation and blood. The wording of a mechanism answer matters here, and the topic on disease and defence concepts practises that habit.
An orienting example
A student runs for the bus. Her breathing gets faster and deeper, and her legs feel heavy when she stops.
Faster breathing is ventilation: muscles change the chest volume and air moves in. Oxygen then diffuses across the thin alveolar wall into the blood, and carbon dioxide diffuses the other way.
In the muscle cells, respiration uses the oxygen to release energy from glucose. If the oxygen supply cannot keep up, some respiration becomes anaerobic and lactic acid is made.
Four ideas appear in one short story. The topic is about keeping them apart.
In what order should you study the lessons?
- Explain ventilation using pressure and volume: it gives the cause-and-effect chain for moving air.
- Connect alveolar structure with diffusion: it explains why the exchange surface is built the way it is.
- Distinguish gas exchange from respiration: it fixes the most common confusion in the topic.
- Compare aerobic and anaerobic models: it gives the equations and what each process releases.
- Interpret a respiration experiment from supplied data: it turns the ideas into rates and conclusions.
What are the common traps?
- Writing “respiration” when the question asks about breathing, or the reverse.
- Saying the lungs “suck in” air, instead of explaining volume and pressure.
- Saying oxygen “is used up” at the alveolus, when the exchange there is diffusion only.
- Writing that anaerobic respiration releases no energy, instead of less energy.
- Reading gas volume change in a respirometer as carbon dioxide production, without noticing the absorber.
How should you use the practice set?
Attempt the mixed practice set after the lessons, on paper, in full sentences. Mark each mistake by type, go back to the matching lesson, and retry a fresh question a few days later.
The next topic, excretion and homeostasis, builds on the blood and gas ideas here.
Students who want someone to challenge each sentence of a written explanation can see how our teachers work in online one-to-one Biology tuition. You can also browse the full Biology learning guide.