KS3 · Biology
Breathing, respiration and gas exchange
You just breathed in. That oxygen has a long way to go, all the way inside your cells. Let's follow it.
Predict, then check
Two jars, two burning candles. One jar holds room air. The other holds air a person has breathed out.
Which candle goes out first?
WHAT YOU'VE LEARNED
A quick recap of today's lesson.
What you need to know
- Breathing is the process of air moving in and out of the lungs.
- Air is a mixture of gases: mostly nitrogen, then oxygen, with carbon dioxide taking up less than 1%.
- Nitrogen is really unreactive, so it isn't involved in breathing, respiration or gas exchange.
- Cells need oxygen for cellular respiration, which provides energy for life processes.
- Cellular respiration is a chemical process that uses sugar as a fuel. It happens in every cell in the body.
Have a goThink of the cells in your hand. Do they do cellular respiration, or is that just a lungs-and-chest job?
Yes, hand cells do it. Respiration happens in every cell.
Respiration isn't a lungs job at all. Every cell in the body uses sugar as a fuel to provide energy for life processes.
- Inside cells, mitochondria combine oxygen from the air with sugar from food to provide energy.
- Carbon dioxide is a waste product of respiration. The blood carries it back to the lungs to be breathed out.
Have a goA cell makes carbon dioxide. Does it go straight into an air sac, or stop somewhere on the way? Say where.
It stops in the blood first. The blood carries it back to the lungs.
The route is cell, then blood, then air sac. The blood does the carrying, and gas exchange hands the carbon dioxide over to the air sac.
- Gas exchange is oxygen diffusing from the lungs into the blood, and carbon dioxide diffusing from the blood into the lungs.
- Tiny air sacs fill with the air you breathe in. Each is surrounded by blood vessels, and gas exchange happens between the two.
Have a goSpot the slip: "Gas exchange happens inside cells." Fix the sentence.
Gas exchange happens between the air sacs and the blood vessels around them.
Cells are where respiration happens. Mixing up those two places is the classic slip.
- So breathing happens in the lungs, respiration happens in cells, and gas exchange happens between air sacs and blood vessels.
- Breathed-out air has less oxygen and more carbon dioxide than breathed-in air, because cells used oxygen and made carbon dioxide.
The big picture
Breathing, gas exchange and cellular respiration are three different processes in three different places. Breathing moves air in and out of the lungs. Gas exchange moves oxygen into the blood and carbon dioxide out of it, between the air sacs and the blood vessels. Cellular respiration happens in every cell, where mitochondria use oxygen and sugar to provide energy and carbon dioxide is made as a waste product.
Key points
Worked example
Problem
Zara sprints for the bus. Three things are happening in her body: (1) air is moving into her lungs, (2) oxygen is moving from an air sac into the blood vessel around it, (3) cells in her legs are combining oxygen and sugar in their mitochondria to provide energy. Name the process in each case.
⚠ Watch out
Thinking breathing and respiration are the same thing. Breathing is air moving in and out of the lungs. Respiration happens in cells and provides energy. They are linked, because breathing in provides cells with the oxygen they need, but they are different processes in different places.
Memory hook
Lungs BREATHE. Air sac and blood vessel SWAP. Cells RESPIRE.
Check yourself
Cover the screen. Name the three processes, say where each one happens, and say which way oxygen and which way carbon dioxide are moving at each place.
Flashcards
(11)What is breathing?
What is the most abundant gas in the air, and is it involved in breathing?
How much of the air is carbon dioxide?
What does cellular respiration do?
Where does cellular respiration happen?
What do mitochondria combine, and why?
Why is carbon dioxide made, and what happens to it?
What is gas exchange?
Where does gas exchange take place?
How are breathing and cellular respiration linked?
How does breathed-out air differ from breathed-in air?
Tap any card to flip it, or use Study as deck to go through them one at a time. In the full lesson these run as a spaced-repetition deck — you rate each card Hard, Good or Easy and the tricky ones keep coming back until they stick.
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How this lesson was checked. This KS3 Biologylesson was published through Lightbulb Learning's human-designed editorial process — the educational standards, accuracy rules and publication checks it must pass were authored and approved by Philip Halpin. It passed subject-specific assessment, automated educational checks and technical publication verification before going live (publication checks completed 9 October 2026). Published pages are monitored, human spot-checking is ongoing across the lesson library, and anything found wrong is corrected or withdrawn. How our lessons are made and checked. Spotted a mistake? Email hello@lightbulblearning.co and we'll review it.