2.7Structures and Functions in Living Organisms
Gas exchange — Edexcel Modular - OL Revision Notes & Topical Questions
10 min read 8 parts Lesson 7 of 12 1 quiz
Lesson summary
Gas exchange Edexcel O Level Modular Biology notes: describe the structure of the thorax, including the ribs, intercostal muscles, diaphragm, trachea, bronchi, bronchioles, alveoli and pleural membranesOxygen diffuses from alveoli into blood.Carbon dioxide diffuses from blood int…
- Leaf adaptations for gas exchange
- Alveolus adaptations
- Hydrogen-carbonate indicator results
- Carbon monoxide
- A tube with a leaf wrapped in foil turns yellow after several hours. Explain
- Explain how the diaphragm and external intercostal muscles cause air to enter the lungs
Section 1 of 8Process & sequence
- Nose / mouth
- Trachea
- Bronchi
- Bronchioles
- Alveoli
- Blood capillaries
Alveoli adaptations
Thin wallsone cell thick
Large surface areamillions of alveoli
Good blood supplymany capillaries
Moistgases dissolve
1
Air enters through the trachea (windpipe).
2
It passes into the bronchus, the large tube leading to each lung.
3
The bronchus branches into bronchioles, the smaller tubes.
4
Bronchioles end at the alveoli, the gas exchange surfaces.
5
Oxygen diffuses across the one-cell-thick wall into the capillary blood; carbon dioxide diffuses the other way.
Air to the bloodwrite these steps in this order; each step is a marking point.
1
Air drawn into the trachea
2
Bronchus carries air to each lung
3
Bronchioles, the smaller tubes
4
Alveoli, the gas exchange surface
5
Oxygen diffuses across the one-cell-thick wall
6
Carbon dioxide diffuses from blood into the alveolus
7
Ventilation keeps the concentration gradient
Why the indicator changesorder these points.
1
Leaf respires, releasing CO₂ in all conditions
2
In bright light photosynthesis removes CO₂ faster than respiration releases it
3
CO₂ concentration falls, so the indicator turns purple
4
In foil there is no light, so only respiration continues
5
CO₂ concentration rises, so the indicator turns yellow
Section 2 of 8Compare
| Gas | Inhaled air | Exhaled air | Change | Why | Test |
|---|---|---|---|---|---|
| Oxygen | 21% | 16% | Decreases | Used in respiration | Glowing splint |
| Carbon dioxide | 0.04% | 4% | Increases | Made in respiration | Limewater turns milky |
| Nitrogen | 78% | 78% | Same | Not used by body | — |
| Water vapour | Variable | Saturated | Increases | Evaporates from lung lining | Condenses on cold mirror |
| Temperature | Air temperature | ~37 °C | Increases | Warmed by body | Thermometer |
| Feature | Inhalation | Normal exhalation |
|---|---|---|
| Diaphragm | Contracts and flattens | Relaxes and domes upward |
| External intercostal muscles | Contract, ribs move up and out | Relax, ribs move down and in |
| Volume of the thorax | Increases | Decreases |
| Pressure in the lungs | Decreases | Increases |
| Air movement | Drawn in | Forced out |
Section 3 of 8Worked examples
1A tube with a leaf wrapped in foil turns yellow after several hours. Explain.
- Foil excludes light, so the leaf cannot photosynthesise but continues to respire.
- Carbon dioxide therefore accumulates around the leaf, and the hydrogen-carbonate indicator turns yellow at a higher carbon dioxide concentration.
2Contracts and flattens
Explain how the diaphragm and external intercostal muscles cause air to enter the lungs.
- The diaphragm contracts and flattens while the external intercostal muscles contract, pulling the ribs up and out.
- The volume of the thorax increases, the pressure in the lungs decreases below atmospheric pressure, and air is drawn in.
3Why does emphysema reduce gas exchange?
- Elastase breaks down the elastic fibres in the alveolar walls, so the alveoli burst into larger air spaces.
- This reduces the surface area available for diffusion, so less oxygen enters the blood.
4State two effects of smoking on the blood vessels and on oxygen transport.
- Nicotine narrows the vessels, raising blood pressure and encouraging clots. Carbon monoxide binds to haemoglobin, so the blood carries less oxygen.
Section 4 of 8Keep going
Gas exchange Edexcel O Level Modular Biology: what to do next
These Gas exchange Edexcel O Level Modular Biology notes covered the key ideas, worked examples and exam technique for this topic. Check your Gas exchange Edexcel O Level Modular Biology understanding with the practice questions below, then reread any Gas exchange Edexcel O Level Modular Biology point you were unsure about before moving on.
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Previous lesson: 2.6 Respiration — Notes
3
Next lesson: 2.8 Transport — Notes
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Course: Edexcel Modular - OL — all units
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Official syllabus: Pearson Edexcel International GCSEs
Section 5 of 8Key terms
Section 6 of 8Syllabus checklist
1
describe the structure of the thorax, including the ribs, intercostal muscles, diaphragm, trachea, bronchi, bronchioles, alveoli and pleural membranes
2
understand the role of the intercostal muscles and the diaphragm in ventilation
3
explain how alveoli are adapted for gas exchange by diffusion between air in the lungs and blood in capillaries
4
understand the biological consequences of smoking in relation to the lungs and the circulatory system, including coronary heart disease
5
practical: investigate breathing in humans, including the release of carbon dioxide and the effect of exercise
Section 7 of 8Command terms
Command terms
Give / State / Name
Recall a fact; one idea per mark.
Define
Give the exact meaning using the specification keywords.
Describe
Give an account of what happens / the pattern in data, quoting values; no reasons needed.
Explain
Give reasons with linked statements (cause → effect). Each linked step is usually a separate marking point.
Suggest
Use knowledge in a new context; more than one answer may be accepted.
Compare
Similarities and differences; each point must refer to both items.
Calculate
Show working; include units; check significant figures. Method marks are given even if the final answer is wrong.
Discuss
Explore the issue from different angles and link the points.
Evaluate / Assess
Weigh strengths and weaknesses and reach a supported judgement.
Deduce
Draw a conclusion from the information given.
Justify
Support a statement/choice with evidence.
Section 8 of 8Exam technique
Keywords the mark scheme looks for in this lesson
Keywords the mark scheme looks for in this lesson
Gas exchange · Alveoli · Diffusion · Capillary · Trachea · Bronchus · Bronchiole · Red blood cells · Plasma · Limewater
Learn each definition in the Key words tab exactly; a missing keyword in a definition usually loses the mark.
How to get full marks (examiner guidance)
1
Use specification keywords exactly (e.g. eukaryotic/prokaryotic, enzyme–substrate complex, active site, complementary).
2Link ideas
Link ideas for “explain” each marking point is usually a cause → consequence step; unlinked facts often score nothing.
3Use the information in the question
“discuss/evaluate” answers must use the data provided and match the number of marks.
4
Convert units carefully (mg → g, cm³ → dm³) and show working for calculations.
5Avoid vague words
“it”, “stuff”, “things”, “goes up”, “energy is made”. Name the substance and the process.
6
Do not extrapolate a graph line beyond the data or to zero unless asked.
7
Six-mark extended responses are marked on content AND a logical, linked line of reasoning.
8Keywords the mark scheme looks for in this lesson
- Trachea — Bronchus — Bronchiole — Alveolus — Diffusion gradient — Ventilation — Stoma / Guard cell — Spongy mesophyll — Hydrogen-carbonate indicator — Nicotine — Carbon monoxide — Tar — COPD
- Name the structures exactly.
- "Windpipe" only scores where the question asks for the tube, and the marking point is normally trachea.
9Mechanism questions
Mechanism questions give the chain, not the conclusion
- Always write muscle action → volume change → pressure change → air movement.
- Skipping the pressure step loses the mark even when "air is drawn in" is correct.
- For adaptations, pair each feature with its reason: one cell thick = short diffusion distance; many alveoli = large surface area; moist lining = gases dissolve; blood supply = maintains the gradient.
10Practical questions
- Learn the four indicator colours and which condition gives them: no leaf = orange (control), bright light = purple, foil = yellow, muslin = orange/slightly purple.
- State the comparison being made, not just the colour.
Marked practiceQuizzes for this lesson
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