2gStructures and Functions in Living Organisms
Gas exchange — Edexcel Linear - OL Revision Notes & Topical Questions
10 min read 8 parts Lesson 7 of 25 2 quizzes
Curriculum Equation Key قوانين الرقم الهيدروجيني والاتزان الأيوني - pH & Acid-Base Equilibrium FormulasFast Exam Solution
pH = -log[H⁺] | pOH = -log[OH⁻] | pH + pOH = 14 | [H⁺][OH⁻] = 10⁻¹⁴
Word Equation & Conditions: الرقم الهيدروجيني = -لوغاريتم تركيز أيون الهيدروجين | pH + pOH = 14
Location & Specifications: الاتزان الأيوني، كيمياء 3 ثانوي، التحصيلي، و AP Chemistry.
Lesson summary
Gas exchange Edexcel O Level Linear 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 into…
- 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
Part 1 of 7Key ideas
1
Oxygen diffuses from alveoli into blood.
2
Carbon dioxide diffuses from blood into alveoli.
3
Smoking damages alveoli and reduces gas exchange.
4Leaf adaptations for gas exchange
Leaf adaptations for gas exchange thin, so the diffusion distance is short; flat, giving a large surface area to volume ratio; stomata, so gases move in and out; air spaces in the loose spongy mesophyll; moist surfaces, so gases dissolve before diffusing.
5Alveolus adaptations
Alveolus adaptations many alveoli give a large surface area; a moist lining; walls only one cell thick; ventilation maintains the concentration gradient; a good blood supply carries the gases away.
6Hydrogen-carbonate indicator results
Hydrogen-carbonate indicator results tube A (no leaf) stays orange; tube B (leaf, bright light) turns purple because carbon dioxide has been used up; tube C (leaf, in foil) turns yellow because carbon dioxide has built up; tube D (leaf, muslin) is orange or slightly purple.
7Nicotine
Nicotine narrows the blood vessels, raising blood pressure, and promotes clotting.
8Carbon monoxide
Carbon monoxide binds to haemoglobin, so less oxygen is carried.
9Tar
Tar is a carcinogen and damages the cilia, contributing to COPD.
10Frequency and the depth
Exercise increases both the frequency and the depth of breathing.
11Internal intercostal muscles
Forced exhalation uses the internal intercostal muscles, which pull the ribs down.
12Water enters the guard cells
Stomata open because water enters the guard cells and they become turgid.
Part 2 of 7Syllabus points
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
Part 3 of 7Process & 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
Part 4 of 7Worked 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.
Part 5 of 7Summary
1
Gas exchange happens in the alveoli.
2
Exhaled air has more carbon dioxide.
3
Air follows trachea → bronchus → bronchiole → alveolus.
4
Alveoli are adapted by large surface area, moist lining, one-cell-thick walls, ventilation and a good blood supply.
5
In daylight a leaf takes in CO₂ and gives out O₂; at night it does the reverse.
6
The hydrogen-carbonate indicator goes purple when CO₂ falls and yellow when CO₂ rises.
7
Smoke: nicotine narrows vessels, carbon monoxide binds haemoglobin, tar is a carcinogen.
Part 6 of 7Keep going
Gas exchange Edexcel O Level Linear Biology: what to do next
These Gas exchange Edexcel O Level Linear Biology notes covered the key ideas, worked examples and exam technique for this topic. Check your Gas exchange Edexcel O Level Linear Biology understanding with the practice questions below, then reread any Gas exchange Edexcel O Level Linear Biology point you were unsure about before moving on.
Gas exchange Edexcel O Level Linear Biology: revision checklist
1
Revise the main Gas exchange Edexcel O Level Linear Biology definitions until you can write them from memory.
2
Practise Gas exchange Edexcel O Level Linear Biology exam questions under timed conditions.
3
Link each Gas exchange Edexcel O Level Linear Biology idea to a diagram or a worked example.
4
List the common Gas exchange Edexcel O Level Linear Biology mistakes and how to avoid them.
5
Explain one Gas exchange Edexcel O Level Linear Biology idea out loud as if teaching a friend.
6
Summarise Gas exchange Edexcel O Level Linear Biology in five key points on a revision card.
2
Previous lesson: 2f Respiration — Notes
3
Next lesson: 2h Transport — Notes
4
Course: Edexcel Linear - OL — all units
5
Official syllabus: Pearson Edexcel International GCSEs
Part 7 of 7Compare: Compare
| 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 |
Key terms
Command 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.
Exam 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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