Biology • Year 11 • Module 2 • Lesson 10
Gas Exchange in Animals
Lock in the core vocabulary, the four features of efficient exchange surfaces, and the structure–function links for insect, fish and mammalian gas exchange.
1. Term–definition match
The eight definitions below are shuffled. Write the matching term from this list in the right-hand column: exchange surface, surface area-to-volume ratio, tracheal system, spiracle, gill lamellae, counter-current exchange, alveolus, ventilation. 8 marks
| # | Definition (shuffled) | Matching term |
|---|---|---|
| 1.1 | A surface adapted to maximise diffusion of substances between an organism and its environment; must be large, thin, moist, and maintain a concentration gradient. | |
| 1.2 | The ratio that falls as organisms grow larger because volume increases faster than surface area, creating the need for specialised exchange organs. | |
| 1.3 | The insect gas exchange system, air enters through openings on the body wall and moves through branching tubes that deliver O₂ directly to cells. | |
| 1.4 | A tiny opening on an insect's body wall through which air enters and exits the tracheal system. | |
| 1.5 | Thin, folded structures in fish gills that provide a large surface area for gas exchange between water and blood. | |
| 1.6 | An arrangement in fish gills where blood and water flow in opposite directions, maintaining a concentration gradient along the entire gill surface. | |
| 1.7 | A tiny air sac in mammalian lungs; millions provide an enormous collective surface area with a thin membrane and rich capillary supply. | |
| 1.8 | The active movement of air (or water) across an exchange surface that refreshes the medium and maintains steep concentration gradients. |
2. Surface area-to-volume ratio, recall and reasoning
Answer each in 1–2 sentences using precise terms from the lesson. 10 marks (2 each)
2.1 What happens to the SA:V ratio as an organism's body size increases? Why is this a problem for gas exchange?
2.2 Why can a flatworm survive without lungs or gills, but a mammal cannot?
2.3 What does specialised gas exchange do that direct diffusion through the body surface cannot?
2.4 Why do large animals need both an exchange surface and a transport system to move gases to all body cells?
2.5 Name the four features shared by all efficient gas exchange surfaces and give one example of each from any animal group covered in the lesson.
3. Feature–function table, efficient exchange surfaces
The table below lists the four features of efficient exchange surfaces. Complete the second column: explain why each feature improves gas exchange rate, and give a specific anatomical example from the lesson. 12 marks (1 why + 1 example per row)
| Feature | Why it improves diffusion rate | Specific example from lesson |
|---|---|---|
| Large surface area |
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| Thin barrier / membrane |
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| Moist surface |
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| Maintained concentration gradient |
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4. Complete the insect tracheal system flowchart
The boxes below show the path that oxygen takes from the atmosphere to an insect body cell, and the path carbon dioxide takes back out. Fill in each blank with the correct anatomical structure or process. 8 marks
Oxygen pathway (atmosphere → cell)
| Step | Structure or process | Your answer |
|---|---|---|
| 1 | Gas enters the insect's body through a small opening called a ___ | |
| 2 | Air moves into a larger tube called the ___ | |
| 3 | Air reaches fine branching tubes called ___ | |
| 4 | O₂ moves from tracheoles directly into cells by the process of ___ |
Carbon dioxide pathway (cell → atmosphere)
| Step | Structure or process | Your answer |
|---|---|---|
| 5 | CO₂ diffuses out of cells into ___ | |
| 6 | CO₂ moves through ___ to the larger tracheae | |
| 7 | Gas exits through ___ | |
| 8 | In insects, haemolymph plays no role in oxygen transport because ___ |
5. True or false, with correction
Circle T or F. If false, write the corrected version. 8 marks (1 T/F + 1 correction where needed)
5.1 In insects, oxygen is carried through the circulatory system (haemolymph) to cells, just as haemoglobin carries oxygen in mammals. T / F
5.2 Counter-current exchange in fish gills means that blood and water flow in opposite directions, maintaining a diffusion gradient along the full gill surface. T / F
5.3 Mammalian alveoli are efficient primarily because of their thick walls, which slow down gas movement and allow more time for oxygen to cross. T / F
5.4 Ventilation and blood flow are both necessary to maintain steep concentration gradients at mammalian alveoli. T / F
Q1, Term–definition matches
1.1 exchange surface • 1.2 surface area-to-volume ratio • 1.3 tracheal system • 1.4 spiracle • 1.5 gill lamellae • 1.6 counter-current exchange • 1.7 alveolus • 1.8 ventilation.
Q2.1, SA:V ratio and body size
As body size increases, volume grows faster than surface area, so the SA:V ratio falls. With a lower SA:V ratio, the outer body surface provides proportionally less membrane area relative to the number of cells that need oxygen, making direct surface diffusion insufficient.
Q2.2, Flatworm vs mammal
A flatworm is small and flat, giving it a high SA:V ratio and short diffusion distances, so every cell lies close enough to the environment for direct diffusion to work. A mammal's large body has a low SA:V ratio and internal cells many millimetres from the surface, diffusion alone would be far too slow to supply them with sufficient oxygen.
Q2.3, What specialised exchange adds
Specialised exchange surfaces concentrate a large, thin, moist membrane area in one internal location (e.g. alveoli, gills) and pair it with ventilation and/or blood flow to maintain steep gradients. This makes diffusion fast enough to meet the demands of a large, metabolically active body, something an unmodified outer body surface cannot achieve.
Q2.4, Why a transport system is also needed
Even with an efficient exchange surface, diffusion alone cannot carry gases from the exchange organ to cells deep in the body fast enough. A transport system (blood or haemolymph + vessels) carries oxygen rapidly to all tissues and returns CO₂ to the exchange organ.
Q2.5, Four features with examples
Large surface areae.g. millions of alveoli in mammalian lungs, or folded gill lamellae in fish. Thin barriere.g. alveolar wall plus capillary wall is only one cell thick each; tracheole wall in insects is very thin. Moist surfacee.g. fluid lining of alveoli; the moist lining of tracheoles in insects. Maintained concentration gradiente.g. ventilation keeps O₂ high in alveolar air; blood flow keeps O₂ low on the blood side; counter-current flow in fish gills.
Q3, Feature–function table
Large surface area: More membrane area means more O₂ molecules can simultaneously cross per second, increasing total diffusion rate. Example: folded gill lamellae in fish / millions of alveoli in lungs.
Thin barrier: Reducing diffusion distance allows gases to cross more quickly (by Fick's law, rate ∝ 1/distance). Example: alveolar membrane + capillary wall each one cell thick; tracheole wall in insects.
Moist surface: Gases (O₂ and CO₂) must dissolve in water before crossing cell membranes; moisture enables this dissolution. Example: mucus/fluid lining of alveoli; fluid in tracheoles.
Maintained concentration gradient: Diffusion only occurs if one side has higher concentration. Ventilation refreshes the air side; blood flow removes O₂ from the blood side, both keep the gradient steep. Example: breathing in mammals; buccal–opercular pumping in fish; body movements in active insects.
Q4, Tracheal system flowchart
O₂ pathway: 1 spiracle • 2 trachea • 3 tracheoles • 4 diffusion.
CO₂ pathway: 5 tracheoles • 6 tracheoles (flowing toward tracheae) • 7 spiracles • 8 oxygen travels directly to cells through the tracheoles, so haemolymph does not need to (and cannot effectively) transport gaseous O₂ to tissues.
Q5, True / false with correction
5.1 False. Correction: In insects, oxygen travels directly to cells through the tracheal tubes (tracheoles), haemolymph does not transport most oxygen. This is a key difference from mammalian gas exchange.
5.2 True.
5.3 False. Correction: Mammalian alveoli are efficient because their walls are extremely thin (one cell thick), minimising diffusion distance so gases can cross quickly, not because the walls are thick.
5.4 True.