Biology • Year 11 • Module 2 • Lesson 15
Gas Exchange Between Internal and External Environments
Lock in the key vocabulary, the partial pressure data table, Fick’s law, and the four universal features of gas exchange surfaces.
1. Term–definition match
Eight definitions are listed below. Write the matching term from this list in the right-hand column: partial pressure, diffusion gradient, Fick’s law, alveolus, ventilation, internal gas exchange, external gas exchange, haemoglobin. 8 marks
| # | Definition | Matching term |
|---|---|---|
| 1.1 | The pressure exerted by one gas in a mixture; directly proportional to its concentration and determines the direction of diffusion. | |
| 1.2 | The difference in concentration (or partial pressure) of a substance across a membrane that drives net diffusion. | |
| 1.3 | The principle that the rate of diffusion is proportional to surface area and concentration gradient, and inversely proportional to membrane thickness. | |
| 1.4 | A tiny air sac in the mammalian lung that provides a large, moist, thin surface for external gas exchange. | |
| 1.5 | The movement of air into and out of the lungs by breathing, which continuously refreshes alveolar air to maintain the diffusion gradient. | |
| 1.6 | The exchange of O₂ and CO₂ between blood and body tissue cells at the systemic capillary beds. | |
| 1.7 | The exchange of O₂ and CO₂ between the air in alveoli and the blood in surrounding pulmonary capillaries. | |
| 1.8 | An iron-containing protein in red blood cells that binds oxygen reversibly, greatly increasing blood’s oxygen-carrying capacity. |
2. Partial pressure data table
Complete the table by filling in the correct partial pressure values and the direction of O₂ and CO₂ movement at each transition. Use the values from the lesson’s data table. 10 marks
| Location | O₂ partial pressure (mmHg) | CO₂ partial pressure (mmHg) | Direction O₂ moves at this step | Direction CO₂ moves at this step |
|---|---|---|---|---|
| Atmospheric air | n/a (not yet in body) | n/a | ||
| Alveolar air | ||||
| Arterial blood (pulmonary vein) | ||||
| Venous blood (at rest) | ||||
| Tissue cells / mitochondria | O₂ used here (cellular respiration) | CO₂ produced here |
3. Fick’s law – fill in the blanks
Complete the Fick’s law statement and the summary table below. 8 marks
3.1 Complete the formula: Rate of diffusion is proportional to __________________ × __________________ divided by __________________.
| Alveolar adaptation | Which Fick variable it affects | Direction of change (increases / decreases / maintains) | Effect on gas exchange rate |
|---|---|---|---|
| ~500 million alveoli giving ~250 m² total surface area | |||
| Alveolar wall 1 cell thick + capillary wall 1 cell thick = ~0.5 μm combined | |||
| Continuous ventilation refreshes alveolar air; blood flow removes loaded O₂ |
4. Four universal features of gas exchange surfaces
Name the four universal features and briefly explain why each is required for effective gas exchange. 8 marks (1 per feature name + 1 per explanation)
Feature 1 name: ________________________
Why required:
Feature 2 name: ________________________
Why required:
Feature 3 name: ________________________
Why required:
Feature 4 name: ________________________
Why required:
5. True or false – with correction
Circle T or F. If the statement is false, write a corrected version. 8 marks (1 for T/F, 1 for correction where needed)
5.1 Oxygen diffuses from alveolar air into blood because blood has low oxygen content (total amount). T / F
5.2 Ventilation is a form of diffusion that pushes air into the alveoli. T / F
5.3 The partial pressure of CO₂ increases as you move from atmospheric air toward tissue cells. T / F
5.4 Without haemoglobin, blood could carry only approximately 3 mL O₂ per 100 mL, compared with ~20 mL with haemoglobin. T / F
Q1, Term–definition matches
1.1 partial pressure • 1.2 diffusion gradient • 1.3 Fick’s law • 1.4 alveolus • 1.5 ventilation • 1.6 internal gas exchange • 1.7 external gas exchange • 1.8 haemoglobin.
Q2, Partial pressure data table
Atmospheric air: O₂ = 159 mmHg; CO₂ = 0.3 mmHg.
Alveolar air: O₂ = 100 mmHg; CO₂ = 40 mmHg. O₂ moves from atmosphere into alveoli (bulk flow via ventilation); CO₂ moves from alveoli into expired air.
Arterial blood: O₂ = 95 mmHg; CO₂ = 40 mmHg. O₂ moves from alveolar air (100) into blood (40 arriving); CO₂ moves from blood (45) into alveolar air (40).
Venous blood at rest: O₂ = 40 mmHg; CO₂ = 45 mmHg. O₂ moves from blood (95) into tissue cells (20–30); CO₂ moves from tissue cells (50+) into blood (40).
Tissue cells / mitochondria: O₂ = 20–30 mmHg; CO₂ = 50+ mmHg.
Marking notes: 1 mark per correct O₂ partial pressure column (all 5 values = 2 marks); 1 mark per correct CO₂ column (2 marks); 1 mark per correct direction pair at alveoli and tissue cells steps (2 marks each = 4 marks). 10 marks total.
Q3, Fick’s law fill-in
3.1 Rate of diffusion is proportional to surface area × concentration gradient divided by membrane thickness.
Row 1: Surface area; increases; rate increases proportionally.
Row 2: Membrane thickness; decreases (minimises diffusion distance); rate increases (inversely proportional to thickness).
Row 3: Concentration gradient; maintains at maximum; rate remains high (gradient never equilibrates).
Q4, Four universal features
Feature 1: Large surface area. More surface = more simultaneous diffusion events = higher total exchange rate (Fick: SA increases rate).
Feature 2: Thin membrane. Shorter diffusion distance = faster exchange (Fick: rate inversely proportional to membrane thickness).
Feature 3: Moist surface. Gases must dissolve in a water layer before diffusing through the membrane; dry surfaces prevent gas exchange entirely.
Feature 4: Maintained concentration gradient. Without gradient maintenance (ventilation + blood flow), alveolar O₂ would fall and blood O₂ would rise until equilibrium, stopping diffusion.
Q5, True / false with correction
5.1 False. Correction: diffusion is driven by partial pressure gradients, not absolute oxygen content. Oxygen diffuses from alveolar air (pO₂ 100 mmHg) into blood (pO₂ 40 mmHg arriving) because the partial pressure is higher on the alveolar side.
5.2 False. Correction: ventilation is bulk flow driven by pressure differences created by the diaphragm, not diffusion. Diffusion only occurs across the thin alveolar membrane once air has arrived.
5.3 True.
5.4 True.