IB Biology · Theme B: Form and function · SL and HL
B3.1 Gas exchange
A one-page summary of B3.1 Gas exchange, the key terms to know, and sample exam questions with answers. For the full lesson, open the illustrated revision slides or practise in the app.
Guiding questions
How are multicellular organisms adapted to carry out gas exchange?
What are the similarities and differences in gas exchange between a flowering plant and a mammal?
What B3.1 covers
Solving the surface-area problem
- B3.1.1Bigger bodies, worse geometry
- B3.1.2What every exchange surface needs
- B3.1.3Keeping the gradient steep
- B3.1.4From windpipe to 250 million alveoli
- B3.1.4The alveolus wall itself
- B3.1.5Breathing in: expanding the thorax
- B3.1.5Breathing out: passive or forced
- B3.1.6Measuring what the lungs can do
- B3.1.7A leaf has the same problem
- B3.1.8A leaf, layer by layer
- B3.1.9Transpiration: the price of open stomata
- B3.1.10Counting stomata
Haemoglobin fine-tunes the exchange
- B3.1.11 · HLFoetal haemoglobin out-competes maternal Hb
- B3.1.12 · HLThe Bohr shift: CO₂ unlocks oxygen
- B3.1.13 · HLWhy the curve is S-shaped
B3.1 Gas exchange: summary
The core problem
- SA:V falls as organisms get bigger.
- Fick's law: area × gradient ÷ distance.
Mammalian lung
- Trachea → bronchi → bronchioles → ~250M alveoli.
- Type I thin wall; type II secretes surfactant.
Ventilation & volumes
- Inspiration active; rest expiration passive.
- Spirometer: TV, IRV, ERV, VC.
Leaf structure
- Cuticle → epidermis → palisade → spongy → vein.
- Stomata + guard cells control gas flow.
Transpiration & density
- Water loss is the unavoidable cost of CO₂ uptake.
- Nail-varnish cast + stage micrometer counts stomata.
Haemoglobin HL
- HbF left-shifted vs HbA — placental transfer.
- Bohr shift: ↑CO₂/↓pH shifts curve right.
Key terms
- Gas exchange
- Diffusion of O₂ into and CO₂ out of an organism, driven by a concentration gradient.
- Ventilation
- Physical movement of air or water over a gas-exchange surface to maintain a gradient.
- Vital capacity
- The maximum volume of air moved in one breath: TV + IRV + ERV.
- Guard cells
- Paired cells controlling a stoma's aperture by active K⁺ transport and osmosis.
- Transpiration
- Evaporative loss of water vapour from a plant, mainly through open stomata.
- Bohr shift HL
- Rightward shift of the O₂ dissociation curve caused by rising CO₂ and falling pH.
- Oxygen dissociation curve HL
- Graph of % haemoglobin saturation against pO₂, S-shaped from cooperative binding.
Sample exam questions
Three of the 43 multiple-choice questions for B3.1. Try each one before opening the answer.
Question 1. Fick's law describes the rate of diffusion across a gas-exchange surface. According to Fick's law, which combination of factors would maximise the rate of gas exchange?
- Small surface area, long diffusion distance, large concentration gradient
- Large surface area, short diffusion distance, large concentration gradient
- Small surface area, short diffusion distance, small concentration gradient
- Large surface area, long diffusion distance, small concentration gradient
Show the answer
Answer: B. Fick's law: rate ∝ (surface area × concentration gradient) ÷ diffusion distance. Maximising surface area and concentration gradient while minimising diffusion distance gives the highest rate. All gas-exchange surfaces in biology are optimised this way.
Question 2. Which of the following correctly describes ventilation of the human lungs during inspiration?
- The diaphragm contracts and flattens, the external intercostal muscles contract, the thoracic volume increases and pressure decreases, drawing air in
- Air is actively pumped into the lungs by the contraction of smooth muscle in the bronchioles
- The diaphragm contracts, the abdominal muscles contract, and air is forced into the lungs under positive pressure
- The diaphragm relaxes and domes upward, the internal intercostal muscles contract, the thoracic volume decreases and air is pushed in
Show the answer
Answer: A. Inspiration is an active process: diaphragm contraction flattens it, external intercostal contraction lifts the ribcage up and out → thoracic volume increases → intrapulmonary pressure drops below atmospheric pressure → air flows down the pressure gradient into the lungs.
Question 3. In a spirometer experiment, soda lime is placed in the breathing circuit to absorb CO₂ as the subject breathes. What does the gradual downward drift of the spirometer trace over several minutes indicate?
- The subject is exhaling more air than they inhale
- A fault in the spirometer apparatus
- The subject's vital capacity is decreasing over time
- The subject's oxygen consumption, since O₂ is being used up by the body while the CO₂ produced is absorbed by the soda lime rather than being returned to the circuit
Show the answer
Answer: D. With soda lime absorbing all exhaled CO₂, the only net change in the closed circuit's air volume is the O₂ consumed by the subject through aerobic respiration but not replaced (since exhaled CO₂ is removed rather than contributing to the circuit's volume). This steady decrease in volume over time, seen as a downward drift on the trace, is used to calculate the rate of oxygen consumption.
Linking questions
Questions that connect B3.1 to other parts of the course, the kind that come up in Paper 2.
- The alveoli are lined with a thin film of water containing surfactant (a phospholipid mixture). How do the properties of phospholipids (B1.1) enable surfactant to reduce surface tension at the air-water interface? (see B1.1)
- Oxygen diffuses from the alveoli into the blood, and CO₂ diffuses in the opposite direction. How does the structure of the cell membrane (B2.1) facilitate the rapid diffusion of these small, nonpolar gas molecules? (see B2.1)
- The circulatory system (B3.2) transports oxygen from the lungs to respiring tissues. Explain how the Bohr effect ensures that oxygen is released preferentially in tissues with the highest metabolic rate. (see B3.2)
- During aerobic respiration (C1.2), cells consume O₂ and produce CO₂. Explain how the ventilation rate is homeostatically regulated to match O₂ delivery and CO₂ removal to the rate of cellular respiration. (see C1.2)
Practise B3.1
Study notes, every question and full markschemes for B3.1 are in the app with Pro. Two lessons are completely free to try: A1.1 Water and B1.1 Carbohydrates and lipids.