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
Exchange surfaces in animals and plants
- B3.1.1Gas exchange: a vital function
- B3.1.2Properties of exchange surfaces
- B3.1.3Keeping the gradient steep
- B3.1.3Gills: ventilation with water
- B3.1.4A branched network of bronchioles
- B3.1.4Alveoli and capillary beds
- B3.1.5The muscles of ventilation
- B3.1.6 · SkillMeasuring lung volumes
- B3.1.7A leaf is adapted for gas exchange
- B3.1.8 · SkillTissues in a leaf
- B3.1.9Transpiration
- B3.1.10 · SkillStomatal density
Haemoglobin and oxygen
- B3.1.11 · HLHaemoglobin
- B3.1.11 · HLFoetal and adult haemoglobin
- B3.1.12 · HLThe Bohr shift
- B3.1.13 · HLWhy the curve is S-shaped
B3.1 Gas exchange: summary
Why gas exchange is hard
- Larger size: lower SA:V and longer distances.
- Surfaces: permeable, thin, moist, large area.
Gradients
- Dense blood vessels and continuous blood flow.
- Ventilation with air (lungs) or water (gills).
Lungs
- Branched bronchioles; many alveoli; capillary beds.
- Surfactant stops the alveoli collapsing.
Ventilation and volumes
- In: external intercostals and diaphragm contract. Forced out: internal intercostals and abdominal muscles.
- VC = TV + IRV + ERV.
Leaves
- Cuticle, epidermis, palisade, spongy mesophyll, stomata, veins.
- Transpiration; stomatal density with replicate counts.
HL · Haemoglobin
- Cooperative binding gives an S-shaped curve; foetal Hb has higher affinity.
- Bohr shift: CO₂ binds allosterically, more O₂ released.
Key terms
- Ventilation
- moving air or water over a gas-exchange surface.
- Alveolus
- a tiny air sac in the lungs where gases are exchanged.
- Bronchiole
- a narrow branch of the airways that carries air to the alveoli.
- Surfactant
- a secretion that stops the alveolus walls sticking together.
- Tidal volume
- the volume of air breathed in or out in one normal breath.
- Vital capacity
- tidal volume + inspiratory reserve + expiratory reserve.
- Stoma
- a pore in the leaf epidermis, opened and closed by guard cells.
- Transpiration
- loss of water vapour from leaves.
- Stomatal density
- the number of stomata per unit area of leaf.
- Haemoglobin
- the protein in red blood cells that transports oxygen.
- Haem group
- the part of each subunit that binds one oxygen.
- Cooperative binding
- each oxygen bound makes the next bind more easily.
- Allosteric site
- a binding site other than the one for oxygen.
- Affinity
- how strongly haemoglobin binds oxygen.
- Bohr shift
- the fall in oxygen affinity caused by more carbon dioxide.
Sample exam questions
Three of the 43 multiple-choice questions for B3.1. Try each one before opening the answer.
Question 1. Which is a property of efficient gas-exchange surfaces?
- Thick and dry
- Thin and moist
- Small and waxy
- Impermeable to gases
Show the answer
Answer: B. Gas-exchange surfaces are permeable, thin, moist and have a large surface area.
Question 2. Which describes inspiration at rest?
- Diaphragm contracts and flattens; volume rises
- Diaphragm relaxes and domes; volume rises
- Intercostals relax; ribs move down
- Abdominal muscles contract; pressure rises
Show the answer
Answer: A. The diaphragm contracts and flattens and the external intercostals raise the ribs, so thorax volume rises and pressure falls.
Question 3. Which feature of alveoli increases the rate of gas exchange?
- Thick walls of cartilage
- A waxy lining
- Few capillaries
- Walls one cell thick
Show the answer
Answer: D. The walls are very thin, so the diffusion distance is short.
Linking questions
Questions that connect B3.1 to other parts of the course, the kind that come up in Paper 2.
- How do multicellular organisms solve the problem of access to materials for all their cells? (see B3.2)
- What is the relationship between gas exchange and metabolic processes in cells? (see C1.2, C1.3)
- 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)
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.