Gas exchange — IB Diploma Biology HL
IB Biology B3.1 gas exchange: exchange surfaces, alveolar lungs, ventilation, lung volumes, leaves, transpiration, stomata and (HL) haemoglobin.
Revision notes
6 short notes, in the order of the specification. Each one in short:
Gas exchange is the diffusion of oxygen into an organism and carbon dioxide out of it across a gas-exchange surface. As organisms get larger their surface area-to-volume ratio falls and diffusion distances rise, so large organisms need surfaces that are permeable, thin, moist and large, with steep gradients kept up by blood flow and ventilation.
The alveolar lungs of a mammal are adapted for gas exchange by a branched network of bronchioles ending in millions of alveoli, giving a very high surface area. Extensive capillary beds surround each alveolus, the walls are thin, and surfactant stops the moist alveoli sticking together and collapsing when air is breathed out.
Ventilation and lung volumes
PracticalVentilation is the movement of air into and out of the lungs, caused by pressure changes in the thorax. Contraction of the diaphragm and external intercostal muscles raises thorax volume so pressure falls and air flows in. Tidal volume, vital capacity and the inspiratory and expiratory reserves are measured with a spirometer.
A leaf is adapted for gas exchange by stomata, opened and closed by guard cells, which let carbon dioxide in and oxygen out, and by air spaces in the spongy mesophyll that give a large moist surface. The waxy cuticle and epidermis reduce water loss, and veins bring water and remove sugars.
Transpiration and stomatal density
PracticalTranspiration is the loss of water vapour from the leaves and stems of plants. It is a consequence of gas exchange because stomata must open to let carbon dioxide in, and water vapour then diffuses out. Its rate rises with light, temperature and wind, and falls with humidity. Stomatal density is stomata per unit area.
Haemoglobin binds oxygen cooperatively: when one oxygen binds to a haem group, the shape of the molecule changes so the other haem groups bind oxygen more easily. This gives an S-shaped dissociation curve. Foetal haemoglobin has a higher affinity than adult haemoglobin, and carbon dioxide lowers affinity, the Bohr shift.
Exam questions
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Quick check questions
What happens to the surface area-to-volume ratio as an organism grows larger?
Show answer
It decreases.
What is the function of surfactant in the alveoli?
Show answer
It reduces surface tension so the alveoli do not stick together and collapse when air is breathed out.
Which three volumes add up to the vital capacity?
Show answer
Tidal volume, inspiratory reserve volume and expiratory reserve volume.
Why does increased humidity reduce the rate of transpiration?
Show answer
It reduces the concentration gradient of water vapour between the air spaces and the outside air.
HL only In which direction does high carbon dioxide shift the oxygen dissociation curve?
Show answer
To the right (lower affinity, more oxygen released): the Bohr shift.
Frequently asked questions
Why do larger organisms need specialised gas exchange surfaces?
Larger organisms need specialised gas exchange surfaces because their surface area-to-volume ratio is smaller and the distance from their centre to the outside is greater. Diffusion across the body surface alone would be far too slow, so they need large, thin, moist, permeable surfaces such as lungs or gills and a transport system.
How are alveoli adapted for gas exchange?
Alveoli are adapted for gas exchange by their huge total surface area, walls one thin layer of cells thick, a moist lining and a dense capillary network around each one. Surfactant lowers surface tension so alveoli do not collapse, and ventilation with continuous blood flow keeps the concentration gradients steep.
What happens to the diaphragm and intercostal muscles when you breathe in?
When you breathe in, the diaphragm contracts and flattens and the external intercostal muscles contract, pulling the ribs up and out. The volume of the thorax increases, so the pressure inside falls below atmospheric pressure and air flows into the lungs.
Why is transpiration a consequence of gas exchange?
Transpiration is a consequence of gas exchange because stomata must open to let carbon dioxide into the leaf for photosynthesis. When they are open, water vapour diffuses out of the moist air spaces down its concentration gradient, so some water loss is unavoidable whenever the leaf exchanges gases.
Why is the oxygen dissociation curve S-shaped?
HL only The oxygen dissociation curve is S-shaped because of cooperative binding. The first oxygen binds to haemoglobin with difficulty, but this changes the molecule's shape so the next ones bind more easily, making the curve steep. It levels off when almost all four haem groups are occupied.
Written and checked against the IB Biology HL specification · Updated October 2026