IB DP Biology: Gas exchange: surfaces, lungs, leaves and haemoglobin (DP Biology)

This guide teaches section B3.1 (Gas exchange) of the IB Biology guide, first assessment 2025. It covers statements B3.1.1 to B3.1.10 for SL and HL, plus B3.1.11 to B3.1.13 (haemoglobin, the Bohr shift and oxygen dissociation curves), which are HL only. For condensed recall use the revision notes, then test yourself with the practice questions. The wider theme is in the Theme B study guide. Course hub: IB DP Biology. Checklist: printable checklist. To find weak areas first, take the free 10-minute diagnostics.

What this unit covers

Guide section	What you must be able to do	Level
B3.1.1	Explain why gas exchange is vital and why it gets harder as size increases	SL and HL
B3.1.2	State the properties of gas-exchange surfaces	SL and HL
B3.1.3	Explain how concentration gradients are maintained in animals	SL and HL
B3.1.4	Explain the adaptations of alveolar lungs	SL and HL
B3.1.5	Explain ventilation using diaphragm, intercostal and abdominal muscles, and ribs	SL and HL
B3.1.6	Measure and calculate lung volumes	SL and HL
B3.1.7	Relate leaf structure to gas exchange	SL and HL
B3.1.8	Draw and label a plan diagram of a dicotyledonous leaf	SL and HL
B3.1.9	Explain transpiration as a consequence of gas exchange and the factors affecting it	SL and HL
B3.1.10	Determine stomatal density from micrographs or leaf casts	SL and HL
B3.1.11	Explain foetal and adult haemoglobin, cooperative binding, allosteric binding of carbon dioxide	HL only
B3.1.12	Explain the Bohr shift	HL only
B3.1.13	Interpret oxygen dissociation curves and explain their S shape	HL only

B3.1.1 Why gas exchange matters

Every cell needs a supply of oxygen for aerobic cell respiration and must remove the carbon dioxide it produces. A photosynthesising cell also needs carbon dioxide and gives off oxygen. In every organism, gas exchange is the movement of these gases across the boundary with the environment, by diffusion.

Size creates the challenge. As an organism gets larger, volume grows more quickly than surface area, which makes the ratio of surface to volume drop. The distance from the centre to the outside also increases. Diffusion alone is then too slow to supply the middle.

Worked example. Compare cubes of side 2 mm and 8 mm as models of two organisms.

2 mm cube: surface area = 6 x 2 x 2 = 24 mm2;  volume = 8 mm3;   ratio = 3.0 mm-1
8 mm cube: surface area = 6 x 8 x 8 = 384 mm2; volume = 512 mm3; ratio = 0.75 mm-1

The side is four times longer, but the ratio is four times smaller. A small organism can rely on its body surface. A large one needs a specialised exchange surface and, usually, a transport system.

B3.1.2 What makes a good exchange surface

An effective surface has four properties:

• Permeable to oxygen and carbon dioxide, so gases cross freely.
• Thin tissue layer, often one cell thick, so the diffusion distance is short.
• Moist, because gases dissolve in a film of water before crossing the cell membrane.
• Large surface area, so many molecules can cross at once.

B3.1.3 Maintaining concentration gradients

Diffusion needs a gradient. If oxygen built up inside the body or carbon dioxide built up outside, exchange would stop. Animals keep the gradient steep in three ways:

• Dense networks of blood vessels carry gases away from the exchange surface and bring more to it.
• Continuous blood flow replaces blood that has already equilibrated, so the surface always meets blood low in oxygen and high in carbon dioxide.
• Ventilation renews the medium next to the surface: air for lungs, water for gills.

B3.1.4 How alveolar lungs are adapted

Limit this to the alveolar lungs of a mammal. Air passes down the trachea into bronchi, then a branched network of bronchioles, ending in millions of alveoli.

Adaptation	How it helps
High surface area	Millions of alveoli give a very large area for diffusion
Branched bronchioles	Carry air close to every alveolus
Extensive capillary beds	Each alveolus is wrapped in capillaries, so blood flow keeps the gradient steep
Thin walls	Alveolar and capillary walls are very thin, so the diffusion path is short
Surfactant	Reduces surface tension in the moist lining, so the alveolar walls do not stick together and the alveoli stay open

B3.1.5 Ventilation of the lungs

Ventilation moves air in and out, which renews the air in the alveoli. It works by changing the volume, and therefore the pressure, of the thorax. Air flows from higher to lower pressure.

Inhalation. The diaphragm contracts and flattens. The external intercostal muscles contract and raise the ribs up and outwards. Thoracic volume increases, pressure falls below atmospheric pressure and air flows in.

Quiet exhalation. The diaphragm and external intercostals relax, the ribs fall and the diaphragm domes upwards. Volume falls, pressure rises above atmospheric pressure and air flows out.

Forced exhalation (exercise, coughing). The internal intercostal muscles contract and pull the ribs down and in. The abdominal muscles contract and push the abdominal organs and diaphragm upwards. Air leaves faster and more completely.

The muscle sets are antagonistic.

B3.1.6 Measuring lung volumes

A spirometer or a similar device records volumes of air breathed. You should be able to make or read measurements of four quantities:

• Tidal volume: air moved in or out in one normal breath.
• Inspiratory reserve volume: extra air that can be breathed in after a normal inhalation.
• Expiratory reserve volume: extra air that can be breathed out after a normal exhalation.
• Vital capacity: the maximum volume that can be moved in one breath, from fullest inhalation to fullest exhalation.

Vital capacity = tidal volume + inspiratory reserve volume + expiratory reserve volume. Air left in the lungs after the strongest exhalation is the residual volume, which this method does not measure.

Worked example. A student at rest has tidal volume 0.45 L, inspiratory reserve volume 2.9 L and expiratory reserve volume 1.0 L, and breathes 14 times per minute.

1. Vital capacity = 0.45 + 2.9 + 1.0 = 4.35 L.
2. Ventilation rate in volume terms = 0.45 x 14 = 6.3 L per minute.

B3.1.7 How a leaf is built for gas exchange

A leaf must take in carbon dioxide for photosynthesis and release oxygen, while losing as little water as possible.

• Waxy cuticle: waterproof, so it reduces water loss from the surface. It also blocks gas entry, so gases use the stomata.
• Epidermis: a transparent layer that protects and lets light through. Stomata are found mainly in the lower epidermis.
• Stomatal guard cells: pairs of cells that open and close each pore. Open stomata let carbon dioxide in and oxygen out. Closed stomata cut water loss.
• Spongy mesophyll: loosely packed cells with large air spaces, so gases diffuse quickly through the leaf and reach every cell. Their moist surfaces are the exchange surface.
• Veins: xylem brings water to the mesophyll to keep the cell surfaces moist, and phloem carries sugars away.

B3.1.8 Distribution of tissues: the plan diagram

A plan diagram maps the position of each tissue in a cross-cut slice through a leaf of a dicot plant. It shows tissue boundaries only. Do not draw individual cells. Use clear lines and label lines that do not cross.

From top to bottom, label: waxy cuticle, upper epidermis, palisade mesophyll, spongy mesophyll with air spaces, vein (xylem and phloem), lower epidermis with a stoma and guard cells, lower cuticle.

Worked example. A leaf section drawn from a micrograph is 66 mm thick in the drawing, at magnification x300.

Actual thickness = image size / magnification = 66 / 300 = 0.22 mm = 220 micrometres.

B3.1.9 Transpiration: the cost of open stomata

Open stomata give water vapour a route out of the moist leaf. Water evaporates from mesophyll cell surfaces into the air spaces and diffuses out through the stomata. This loss is transpiration. It is the cost of gas exchange.

Factors that raise the rate:

• Higher light intensity: stomata open wider, so more vapour escapes.
• Higher temperature: faster evaporation and diffusion.
• Lower humidity: a steeper vapour gradient between leaf and air.
• More wind: moist air next to the leaf is swept away, keeping the gradient steep.

A potometer estimates the rate by following a bubble in a capillary tube.

Worked example. The capillary radius is 0.50 mm. The bubble moves 36 mm in 6 minutes.

volume = pi x r2 x distance = pi x 0.50 x 0.50 x 36 = 28.3 mm3
rate = 28.3 / 6 = 4.71 mm3 per minute

B3.1.10 Stomatal density

Stomatal density is the number of stomata per unit area of leaf. Make a leaf cast by painting clear varnish on the surface, peeling it off when dry and viewing it at high power. Or count stomata on a micrograph.

Worked example. The field of view has diameter 0.50 mm. Five fields give counts of 14, 17, 15, 16 and 13.

mean count = 75 / 5 = 15.0
area = pi x 0.25 x 0.25 = 0.196 mm2
density = 15.0 / 0.196 = 76 stomata per mm2

Nature of science: reliability. Counts vary between fields, because biological material varies. Repeating the count in several fields and taking a mean reduces the effect of chance. More replicates give a more reliable estimate.

B3.1.11 Haemoglobin (HL only)

Haemoglobin is a protein of four polypeptide chains, each with a haem group that binds one oxygen molecule. In the lungs it loads oxygen. In the tissues it unloads it.

Cooperative binding. When the first oxygen binds, the shape of the molecule changes slightly and the other haem groups bind oxygen more easily. Each further oxygen binds more readily still.

Allosteric binding of carbon dioxide. Carbon dioxide binds at a site other than the haem group. This changes the shape of haemoglobin and reduces its affinity for oxygen.

Foetal haemoglobin is built from different polypeptide chains from adult haemoglobin and has a higher affinity for oxygen than adult haemoglobin. At the same oxygen concentration it is more saturated, so in the placenta oxygen passes from maternal blood to the foetus.

B3.1.13 Oxygen dissociation curves (HL only)

An oxygen dissociation curve shows how full haemoglobin is with oxygen (percentage saturation) at each oxygen partial pressure. These values are supplied as data.

Oxygen (kPa)	1	2	3	5	8	13
Adult, normal	3	18	40	72	90	97
Foetal	8	35	62	87	96	99
Adult, higher carbon dioxide	1	10	25	57	83	95

Why the curve is S-shaped. At low oxygen, the first molecule binds with difficulty, so the curve rises slowly. Once it binds, binding becomes easier, so the curve climbs steeply. At high oxygen, nearly all haem groups are occupied, so few sites remain and the curve flattens.

A curve further left means higher affinity. Foetal haemoglobin is left of adult.

B3.1.12 The Bohr shift (HL only)

More carbon dioxide causes haemoglobin to release more oxygen, shifting the curve to the right. In the table, at 5 kPa the saturation is 72% normally but 57% with more carbon dioxide.

Worked example. Lungs at 13 kPa, resting tissue at 5 kPa.

• Normal: 97 - 72 = 25 percentage points released.
• Higher carbon dioxide: 95 - 57 = 38 percentage points released.

Actively respiring tissue makes more carbon dioxide, so it gets more oxygen where the demand is greatest. In the lungs, where carbon dioxide is low, loading is almost unaffected (97 against 95).

Common errors

• Saying oxygen is “pumped” across exchange surfaces. It diffuses.
• Writing that surfactant makes the lungs moist. It reduces surface tension.
• Mixing up the diaphragm: it flattens on inhalation and domes on exhalation.
• Drawing cells in a plan diagram.
• Using a left shift for the Bohr shift. It is a right shift.

Official syllabus

IB Diploma Programme Biology guide, first assessment 2025, published by the International Baccalaureate Organization. Section B3.1, Gas exchange.