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

These notes condense section B3.1 (Gas exchange) of the IB Biology guide, first assessment 2025. B3.1.1 to B3.1.10 are SL and HL. B3.1.11 to B3.1.13 are HL only and are marked. For full explanations use the study guide, and practise with the practice questions. Related: Theme B revision notes. Hub: IB DP Biology. Checklist: printable checklist. Unsure where to start? Take the free 10-minute diagnostics.

Key terms

Term	Meaning
Gas exchange	Diffusion of oxygen and carbon dioxide across a surface between organism and environment
Surface area-to-volume ratio	Surface area divided by volume; falls as size rises
Ventilation	Moving air or water over the exchange surface to renew it
Surfactant	Substance that lowers surface tension in the alveolar lining
Tidal volume	Air moved in one normal breath
Vital capacity	Largest volume moved in one breath
Transpiration	Loss of water vapour from a plant, mainly through stomata
Stomatal density	Number of stomata per unit area of leaf
Cooperative binding (HL only)	Binding of one oxygen makes the next bind more easily
Bohr shift (HL only)	Rightward shift of the dissociation curve when carbon dioxide rises

B3.1.1 to B3.1.3 Surfaces and gradients

• Size problem: volume grows faster than surface area, so the ratio falls and the centre is further from the outside.

• Reminder: cube of side 3 mm has ratio 54/27 = 2.0 per mm. Side 6 mm gives 216/216 = 1.0 per mm. Double the side halves the ratio.

• Four properties of a gas-exchange surface: permeable, thin, moist, large surface area.

• Three ways gradients are kept steep: dense blood vessel networks, continuous blood flow, ventilation (air for lungs, water for gills).

• Gills: water is the medium. A one-way flow of water over the gill surface keeps oxygen available at the exchange surface.

• Single-celled organisms and very small animals can exchange gases across the whole body surface because the ratio is high and the distance is short.

Must-know distinction. Ventilation renews the air or water at the surface. Blood flow renews the fluid on the other side. Both are needed.

B3.1.4 and B3.1.5 Lungs and ventilation

Air path: trachea, bronchi, bronchioles, alveoli. At the alveoli, oxygen moves by diffusion from the air into the blood, while carbon dioxide moves the other way into the air that will be exhaled.

Alveolar lung adaptations to recall: surfactant, branched bronchioles, extensive capillary beds, high surface area, thin walls.

Stage	Diaphragm	Intercostals and abdominal muscles	Ribs	Thorax volume	Pressure
Inhalation	Contracts, flattens	External intercostals contract	Up and out	Rises	Falls below atmospheric
Quiet exhalation	Relaxes, domes up	External intercostals relax	Down	Falls	Rises above atmospheric
Forced exhalation	Pushed up	Internal intercostals and abdominal muscles contract	Pulled down and in	Falls more	Rises more

Method: explain a ventilation question.

1. Name the muscles that contract.
2. State the movement of ribs and diaphragm.
3. State the change in thoracic volume.
4. State the change in pressure.
5. State the direction of air flow.

B3.1.6 Lung volumes

Quantity	How to get it
Tidal volume	Read from the resting trace
Inspiratory reserve volume	Extra breath in above tidal
Expiratory reserve volume	Extra breath out below tidal
Vital capacity	Tidal + inspiratory reserve + expiratory reserve
Ventilation per minute	Tidal volume x breaths per minute

Residual volume stays in the lungs and is not part of vital capacity.

Worked reminder. A trace shows tidal volume 0.60 L and inspiratory reserve volume 2.8 L. Vital capacity is 4.5 L. Expiratory reserve volume = 4.5 - 0.60 - 2.8 = 1.1 L.

Exercise. Tidal volume and breathing rate both rise, so ventilation per minute rises. Forced exhalation uses the reserve volumes, so the expiratory reserve volume shrinks on a trace.

B3.1.7 to B3.1.10 Leaves

Structure	Role
Waxy cuticle	Cuts water loss
Epidermis	Protective and transparent; lower one holds stomata
Guard cells	Open and close stomata
Spongy mesophyll and air spaces	Fast diffusion through the leaf; moist exchange surface
Veins	Xylem supplies water, phloem removes sugars

Plan diagram checklist. Tissue outlines only. No cells. Clear title. Label lines that do not cross. Order from top: cuticle, upper epidermis, palisade mesophyll, spongy mesophyll, vein, lower epidermis with stoma.

Plants and animals compared. Both rely on diffusion across a thin, moist surface. Animals use ventilation and blood flow to keep gradients steep. A leaf uses air spaces and stomata, and it loses water as a side effect.

Transpiration factors. Rate rises with light, temperature and wind, and falls as humidity rises. Reason: each changes stomatal opening or the vapour gradient.

Worked reminder (potometer). Capillary radius 0.40 mm, bubble moves 25 mm in 5 minutes. Volume = pi x 0.40 x 0.40 x 25 = 12.6 mm3. Rate = 12.6 / 5 = 2.51 mm3 per minute.

Worked reminder (stomatal density). Field diameter 0.60 mm. Counts in four fields: 31, 28, 33, 30. Mean = 122 / 4 = 30.5. Area = pi x 0.30 x 0.30 = 0.283 mm2. Density = 30.5 / 0.283 = 108 per mm2.

Worked reminder (magnification). A drawing of a leaf section is 45 mm thick and the real thickness is 0.15 mm. Magnification = 45 / 0.15 = x300. To find real size, divide the drawn size by the magnification.

Reliability. Count several fields and use the mean. Variation between fields shows why repeats are needed.

B3.1.11 to B3.1.13 Haemoglobin (HL only)

• Four polypeptides, each with a haem group binding one oxygen molecule.
• Cooperative binding gives the S-shaped curve: slow start, steep middle, flat top.
• Carbon dioxide binds at another site (allosteric) and lowers oxygen affinity.
• Foetal haemoglobin has higher oxygen affinity. Its curve lies to the left of the adult curve, so it loads oxygen from maternal blood in the placenta.
• Bohr shift: more carbon dioxide moves the curve right, so more oxygen is released. This helps actively respiring tissue.

Curve direction.

Shift	Affinity	Effect at the tissues
Left	Higher	Less oxygen released
Right	Lower	More oxygen released

Worked reminder (Bohr shift). At 5 kPa a curve gives 72% saturation. With more carbon dioxide it gives 57%. Difference = 72 - 57 = 15 percentage points more oxygen released at that oxygen level.

Method: compare two curves.

1. Read both saturations at the same oxygen level.
2. Subtract to find the difference.
3. State which curve is to the right or left.
4. Link to affinity and to the biological advantage.

Quick self-test

1. A cube has sides of 4 mm. Work out its surface area to volume ratio.
2. State four properties of a gas-exchange surface.
3. Explain why blood flow helps maintain the gradient in the lungs.
4. State the role of surfactant.
5. Name the muscles that contract in forced exhalation.
6. A person has tidal volume 0.5 L, inspiratory reserve volume 3.0 L and expiratory reserve volume 1.1 L. Calculate vital capacity.
7. The same person breathes 12 times per minute. Calculate ventilation per minute.
8. Explain why a leaf loses water when it takes in carbon dioxide.
9. HL only. Explain why the oxygen dissociation curve is S-shaped.
10. HL only. At 4 kPa, saturation is 59% normally and 42% with more carbon dioxide. Describe and explain this change.

Answers

1. 6 x 16 / 64 = 96 / 64 = 1.5 per mm.
2. Permeable, thin, moist, large surface area.
3. Blood carries oxygen away and brings carbon dioxide, so the concentration gradient stays steep.
4. Reduces surface tension so alveoli do not stick together and stay open.
5. Internal intercostals and abdominal muscles.
6. 0.5 + 3.0 + 1.1 = 4.6 L.
7. 0.5 x 12 = 6.0 L per minute.
8. Open stomata let carbon dioxide in, but water vapour diffuses out from the moist mesophyll surfaces.
9. The first oxygen binds slowly; it changes shape so further molecules bind more easily; at high oxygen few sites remain, so the curve flattens.
10. Saturation falls by 17 percentage points. Carbon dioxide lowers affinity (Bohr shift), so more oxygen is released to tissue.

Where marks are usually lost

• Writing “air is sucked in” without stating the fall in pressure.
• Describing exhalation as always active. Quiet exhalation is passive.
• Stating that stomata “let in” gases without naming diffusion.
• Missing units on a lung volume, volume per minute or density.
• Using the radius, not the diameter, when finding a field of view area.
• Drawing individual cells on a plan diagram.
• Answering “high surface area” without saying what provides it.
• Saying haemoglobin “likes” oxygen instead of using affinity.
• Reversing left and right when describing a shift of the curve.

Official syllabus

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