IB Biology HL B3.1 Gas exchange Questions

Practise IB Biology HL B3.1 by linking respiratory surfaces and ventilation to oxygen transport, dissociation curves and transpiration evidence.

120 questions in this syllabus scope, with question parts counted together.

Exam analysis

HL · 2010–2025

Papers with this topic50 / 117in the analysed sample

Latest appearanceNovember 2025

Tagged question parts77

Paper types5

Most tested objectives

B3.1.9—Transpiration17 papers
B3.1.12 (HL)—Bohr shift10 papers
B3.1.5—Lung ventilation8 papers
B3.1.4—Mammalian lung adaptations6 papers
B3.1.8—Leaf tissue distribution5 papers

Common question formats

Process explanation
Definition or recall
Graph interpretation
Diagram interpretation
Calculation
Experimental design
Comparison

Recent exam appearances

November 2025Paper 1A HL · TZ1Q23[ 1 ]

May 2025Paper 2 HL · TZ3Q4(d), 6(a), 6(b)[ 6 ]

May 2025Paper 1B HL · TZ2Q1(a), 1(b), 1(c), 1(d)[ 5 ]

117 papers analysed · Tagging incomplete

Exam points

Explain how lower oxygen partial pressure at high altitude reduces the diffusion gradient for gas exchange.
Relate alveolar area, thin walls, capillaries and surfactant to efficient gas exchange.
Trace ventilation muscles, thoracic pressure and airflow, interpreting pressure or spirometry data.
Calculate spirometric volumes and compare ventilation responses as exercise intensity increases.
Explain stomatal control, guard-cell responses and leaf adaptations that limit water loss.
Interpret leaf tissue distribution and internal air spaces as routes for light capture and gas diffusion.
Define transpiration, predict environmental water loss and evaluate potometer measurements.
Compare stomatal density and indicator patterns between leaf surfaces.
Explain haemoglobin subunits, haem binding, cooperative loading and fetal oxygen affinity.
Explain the Bohr shift from carbon dioxide and pH, predicting oxygen unloading from curve shifts.
Read oxygen dissociation curves for saturation, pO2, plateaus, cooperativity and relative affinity.

Covered syllabus points

B3.1.1—Gas exchange as vital function B3.1.2—Properties of gas-exchange surfaces B3.1.3—Maintaining concentration gradients B3.1.4—Mammalian lung adaptations B3.1.5—Lung ventilation B3.1.6—Lung volume measurements B3.1.7—Leaf gas exchange adaptations B3.1.8—Leaf tissue distribution B3.1.9—Transpiration B3.1.10—Stomatal density B3.1.11 (HL)—Haemoglobin adaptations B3.1.12 (HL)—Bohr shift B3.1.13 (HL)—Oxygen dissociation curves

Exam points

Identify alveoli as the gas-exchange surface and trace oxygen into blood and carbon dioxide to alveoli.
Explain how ventilation and blood flow maintain oxygen and carbon-dioxide gradients across alveoli.
Explain why many alveoli and thin walls create large surface area and short diffusion paths.

[Maximum number: 2]

Scientists studied the relationship of the age of asthma onset and body weight. A study was undertaken of 504 people with asthma in categories of obese, overweight and normal.

Late-onset asthma was defined as beginning at 12 years of age or above. The results are shown in the table.

Outline, using your own knowledge, the effects of asthma on the gas exchange system of the people in this study.

Exam points

Explain how alveolar surface area, thin walls and a moist lining aid gas diffusion.
Explain how blood flow and ventilation maintain concentration gradients across alveoli.

[Maximum number: 3]

The graph shows the relationship between partial pressure of oxygen in air and altitude.

Explain the consequence of high altitude for gas exchange in humans.

Exam points

Explain how lower oxygen partial pressure at altitude reduces oxygen diffusion into blood.

[Maximum number: 3]

The graph shows the relationship between partial pressure of oxygen in air and altitude.

Explain the consequence of high altitude for gas exchange in humans.

a. lower oxygen partial pressure reduces oxygen availability (for the blood);

b. less oxygen diffuses/is exchanged with the blood OR less oxygen diffuses across the alveolar membrane/alveoli;

c. less saturation of hemoglobin with oxygen;

d. more $\mathrm{CO}_{2}$ remains in the blood OR less $\mathrm{CO}_{2}$ is exhaled;

e. breathing rate/depth is increased (in the short term);

f. (to compensate for the lack of oxygen) the body will produce more erythrocytes/RBCs/hemoglobin;

Mp a: Do not accept lower oxygen concentration at high altitude. Partial pressure must be mentioned.

Mp c: Do not accept increased hemoglobin affinity for oxygen instead of saturation.

Mp d: Allow higher $\mathrm{CO}_{2}$ decreasing hemoglobin affinity for oxygen.

Marking guidance:

3 max

Exam points

Link many alveoli, thin walls and capillary beds to large area and short diffusion paths for gas exchange.
Explain how ventilation and blood flow maintain alveolar gas gradients for diffusion.
Explain how surfactant moistens alveoli, lowers surface tension and prevents collapse.

[Maximum number: 1]

The diagram shows a section through an alveolus.

What are the functions of the following labelled structures?

Type I pneumocyte

Type II pneumocyte

Capillary

Gas exchange

Produces surfactant

Transports carbon dioxide from alveolus

Produces surfactant

Gas exchange

Transports oxygen to alveolus

Gas exchange

Produces surfactant

Transports carbon dioxide to alveolus

Produces surfactant

Gas exchange

Transports oxygen from alveolus

Exam points

Link diaphragm and external intercostal contraction to thoracic expansion, lower pressure and inward airflow.
Explain expiration as thoracic volume falls and pressure rises, driving air out of the lungs.
Identify internal intercostal and abdominal contraction as the cause of forced expiration.
Interpret ventilation or spirometry data by linking pressure changes and muscle action to airflow.

[Maximum number: 2]

The graph shows the relationship between blood pH in the arteries and volume of air inhaled and exhaled per minute.

State which muscles would contract more vigorously, or more frequently, to increase the volume of air inhaled per minute.

Exam points

Identify a spirometer as an instrument for measuring tidal volume and lung capacity.
Read normal-breathing traces to identify tidal volume and calculate ventilation rate with units.
Determine vital capacity from the difference between maximum and minimum volumes on a forced-breath trace.

[Maximum number: 5]

The graph shows the ventilation rate and tidal volume of a well-trained runner during exercise on a treadmill. The tidal volume is the volume of air being moved in and out of the lungs in each breath.
- Ventilation rate volume 50 Tidal

(a)

State the apparatus used to measure the tidal volume.

[ 1 ]

(b)

Calculate the total volume of air inhaled during one minute during the highest velocity of the treadmill in this test, giving the units.

[ 2 ]

a. tidal volume x ventilation rate
OR
litres breath ${ }^{-1} \times$ breath $\min ^{-1}$
b. $186 \mathrm{~L}\left(\min ^{-1}\right)$

(c)

Compare and contrast the effect of increasing treadmill speed on the ventilation rate and tidal volume in this runner.

[ 2 ]

a. both ventilation rate and tidal volume increase with increase in treadmill speed / intensity of exercise
b. at low treadmill speed/below $14.2 \mathrm{kmh}^{-1}$ , the tidal volume increases more steeply than ventilation rate
OR
at high treadmill speed, ventilation rate increases more steeply than tidal volume
OR
tidal volume plateaus while ventilation rate does not

Exam points

Compare daily stomatal-opening graphs to identify when different plants can take up carbon dioxide.
Explain how guard cells alter stomatal opening to regulate gas exchange and limit water loss.
Explain how spongy mesophyll and air spaces support diffusion of gases through a leaf.

[Maximum number: 1]

The graph shows the percentage of stomata that are open in two different species of plants over a 24-hour period.

What does this graph show about plants X and Y ?

Plant X absorbs most carbon dioxide at night.

Plant Y is adapted to desert conditions.

Plant X photosynthesises most at midday.

Plant Y only respires during the day.

Exam points

Draw and label the relative positions of leaf tissues, including epidermis, mesophyll, xylem and phloem.
Relate leaf tissue positions to light capture, photosynthesis and transport of water and assimilates.
Explain how stomata and loosely packed spongy mesophyll allow leaf gas exchange.

[Maximum number: 5]

The leaves of plants are adapted for photosynthesis. Draw a labelled plan diagram of a leaf to show the distribution of tissues in a leaf.

Exam points

Describe transpiration as evaporation from mesophyll walls followed by water-vapour diffusion through stomata.
Predict how humidity, temperature, wind and light change transpiration using graphs or data.
Use bubble movement to estimate potometer water-uptake rate and distinguish it from water lost by the shoot.
Design potometer tests by varying one factor and controlling leaf area, light or temperature.
Use leaf-removal or leafless-control data to infer transpiration and xylem water movement.

[Maximum number: 1]

What increases the rate of transpiration?

Increasing the humidity

Decreasing the temperature

Decreasing the light intensity

Increasing the wind speed

Exam points

Predict how stomatal density differs between upper and lower leaf surfaces from a measured density.
Interpret cobalt-chloride colour patterns to compare stomatal distribution and transpiration across leaf surfaces.

[Maximum number: 1]

The stomatal density is the number of stomata per area of leaf epidermis. A study was done on the stomatal density of the lower epidermis of leaves from Protium decandrum, a tree found in the Amazon forest.

The image shows the epidermis stripped from the lower surface of a leaf.

The mean stomatal density for the lower epidermis of P. decandrum was around 600 per $\mathrm{mm}^{2}$ . Predict how the stomatal density for the upper epidermis would compare.

Exam points

Explain that four haemoglobin subunits each contain a haem group that binds oxygen reversibly.
Explain how cooperative oxygen binding raises affinity at high pO₂ and lowers it for release at low pO₂.
Compare foetal and adult oxygen dissociation curves to explain higher affinity and placental transfer.

[Maximum number: 2]

The diagram shows the liver.

Iron is stored in the liver and carried to the bone marrow to produce hemoglobin in new red blood cells. Outline the role of hemoglobin in the transport of gases.

a	both $\mathrm{O}_{2}$ and $\mathrm{CO}_{2}$ bind (reversibly) to hemoglobin;
b	carries oxygen as oxyhemoglobin (in RBC) OR carries carbon dioxide as carboxyhemoglobin;
c	binding of oxygen to hemoglobin increases affinity of hemoglobin for oxygen;
d	lower $\mathrm{pO}_{2}$ decreases the affinity for $\mathrm{O}_{2}$ /vice versa;
e	fetal hemoglobin has greater affinity for oxygen than adult hemoglobin;

Exam points

Explain how CO₂ from respiring tissues lowers blood pH and haemoglobin affinity for oxygen.
Identify the Bohr shift as a rightward oxygen dissociation curve shift when CO₂ rises.
Use a right-shifted curve to explain increased O₂ release to active tissues at the same pO₂.

[Maximum number: 1]

Two oxygen dissociation curves in an adult human are shown.

What is the explanation for the shift of the dissociation curve to the right-hand curve?

Carbon dioxide

Affinity of haemoglobin for oxygen

dissociates from haemoglobin

increases

dissociates from haemoglobin

decreases

associates with haemoglobin

increases

associates with haemoglobin

decreases

Exam points

Read curve axes and use pO₂ to find haemoglobin saturation, including plateau values.
Explain how cooperative oxygen binding between haemoglobin subunits produces a sigmoid curve.
Compare adult, foetal and myoglobin curves to infer affinity and oxygen loading or tissue release.

[Maximum number: 1]

The graph shows the oxygen dissociation curves for hemoglobin in a fetus, a healthy adult and an adult with emphysema.

State where the curve for myoglobin would be drawn on the diagram.

Exam analysis

Most tested objectives

Common question formats

Recent exam appearances

Covered syllabus points

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