B3.1 Gas exchange
Practice exam-style IB Biology questions for Gas exchange, aligned with the syllabus and grouped by topic.
Question 1
A cube-shaped organism increases in linear dimension while keeping the same shape. What change makes gas exchange across its outer surface less effective?
A.
Surface area increases faster than volume.
B.
Diffusion becomes an active process.
C.
The concentration gradient becomes reversed.
D.
Volume increases faster than surface area.
Question 2
What property of an efficient gas-exchange surface directly reduces the diffusion distance?
A.
Moist surface
B.
Thin tissue layer
C.
Large surface area
D.
Dense blood supply
Question 3
How many oxygen molecules can one haemoglobin molecule carry when fully saturated?
A.
One
B.
Eight
C.
Two
D.
Four
Question 4
What maintains a steep oxygen concentration gradient between alveolar air and blood?
A.
Thickening of the alveolar wall during inspiration
B.
Conversion of oxygen into carbon dioxide in capillaries
C.
Blood flow bringing deoxygenated blood to alveoli and removing oxygenated blood
D.
Closure of bronchioles after each breath
Question 5
What is the role of surfactant in mammalian alveoli?
A.
It contracts to force air out of the lungs.
B.
It increases the thickness of the alveolar epithelium.
C.
It transports oxygen in red blood cells.
D.
It reduces surface tension so alveoli are less likely to collapse.
Question 6
During normal inspiration, what happens to the diaphragm and external intercostal muscles?
A.
Diaphragm contracts and external intercostal muscles relax.
B.
Diaphragm relaxes and external intercostal muscles relax.
C.
Diaphragm relaxes and external intercostal muscles contract.
D.
Diaphragm contracts and external intercostal muscles contract.
Question 7
What is the function of guard cells in a leaf epidermis?
A.
They change shape to alter the size of stomatal pores.
B.
They absorb most light for photosynthesis.
C.
They transport sucrose from the leaf to sinks.
D.
They form the waterproof cuticle over the leaf.
Question 8
Foetal haemoglobin differs from adult haemoglobin by having a higher affinity for oxygen. What is the advantage of this in the placenta?
A.
It makes foetal and maternal blood mix directly.
B.
It prevents carbon dioxide from diffusing to maternal blood.
C.
It eliminates the need for haem groups.
D.
It allows oxygen to transfer from maternal blood to foetal blood.
Question 9
On an oxygen dissociation curve, a left-shifted curve compared with adult haemoglobin indicates what property?
A.
Lower maximum number of haem groups
B.
Lower oxygen affinity
C.
Higher oxygen affinity
D.
No cooperative binding
Question 10
Define gas exchange.
[1]
Outline why large multicellular animals cannot rely only on diffusion across the outer body surface.
[1]
Question 11
A spirometer trace shows repeated resting breaths. What lung volume is measured as the vertical difference between the peak and trough of one normal breath?
A.
Expiratory reserve volume
B.
Tidal volume
C.
Inspiratory reserve volume
D.
Vital capacity
Question 12
Air around a leaf becomes more humid while other conditions remain constant. What is the expected effect on transpiration rate?
A.
It decreases because the water vapour concentration gradient is reduced.
B.
It decreases because carbon dioxide cannot dissolve in water.
C.
It increases because evaporation from mesophyll cell walls rises.
D.
It increases because stomatal density immediately rises.
Question 13
What causes the S-shaped oxygen dissociation curve of adult haemoglobin?
A.
Cooperative binding between haem groups
B.
Diffusion of oxygen through plasma only
C.
Irreversible binding of oxygen to iron
D.
Collapse of alveoli at low oxygen concentration
Question 14
What is the Bohr shift?
A.
The movement of haemoglobin from plasma into red blood cells
B.
An increase in oxygen affinity caused by decreased pH
C.
A decrease in haemoglobin oxygen affinity caused by increased carbon dioxide
D.
The binding of oxygen permanently to haemoglobin
Question 15
Carbon dioxide binding to haemoglobin is described as allosteric because it does what?
A.
It binds to the same haem iron atom as oxygen.
B.
It prevents haemoglobin entering red blood cells.
C.
It binds at one site and changes oxygen affinity at another site.
D.
It changes haemoglobin into an enzyme.
Question 16
Efficient gas-exchange surfaces are usually moist and thin.
Explain the advantage of a moist surface.
[1]
Explain the advantage of a thin surface.
[1]
State one other property of an efficient gas-exchange surface.
[1]
Question 17
In mammalian lungs, oxygen diffuses from alveolar air into blood.
State the direction of carbon dioxide diffusion at the alveolus.
[1]
Explain how ventilation helps maintain the oxygen concentration gradient.
[1]
Explain how blood flow helps maintain the oxygen concentration gradient.
[1]
Question 18
The diagram shows part of a mammalian alveolus and a surrounding capillary.
Identify one structural feature that gives alveoli a large total surface area.
[1]
State the function of the capillary network around alveoli.
[1]
State the role of surfactant in alveoli.
[1]
Question 19
State the number of haem groups in one haemoglobin molecule.
[1]
Outline cooperative binding of oxygen by haemoglobin.
[1]
Question 20
Distinguish between cooperative binding and allosteric binding in haemoglobin. [2]
Question 21
A spirometer trace was recorded from a resting adult and then during a maximal breath.
Determine the tidal volume from one normal breath.
[1]
Determine the vital capacity from the maximal inhalation and maximal exhalation.
[1]
Suggest why the breathing rate changes after exercise.
[1]
Question 22
The graph shows changes in pressure in the thoracic cavity during one breathing cycle.
Identify the interval during which inspiration occurs.
[1]
State the change in thoracic volume during this interval.
[1]
Explain why air moves into the lungs during this interval.
[1]
State one muscle action that produces this change.
[1]
Question 23
A rise in carbon dioxide concentration near haemoglobin would produce which change in an oxygen dissociation curve?
A.
A shift to the left, indicating lower oxygen affinity
B.
A shift to the right, indicating lower oxygen affinity
C.
No change in saturation at any oxygen partial pressure
D.
A vertical line at all oxygen partial pressures
Question 24
Why is the Bohr shift beneficial in actively respiring muscle?
A.
It increases oxygen loading in tissues with low oxygen demand.
B.
It increases oxygen unloading where carbon dioxide production is high.
C.
It prevents aerobic respiration from producing carbon dioxide.
D.
It stops haemoglobin binding carbon dioxide in the lungs.
Question 25
Describe the movement of the diaphragm during inspiration.
[1]
Describe the action of the external intercostal muscles during inspiration.
[1]
Explain why air enters the lungs during inspiration.
[1]
Question 26
A student uses a spirometer. A normal breath has a peak volume reading of 2.9 dm³ and a trough reading of 2.4 dm³. After a maximum inhalation, the reading is 5.7 dm³. After a maximum exhalation, it is 1.2 dm³.
Calculate the tidal volume.
[1]
Calculate the vital capacity.
[1]
State why repeated rebreathing from a closed spirometer requires carbon dioxide absorption.
[1]
Question 27
The figure shows a transverse section through a dicotyledonous leaf.
Identify the tissue usually found just below the upper epidermis.
[1]
State the main gas-exchange function of the spongy mesophyll.
[1]
In a vein, state the usual position of xylem relative to phloem.
[1]
State one feature expected in a plan diagram of this section.
[1]
Question 28
A student counts stomata in five microscope fields from a leaf cast. The mean number of stomata per field is 18. The area of one field of view is 0.060 mm².
Calculate the stomatal density in mm⁻².
[1]
State why several fields of view should be counted.
[1]
State one reason why stomatal density may differ between the upper and lower epidermis.
[1]
Question 29
Foetal haemoglobin has a higher affinity for oxygen than adult haemoglobin.
State what is meant by higher oxygen affinity.
[1]
Explain how this difference helps oxygen transfer in the placenta.
[1]
Question 30
Define the Bohr shift.
[1]
Explain why the Bohr shift increases oxygen delivery to actively respiring tissues.
[1]
Question 31
The graph shows oxygen dissociation curves for haemoglobin under two carbon dioxide concentrations.
Identify the curve representing higher carbon dioxide concentration.
[1]
Explain the effect of higher carbon dioxide on haemoglobin saturation at the same oxygen partial pressure.
[1]
Question 32
A muscle changes from rest to vigorous activity.
Predict the change in carbon dioxide concentration around its capillaries.
[1]
Predict the change in haemoglobin oxygen affinity.
[1]
State the benefit of this change.
[1]
Question 33
Compare the oxygen dissociation curves of foetal haemoglobin and adult haemoglobin. [3]
Question 34
A student measured water uptake by a leafy shoot using a potometer under different relative humidities.
Describe the relationship between relative humidity and water uptake rate.
[1]
Explain why water uptake rate is used as an estimate of transpiration rate.
[1]
Explain the relationship shown using water vapour gradients.
[1]
Suggest one controlled variable for this investigation.
[1]
Question 35
Micrographs were taken from leaf surface casts of three plants of the same species.
| Plant | Field area / mm² | Field 1 / stomata | Field 2 / stomata | Field 3 / stomata | Field 4 / stomata | Field 5 / stomata |
|---|---|---|---|---|---|---|
| A | 0.20 | 36 | 40 | 38 | 39 | 37 |
| B | 0.20 | 48 | 50 | 49 | 51 | 52 |
| C | 0.20 | 42 | 41 | 44 | 40 | 43 |
Calculate stomatal density for one plant using the field area provided.
[1]
Identify the plant with the greatest mean stomatal density.
[1]
State one source of biological variation in these data.
[1]
Explain why repeated fields of view improve reliability.
[1]
Question 36
The graph shows oxygen dissociation curves for adult and foetal haemoglobin.
Identify the curve for foetal haemoglobin.
[1]
Compare the saturation of adult and foetal haemoglobin at the same oxygen partial pressure in the placenta.
[1]
Explain how the difference between the curves supports oxygen transfer to the fetus.
[1]
Question 37
The table shows the mean number of oxygen molecules bound per haemoglobin molecule at increasing oxygen partial pressures.
| pO₂ / kPa | O₂ bound / molecules Hb⁻¹ |
|---|---|
| 0.0 | 0.0 |
| 1.0 | 0.1 |
| 2.0 | 0.7 |
| 3.0 | 1.6 |
| 4.0 | 2.4 |
| 5.0 | 2.9 |
| 6.0 | 3.3 |
| 8.0 | 3.6 |
| 10.0 | 3.8 |
| 13.3 | 3.9 |
Describe the change in oxygen binding as partial pressure increases.
[1]
Identify the range of partial pressures where binding increases most steeply.
[1]
Explain the steep increase using cooperative binding.
[1]
Question 38
Explain the S-shaped form of the oxygen dissociation curve for adult haemoglobin. [4]
Question 39
Two leaf types were compared for gas exchange and water conservation.
| Leaf type | Stomatal density / mm^-2 | Mean aperture / µm | Cuticle thickness / µm | Transpiration rate / mmol m^-2 s^-1 |
|---|---|---|---|---|
| A | 95 | 2.4 | 8.1 | 1.7 |
| B | 210 | 5.8 | 3.2 | 4.9 |
Identify which leaf type is likely to have the lower transpiration rate.
[1]
Use the data to support your answer.
[1]
Explain one trade-off between stomatal opening and photosynthesis.
[1]
Suggest one additional measurement that would strengthen the conclusion.
[1]
Question 40
Haemoglobin saturation was measured at different partial pressures of oxygen under low and high carbon dioxide concentrations.
Identify the curve recorded at high carbon dioxide concentration.
[1]
Determine the approximate difference in saturation between the two curves at a stated tissue oxygen partial pressure.
[1]
Explain why high carbon dioxide changes haemoglobin saturation.
[1]
Suggest why this change is advantageous during exercise.
[1]
Question 41
Blood samples were taken from a resting muscle and an active muscle. The samples differed in carbon dioxide concentration, pH and haemoglobin oxygen saturation.
| Sample | CO₂ concentration / mmol dm⁻³ | pH | Hb O₂ saturation / % |
|---|---|---|---|
| A | 26.0 | 7.18 | 42 |
| B | 21.5 | 7.38 | 75 |
Identify the sample from the active muscle.
[1]
Explain the relationship between carbon dioxide concentration and pH.
[1]
Explain the relationship between pH and oxygen saturation.
[1]
Suggest why venous blood from active muscle contains less oxygen.
[1]
Question 42
Outline two properties shared by efficient gas-exchange surfaces.
[1]
Explain how mammalian alveolar lungs are adapted for rapid gas exchange.
[1]
Question 43
Describe the actions of the diaphragm and external intercostal muscles during inspiration.
[1]
Explain how normal inspiration and forced expiration ventilate the lungs.
[1]
Question 44
Describe how stomatal density can be determined from a leaf surface cast.
[1]
Evaluate why repeated measurements are needed when estimating stomatal density.
[1]
Question 45
Oxygen dissociation curves were obtained from two mammal species living at different altitudes.
Identify the species with haemoglobin of higher oxygen affinity.
[1]
Use the graph to justify your answer.
[1]
Suggest an advantage of this curve position for an animal living where oxygen partial pressure is low.
[1]
Evaluate one possible disadvantage of very high oxygen affinity in tissues.
[1]
Question 46
Outline the function of stomata and guard cells in leaves.
[1]
Discuss how leaf structure allows gas exchange while reducing water loss.
[1]
Question 47
Outline the structure of haemoglobin in relation to oxygen binding.
[1]
Explain cooperative binding and its importance for oxygen transport.
[1]
Question 48
Define allosteric binding and the Bohr shift in haemoglobin.
[1]
Discuss how carbon dioxide affects oxygen transport by haemoglobin in active tissues and in the lungs.
[1]
Question 49
Outline why foetal haemoglobin must differ functionally from adult haemoglobin.
[1]
Evaluate the importance of higher oxygen affinity in foetal haemoglobin for gas exchange in the placenta.
[1]
Question 50
State what is represented on an oxygen dissociation curve.
[1]
Compare and contrast adult haemoglobin, foetal haemoglobin and haemoglobin under high carbon dioxide conditions using oxygen dissociation curves.
[1]
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