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

These are original questions written for Marlbridge, for revision and practice on this content. They are not reproduced past-paper questions, and they do not replicate the exam’s exact structure, question count or mark tariffs – the IB holds copyright in its own papers. Use these alongside the official past papers available through your school or the IB store.

These questions cover section B3.1 (Gas exchange) of the IB Biology guide, first assessment 2025. Questions 9 to 11 test B3.1.11 to B3.1.13 and are HL only; the rest are SL and HL. Read the study guide first and keep the revision notes beside you. Hub: IB DP Biology. Checklist: printable checklist. To find weak areas first, take the free 10-minute diagnostics.

Questions

1. State two properties that all gas-exchange surfaces share. [2]

2. A student models two organisms as cubes of side 5 mm and 15 mm.

(a) Calculate the surface area-to-volume ratio of each cube. [2] (b) Explain why the larger organism is more likely to need a specialised gas-exchange system. [2]

3. Explain how four features of mammalian alveolar lungs make gas exchange efficient. [4]

4. Describe how muscles and ribs produce inhalation, and how forced exhalation differs from quiet exhalation. [6]

5. A resting student has a tidal volume of 0.55 L, an inspiratory reserve volume of 3.05 L and a vital capacity of 4.70 L. The student breathes 15 times per minute.

(a) Calculate the expiratory reserve volume. [2] (b) Calculate the volume of air breathed per minute at rest. [1] (c) During exercise, tidal volume is 1.8 L at 24 breaths per minute. Calculate how many times greater the volume breathed per minute is than at rest. [2] (d) Explain why a spirometer cannot give a value for total lung volume. [2]

6. A student draws a plan diagram of a transverse section of a dicotyledonous leaf.

(a) State two features of a plan diagram. [2] (b) Outline how three leaf structures help gas exchange while limiting water loss. [3]

7. A potometer has a capillary of radius 0.60 mm. In still air a bubble moves 20 mm in 4 minutes. With a fan blowing it moves 33 mm in 4 minutes.

(a) Calculate the volume of water taken up per minute in still air. [3] (b) Calculate the percentage increase in rate with the fan. [2] (c) Explain the effect of the fan. [2]

8. A student makes leaf casts from both surfaces of a leaf and counts stomata in five fields of view, each of diameter 0.45 mm.

Field	1	2	3	4	5
Lower surface	21	25	23	24	22
Upper surface	2	0	1	1	2

(a) Calculate the stomatal density of the lower surface in stomata per mm2. [3] (b) Calculate the stomatal density of the upper surface. [2] (c) Explain why five fields were counted rather than one. [2] (d) Suggest why the upper surface has fewer stomata. [1]

9. (HL only) Explain why the oxygen dissociation curve of adult haemoglobin is S-shaped. [5]

10. (HL only) The table shows percentage saturation of adult and foetal haemoglobin.

Oxygen (kPa)	2	3	4	5	6
Adult	18	40	59	72	81
Foetal	35	62	78	87	91

(a) Calculate the difference in saturation at 4 kPa. [1] (b) Describe how the foetal curve compares with the adult curve. [2] (c) Explain how this difference benefits the foetus. [3] (d) Predict how foetal and adult saturation compare at 1 kPa. [1]

11. (HL only) Exercising muscle has an oxygen concentration of 3 kPa. The lungs have 13 kPa. Saturation at 3 kPa is 40% normally and 25% with raised carbon dioxide. At 13 kPa it is 97% and 95%.

(a) Calculate the percentage points of saturation lost between lungs and muscle in each condition. [2] (b) Explain how carbon dioxide causes this change. [3] (c) Explain the benefit to the muscle. [2] (d) Explain why loading in the lungs is almost unchanged. [2]

12. Compare how concentration gradients are maintained at fish gills and mammalian alveoli. [4]

Answers

1. Any two of: permeable to the gases; thin, often one cell thick; moist; large surface area. [1][1] [2] Examiner insight: Answers gain credit when each property is a separate, named point; “efficient” alone is too vague to be credited.

2. (a) 5 mm: 6 x 25 = 150 mm2, volume 125 mm3, ratio 1.2 per mm [1]. 15 mm: 6 x 225 = 1350 mm2, volume 3375 mm3, ratio 0.4 per mm [1]. [2] (b) Ratio is lower in the larger organism, so less surface per unit volume [1]. Diffusion distance to the centre is greater, so diffusion alone cannot supply every cell [1]. [2] Examiner insight: Quote the ratio values from part (a) in part (b); comparing them directly makes the explanation clear.

3. Any four, each with a reason: high surface area from many alveoli gives more area for diffusion; branched bronchioles carry air close to every alveolus; extensive capillary beds keep blood flowing and the gradient steep; surfactant reduces surface tension so alveoli stay open; thin walls give a short diffusion path. [1][1][1][1] [4] Examiner insight: A feature without its function is a weaker answer; use “so that” or “because” every time.

4. Inhalation: diaphragm contracts and flattens [1]; external intercostals contract, moving ribs up and out [1]; thoracic volume increases and pressure falls below atmospheric, so air flows in [1]. Quiet exhalation is passive: muscles relax and the diaphragm domes [1]. Forced exhalation: internal intercostals contract, pulling ribs down and in [1]; abdominal muscles contract, pushing the diaphragm up [1]. [6] Examiner insight: Answers tend to gain credit when they link muscle action to volume and pressure change, not just to rib movement.

5. (a) ERV = 4.70 - 0.55 - 3.05 [1] = 1.10 L [1]. [2] (b) 0.55 x 15 = 8.25 L per minute [1]. [1] (c) 1.8 x 24 = 43.2 L per minute [1]; 43.2 / 8.25 = 5.24 times [1]. [2] (d) Some air stays in the lungs after the strongest exhalation [1]; this residual volume cannot be breathed out into the spirometer [1]. [2] Examiner insight: Show the working line and include units; a correct value with no units is a common reason for lost credit.

6. (a) Any two of: shows tissue outlines only, no individual cells; clear continuous lines; labelled with non-crossing label lines; a title. [1][1] [2] (b) Any three: waxy cuticle reduces water loss; stomata with guard cells open to allow gases in and close to reduce water loss; air spaces in spongy mesophyll let gases diffuse to cells; transparent epidermis lets light through. [1][1][1] [3] Examiner insight: A strong answer links each structure to both aims, gas exchange and water saving.

7. (a) Volume = pi x 0.60 x 0.60 x 20 [1] = 22.6 mm3 [1]; rate = 22.6 / 4 = 5.65 mm3 per minute [1]. [3] (b) Fan rate = pi x 0.36 x 33 / 4 = 9.33 mm3 per minute [1]; increase = (9.33 - 5.65) / 5.65 x 100 = 65% [1]. [2] (c) Wind removes humid air next to the leaf [1]; this steepens the water vapour gradient, so transpiration increases [1]. [2] Examiner insight: Percentage change is relative to the original value, so divide by the still-air rate.

8. (a) Mean = 115 / 5 = 23.0 [1]; area = pi x 0.225 x 0.225 = 0.159 mm2 [1]; density = 23.0 / 0.159 = 145 per mm2 [1]. [3] (b) Mean = 6 / 5 = 1.2 [1]; density = 1.2 / 0.159 = 7.5 per mm2 [1]. [2] (c) Counts vary from field to field [1]; a mean of repeats reduces the effect of chance and increases reliability [1]. [2] (d) The upper surface is more exposed to sun and heat, so fewer stomata reduce water loss. [1] [1] Examiner insight: The field of view diameter must be halved before finding the area; using the diameter as a radius is a common slip.

9. Haemoglobin has four haem groups, each binding one oxygen [1]. At low oxygen the first molecule binds slowly, so the curve rises gently [1]. Binding changes the shape of the molecule [1]. This makes the other haem groups bind oxygen more easily (cooperative binding), so the curve steepens [1]. At high oxygen most sites are full, so the curve flattens [1]. [5] Examiner insight: Name cooperative binding and describe all three regions of the curve.

10. (a) 78 - 59 = 19 percentage points [1]. [1] (b) The foetal curve lies to the left [1]; saturation is higher at every oxygen level shown [1]. [2] (c) Foetal haemoglobin has a higher affinity for oxygen [1]. In the placenta, maternal blood gives up oxygen at low oxygen levels [1]; foetal haemoglobin picks it up, keeping the gradient from mother to foetus [1]. [3] (d) Foetal saturation is higher, because its curve lies to the left throughout. [1] [1] Examiner insight: “Higher affinity” is more precise than “better at” and should be used in explanations.

11. (a) Normal: 97 - 40 = 57 [1]; raised carbon dioxide: 95 - 25 = 70 [1]. [2] (b) Carbon dioxide binds to haemoglobin at a site other than the haem group (allosteric) [1]; shape changes, lowering affinity for oxygen [1]; curve shifts right, so more oxygen is released at the same oxygen level (Bohr shift) [1]. [3] (c) Respiring muscle makes more carbon dioxide [1]; it receives more oxygen where demand is highest [1]. [2] (d) Carbon dioxide is low in the lungs as it is breathed out [1]; haemoglobin loads almost fully at high oxygen, so the shift hardly changes loading [1]. [2] Examiner insight: State the direction of the shift and tie it to the tissue, not to the curve alone.

12. Gills: water flows continuously over the gills, renewing oxygen at the surface [1]; blood flows through capillaries in the gills, carrying oxygen away [1]. Alveoli: ventilation renews air [1]; extensive capillary beds with continuous blood flow remove oxygen from the alveoli [1]. [4] Examiner insight: Comparison questions are best answered with a point for each, noting the medium (water or air).

Where marks are usually lost

• Writing that gases are “pumped” or “sucked” instead of diffusing or flowing down a pressure gradient.
• Giving a feature of the lung with no function.
• Forgetting units on volumes, rates and densities.
• Using the field-of-view diameter as the radius.
• Calculating percentage change from the new value, not the original.
• Saying “residual volume” is measured by the spirometer.
• Describing a plan diagram with cells drawn.
• Mixing up left and right when comparing curves.
• Not naming cooperative binding in the S-shape answer.

Next steps

• Revise with the revision notes and re-read the study guide.
• Course hub: IB DP Biology. Checklist: printable checklist.
• Try all free 10-minute diagnostics to find your weakest areas.
• Book a free trial class.

Official syllabus

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