OCR A Level Biology: Exchange and Transport (H420) – 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 – examination boards hold copyright in their own papers. Use these alongside the official past papers from your board or school.

These questions follow Module 3 (Exchange and transport, sections 3.1.1 to 3.1.3) of the OCR A Level Biology A (H420) specification, Version 4.1 (April 2026). Every number is invented for practice. Read the study guide first, use the revision notes for recall, and see the course hub and the printable checklist. Try the free diagnostics to find weak spots.

Questions

1. A small invertebrate is a cuboid measuring 2 mm × 2 mm × 8 mm.

(a) Calculate its surface area to volume ratio. [2] (b) Explain why a much larger animal of similar shape cannot rely on its body surface for gas exchange. [3]

2. Describe how the tissues in the wall of the trachea and of a bronchiole differ, and give the function of each tissue you name. [4]

3. A student at rest breathes through a spirometer. The mean tidal volume is 0.45 dm³ and the breathing rate is 16 breaths per minute. The trace falls by 0.62 dm³ over 2.5 minutes as oxygen is used and carbon dioxide is absorbed.

(a) Describe the changes that cause air to enter the lungs during inspiration. [3] (b) Calculate the ventilation rate. [2] (c) Calculate the rate of oxygen uptake, in dm³ per minute. [2]

4. In one gill lamella, the oxygen saturation of water falls from 90% to 50% along its length. Blood flows through the lamella in the opposite direction, entering at 40% saturation and leaving at 80%.

(a) Explain how the buccal cavity and operculum keep water flowing over the gills. [3] (b) Explain why this arrangement is better than parallel flow, in which blood and water would reach about 65% saturation each. [3]

5. Describe how oxygen reaches the muscle cells of a flying insect, including how the system is ventilated. [4]

6. (a) State what is meant by a closed, single circulatory system and name one animal that has one. [2] (b) Explain the advantage of a double circulation in a mammal. [2] (c) Describe how the wall of an artery differs from that of a vein, linking each difference to function. [3]

7. At the arteriole end of a capillary, hydrostatic pressure is 4.0 kPa and oncotic pressure is 3.0 kPa. At the venule end, hydrostatic pressure is 1.5 kPa and oncotic pressure is still 3.0 kPa. Ignore any pressure in the tissue fluid.

(a) Calculate the net pressure at each end, stating its direction. [2] (b) Explain why the oncotic pressure stays constant. [1] (c) State two ways in which tissue fluid differs from blood plasma, and describe the role of lymph. [3]

8. (a) Describe what happens to the valves and the pressures during ventricular systole. [3] (b) Explain how the heart wall contracts in a coordinated way, naming the structures involved. [3] (c) At rest a person has a heart rate of 64 beats per minute and a stroke volume of 85 cm³. After training these are 52 and 110. Calculate each cardiac output. [2]

9. An ECG shows R waves 0.50 s apart.

(a) Calculate the heart rate and name the condition it indicates. [2] (b) Describe how an ECG trace of fibrillation and of an ectopic beat would each look. [2]

10. (a) Explain how carbon dioxide is carried by red blood cells, naming the enzyme and the chloride shift. [4] (b) Fetal haemoglobin has a dissociation curve to the left of the adult curve. Explain the significance. [2] (c) Explain the Bohr effect and its value to a respiring muscle. [2]

11. In a potometer the capillary tube has a radius of 0.40 mm. An air bubble moves 45 mm in 6.0 minutes.

(a) Calculate the rate of water uptake in mm³ per minute. [3] (b) Predict, with a reason, the effect of moving the shoot into moving air. [2] (c) Give one structural adaptation of marram grass and explain how it reduces water loss. [2]

12. (a) Distinguish the apoplast pathway from the symplast pathway. [2] (b) Cell A has a water potential of −500 kPa and the neighbouring cell B has −650 kPa. State the direction of net water movement and why. [2] (c) Explain how sucrose is loaded into phloem at a source and how this moves it to a sink. [4]

Answers

1. (a) Surface area = 2 × (2×2) + 4 × (2×8) = 8 + 64 = 72 mm² [1]. Volume = 32 mm³, so the ratio is 2.25 mm⁻¹ [1] [2] (b) A larger animal has a smaller SA:V ratio [1], so diffusion distance is longer and surface area is too small for its volume [1]; active cells use oxygen faster than diffusion can supply it, so it needs a specialised surface and transport system [1] [3] Examiner insight: Showing area and volume separately before dividing makes a slip easier to credit than a bare ratio.

2. Trachea has C-shaped cartilage to keep the airway open [1]; bronchioles have no cartilage but more smooth muscle to alter the diameter [1]. Ciliated epithelium in both moves mucus upward [1]; goblet cells in the trachea secrete mucus that traps particles [1]. Elastic fibres in both recoil to help expiration [4] Examiner insight: Answers that name a tissue without its function, or its location, usually miss the linked mark.

3. (a) The external intercostal muscles and diaphragm contract [1], the ribs move up and out and the diaphragm flattens [1], volume rises and pressure falls below atmospheric, so air flows in [1] [3] (b) 0.45 × 16 [1] = 7.2 dm³ per minute [1] [2] (c) 0.62 ÷ 2.5 [1] = 0.25 dm³ per minute (0.248) [1] [2] Examiner insight: Quote the unit with each calculated answer; a number alone is easy to lose.

4. (a) The floor of the mouth is lowered with the operculum shut, so buccal volume rises and pressure falls, drawing water in [1]. The floor then rises and the operculum opens [1], raising pressure so water flows over the gills and out [1] [3] (b) In counter-current flow, water always meets blood of lower saturation [1], so a diffusion gradient (10 percentage points here) is kept along the whole lamella [1]; blood leaves at 80%, above the roughly 65% parallel flow could reach [1] [3] Examiner insight: Use the figures given; a generic statement about “maintaining a gradient” gains less than a quoted comparison.

5. Air enters through spiracles [1] into tracheae and then tracheoles [1], which end in fluid in contact with muscle cells, where oxygen dissolves and diffuses [1]. Thoracic and abdominal movements change body volume to pump air [1] [4] Examiner insight: Mention the tracheal fluid; many answers stop at the spiracles.

6. (a) Blood stays within vessels [1] and passes through the heart once per circuit; a fish is an example [1] [2] (b) Blood returns to the heart after the lungs and is pressurised again [1], so it reaches the body at high pressure [1] [2] (c) An artery has a thick wall with elastic fibres and muscle to withstand and smooth pressure surges [1]; a vein has a thinner wall, a wide lumen [1] and valves to prevent backflow at low pressure [1] [3] Examiner insight: A comparison needs both vessels in each sentence, not two separate descriptions.

7. (a) Arteriole end: 4.0 − 3.0 = 1.0 kPa outwards [1]. Venule end: 1.5 − 3.0 = 1.5 kPa inwards [1] [2] (b) Plasma proteins are too large to leave the capillary [1] [1] (c) Tissue fluid contains fewer large proteins than plasma [1] and has no red cells [1]; lymph vessels return excess fluid to the blood, draining into veins [1] [3] Examiner insight: Show the subtraction and the sign or direction; the direction is a separate point from the number.

8. (a) Ventricular pressure rises above atrial pressure, closing the atrioventricular valves [1]; when it exceeds aortic pressure, the semilunar valves open [1] and blood is ejected [1] [3] (b) The SAN initiates a wave of excitation across the atria [1]; it reaches the AVN, which delays it [1], then purkyne tissue carries it down the septum to the apex and up the walls; cardiac muscle is myogenic [1] [3] (c) 64 × 85 = 5440 cm³ per minute [1]; 52 × 110 = 5720 cm³ per minute [1] [2] Examiner insight: Give pressure comparisons in words such as “higher than”, not just “high”.

9. (a) 60 ÷ 0.50 = 120 beats per minute [1], which is tachycardia [1] [2] (b) Fibrillation shows irregular, rapid and disorganised waves with no clear pattern [1]; an ectopic beat is an early extra beat followed by a pause [1] [2] Examiner insight: Read the time scale first; heart rate errors usually start there.

10. (a) Most carbon dioxide diffuses into red cells and, with water, forms carbonic acid, catalysed by carbonic anhydrase [1], which dissociates to H⁺ and hydrogencarbonate ions [1]. Hydrogencarbonate leaves and chloride ions enter, the chloride shift [1]; H⁺ binds haemoglobin forming haemoglobinic acid [1] [4] (b) The fetus can take oxygen from the mother’s blood [1] because its haemoglobin has a higher affinity at the same partial pressure [1] [2] (c) Higher carbon dioxide shifts the curve right [1], so haemoglobin unloads more oxygen in active tissue [1] [2] Examiner insight: “Higher affinity” is more precise than “better at binding” and compares like with like.

11. (a) Volume = π × 0.40² × 45 = 22.6 mm³ [1]; ÷ 6.0 [1] = 3.77 mm³ per minute [1] [3] (b) Uptake rises [1] because the humid air near the leaf is removed, steepening the gradient [1] [2] (c) Rolled leaves trap humid air, reducing the gradient [1]; so less water vapour is lost [1] [2] Examiner insight: A prediction needs a reason; the direction alone is a single point.

12. (a) The apoplast pathway is through cell walls and spaces [1]; the symplast pathway is through cytoplasm via plasmodesmata [1] [2] (b) From A to B [1], because water moves down a water potential gradient to the more negative value [1] [2] (c) Companion cells use ATP to actively pump H⁺ out [1]; H⁺ re-enters with sucrose by co-transport [1]; sucrose entering sieve tubes lowers water potential so water enters by osmosis [1], raising pressure which moves sap to the sink [1] [4] Examiner insight: Name the energy-requiring step explicitly.

Where marks are usually lost

• Writing “thin” without saying that distance for diffusion is shorter.
• Calling a tissue’s function by its name only, such as “cartilage supports”.
• Saying a gradient is “kept” without naming the process that maintains it.
• Forgetting units or the direction of net movement in tissue fluid calculations.
• Confusing hydrostatic pressure with oncotic pressure.
• Mixing up which valves close and open in each stage of the cardiac cycle.
• Describing the Bohr effect without saying which way the curve moves.
• Writing “water moves from high to low water potential” with the wrong sign for negative values.

Next steps

• Revise with the revision notes and study guide.
• Course hub: OCR A Level Biology and checklist.
• Take all free 10-minute diagnostics.
• Book a free trial class.

Frequently asked questions

Why is “the membrane is thin” not enough for the mark? You need to link the feature to its effect, for example that a thin wall gives a shorter diffusion distance and so faster diffusion. Naming a feature without explaining how it affects diffusion is one of the most common ways to lose marks in Module 3.

What is the difference between hydrostatic pressure and oncotic pressure? Hydrostatic pressure is the pressure of the blood pushing fluid out of the capillaries, while oncotic pressure is the tendency of plasma proteins to draw water back in by osmosis. Confusing the two, or forgetting to state the direction of net movement, loses marks in tissue fluid questions.

How should you describe the Bohr effect in an exam? State which way the oxygen dissociation curve moves: higher carbon dioxide concentration shifts it to the right, so haemoglobin releases oxygen more readily in respiring tissue. Describing the effect without the direction of the shift is incomplete.

How do you avoid sign errors with water potential? Water potential values are negative, so the less negative value is the higher water potential. Water moves from a higher (less negative) to a lower (more negative) water potential, and writing “high to low” with the signs mixed up is a common mistake.

Which part of the H420 course do these questions cover? They cover Module 3 Exchange and transport of OCR A Level Biology A (H420): exchange surfaces, gas exchange, the heart, haemoglobin and transport in plants.

Official syllabus

OCR Advanced GCE in Biology A (H420), Version 4.1 (April 2026), for first assessment in 2025, published by OCR.