Gas Exchange & Transport – A-Level Biology Revision Notes
Complete revision notes on gas exchange surfaces, ventilation mechanisms, spirometry, oxygen dissociation curves, the Bohr effect, CO₂ transport and the chloride shift. Written by a Chartered Biologist and former WJEC/Eduqas & Edexcel examiner with 25+ years of teaching experience who knows exactly how these questions are marked.
Last updated: February 2026
Why Gas Exchange Questions Expose Weak Answers – An Examiner’s View
Gas exchange and transport is one of the broadest topics in A-Level Biology, spanning everything from alveolar structure to the oxygen dissociation curve, from insect tracheal systems to the chloride shift. It is also one of the most harshly marked. The AQA 2023 Paper 3 examiner report noted that gas exchange essays were “generally at GCSE level” and that students regularly used terms like “thin membrane” – language that earns nothing at A-Level.
Having examined these questions for both WJEC/Eduqas and Edexcel, I can tell you that the core problem is not that students lack knowledge. Most can describe alveoli and mention surface area. The problem is precision. Mark schemes require adaptations in pairs – the feature AND its consequence. Writing “alveoli have a large surface area” without adding “so there is a greater rate of diffusion” scores zero for that marking point. Writing “thin walls” instead of “squamous epithelium, one cell thick, providing a short diffusion pathway” loses you marks every time.
The WJEC/Eduqas 2024 Component 3 examiner report on the QER question comparing human, fish and insect ventilation confirmed this – marks were lost due to “vague answers and lack of scientific terminology” throughout. On this page, I will take you through every aspect of gas exchange and transport your exam board requires, showing you exactly what examiners look for at each marking point.
Gas Exchange Surfaces – Adaptations That Must Be Written in Pairs
Every organism that exchanges gases has evolved structures that maximise the rate of diffusion. The key principle is Fick’s law: the rate of diffusion is proportional to the surface area and the concentration gradient, and inversely proportional to the thickness of the exchange surface.
Mammalian Lungs – Alveolar Adaptations
The alveoli are the gas exchange surfaces in the lungs. Mark schemes award adaptations only when you state the feature and its consequence as a pair. Writing just the feature earns nothing:
| Adaptation (Feature) | How It Increases Diffusion Rate (Consequence) |
|---|---|
| Many alveoli (approximately 300 million per lung) | Provides a very large surface area for gas exchange |
| Squamous epithelium – alveolar walls are one cell thick | Provides a short diffusion pathway/distance |
| Dense network of capillaries surrounding each alveolus | Maintains the concentration gradient by carrying oxygenated blood away rapidly |
| Ventilation (breathing) constantly refreshes air in the alveoli | Maintains a steep concentration gradient for O₂ and CO₂ |
| Surfactant lines the inner surface of alveoli | Reduces surface tension, prevents alveoli collapsing during expiration (WJEC/Eduqas explicit) |
Gas Exchange in Fish – Countercurrent Flow
Fish exchange gases across their gill filaments, which are further subdivided into lamellae to increase surface area. The key mechanism is countercurrent flow: water flows across the gills in the opposite direction to blood flow within the lamellae.
The mark scheme for countercurrent flow requires three linked points: (1) water and blood flow in opposite directions; (2) blood always passes water with a higher oxygen concentration; (3) the concentration gradient is maintained along the entire length of the lamella, preventing equilibrium. If you only state “opposite directions” without explaining why this maintains the gradient, you score 1 of 3 marks.
Fish also ventilate using a buccal-opercular pump mechanism. The WJEC 2024 examiner report noted that some students were “not familiar with the term buccal cavity” and used “mouth cavity” instead – while accepted, the specification term is preferred.
Gas Exchange in Insects – The Tracheal System
Insects have an entirely different gas exchange system that does not involve blood. Air enters through spiracles on the body surface, passes through a branching network of tracheae (lined with chitin to prevent collapse), and reaches the tissues via tiny, thin-walled tracheoles that penetrate directly into cells.
The mark scheme requires adaptations in pairs, just like alveoli:
- Tracheole walls are thin → rapid diffusion of gases
- Tracheoles enter/penetrate tissues directly → diffusion occurs directly into cells (no blood transport needed)
- Tracheoles are highly branched → short diffusion distance and large surface area
Gas Exchange in Plants
Plants exchange gases through stomata (pores in the leaf epidermis, primarily on the lower surface) and across the surfaces of spongy mesophyll cells. Guard cells control stomatal opening and closing. OCR B uniquely requires the mechanism: guard cells take up K⁺ ions, lowering water potential, causing water to enter by osmosis, making guard cells turgid and opening the stoma.
The air spaces between spongy mesophyll cells create a large internal surface area for gas exchange. CO₂ dissolves into the moisture on cell surfaces before diffusing into cells for photosynthesis.
The Mechanism of Ventilation – Mark-by-Mark Breakdown
Ventilation (breathing) questions are typically worth 3–6 marks and require a precise sequence. The most damaging misconception is the causation reversal: students believe that lungs expand because air rushes in. The correct sequence is that muscles contract → thoracic cavity volume increases → pressure drops below atmospheric → air enters down a pressure gradient.
Inspiration (Breathing In)
Muscles Contract
The diaphragm (muscle) contracts and flattens AND the external intercostal muscles contract, pulling the ribcage up and out. You MUST name both – naming only one loses a mark. Writing just “intercostal muscles” without specifying “external” risks losing the mark.
Volume Increases, Pressure Decreases
The volume of the thoracic cavity increases. This causes the pressure inside to decrease below atmospheric pressure. The WJEC 2024 report noted that writing “volume of the lungs increasing” instead of “thoracic cavity” is a persistent error.
Air Enters Down a Pressure Gradient
Air moves into the lungs down a pressure gradient. You must write “down” the gradient – mark schemes note that “along” a gradient is ignored.
Expiration at Rest (Breathing Out – Passive)
Muscles Relax
The diaphragm relaxes and returns to its domed shape. The external intercostal muscles relax and the ribcage moves down and in under gravity. At rest, expiration is passive – no muscle contraction is needed. The elastic recoil of the lung tissue aids this process.
Volume Decreases, Pressure Increases
Thoracic cavity volume decreases, pressure increases above atmospheric pressure, and air is forced out down the pressure gradient.
Spirometry – Reading Traces and Calculating PVR
Spirometer trace interpretation appears regularly across all boards, though the depth varies considerably. The key equation all boards require is:
Reading a Spirometer Trace
The counterintuitive aspect that catches students: breathing in moves the trace down (you draw air from the closed chamber, reducing its volume), and breathing out moves the trace up. The overall downward slope of the trace represents oxygen consumption – because soda lime absorbs CO₂, the decline in chamber volume is solely due to O₂ being used up.
- Tidal volume: the amplitude (distance between peak and trough) of normal breathing
- Vital capacity: the maximum volume from the deepest possible inhalation to the fullest possible exhalation
- Breathing rate: count the number of complete breathing cycles in a given time period
- O₂ consumption rate: measure the gradient of the downward slope over time
Struggling with Gas Exchange or Dissociation Curves? Let’s Fix That
Gas exchange questions demand paired adaptations, precise terminology and mark-scheme language. Dissociation curves require a deep understanding of cooperative binding and the Bohr effect. In a tutoring session, I will take you through exactly how I mark these questions – showing you what earns each mark and what costs you marks.
Tyrone • CBiol MRSB • Former WJEC/Eduqas & Edexcel Examiner • 25+ Years Teaching
Book a Free 20-Minute ConsultationOxygen Dissociation Curves – Understanding the S-Shaped Curve
The oxygen dissociation curve is one of the most frequently examined graphs in A-Level Biology. Understanding why it is S-shaped (sigmoid) – not just memorising the shape – is what separates strong answers from weak ones.
Why the Curve Is S-Shaped: Cooperative Binding
Haemoglobin is a quaternary protein with four subunits (2α and 2β polypeptide chains in adult HbA), each containing a haem group with an iron (Fe²⁺) ion that binds one oxygen molecule. The maximum capacity is therefore four O₂ molecules per haemoglobin.
The S-shape results from cooperative binding: when the first oxygen molecule binds to a haem group, it causes a conformational change (the protein shifts from the T-state to the R-state) that increases the affinity of the remaining haem groups for oxygen. This means the second, third and fourth oxygen molecules bind progressively more easily.
The curve has three important regions:
- Lower-left (shallow region): At low pO₂, haemoglobin is in the T-state with low affinity. The first O₂ binds slowly.
- Middle (steep region, pO₂ ≈ 2–6 kPa): Cooperative binding is in full effect. Small increases in pO₂ cause large increases in percentage saturation. This region corresponds to respiring tissues.
- Upper-right (plateau, pO₂ ≈ 10–13 kPa): Haemoglobin approaches full saturation. This corresponds to the lungs. The plateau means haemoglobin remains nearly fully saturated even if alveolar pO₂ drops slightly.
Key Terms for Dissociation Curves
Partial pressure (pO₂): The pressure of oxygen in a gas mixture, measured in kilopascals (kPa). This is the correct A-Level term – using “oxygen concentration” risks losing marks.
Percentage saturation: The proportion of haemoglobin molecules carrying oxygen, expressed as a percentage.
Loading tension: The pO₂ at which haemoglobin becomes nearly fully saturated (≈98%), corresponding to the lungs (≈16 kPa).
Unloading tension: The pO₂ at which haemoglobin is 50% saturated, corresponding to the respiring tissues (≈9 kPa for WJEC, ≈3.5 kPa P50 standard measure).
The Bohr Effect – The Complete Causal Chain for Full Marks
The Bohr effect is typically worth 3–4 marks and requires a precise causal chain. Missing any step in the sequence costs you a mark. Here is the complete mechanism as mark schemes expect it:
CO₂ Produced at Respiring Tissues
Active/respiring tissues produce CO₂ as a waste product of aerobic respiration. This increases the partial pressure of CO₂ (pCO₂) at those tissues.
CO₂ Enters Red Blood Cells and Forms H⁺ Ions
CO₂ diffuses into red blood cells and reacts with water: CO₂ + H₂O → H₂CO₃ (carbonic acid), catalysed by the enzyme carbonic anhydrase. Carbonic acid dissociates: H₂CO₃ → H⁺ + HCO₃⁻. This lowers the pH.
H⁺ Ions Reduce Haemoglobin’s Affinity for O₂
H⁺ ions bind to haemoglobin, causing a conformational change that reduces its affinity for oxygen. The word “affinity” is critical – writing “holds less oxygen” without using “affinity” can lose the mark.
Curve Shifts RIGHT – More O₂ Released
The dissociation curve shifts to the right. At any given pO₂, the percentage saturation is lower, meaning more oxygen dissociates from haemoglobin and is delivered to the tissues that need it most.
Fetal Haemoglobin – Why the Curve Shifts Left
Fetal haemoglobin (HbF) has a different structure from adult haemoglobin (HbA). It contains 2α and 2γ chains (not 2α and 2β). The γ-chain has a reduced binding affinity for 2,3-DPG (a molecule that stabilises the T-state and reduces O₂ affinity). Because HbF binds 2,3-DPG less effectively, it has a higher oxygen affinity than adult haemoglobin.
On the dissociation graph, the fetal haemoglobin curve sits to the left of the adult curve. At the placenta, where pO₂ is relatively low, maternal HbA unloads oxygen while fetal HbF loads it – oxygen transfers from mother to fetus at the same pO₂.
The mark scheme answer requires three linked points: (1) fetal haemoglobin has a higher affinity for oxygen than adult haemoglobin; (2) at the low pO₂ at the placenta; (3) oxygen transfers from maternal to fetal blood.
Other Organisms with Shifted Curves
Examiners frequently test whether students can apply dissociation curve logic to unfamiliar organisms:
- Llama: Lives at high altitude (low atmospheric pO₂). Has a higher O₂ affinity than human Hb – curve shifted left. Can load O₂ efficiently at lower pO₂.
- Lugworm: Lives in anaerobic mud burrows. Very high O₂ affinity – curve far left. Loads O₂ from stagnant, low-oxygen water.
- Myoglobin (Edexcel B requirement): Single polypeptide with one haem group. Cannot exhibit cooperative binding so the curve is hyperbolic (not sigmoid), shifted far left. Acts as an O₂ store in muscle, releasing oxygen only when pO₂ drops very low during intense exercise.
Carbon Dioxide Transport – The Three Methods and the Chloride Shift
CO₂ is transported from respiring tissues back to the lungs by three simultaneous methods. You must know all three and their approximate proportions:
As Hydrogen Carbonate Ions (HCO₃⁻) – ~70–85% (DOMINANT)
Inside red blood cells, carbonic anhydrase catalyses: CO₂ + H₂O → H₂CO₃ → H⁺ + HCO₃⁻. The H⁺ is buffered by haemoglobin (forming haemoglobonic acid, HHb), preventing dangerous pH changes. The HCO₃⁻ diffuses out of the RBC into the plasma via the Band 3 (AE1) anion exchanger.
As Carbaminohaemoglobin – ~10–20%
CO₂ binds directly to the amino groups of globin chains (NOT to the haem group). This is carbaminohaemoglobin – do not confuse with carboxyhaemoglobin (CO binding, which is toxic and irreversible).
Dissolved in Plasma – ~5–7%
A small proportion of CO₂ dissolves directly in the blood plasma. CO₂ is approximately 20 times more soluble than O₂ in water, but this still accounts for only a small fraction.
The Chloride Shift
When HCO₃⁻ diffuses out of the red blood cell, the cell would become positively charged. To maintain electrical neutrality, chloride ions (Cl⁻) move into the red blood cell – this is the chloride shift (also called the Hamburger phenomenon). Water follows the chloride ions osmotically, making venous red blood cells slightly larger than arterial ones.
At the lungs, the entire process reverses: Cl⁻ exits the RBC, HCO₃⁻ re-enters, combines with H⁺ to form H₂CO₃, which breaks down to CO₂ + H₂O, and CO₂ diffuses into the alveoli to be exhaled.
Exam Board Comparison – What YOUR Board Requires
The differences between boards on gas exchange and transport are the most extreme of any A-Level Biology topic. Edexcel A covers almost none of the transport content, while OCR A requires everything including the Haldane effect. Check this table before you revise.
| Content Area | AQA | OCR A | OCR B | Edexcel A | Edexcel B | WJEC/Eduqas |
|---|---|---|---|---|---|---|
| Fick’s law (named) | Principle only | ✓ Explicit | Principle only | ✓ Explicit | Principle only | Principle only |
| Insect tracheal system | ✓ | ✓ | ✗ | ✗ | ✓ | ✓ |
| Fish/countercurrent | ✓ | ✓ | ✗ | ✗ | ✓ | ✓ |
| Spirometry depth | PVR only | Full + volumes | FEV₁ & PEFR | Basic | Moderate | Full + reserves |
| Lung disease | ✓ Major | Limited | ✓ Clinical | Limited | Limited | Limited |
| O₂ dissociation curves | ✓ | ✓ | ✓ | ✗ | ✓ | ✓ |
| Bohr effect | ✓ | ✓ | ✓ | ✗ | ✓ | ✓ |
| Fetal haemoglobin | ✓ | ✓ | ✓ | ✗ | ✓ | ✓ |
| Myoglobin | ✗ | Past papers | ✗ | ✗ | ✓ Required | ✗ |
| Chloride shift | ✗ | ✓ Explicit | ✗ | ✗ | ✓ | ✓ |
| Carbaminohaemoglobin | Limited | ✓ Explicit | ✗ | ✗ | ✓ | ✓ |
| Carbonic anhydrase | ✓ | ✓ Explicit | ✗ | ✗ | ✓ | ✓ |
12 Common Mistakes Examiners See Every Year
These are not hypothetical errors. These are mistakes I see repeatedly when marking gas exchange and transport scripts, confirmed by examiner reports from AQA (2023), WJEC/Eduqas (2024) and Edexcel. Eliminating these twelve errors will significantly improve your marks.
| # | The Mistake | The Correction |
|---|---|---|
| 1 | Writing “thin membrane” for alveolar walls | Write “squamous epithelium, one cell thick” followed by “short diffusion pathway.” AQA 2023 flagged this as GCSE-level language. |
| 2 | Believing lungs expand because air rushes in | Muscles contract → thoracic cavity volume increases → pressure drops → air enters down a pressure gradient. Pressure change causes air movement, not the reverse. |
| 3 | Writing “intercostal muscles” without specifying type | External intercostals for inspiration. Internal intercostals for forced expiration only. Normal expiration at rest is passive (elastic recoil). |
| 4 | Volume of “lungs” increases during inspiration | Volume of the thoracic cavity increases. The WJEC 2024 examiner report flagged this as a persistent error. |
| 5 | Mentioning blood in insect gas exchange | Triggers a maximum 2 marks penalty. The tracheal system delivers O₂ directly to cells. Haemolymph does NOT transport oxygen. |
| 6 | Writing “oxygen concentration” instead of partial pressure | The correct A-Level term is partial pressure (pO₂), measured in kPa. Using “concentration” risks losing marks on dissociation curve questions. |
| 7 | Incomplete Bohr effect causal chain | Must include: increased CO₂ → carbonic acid → H⁺ ions → reduced affinity (this word is critical) → curve shifts right → more O₂ unloaded. |
| 8 | Stating countercurrent is just “opposite directions” | Must explain that this maintains the gradient along the entire length and prevents equilibrium. Stating direction alone scores 1 of 3. |
| 9 | Confusing carbaminohaemoglobin with carboxyhaemoglobin | Carbamino = CO₂ bound to globin (normal, reversible). Carboxy = CO bound to haem (toxic, irreversible). CO₂ binds globin, NOT haem. |
| 10 | Spirometer trace: breathing in moves trace up | Breathing IN moves the trace DOWN (air drawn from chamber reduces volume). Overall downward slope = O₂ consumption. |
| 11 | Confusing the Bohr shift with the chloride shift | Bohr shift = CO₂/pH affects Hb’s affinity for O₂ (curve shifts right). Chloride shift = HCO₃⁻/Cl⁻ exchange across RBC membrane for electrical neutrality. Different phenomena. |
| 12 | Incomplete CO₂ transport answer | Must state all three methods: dissolved in plasma (~5%), as carbaminohaemoglobin (~10–20%), and as HCO₃⁻ (~70–85%, the dominant mechanism via carbonic anhydrase). |
Frequently Asked Questions – Gas Exchange & Transport
The alveoli have five key adaptations, each of which must be stated as a feature-consequence pair for full marks. Many alveoli provide a large surface area for diffusion. Walls made of squamous epithelium (one cell thick) provide a short diffusion pathway. A dense capillary network maintains the concentration gradient by carrying oxygenated blood away quickly. Ventilation constantly refreshes air to maintain a steep O₂ and CO₂ gradient. Surfactant reduces surface tension, preventing alveolar collapse. At A-Level, never write “thin walls” – you must use “squamous epithelium” or “one cell thick.”
The S-shape (sigmoid curve) is caused by cooperative binding. Haemoglobin has four haem groups, and when the first oxygen molecule binds, it causes a conformational change (T-state to R-state) that increases the affinity of the remaining haem groups for oxygen. This means the first O₂ binds slowly (shallow lower portion), the second and third bind rapidly as affinity increases (steep middle section), and at high pO₂ haemoglobin approaches full saturation (plateau). The plateau ensures efficient loading in the lungs even if pO₂ varies, while the steep section ensures efficient unloading at respiring tissues.
The Bohr effect describes how increased CO₂ and lower pH at respiring tissues reduce haemoglobin’s affinity for oxygen, shifting the dissociation curve to the right. The mechanism is: CO₂ enters red blood cells, carbonic anhydrase converts it to carbonic acid, which dissociates into H⁺ and HCO₃⁻. The H⁺ ions bind to haemoglobin, causing a conformational change that reduces its oxygen affinity. This means more oxygen is released precisely where it is needed most – at the most metabolically active tissues. During exercise, more CO₂ production shifts the curve further right, increasing oxygen delivery proportionally.
In fish gills, water flows across the gill lamellae in the opposite direction to blood flow within the capillaries – this is countercurrent flow. The key advantage is that blood always encounters water with a higher oxygen concentration than itself at every point along the lamella. This maintains a concentration gradient along the entire length of the exchange surface, preventing equilibrium from being reached. As a result, fish can extract up to 80% of dissolved oxygen from water, far more efficient than a concurrent (parallel) flow system which would only achieve about 50%.
Fetal haemoglobin (HbF) contains two alpha and two gamma polypeptide chains, whereas adult haemoglobin (HbA) has two alpha and two beta chains. The gamma chains give HbF a higher affinity for oxygen than HbA because they bind 2,3-DPG less effectively (2,3-DPG normally reduces oxygen affinity). On a dissociation curve graph, the fetal curve sits to the left of the adult curve. At the placenta, where oxygen partial pressure is low, this difference allows fetal haemoglobin to take up oxygen that adult haemoglobin is releasing, ensuring oxygen transfer from maternal to fetal blood.
CO₂ is transported by three methods simultaneously. The dominant method (~70–85%) is as hydrogen carbonate ions (HCO₃⁻): CO₂ enters red blood cells where carbonic anhydrase converts it to carbonic acid, which dissociates into H⁺ and HCO₃⁻. The HCO₃⁻ then moves into the plasma. About 10–20% is transported as carbaminohaemoglobin, where CO₂ binds to the amino groups on the globin chains of haemoglobin (not the haem group). The remaining ~5–7% dissolves directly in the blood plasma. At the lungs, all three processes reverse to release CO₂ for exhalation.
Pulmonary ventilation rate (PVR) = tidal volume × breathing rate, expressed in dm³ min⁻¹. From a spirometer trace: tidal volume is the amplitude of a normal breathing cycle (the distance between the peak and trough of a regular breath). Breathing rate is calculated by counting the number of complete cycles in a given time period and converting to per minute. Remember that on a spirometer trace, breathing in moves the line downward (drawing air from the chamber) and breathing out moves it upward. The overall downward slope represents oxygen consumption because soda lime absorbs CO₂.
The curve shifts to the right when haemoglobin’s affinity for oxygen decreases, causing more oxygen to be released at the tissues. This occurs with: increased CO₂ partial pressure (the Bohr effect), decreased pH (due to H⁺ ions from carbonic acid or lactic acid), and increased temperature (at exercising tissues). All three factors increase during exercise, shifting the curve progressively rightward so that the most metabolically active tissues receive proportionally more oxygen. A left shift (increased affinity) occurs in the lungs where CO₂ is removed, pH rises, and at the lower temperatures of fetal haemoglobin, llama haemoglobin and myoglobin.
These revision notes are written by an independent A-Level Biology tutor and are not affiliated with or endorsed by AQA, OCR, Edexcel (Pearson), WJEC, Eduqas, Cambridge International (CIE) or the IB Organisation. All exam board names are trademarks of their respective owners. Content is based on publicly available specifications and examiner reports.