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This lesson covers the structure and function of haemoglobin, the oxygen dissociation curve, and factors that affect oxygen transport, as required by the Edexcel A-Level Biology specification (9BI0). You need to understand cooperative binding, the sigmoid shape of the dissociation curve, the Bohr effect, and how different haemoglobins are adapted to different environments.
Haemoglobin (Hb) is a globular protein found in red blood cells (erythrocytes). Each molecule consists of:
Haemoglobin is an example of a protein with quaternary structure — it is composed of multiple polypeptide subunits held together by hydrophobic interactions, hydrogen bonds, and ionic bonds.
Key Definition: Haemoglobin — a globular protein with quaternary structure, composed of four polypeptide subunits (2α + 2β), each containing a haem group with an iron (Fe²⁺) ion that reversibly binds one oxygen molecule.
In the lungs, where the partial pressure of oxygen (pO₂) is high:
Hb + 4O₂ → HbO₈ (oxyhaemoglobin)
Written more precisely: Hb + 4O₂ ⇌ Hb(O₂)₄
Haemoglobin loads (picks up) oxygen and becomes oxyhaemoglobin. Oxyhaemoglobin is bright red.
In the body tissues, where pO₂ is lower (especially in active, respiring tissues):
Hb(O₂)₄ → Hb + 4O₂
Haemoglobin unloads (releases) oxygen. Deoxyhaemoglobin is a darker purplish-red.
Cooperative binding is a crucial property of haemoglobin.
This cooperative behaviour explains the S-shaped (sigmoid) shape of the oxygen dissociation curve.
Exam Tip: Cooperative binding means that haemoglobin is very efficient at both loading oxygen in the lungs (where pO₂ is high) and unloading oxygen in the tissues (where pO₂ is low). Make sure you explain the conformational change as the mechanism.
The oxygen dissociation curve is a graph showing the relationship between the partial pressure of oxygen (pO₂) on the x-axis and the percentage saturation of haemoglobin with oxygen on the y-axis.
The curve is S-shaped (sigmoid), not a straight line. This is due to cooperative binding.
| Region | pO₂ | What happens |
|---|---|---|
| Flat at the bottom (low pO₂) | Very low (tissues at rest) | Haemoglobin has low affinity; first O₂ is hard to bind |
| Steep middle section | Moderate | After the first O₂ binds, affinity increases sharply; haemoglobin rapidly loads O₂ |
| Flat at the top (high pO₂) | High (e.g. lungs) | Haemoglobin is nearly fully saturated (~97–98%); further increases in pO₂ produce little extra saturation |
Exam Tip: Make sure you can read values off the oxygen dissociation curve. The difference between saturation in the lungs and in the tissues tells you how much oxygen is delivered.
The Bohr effect describes the shift of the oxygen dissociation curve to the right when the partial pressure of carbon dioxide (pCO₂) increases (or when pH decreases).
As HCO₃⁻ ions are produced inside the red blood cell, they diffuse out into the plasma. To maintain electrical neutrality, chloride ions (Cl⁻) diffuse into the red blood cell from the plasma. This is the chloride shift.
On the oxygen dissociation curve:
| Condition | Curve position | Meaning |
|---|---|---|
| High pCO₂ (active tissues) | Shifted to the right | Lower affinity for O₂ → more O₂ released |
| Low pCO₂ (lungs) | Shifted to the left (or returns to normal position) | Higher affinity for O₂ → more O₂ loaded |
Key Definition: Bohr effect — the rightward shift of the oxygen dissociation curve in response to increased pCO₂ (or decreased pH), resulting in haemoglobin releasing more oxygen to actively respiring tissues.
CO₂ is transported in the blood in three ways:
| Method | Percentage | Detail |
|---|---|---|
| As hydrogencarbonate ions (HCO₃⁻) | ~85% | CO₂ + H₂O → H₂CO₃ → H⁺ + HCO₃⁻ (in red blood cells, catalysed by carbonic anhydrase) |
| Bound to haemoglobin (carbaminohaemoglobin) | ~10% | CO₂ binds to the amino groups of the globin chains (not to the haem group) |
| Dissolved in plasma | ~5% | A small proportion dissolves directly in the plasma |
Foetal haemoglobin (HbF) has a higher affinity for oxygen than adult haemoglobin (HbA).
Foetal haemoglobin contains two α chains and two γ (gamma) chains (instead of β chains). The γ chains have a lower sensitivity to the allosteric effector 2,3-bisphosphoglycerate (2,3-BPG), which normally reduces oxygen affinity. As a result, HbF binds O₂ more tightly.
Different organisms have haemoglobins (or other respiratory pigments) adapted to their specific environments:
| Organism | Dissociation curve | Reason |
|---|---|---|
| Lugworm (lives in anaerobic mud) | Shifted to the left (higher affinity) | Must load oxygen even at very low pO₂ in its environment |
| Llama (lives at high altitude) | Shifted to the left | Lower atmospheric pO₂ at high altitude; higher affinity ensures adequate O₂ loading |
| Fish (active, e.g. mackerel) | Similar to human or slightly left | Needs to extract dissolved O₂ from water efficiently |
| Human adult | Normal position | Balanced for efficient loading in lungs and unloading in tissues |
Exam Tip: When comparing dissociation curves of different haemoglobins, always state which way the curve is shifted and explain why. A curve shifted to the left means higher affinity (loads oxygen more readily at lower pO₂). A curve shifted to the right means lower affinity (releases oxygen more readily).
Myoglobin is a monomeric (single polypeptide chain) oxygen-binding protein found in muscle tissue. Its dissociation curve is a hyperbola (not sigmoid), shifted to the left of adult haemoglobin.
| Feature | Detail |
|---|---|
| Haemoglobin structure | 4 subunits (2α + 2β), each with a haem group containing Fe²⁺ |
| Max O₂ capacity | 4 O₂ molecules per Hb molecule |
| Cooperative binding | First O₂ causes conformational change, making subsequent binding easier |
| Dissociation curve shape | Sigmoid (S-shaped) |
| Bohr effect | High CO₂/low pH → curve shifts right → more O₂ released to tissues |
| CO₂ transport | 85% as HCO₃⁻, 10% as carbaminohaemoglobin, 5% dissolved |
| Foetal haemoglobin | Higher affinity (curve shifted left) → extracts O₂ from maternal blood |
| Myoglobin | Monomer; very high affinity; O₂ store in muscle |
Haemoglobin and oxygen transport are frequently examined at A-Level — make sure you can sketch and interpret dissociation curves, explain the Bohr effect, and compare different haemoglobins.
This material sits at the gas-transport core of Edexcel 9BI0 Topic 7 (Run for your life — Exchange and Transport). Candidates must relate Hb's quaternary structure (2α + 2β, each chain carrying a haem-Fe²⁺ prosthetic group) to its function as the principal vehicle of O2 transport, interpret the sigmoidal dissociation curve via cooperative binding, explain the Bohr shift in terms of pCO2/pH effects on affinity, and compare adult Hb with foetal Hb (HbF) and myoglobin. Synoptic with Topic 1 (proteins) — Hb is the canonical quaternary-structure example with a non-protein prosthetic group; with lesson 6 — the erythrocyte concentrates Hb and houses carbonic anhydrase; with lesson 5 (cardiac cycle) — peak left-ventricular pressure drives Hb-loaded blood to the systemic circulation; with Topic 5 (respiration) — active tissue produces the CO2 that shifts the curve right; and with Topic 8 — chronic hypoxia triggers renal EPO secretion, raising erythrocyte numbers. Refer to the official Pearson Edexcel 9BI0 specification for exact wording.
Question (8 marks):
Figure 1 (not shown) shows three oxygen-dissociation curves on the same axes (pO2 kPa vs % saturation): curve P (HbA), curve Q (HbF) and curve R (HbA at low pH, e.g. pH 7.2 instead of 7.4).
(a) At pO2 = 4 kPa, curve P shows ~50% saturation and curve Q shows ~75%. Calculate the difference in O2 released between the two haemoglobins as blood passes from pO2 = 13 kPa (both ~95% saturated) to pO2 = 4 kPa, and state which releases more. (3)
(b) Explain, in structural and functional terms, why HbF's curve lies left of HbA's, and why this is essential in utero. (3)
(c) Explain why curve R lies right of curve P, and identify a tissue in vivo in which Hb behaves like curve R. (2)
Solution with mark scheme:
(a) M1 (AO2) — HbA: 95−50=45 percentage points of saturation released. HbF: 95−75=20 percentage points released.
M1 (AO2) — HbA therefore releases more O2 on this journey (45 vs 20 percentage points; ~2.25× more).
A1 (AO3) — Expected direction: HbA exists to unload at tissue pO2, HbF exists to load at low placental pO2. Many candidates lose marks here by reading the lower-saturation curve as "worse" rather than "better at unloading".
(b) M1 (AO1) — HbF substitutes two γ chains for the two β chains, giving 2α + 2γ.
M1 (AO2) — γ chains bind the allosteric inhibitor 2,3-BPG less tightly than β chains, so HbF retains higher intrinsic O2 affinity — its curve lies left of HbA's.
A1 (AO3) — In utero, O2 is acquired by diffusion across the placenta from maternal blood. For O2 to flow mother → foetus, HbF must bind O2 at a pO2 at which HbA is releasing it — i.e. HbF must have higher affinity. A common pitfall is asserting the placenta "pumps" O2; transfer is purely diffusive down the partial-pressure gradient set by the affinity difference.
(c) M1 (AO2) — Lower pH (higher H+) protonates basic R-groups in the globin chains, stabilising the low-affinity (T) conformation of haemoglobin. Affinity for O2 falls and the curve shifts right (Bohr shift).
A1 (AO2) — In vivo, this occurs in actively respiring tissue (e.g. exercising skeletal muscle), where mitochondrial CO2 output drives the carbonic-anhydrase reaction in erythrocytes, raising H+ locally. The curve-R behaviour is therefore the physiologically useful state in tissue capillaries.
Total: 8 marks (M5 A3).
Question (6 marks): Explain how the cooperative binding of oxygen to haemoglobin, together with the Bohr effect, allows adult haemoglobin to load oxygen efficiently in the lungs and to release a high fraction of that oxygen in actively respiring muscle.
Mark scheme decomposition by AO:
| Mark | AO | Earned by |
|---|---|---|
| 1 | AO1.1 | Stating that haemoglobin is a quaternary protein of 2α + 2β subunits, each containing a haem group with Fe²⁺ that binds one O2, giving four binding sites in total |
| 2 | AO1.2 | Stating that binding of the first O2 causes a conformational change (T → R) that raises the affinity of the remaining sites — cooperative binding |
| 3 | AO2.1 | Linking cooperativity to the sigmoidal shape of the dissociation curve: the steep middle section is where small changes in pO2 produce large changes in saturation |
| 4 | AO2.1 | Applying this to the lungs: at pO2 ~13 kPa, Hb sits on the upper plateau (~97% saturated), so loading is near-maximal and insensitive to small pO2 fluctuations |
| 5 | AO2.7 | Applying this to active muscle: high CO2 → carbonic anhydrase → H+ → Bohr shift to the right, lowering affinity at given pO2 and raising the fraction of O2 released |
| 6 | AO3.1 | Synthesis: cooperativity makes Hb a switch between loading and unloading states; the Bohr effect biases that switch toward unloading exactly where O2 demand is highest, so delivery is matched to demand without changing erythrocyte number or cardiac output |
Total: 6 marks (AO1 = 2, AO2 = 3, AO3 = 1). Edexcel oxygen-transport questions of this type reliably split AO marks roughly 30/50/20 across AO1/AO2/AO3.
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