Explain how haemoglobin's quaternary structure enables it to transport oxygen efficiently in the blood.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Model answer (4 marks)
Haemoglobin is a tetramer, consisting of four polypeptide subunits that form a quaternary structure.
Each subunit contains a haem prosthetic group with a central Fe²⁺ ion that can bind one O₂ molecule.
Because there are four subunits, a single haemoglobin molecule can carry up to four O₂ molecules, greatly increasing the oxygen‑carrying capacity of the blood.
Binding of O₂ to one subunit induces a conformational change that raises the affinity of the remaining subunits for O₂ (cooperative binding). This allows haemoglobin to load O₂ efficiently in the high‑O₂ environment of the lungs and to release it readily in the low‑O₂ environment of tissues.
Each subunit contains a haem prosthetic group with a central Fe²⁺ ion that can bind one O₂ molecule.
Because there are four subunits, a single haemoglobin molecule can carry up to four O₂ molecules, greatly increasing the oxygen‑carrying capacity of the blood.
Binding of O₂ to one subunit induces a conformational change that raises the affinity of the remaining subunits for O₂ (cooperative binding). This allows haemoglobin to load O₂ efficiently in the high‑O₂ environment of the lungs and to release it readily in the low‑O₂ environment of tissues.
Examiner tips
- Mention the tetrameric quaternary structure and the haem group in each subunit.
- Explain the four‑fold capacity and cooperative binding to show efficient transport.
- Use terms like ‘cooperative binding’ and ‘affinity’ as they are key words in the mark scheme.
- Keep the answer concise – 4 marks, so one sentence per point is sufficient.
Common mistakes
- Confusing haemoglobin with myoglobin (single subunit).
- Failing to mention cooperative binding or the increase in affinity.
- Using vague phrases like ‘more oxygen’ instead of specifying the four subunits and haem groups.
Mark scheme (4 marks)
- Haemoglobin is made up of four polypeptide subunits (quaternary structure) held together by interactions between the subunits.
- Each subunit contains a haem (prosthetic) group with a central iron ion that binds one oxygen molecule.
- Having four subunits allows haemoglobin to carry up to four oxygen molecules simultaneously, increasing the oxygen-carrying capacity of the blood.
- Cooperative binding occurs because binding of oxygen to one subunit changes the shape of the other subunits, increasing their affinity for oxygen, so haemoglobin loads oxygen efficiently at the lungs and unloads it in respiring tissues.
Key terms in this question
Related
- All IB DP Biology Standard Level (2023 syllabus) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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