Relate haemoglobin structure to oxygen transport
Haemoglobin is a globular protein with two alpha chains, two beta chains and haem groups containing iron for reversible oxygen binding.
AS Level · Cambridge International Biology 9700, 2028–2030. These lessons explain syllabus concepts; self-checked practice does not measure practical competence or qualify as teacher-reviewed assessment.
Before you begin
- Recognise that biological molecules have structures that relate to their roles in cells and organisms.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
What is the globin-chain composition of haemoglobin?
What you will learn
- Describe haemoglobin quaternary structure from two α-globin and two β-globin chains
- Explain the role of haem iron in reversible oxygen binding
Four globin chains form quaternary structure
A haemoglobin molecule is a globular protein with quaternary structure. It contains two alpha (α-globin) polypeptide chains and two beta (β-globin) polypeptide chains, associated with haem groups.
Each haem group contains an iron ion. The iron can bind oxygen reversibly, allowing haemoglobin to load oxygen where partial pressure is high and unload it where conditions favour dissociation. The iron is part of the haem group; it is not a separate globin chain.
Structure enables transport
A compact soluble globular structure supports haemoglobin’s role in blood. Its four globin subunits each associate with a haem group, so one molecule can bind up to four oxygen molecules when oxygen is available. Binding is reversible rather than a permanent chemical destruction of oxygen.
Relate iron in haem to oxygen binding. Do not say that haemoglobin is itself a red blood cell: it is a protein contained in red blood cells. The dissociation curve and pH effects are studied under gas transport.
Pause and explain
What role does iron in haem have?
Worked example
A mutation changes an iron-binding haem group in one globin subunit. Explain a likely effect on haemoglobin function without claiming an exact clinical outcome.
Show the worked solution
- Haem iron binds oxygen reversibly.
- Changing the site can affect oxygen binding.
- State a functional possibility, not a diagnosis.
Answer The haem group’s iron is involved in reversible oxygen binding. A change affecting the haem group could alter oxygen binding by that subunit and therefore affect haemoglobin’s transport role. The exact effect cannot be concluded without further evidence.
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Write your answer and reasoning first. Use a hint only if you are stuck. The model solution is for self-checking; your written work is not automatically marked.
Describe haemoglobin’s quaternary structure.
Give me a hint
Name the two chain types and their numbers.
Compare my reasoning
- Two α-globin chains are present.
- Two β-globin chains associate with haem groups.
Haemoglobin has quaternary structure formed from two α-globin polypeptide chains and two β-globin polypeptide chains, associated with haem groups that contain iron.
Look for these in your work
- Chain types and counts
- Haem included
Explain why iron is important to haemoglobin’s function.
Give me a hint
Link the haem group to oxygen binding.
Compare my reasoning
- Iron is within each haem group.
- It binds oxygen reversibly.
Iron in each haem group can bind oxygen reversibly. This enables haemoglobin to load and unload oxygen during transport.
Look for these in your work
- Iron location
- Reversible binding connected to function
A student writes “haemoglobin uses up oxygen when it binds it.” Correct the statement.
Give me a hint
Use the word reversible.
Compare my reasoning
- Oxygen binding to haemoglobin is reversible.
- Haemoglobin can load and later unload oxygen.
Oxygen binds reversibly to iron in haem groups. Haemoglobin can load oxygen in one location and unload it in another; it does not permanently use up the oxygen.
Look for these in your work
- Reversibility stated
- Transport consequence explained
Common mistakes
- Describing the four globin chains as four different types
- Saying iron is a separate polypeptide chain
- Confusing reversible binding with oxygen being used up
What can you explain now?
What is the globin-chain composition of haemoglobin?
Compare with the explanation
Two alpha and two beta chains
These four chains form the quaternary structure.
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Make it stick
How do haemoglobin’s globin chains and haem iron support oxygen transport?
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