5.2.8Unit 5. Transition Metals and Organic Nitrogen Chemistry
Haemoglobin and Ligand Exchange — IAL Edexcel Linear (Chemistry) Revision Notes & Topical Questions
5 min read 5 parts Lesson 20 of 43 Unit quiz
Part 1 of 5Structure of Haemoglobin
Haem group
- Each haem contains an iron(II) ion held at the centre of a flat ring-shaped ligand, which binds through four nitrogen atoms.
Fifth position
- A nitrogen atom of a histidine side chain in the globin protein occupies the fifth coordination site.
Sixth position
- The remaining site holds a water molecule or, in oxygenated blood, an oxygen molecule.
Four haem groups
- One haemoglobin molecule contains four haem groups, so it can carry up to four oxygen molecules.
Part 2 of 5Oxygen Transport
1
In the lungs, where oxygen concentration is high, O₂ replaces the water ligand on the iron in each haem group.
2
The complex formed is bright red oxyhaemoglobin and the iron(II) remains in the same oxidation state.
3
In tissues where oxygen concentration is low the equilibrium shifts back and oxygen is released.
4
The overall process can be shown as Hb + 4O₂ ⇌ Hb(O₂)₄ in a simplified form.
5
This requires O₂ to bind strongly enough to be picked up but weakly enough to be released, which is a reversible ligand exchange.
Part 3 of 5Carbon Monoxide Poisoning
Carbon Monoxide Poisoning
Binding
- CO is a ligand that binds to iron(II) in haem much more strongly than O₂ does.
Result
- Carboxyhaemoglobin forms and its binding is essentially irreversible at normal oxygen levels.
Effect
- Fewer haem sites are available for oxygen, so tissues become starved of oxygen and the person may suffocate.
Treatment
- The patient is given high concentrations of oxygen to compete with and displace CO over time.
| Ligand | Bond to Fe²⁺ in haem | Result |
|---|---|---|
| H₂O | Weak | Replaced easily by O₂ |
| O₂ | Moderate, reversible | Oxygen transport |
| CO | Very strong | Blocks oxygen transport |
Part 4 of 5Worked Example
Question How many oxygen molecules can 2.0 mol of haemoglobin bind in total?
- Each haemoglobin molecule has four haem groups, each binding one O₂.
- Moles of O₂ bound = 2.0 x 4 = 8.0 mol.
Answer 8.0 mol of O₂
Part 5 of 5Exam Checklist & Tips
5.2.8 – Key Points for the Exam
- Haem The iron(II)-containing group in haemoglobin.
- Oxyhaemoglobin Haemoglobin bound to oxygen.
- Carboxyhaemoglobin Haemoglobin bound to carbon monoxide.
- Ligand exchange Substitution of one ligand by another at a metal ion.
Common Mistakes
- Saying that iron(III) carries oxygen in haemoglobin.
- Describing the binding of O₂ as permanent.
- Claiming that CO and O₂ bind to different metals.
- Assuming oxidation state changes when O₂ binds.
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