describe and explain the oxygen dissociation curve of adult haemoglobin
Transport of oxygen and carbon dioxide
Objective:
Describe and explain the oxygen dissociation curve of adult haemoglobin.
What is the oxygen dissociation curve?
Think of haemoglobin (Hb) as a team of four friends who love to carry a ball (oxygen). The oxygen dissociation curve shows how happily the team carries the ball depending on how many balls are around.
When there are many balls (high PO₂), the friends are excited and all grab the ball together – the curve climbs steeply. When balls are scarce (low PO₂), only a few friends want to carry it – the curve flattens.
Key features of the curve
- Sigmoidal shape – looks like a gentle S, indicating cooperative binding.
- Cooperativity – the first oxygen molecule makes it easier for the next one to bind.
- p50 – the PO₂ at which 50 % of Hb sites are occupied (≈ 26 mmHg in adults).
- Hill coefficient (n) – measures steepness; adult Hb ≈ 2.7.
Mathematical description
The Hill equation links oxygen saturation (Y) to partial pressure:
$$\frac{Y}{1-Y} = \left(\frac{P_{O_2}}{p_{50}}\right)^n$$Where:
- Y = % saturation (0–1)
- PO₂ = partial pressure of oxygen (mmHg)
- p50 = 26 mmHg (adult)
- n = 2.7 (adult)
Factors that shift the curve
- 🔬 pH (Bohr effect) – lower pH (more CO₂) shifts curve right (harder to release O₂).
- 🩸 CO₂ concentration – same as pH, right shift.
- 🌡️ Temperature – higher temp shifts right.
- 🧪 2,3‑BPG – binds to Hb, right shift.
- 🧬 Allosteric effectors – e.g., ATP, ADP.
Exam tip:
Remember: a right shift means Hb releases O₂ more easily (useful in tissues), while a left shift means Hb holds onto O₂ (useful in lungs). Use the Bohr effect as a mnemonic.
Typical adult haemoglobin values
| Parameter | Value |
|---|---|
| p50 (mmHg) | ≈ 26 |
| Hill coefficient (n) | ≈ 2.7 |
| Left shift (high pH) | ↑ O₂ affinity |
| Right shift (low pH, ↑CO₂) | ↓ O₂ affinity |
Analogy: The oxygen tug‑of‑war
Imagine a tug‑of‑war game where each team member (Hb subunit) pulls on a rope (oxygen). When the rope is heavy (high PO₂), everyone pulls hard and the rope moves quickly – the curve rises steeply. If the rope is light (low PO₂), only a few pull, and the movement is slow – the curve flattens. The team’s coordination (cooperativity) determines how fast the rope moves.
Conclusion
The oxygen dissociation curve is a visual summary of how adult haemoglobin behaves in different environments. By understanding its shape, key parameters, and the factors that shift it, you can predict how oxygen is picked up in the lungs and released in tissues – a crucial skill for A‑Level biology exams.
Revision
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