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2.6.10Inquiry Process

The Oxygen Dissociation Curve — IB Biology HL Revision Notes & Topical Questions

5 min read 5 parts Lesson 72 of 160

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Part 1 of 5Reading the Curve

2.6.10 The Oxygen Dissociation Curve

The curve links oxygen availability to how much haemoglobin is loaded.

Axes
  • The x-axis shows the partial pressure of oxygen in kilopascals, and the y-axis shows the percentage saturation of haemoglobin with oxygen.
Sigmoid shape
  • The S-shaped curve is steep in the middle and flat at each end, which reflects cooperative binding.
Lungs
  • At the high partial pressure in the alveoli, around 13 kPa, haemoglobin is nearly fully saturated.
Tissues
  • At resting tissue partial pressures, around 4 to 5 kPa, saturation falls steeply, so a large amount of oxygen is unloaded.

Part 2 of 5Why the Shape Is Useful

1
At low partial pressures the curve is shallow because the first oxygen binds slowly.
2
Once one oxygen has bound, affinity rises and the curve climbs steeply over a narrow range of pressure.
3
The steep section matches the partial pressures found in respiring tissues, so a small drop in pO₂ releases much oxygen.
4
At high partial pressure the curve levels off because nearly all binding sites are occupied.
5
This means haemoglobin loads fully in the lungs yet unloads readily where oxygen is scarce.

Part 3 of 5Curves Compared

Curves Compared
Shift to the left
  • Curves for fetal haemoglobin or myoglobin lie to the left, indicating higher affinity and loading at lower pO₂.
Shift to the right
  • High CO₂ or low pH moves the curve right, indicating lower affinity and more unloading.
Myoglobin
  • Its curve is hyperbolic rather than sigmoid because it has only one binding site and cannot show cooperativity.
Altitude
  • At high altitude lower atmospheric oxygen means lower saturation in the lungs, and people respond by making more red blood cells.
Curve positionAffinityExample
LeftHigherFetal haemoglobin, myoglobin
RightLowerActive muscle with high CO₂

Part 4 of 5Worked Example

Question

  • Haemoglobin is 97% saturated in the lungs and 40% saturated in a resting tissue.
  • Estimate the percentage of its oxygen load that is released in the tissue.

  1. Oxygen released = 97 − 40 = 57 percentage points of saturation.
  2. Fraction of the load released = 57 ÷ 97 = 0.588.
  3. Convert to a percentage0.588 × 100 = 58.8%.

Answer About 59% of the oxygen carried is unloaded in the tissue.

Part 5 of 5Exam Checklist & Tips

2.6.10 – Key Points for the Exam

  • Oxygen dissociation curve A graph of haemoglobin saturation against the partial pressure of oxygen.
  • Partial pressure The pressure exerted by one gas in a mixture, proportional to its concentration.
  • Saturation The percentage of haem groups carrying oxygen.
  • Sigmoid
    • S-shaped;
      • describes a curve with a slow start, steep middle and flat end.

Common Mistakes

  • Describing the curve as a straight line; it is S-shaped because of cooperativity.
  • Mixing up left and right shifts and their meaning for affinity.
  • Reading percentage saturation as the amount of oxygen in millilitres.
  • Saying myoglobin has a sigmoid curve; it is hyperbolic.

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