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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

📚 IB Biology HL Hub / Topical Revision
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Curriculum Equation Key قوانين الرقم الهيدروجيني والاتزان الأيوني - pH & Acid-Base Equilibrium FormulasFast Exam Solution
pH = -log[H⁺] | pOH = -log[OH⁻] | pH + pOH = 14 | [H⁺][OH⁻] = 10⁻¹⁴
Word Equation & Conditions: الرقم الهيدروجيني = -لوغاريتم تركيز أيون الهيدروجين | pH + pOH = 14
Location & Specifications: الاتزان الأيوني، كيمياء 3 ثانوي، التحصيلي، و AP Chemistry.
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Part 1 of 5Reading the Curve

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

Axes
  • 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
  • Sigmoid shape The S-shaped curve is steep in the middle and flat at each end, which reflects cooperative binding.
Lungs
  • Lungs At the high partial pressure in the alveoli, around 13 kPa, haemoglobin is nearly fully saturated.
Tissues
  • 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
  • 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
  • Shift to the right High CO₂ or low pH moves the curve right, indicating lower affinity and more unloading.
Myoglobin
  • Myoglobin Its curve is hyperbolic rather than sigmoid because it has only one binding site and cannot show cooperativity.
Altitude
  • 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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