Explain how haemoglobin's oxygen dissociation curve enables efficient loading of oxygen in the lungs and unloading of oxygen in respiring tissues.

IB DP Biology Standard Level (2023 syllabus) — B3.2 Transport · Explain · 4 marks · View as Markdown

Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).

Haemoglobin is a protein found in red blood cells that reversibly binds oxygen. The relationship between the partial pressure of oxygen (pO₂) and the percentage saturation of haemoglobin is represented by a characteristic S-shaped (sigmoidal) curve.

Model answer (4 marks)

Haemoglobin is almost fully saturated with O₂ in the lungs where pO₂ is high, so the curve rises steeply and loading is maximised.
In tissues the pO₂ is low; the curve falls steeply, so haemoglobin releases O₂ readily, supplying cells.
Because the curve is sigmoidal, a small drop in pO₂ produces a large drop in saturation – cooperative binding makes unloading efficient.
In tissues with high CO₂ and low pH the curve shifts right (Bohr effect), further promoting O₂ release where it is needed.

Examiner tips

Use the four key points in order: loading, unloading, sigmoidal effect, Bohr shift.
Show the relationship between pO₂ and saturation – mention steepness of curve in lungs and tissues.
Mention cooperative binding as the reason for the sigmoidal shape.
Include the Bohr effect as a right‑shift that aids unloading.

Common mistakes

Confusing the direction of the curve shift (left vs right) for the Bohr effect.
Failing to mention cooperative binding as the cause of the sigmoidal shape.
Not linking high pO₂ in lungs to almost full saturation or low pO₂ in tissues to release of O₂.

Mark scheme (4 marks)

Haemoglobin becomes almost fully saturated with oxygen at the high pO₂ found in the lungs/alveolar capillaries, enabling maximum oxygen loading.
In respiring tissues, the pO₂ is low, so haemoglobin readily releases / dissociates from oxygen, supplying it to cells for aerobic respiration.
The sigmoidal/S-shaped nature of the curve means that a small drop in pO₂ in the tissues causes a large release of oxygen (cooperative binding/allostery), making unloading efficient.
A shift of the curve to the right (Bohr effect) in tissues with high CO₂ / low pH further promotes oxygen unloading where it is most needed.

Key terms in this question

haemoglobin · oxygen dissociation curve