Explain how the Bohr effect enables efficient delivery of oxygen to actively 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).

Model answer (4 marks)

Actively respiring tissues produce more CO₂, lowering local pH (increasing acidity). The lower pH raises H⁺ concentration, which reduces haemoglobin’s affinity for O₂. Consequently the O₂–haemoglobin dissociation curve shifts to the right, so at any given PO₂ more O₂ is unloaded. This ensures that tissues with the highest metabolic demand receive the most oxygen, matching supply to demand.

Examiner tips

  • Use the phrase ‘shift to the right’ to show understanding of the curve.
  • Explain the link between CO₂, pH and H⁺ concentration.
  • Show that unloading occurs at the same PO₂ in tissues.
  • Mention that this matches supply to demand.”]

Mark scheme (4 marks)

  1. Actively respiring tissues produce more carbon dioxide, lowering local pH (increasing acidity).
  2. Lower pH (higher H⁺ concentration) causes haemoglobin to have a reduced affinity for oxygen.
  3. The oxygen–haemoglobin dissociation curve shifts to the right, meaning more oxygen is unloaded at any given partial pressure of oxygen.
  4. This ensures that tissues with the highest metabolic demand receive the most oxygen, matching oxygen supply to demand.

Key terms in this question

Bohr effect

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