Explain how the Bohr shift enables active tissues to obtain more oxygen from haemoglobin during periods of high metabolic activity.

IB DP Biology Higher 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)

Active tissues produce more CO₂ and lactic acid, lowering the local pH. The increased H⁺/CO₂ causes haemoglobin to change conformation, reducing its affinity for O₂. Consequently the oxygen dissociation curve shifts to the right, so haemoglobin releases O₂ at a higher PO₂ than normal. This allows more O₂ to be unloaded in the active tissues where it is needed for aerobic respiration.

Examiner tips

Use the term ‘Bohr shift’ and link CO₂/lactic acid to lower pH; mention the right‑shift of the dissociation curve; explain that unloading occurs at a higher PO₂; keep answer concise and use correct terminology.

Common mistakes

Failing to mention the Bohr shift explicitly; confusing the shift with a left shift; not linking CO₂/lactic acid to pH change; using vague phrases like ‘more oxygen’ without explaining the mechanism.

Mark scheme (4 marks)

Active tissues produce more carbon dioxide (and lactic acid), lowering the local pH / increasing CO₂ concentration.
Lower pH (higher H⁺ / CO₂) causes haemoglobin to change shape / reduces haemoglobin's affinity for oxygen.
The oxygen dissociation curve shifts to the right, meaning haemoglobin releases / unloads oxygen at a higher partial pressure of O₂ than it otherwise would.
This ensures that more oxygen is delivered to (unloaded at) the active tissues where it is most needed for aerobic respiration.

Key terms in this question

Bohr shift