Explain the Bohr shift
Describe how carbon dioxide and pH alter haemoglobin's oxygen affinity and promote unloading in active tissues.
AS Level; Cambridge International Biology 9700, 2028–2030. Written practice is self-checked; microscope recognition, biological drawing and supervised practical competence require practical work, while human subject-teacher review remains pending.
Before you begin
- Know that mass transport moves substances between exchange surfaces and cells, and that diffusion occurs down a concentration or partial-pressure gradient.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
What happens to haemoglobin's oxygen affinity at lower pH in active tissue?
What you will learn
- Describe a Bohr shift
- Explain its importance to oxygen delivery in respiring tissues
Build the explanation
Respiring tissues release carbon dioxide. The resulting increase in hydrogen-ion concentration lowers pH. Under these conditions, haemoglobin's affinity for oxygen decreases, so at the same oxygen partial pressure its saturation is lower. The oxygen dissociation curve shifts to the right: haemoglobin unloads more oxygen in the tissues. In the lungs, carbon dioxide is removed, pH rises relative to active tissue blood, and affinity increases to support oxygen loading.
Apply it carefully
A Bohr shift is an affinity-related shift of the curve, not simply movement along it because oxygen partial pressure changed. State the changed conditions (more CO₂/lower pH), the direction of affinity change, and the tissue benefit. Avoid saying oxygen itself causes the right shift or treating the phenomenon as a direct effect of temperature unless specified.
Pause and explain
What does a rightward Bohr shift indicate?
Worked example
At equal oxygen partial pressure, blood with lower pH has lower haemoglobin saturation. What does this show?
Show the worked solution
- The partial pressure is controlled, so the difference is not movement along one unchanged curve.
- Lower pH has reduced haemoglobin affinity.
Answer The lower pH has decreased haemoglobin's oxygen affinity, producing a rightward Bohr shift and promoting oxygen unloading.
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Describe the Bohr shift and explain why it benefits an active tissue.
Give me a hint
Include CO₂, pH, affinity and unloading.
Compare my reasoning
- Respiring tissue CO₂ increases H⁺ and lowers pH.
- Haemoglobin affinity falls; the curve shifts right and more oxygen is unloaded.
Respiring tissue releases CO₂, which contributes to a lower pH. Haemoglobin's oxygen affinity decreases, shifting the dissociation curve to the right. At a given oxygen partial pressure it has lower saturation, so more oxygen is delivered to the active tissue.
Look for these in your work
- CO₂/pH condition stated
- Affinity and right shift stated
- Unloading benefit linked
A student reports a lower saturation at equal oxygen partial pressure and concludes that oxygen partial pressure caused it. Explain the better interpretation.
Give me a hint
Equal partial pressure rules out simple movement along an unchanged curve.
Compare my reasoning
- At equal partial pressure, changed pH/CO₂ can alter affinity.
- Lower saturation is consistent with a rightward Bohr shift.
Because oxygen partial pressure is equal, the lower saturation indicates changed haemoglobin affinity rather than movement along one unchanged curve. Increased CO₂ and lower pH can cause a rightward Bohr shift.
Look for these in your work
- Controlled variable used
- Affinity change distinguished
- Bohr shift interpreted
Compare haemoglobin affinity in a highly active tissue and the lungs, then relate this to oxygen delivery.
Give me a hint
Contrast local CO₂/pH and partial-pressure conditions.
Compare my reasoning
- Active tissue: more CO₂, lower pH, lower affinity, more unloading.
- Lungs: CO₂ is removed, pH is relatively higher, affinity supports loading at high oxygen partial pressure.
In active tissue, CO₂ accumulation lowers pH and decreases haemoglobin affinity, promoting oxygen unloading. In the lungs, CO₂ is excreted and oxygen partial pressure is high, supporting a higher affinity state and oxygen loading.
Look for these in your work
- Tissue condition and effect correct
- Lung condition and effect correct
- Loading/unloading linked
Common mistakes
- Confusing a right shift with movement along the curve
- Claiming lower pH increases oxygen affinity
- Ignoring the role of respiring tissue CO₂
What can you explain now?
What happens to haemoglobin's oxygen affinity at lower pH in active tissue?
Compare with the explanation
It decreases
More CO₂ and H⁺ in active tissues reduce oxygen affinity and favour unloading.
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Make it stick
How do CO₂ and pH change haemoglobin affinity, and why is that useful in active tissue?
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