2.6.9Inquiry Process
The Bohr Shift — IB Biology HL Revision Notes & Topical Questions
5 min read 4 parts Lesson 71 of 160
Part 1 of 4Carbon Dioxide in the Blood
The key idea is that more carbon dioxide means more oxygen unloading.
1
Respiring cells produce carbon dioxide, which diffuses into the blood plasma and into red blood cells.
2
Inside the red blood cells the enzyme carbonic anhydrase speeds the reaction CO₂ + H₂O ⇌ H₂CO₃.
3
Carbonic acid dissociates: H₂CO₃ ⇌ H⁺ + HCO₃⁻, which lowers the pH inside the cell.
4
Hydrogen ions bind to haemoglobin, helping to buffer pH and changing the shape of the protein.
5
The altered shape reduces its affinity for oxygen, so oxygen is released more readily.
Part 2 of 4The Shift and Its Meaning
Rightward shift
- At a higher partial pressure of carbon dioxide (lower pH), the oxygen dissociation curve moves to the right.
Same pO₂, lower saturation
- At any given partial pressure of oxygen the haemoglobin is less saturated, so more oxygen is released.
Active tissues
- Working muscle produces more carbon dioxide and acid, so the shift supplies extra oxygen exactly where demand is greatest.
Lungs
- In the lungs carbon dioxide leaves the blood, pH rises, affinity increases, and oxygen loads efficiently.
| Condition | Haemoglobin affinity | Result |
|---|---|---|
| High CO₂, low pH (muscle) | Lower | More O₂ released |
| Low CO₂, higher pH (lung) | Higher | More O₂ loaded |
Part 3 of 4Wider Consequences
Wider Consequences
Carbon dioxide transport
- Most carbon dioxide is carried as hydrogencarbonate ions in plasma, a smaller part as carbamino compounds,
- and a little dissolved.
Chloride shift
- Hydrogencarbonate leaves the red cell in exchange for chloride ions, which keeps the charge balanced.
Buffering
- Haemoglobin accepts hydrogen ions, which prevents large pH changes in blood.
Exercise
- As exercise intensifies, the shift becomes stronger and oxygen delivery rises without the need for higher saturation in lungs.
Part 4 of 4Exam Checklist & Tips
2.6.9 – Key Points for the Exam
- Bohr shift The decrease in haemoglobin's oxygen affinity caused by increased carbon dioxide or lower pH.
- Carbonic anhydrase An enzyme in red blood cells that catalyses the conversion of carbon dioxide and water to carbonic acid.
- Hydrogencarbonate ion HCO₃⁻, the main form in which carbon dioxide is transported in blood.
- Dissociation The release of oxygen from haemoglobin.
Common Mistakes
- Saying the curve shifts left in active tissues; it shifts right.
- Stating that more carbon dioxide makes haemoglobin hold oxygen more tightly.
- Forgetting that hydrogen ions, not carbon dioxide itself, cause most of the effect on shape.
- Writing that red blood cells make carbon dioxide; they receive it from respiring cells.
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