The oxygen dissociation curve is S-shaped because haemoglobin displays cooperative binding: binding of one oxygen molecule changes haemoglobin’s shape and increases its affinity for subsequent oxygen molecules. This produces slow loading at low oxygen partial pressure, rapid loading in the middle, and a plateau near full saturation.
This is positive cooperativity. Haemoglobin has four polypeptide subunits, each containing a haem group that can bind one oxygen molecule. When the first oxygen molecule binds, it causes an allosteric conformational change in haemoglobin’s quaternary structure.
This change makes the remaining haem groups more accessible and increases their affinity for oxygen. Therefore, the second and third oxygen molecules bind more readily than the first. As haemoglobin approaches saturation, fewer unoccupied binding sites remain, so the curve levels off.
| Region of curve | Explanation |
|---|---|
| Low oxygen partial pressure | Haemoglobin initially has a relatively low affinity for oxygen, so little oxygen binds and the curve rises slowly. |
| Intermediate oxygen partial pressure | Binding of oxygen increases the affinity of remaining sites, causing a steep rise in percentage saturation. |
| High oxygen partial pressure | Most haem groups are occupied, so haemoglobin approaches 100% saturation and the curve plateaus. |
This shape is biologically important. In the lungs, where oxygen partial pressure is high, haemoglobin becomes highly saturated. In actively respiring tissues, oxygen partial pressure is lower, so haemoglobin unloads oxygen; the steep part of the curve allows a relatively small decrease in oxygen partial pressure to cause substantial oxygen release.
A common misconception is that the curve is S-shaped simply because more oxygen is available at higher partial pressures. Greater availability increases collisions, but the distinctive sigmoid shape specifically results from cooperative binding and changing haemoglobin affinity.
Exam technique: For an AHL “explain” question, link the full sequence: first oxygen binds → conformational change → increased affinity of remaining haem groups → rapid further binding → plateau as binding sites become occupied. Do not merely state that haemoglobin has four binding sites.