Define oxygen dissociation curve. Can you suggest any reason for its sigmoidal pattern?

• The oxygen dissociation (oxyhemoglobin) curve is a graph showing how the percentage of hemoglobin bound to oxygen (O₂ saturation) changes with oxygen partial pressure (pO₂)

• It has a sigmoid (S-shaped) pattern because of cooperative binding:

  • Hemoglobin has four subunits. Binding of the first O₂ molecule is relatively difficult.

  • After the first molecule binds, hemoglobin changes shape, making it easier for the next molecules to bind.

  • This increases affinity with each successive O₂, causing a steep middle section of the curve

• As saturation nears 100%, there are fewer unoccupied sites, so the curve flattens at high pO₂

In summary:
The sigmoidal shape arises because hemoglobin binds oxygen more easily after the initial molecule attaches, leading to accelerated binding at mid-level pO₂ and a plateau as it becomes fully saturated.