Define oxygen dissociation curve. Can you suggest any reason for its sigmoidal pattern?
The Oxygen dissociation curve is defined as a sigmoid curve obtained when the percentage saturation of haemoglobin with O₂ is plotted against the partial pressure of O₂ (pO₂). This curve is highly useful in studying the effect of factors like pCO₂, H⁺ concentration, and temperature on the binding of O₂ with haemoglobin.
The sigmoidal pattern of the curve arises because the binding of oxygen with haemoglobin is primarily related to the partial pressure of O₂. Since each haemoglobin molecule can carry a maximum of four molecules of O₂, the progressive binding of oxygen molecules results in a characteristic sigmoid shape rather than a linear relationship.
Explanation
The answer defines the curve using the specific description in the text: a sigmoid curve resulting from plotting haemoglobin saturation against pO₂. For the reason behind the sigmoidal pattern, the text explicitly links the curve's shape to the relationship between pO₂ and saturation, noting that haemoglobin can carry four molecules of O₂. While the text does not use the term 'cooperative binding', it establishes the physiological basis (the four-molecule capacity and pO₂ relationship) for the shape.