3.4.4 The Oxygen Dissociation Curve

Key Features of the Curve:

1. Initial Shallow Slope:

• It is difficult for the first oxygen molecule to bind to haemoglobin.
• Once bound, haemoglobin undergoes a shape change (conformational change), increasing its affinity for oxygen.

2. Steep Gradient:

• After the first molecule binds, it becomes easier for the second and third oxygen molecules to bind, a process called positive cooperativity.
• This rapid increase in saturation is shown by the steep part of the curve.

3. Flattening of the Curve:

• As haemoglobin approaches full saturation, it becomes harder for the fourth oxygen molecule to bind because most binding sites are occupied.

Effects of Partial Pressure:

1. High $pO₂$:

• Found in the lungs.
• Haemoglobin has a high affinity for oxygen, so it becomes highly saturated.

2. Low $pO₂$:

• Found in respiring tissues.
• Haemoglobin's affinity for oxygen decreases, causing oxygen to be unloaded to cells where it is needed for respiration.

Adaptations in Oxygen Affinity:

1. Fetal Haemoglobin:

• Higher affinity for oxygen than adult haemoglobin.
• Ensures that oxygen is effectively transferred from the mother's blood to the foetus across the placenta, even at low $pO₂$.

2. High-Altitude Animals:

• Their haemoglobin is adapted to have a higher oxygen affinity to facilitate loading at low $pO₂$

3. Active Organisms:

• Haemoglobin with a lower oxygen affinity allows more efficient unloading in tissues with high metabolic activity.

Bohr Effect:

• The Bohr effect explains how carbon dioxide ($pCO₂$) and acidity (low pH) influence the dissociation curve:
  • High $pCO₂$ in respiring tissues lowers haemoglobin's affinity for oxygen.
  • This shifts the dissociation curve to the right, enabling more oxygen to be unloaded where it is needed.

Key Comparisons:

Condition	Effect on Curve	Biological Advantage
Increased $pCO₂$	Curve shifts right	More oxygen unloaded to respiring tissues.
Foetal Haemoglobin	Curve shifts left	Higher affinity ensures oxygen uptake.
High Altitude (Low $pO₂$)	Curve shifts left	Higher affinity facilitates oxygen loading.

Summary of the Curve:

1. The curve is S-shaped (sigmoidal) due to cooperative binding.
2. Loading occurs in the lungs where $pO₂$ is high.
3. Unloading occurs in tissues where $pO₂$ is low or $pCO₂$ is high.
4. The position of the curve reflects haemoglobin's oxygen affinity under different conditions.

Key Terms:

• Positive Cooperativity: Easier binding of oxygen molecules after the first molecule binds.
• Bohr Effect: Reduced oxygen affinity in haemoglobin due to high $pO₂$ or low pH.
• Partial Pressure ($pO₂$): A measure of oxygen concentration.