3.4.2 Haemoglobin & Oxygen

Structure of Haemoglobin:

1. Protein Composition:

• Haemoglobin is a water-soluble globular protein made of:
• Two alpha polypeptide chains.
• Two beta polypeptide chains.

2. Haem Group:

• Each polypeptide contains a haem group, which includes an iron ion $(Fe²⁺)$.
• The haem group can bind to one oxygen molecule.

3. Oxygen Capacity:

• Each haemoglobin molecule can bind to four oxygen molecules, forming oxyhaemoglobin.

Affinity for Oxygen:

1. Definition:

• The affinity of haemoglobin refers to its ability to bind to oxygen.

2. Partial Pressure of Oxygen ($pO₂$):

• Partial pressure is a measure of oxygen concentration.
• The higher the partial pressure of oxygen, the greater the affinity of haemoglobin for oxygen.

Oxygen Binding and Release:

1. In the Lungs:

• High $pO₂$ in the alveoli increases haemoglobin's affinity for oxygen.
• Oxygen binds to haemoglobin in a process called loading or association.

2. In Respiring Tissues:

• Low $pO₂$ in tissues reduces haemoglobin's affinity for oxygen.
• Oxygen is released in a process called unloading or dissociation.
• This ensures oxygen is delivered to cells where it is needed for respiration.

Oxygen Dissociation Curve:

1. S-Shaped (Sigmoidal) Curve:

• The curve shows the relationship between and haemoglobin's saturation with oxygen. $pO₂$

2. Key Features:

• Low $pO₂$: Haemoglobin has a low affinity for oxygen (less saturated).
• Increasing $pO₂$: Haemoglobin's affinity for oxygen increases as the first oxygen molecule binds, making it easier for the remaining oxygen molecules to bind (cooperative binding).
• High $pO₂$: Haemoglobin becomes saturated with oxygen.

Role of Haemoglobin in Gas Transport:

1. Loading:

• Haemoglobin binds oxygen in the lungs where $pO₂$ is high.
• Forms oxyhaemoglobin.

2. Unloading:

• Haemoglobin releases oxygen in tissues where $pO₂$ is low.
• Oxygen diffuses into respiring cells.

3. Transport of Carbon Dioxide:

• Haemoglobin also helps transport carbon dioxide back to the lungs for exhalation.

Adaptations of Haemoglobin in Different Organisms:

1. High-Altitude Organisms:

• Haemoglobin has a higher affinity for oxygen to cope with lower $pO₂$ in the environment.

2. Active Organisms:

• Haemoglobin has a lower affinity for oxygen, allowing oxygen to be unloaded more easily for rapid respiration.

3. Foetal Haemoglobin:

• Higher affinity for oxygen compared to adult haemoglobin, enabling the foetus to extract oxygen from the mother's blood.

Key Terms:

• Haemoglobin: Protein in red blood cells that carries oxygen.
• Oxyhaemoglobin: Haemoglobin bound to oxygen.
• Loading/Association: Binding of oxygen to haemoglobin.
• Unloading/Dissociation: Release of oxygen from haemoglobin.
• Partial Pressure ($pO₂$): Measure of oxygen concentration.