Haemoglobin and oxygen transportEdexcel International A Level Biology: Revision notes

Haemoglobin (Hb) is a quaternary protein made of four polypeptide chains, each holding a haem group with one Fe²⁺ ion. Each haem group binds one O₂ molecule, so one Hb molecule can carry four O₂ molecules:

Hb + 4O₂ ⇌ HbO₈ (oxyhaemoglobin)

The reaction is reversible. Association (loading) happens in the lungs where the partial pressure of oxygen (pO₂), measured in kPa, is high. Dissociation (unloading) happens in respiring tissues where pO₂ is low. Affinity is how readily haemoglobin binds oxygen.

The oxygen dissociation curve plots the percentage saturation of haemoglobin against pO₂. It is S-shaped (sigmoid) because of cooperative binding:

• At low pO₂ the first O₂ binds with difficulty to the compact molecule, so the curve is shallow.
• The first O₂ changes the shape (tertiary/quaternary structure) so other haem groups are exposed and bind more easily: the curve is steep.
• At high pO₂ most haem groups are occupied and the last O₂ binds with difficulty, so the curve levels off.

In the lungs (about 13 kPa) Hb is about 95–98% saturated. In tissues at rest pO₂ falls onto the steep section, so a small drop in pO₂ releases a large amount of O₂.

A curve shifted to the left means a higher affinity: Hb loads O₂ at a lower pO₂ but unloads it less readily. A curve shifted to the right means a lower affinity: Hb unloads O₂ more readily at the same pO₂.

The Bohr effect is the shift to the right when the carbon dioxide concentration rises. CO₂ forms carbonic acid, releasing H⁺ ions that lower the pH. H⁺ ions bind to haemoglobin and change its shape, lowering its affinity for O₂. Active tissues such as exercising muscle produce more CO₂, so Hb releases more O₂ exactly where it is needed.

Carbon dioxide from respiring cells is carried to the lungs in three ways:

• About 5% dissolved in the plasma.
• About 10–20% bound to haemoglobin as carbaminohaemoglobin.
• About 75–85% as hydrogencarbonate ions (HCO₃⁻) in the plasma.

Inside red blood cells carbonic anhydrase catalyses CO₂ + H₂O ⇌ H₂CO₃ ⇌ H⁺ + HCO₃⁻. HCO₃⁻ ions diffuse out into the plasma and Cl⁻ ions move in to keep the electrical balance (the chloride shift). H⁺ ions bind to haemoglobin (forming haemoglobinic acid), so Hb acts as a buffer. In the lungs the reactions reverse and CO₂ diffuses out to be exhaled.

Fetal haemoglobin has a higher affinity for oxygen than adult haemoglobin, so its dissociation curve is to the left. In the placenta the maternal blood has a low pO₂, so adult Hb unloads O₂ while fetal Hb is more saturated at the same pO₂ and loads it. Oxygen therefore passes from mother to fetus.

After birth the baby breathes air, so a high affinity is no longer needed. Fetal Hb is gradually replaced by adult Hb, which unloads O₂ more readily in the baby's active tissues.