Haemoglobin and Oxygen Transport

This lesson covers the structure and function of haemoglobin, the oxygen dissociation curve, and factors that affect oxygen transport, as required by the Edexcel A-Level Biology specification (9BI0). You need to understand cooperative binding, the sigmoid shape of the dissociation curve, the Bohr effect, and how different haemoglobins are adapted to different environments.

---

Structure of Haemoglobin

Haemoglobin (Hb) is a globular protein found in red blood cells (erythrocytes). Each molecule consists of:

• Four polypeptide chains: two alpha (α) chains and two beta (β) chains.
• Each polypeptide chain contains a haem group — a non-protein (prosthetic) group that contains a single iron (Fe²⁺) ion.
• Each haem group can bind one molecule of oxygen (O₂).
• Therefore, each haemoglobin molecule can carry a maximum of four O₂ molecules (8 oxygen atoms).

Quaternary Structure

Haemoglobin is an example of a protein with quaternary structure — it is composed of multiple polypeptide subunits held together by hydrophobic interactions, hydrogen bonds, and ionic bonds.