Describe the structure of the hemoglobin molecule.

A hemoglobin molecule has a quaternary structure of four polypeptide chains (2 alpha and 2 beta chains) each polypeptide chain is linked to a haem group. An iron atom in the ferrous form (Fe²⁺) is located within each haem group.

(ii) Explain why the affinity of hemoglobin for oxygen increases when it already possesses oxygen.

Each haem group in the haemoglobin molecule can combine with one molecule of oxygen. When this happens a conformational change occurs that exposes the rest of the haem groups to bind with oxygen molecules.