Which of the following describes the structure of haemoglobin?

• A. Four polypeptide chains — 2 alpha and 2 beta, each with a haem group containing Fe2+
• B. Two polypeptide chains
• C. Four polypeptide chains — 2 alpha and 2 gamma
• D. Four polypeptide chains — 4 alpha

Answer: (A)

Hemoglobin is a tetramer, made of four polypeptide chains arranged in a quaternary structure. In adults, it consists of two alpha and two beta chains, and each chain contains a heme group with an iron ion (Fe2+) that binds oxygen reversibly. This setup enables cooperative binding and release of oxygen as it moves through the body. For contrast, fetal hemoglobin uses two gamma chains instead of beta, forming α2γ2, which changes its oxygen affinity. The other descriptions don’t fit because hemoglobin isn’t just two chains (that’s myoglobin), it isn’t four alpha chains, and HbF’s composition differs from the adult form.