Hemoglobin's structure and oxygen transport

Steps:

• Identify hemoglobin as a protein with four polypeptide chains (two alpha, two beta).
• Note each chain binds one heme group containing an iron ion (Fe²⁺).
• Recognize that each iron ion reversibly binds one oxygen molecule, allowing four O₂ per hemoglobin.
• Confirm cooperative binding leads to 50% saturation at physiological pO₂ for efficient transport.

Why A is correct:

• Accurately describes four polypeptide chains, each with a heme-associated iron ion that binds oxygen, including 50% saturation for cooperative binding.

Why the others are wrong:

• B: Incorrectly specifies only two alpha chains and vague oxygen transport without heme or full structure.
• C: Mentions iron in heme and two beta chains but wrongly states transport of four atoms instead of four O₂ molecules.
• D: Erroneously places iron in hydrophobic R-groups and claims eight oxygen bindings, ignoring true heme location and capacity.

Final answer: A