This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: General physiology, Subject name: Physiology
[{'content': 'Each hemoglobin contains \nSelect and explain. \nA. 2 polypeptide chains with 2 heme, B. 4 polypeptide chains with 2 heme, C. 2 polypeptide chains with 4 heme, D. 4 polypeptide chains with 4 heme', 'role': 'user'}, {'content': "Answer: D \nExplanation: Hemoglobin is a globular molecule made up of four subunits. Each subunit contains a heme moiety conjugated to a polypeptide. Heme is an iron-containing porphyrin derivative. The polypeptides are referred to collectively as the globin poion of the hemoglobin molecule. There are two pairs of polypeptides in each hemoglobin molecule. Reference: Ganong's review of Medical Physiology; 25th ed; Page: 557", 'role': 'assistant'}]