Final answer:
A hemoglobin molecule can transport four oxygen molecules, with each of its four iron-containing heme units binding one oxygen molecule.
Step-by-step explanation:
A molecule of hemoglobin can hold four molecules of oxygen in the blood for transport. This is due to the structure of hemoglobin, which is composed of four subunits. Each of these subunits contains one iron ion within a molecule of heme, and this iron ion has the ability to bind to one molecule of oxygen. Therefore, hemoglobin is capable of carrying up to four oxygen molecules. This ability of hemoglobin to bind oxygen is essential for transporting oxygen from the lungs to various parts of the body where it is needed for cellular respiration.