Final answer:
Iron is the main element of hemoglobin, which is essential for binding and transporting oxygen in the blood. Hemoglobin's structure changes with oxygen binding, leading to the distinctive color of oxygenated and deoxygenated blood.
Step-by-step explanation:
The main element of hemoglobin is iron (D). Hemoglobin is a protein located inside red blood cells, responsible for carrying oxygen to body cells and carbon dioxide back to the lungs. This essential protein has a quaternary structure comprising four subunits: two alpha and two beta. Each of these subunits harbors a heme group that contains one iron atom. It is this iron atom that can bind to an oxygen molecule.
When oxygen binds to the iron, hemoglobin changes its three-dimensional shape, which allows it to pick up oxygen in the lungs and release it into tissues where the oxygen concentration is low. Conversely, in areas of high oxygen concentration, like the lungs, hemoglobin binds to oxygen, and this bond causes the bright red color that characterizes oxygenated blood. In its deoxygenated form, when oxygen is released, hemoglobin causes the blood to appear darker red.