Final answer:
Hemoglobin is the iron-containing protein in red blood cells responsible for oxygen transport in the blood. It has four subunits with heme groups, allowing reversible oxygen binding and giving blood its red color.
Step-by-step explanation:
The protein in red blood cells (RBCs) that contains iron (Fe) and is responsible for the transport of oxygen (O2) in the blood is known as hemoglobin. Hemoglobin has a complex structure, consisting of four symmetrical subunits and four heme groups. Each heme group contains one iron ion which can bind oxygen reversibly; this binding is essential for the oxygen transport function of hemoglobin. When oxygen is bound, it gives blood its characteristic red color. Not only does hemoglobin carry oxygen to the cells, but it also transports carbon dioxide back to the lungs.
Moreover, hemoglobin's structure includes two types of protein chains known as alpha and beta chains. Each chain is associated with a heme group that holds the iron ion responsible for oxygen binding. There are two alpha subunits and two beta subunits in each hemoglobin molecule. This quaternary structure is vital for the molecule's ability to bind and release oxygen effectively.