Final answer:
The protein responsible for oxygen transport in red blood cells is hemoglobin, which is comprised of four subunits that each bind to a heme group containing iron. Hemoglobin can carry four oxygen molecules, causing the color of blood to change between bright red when oxygenated and darker red when deoxygenated.
Step-by-step explanation:
The protein molecules found inside red blood cells that are responsible for carrying oxygen molecules to cells and carbon dioxide to the lungs is hemoglobin. Hemoglobin consists of four symmetrical subunits and four heme groups. Each heme group contains iron, which is essential for binding oxygen. Notably, the presence of iron in hemoglobin also gives blood its characteristic red color.
Hemoglobin, or Hb, includes two alpha subunits and two beta subunits. Each subunit wraps around a central heme group that shelters an iron ion capable of binding one oxygen molecule. This molecular structure allows each hemoglobin molecule to transport four oxygen molecules. When hemoglobin is fully oxygenated in arterial blood, it appears bright red, but it becomes darker red when it releases oxygen in the veins.
In addition to oxygen transport, hemoglobin is crucial in a single erythrocyte, where it can be found in vast numbers. A single red blood cell may contain approximately 300 million hemoglobin molecules, with the potential to carry over 1 billion oxygen molecules.