Which protein within red blood cells binds and transports oxygen?

• A. Hemoglobin
• B. Albumin
• C. Fibrinogen
• D. Myoglobin

Answer: (A)

Inside red blood cells, the protein that binds and transports oxygen is hemoglobin. Hemoglobin has four heme groups with iron that reversibly binds one O2 each, so a single molecule can carry up to four oxygen molecules. This binding is cooperative, meaning when one oxygen molecule binds, the others are more easily bound, which helps pick up oxygen in the lungs where oxygen pressure is high and release it in tissues where it’s needed. In contrast, albumin is a plasma protein that shuttles various substances in the blood, fibrinogen is involved in blood clotting, and myoglobin stores oxygen in muscle tissue rather than transporting it in blood.