What protein inside red blood cells binds and carries oxygen?

• A. Hemoglobin (Hgb)
• B. Myoglobin
• C. Albumin
• D. Fibrinogen

Answer: (A)

Hemoglobin is the protein inside red blood cells that binds and carries oxygen. It has four polypeptide chains, each with a heme group containing iron that reversibly binds one O2 molecule, so one hemoglobin molecule can carry up to four oxygen molecules. This binding is cooperative: binding of the first oxygen increases the affinity for the next, enabling efficient uptake in the lungs and release in tissues. Hemoglobin’s oxygen transport is finely tuned by factors like pH and carbon dioxide levels, which shift its affinity to meet tissues’ needs.

Myoglobin also binds oxygen, but it resides in muscle cells and contains a single heme group, giving it a high but non-cooperative affinity that serves as an oxygen reserve for muscles rather than a transporter in the bloodstream. Albumin is a major plasma protein that helps maintain blood volume and transports various substances, but it does not bind oxygen. Fibrinogen is a plasma protein involved in clot formation, not oxygen transport.