Which component of blood binds and carries oxygen in red blood cells?

• A. Serum
• B. Hemoglobin
• C. Platelets
• D. Lymph

Answer: (B)

The main idea is that a protein inside red blood cells binds and carries oxygen. Hemoglobin fits this role because it contains iron in its heme groups, and each heme can bind one molecule of oxygen. A single hemoglobin molecule can thus carry up to four oxygen molecules from the lungs to tissues where oxygen is released for use by cells. In the lungs, oxygen loads onto hemoglobin to form oxyhemoglobin, and in tissues it unloads for cellular respiration. The other listed components don’t transport oxygen: serum is the fluid part of blood without cells and clotting factors, platelets are involved in clotting, and lymph is a fluid/immune tissue not a transport carrier for oxygen.