Which protein transports iron in the bloodstream?

• A. Hemoglobin
• B. Cytochromes
• C. Ferritin
• D. Transferrin

Answer: (D)

Iron must be kept soluble in blood and delivered to cells that need it, so a plasma transport protein called transferrin handles this job. Transferrin binds ferric iron (Fe3+) at two sites, forming holotransferrin, and circulates in the bloodstream delivering iron by attaching to transferrin receptors on cell surfaces. The complex is taken into the cell by endocytosis, iron is released in the acidic endosome, and transferrin is recycled back to the plasma. This system keeps iron from catalyzing damaging free radicals and makes delivery to tissues tightly regulated.

Hemoglobin carries iron as part of heme within red blood cells for oxygen transport and isn’t a circulating iron transporter. Ferritin stores iron inside cells, serving as a reservoir rather than a transport vehicle in the blood. Cytochromes contain iron as prosthetic groups involved in electron transfer within cells, not in plasma iron transport.