Final answer:
Transferrin is a glycoprotein that primarily transports iron (Fe³⁺) in the blood to tissues for use in hemoglobin synthesis or storage. It is not directly involved in iron absorption, storage, or excretion, but rather facilitates the movement of iron in its oxidized form from the site of absorption to where it is needed in the body.
Step-by-step explanation:
The principal role of transferrin is to bind iron in the blood and transport it to various tissues, including the liver and bone marrow, where it can be stored or incorporated into hemoglobin and other iron-containing proteins. When dietary iron is absorbed by the intestine, it binds to ferritin within mucosal cells, creating iron-ferritin complexes which serve as a temporary storage mechanism. If the body requires more iron, the iron-bound ferritin is lost as the intestinal cells slough off, but the circulating iron in the plasma remains bound to transferrin. Transferrin then carries the iron in the ferric state (Fe³⁺) through the bloodstream where it can be used for the synthesis of hemoglobin or stored in ferritin for future use.
In the transport process, ferrous iron (Fe²⁺) is absorbed and rapidly oxidized to ferric iron (Fe³⁺) which is then bound to transferrin. Transferrin delivers Fe³⁺ to cells where it can be reduced back to Fe²⁺ and utilized. This process shows that transferrin's main function is in the transport of iron in the blood rather than absorption, storage, or excretion. Therefore, the correct answer to the student's question about the role of transferrin in the transport of iron in the blood is option 3: Transferrin helps in the transport of iron in the blood.