Once free iron is bound to apoferritin, it dissociates to bind with what protein in the bloodstream?

The correct answer is Transferrin. Once free iron is bound to apoferritin, it is stored as ferritin. However, when iron needs to be transported in the bloodstream, it dissociates from ferritin and binds to transferrin, which is the primary iron transport protein in circulation. Transferrin serves to shuttle iron from sites of absorption (like the intestines) and release it to tissues, including the bone marrow and liver, where it is critical for processes such as erythropoiesis (red blood cell production).

Iron has to be in a soluble and nontoxic form in the bloodstream, and transferrin fulfills this role effectively by binding to iron and allowing its safe transport. Once in circulation, transferrin can be taken up by cells that require iron through specific receptors, ensuring that iron is delivered where it is needed in a tightly regulated manner.