The transferrin receptor cycle. Iron-bound transferrin binds to the transferrin receptor (TfR) on the surface of cells. Receptor-mediated endocytosis occurs, leading to the formation of a vesicle called an endosome. As the lumen of the endosome is acidified by the action of proton pumps, iron is released from transferrin, passes through channels in the membrane, and is utilized by the cell. The complex between iron-free transferrin and the transferrin receptor is returned to the plasma membrane for another cycle.
[Information from L. Zecca et al., Nat. Rev. Neurosci. 5:863–873, 2004, Fig.1.]