Iron is a large, charged ion that cannot freely diffuse across cellular membranes. Transmembrane transfer requires specific carrier proteins. There are two general ways in which cells transport iron. Some cells, such as intestinal epithelial cells, hepatocytes and macrophages, are equipped both to take in (import) iron and to release (export) it. These cell types are involved in the acquisition, storage and mobilization of iron. Other cells, particularly erythroid precursors, import iron but do not release it unless the cells are destroyed.
Approximately 25 mg of iron are needed every day to support hemoglobin production in maturing erythrocytes. This amount is much greater than the 1-2 mg entering the body each day through the intestine. Obviously, the iron for erythro-poiesis must be acquired from supplies already existing in the body. The source is reticuloendothelial macrophages, which serve the function of recycling iron from old erythrocytes and making it available to developing erythroid precursors.
Cells use at least three mechanisms to take up iron. Intestinal absorptive cells have cell surface transporters that carry ferrous (Fe2+) ions directly across the membrane. Erythroid precursors have modified their transport pathway by adding a step to concentrate iron in a subcellular compartment, and then transferring it across the membrane of that compartment into the cytoplasm. Hepatocytes probably use both of these mechanisms. Finally, recycling macrophages acquire iron through the phagocytosis of aged or damaged erythrocytes, lysing the cells and dissembling their hemoglobin.
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