Is the iron in your body the same as the metal iron found in everyday objects? Both refer to the same chemical element, iron (Fe). While the fundamental element is identical, its forms and roles differ vastly depending on whether it exists as a bulk metal or within a biological system.
Properties of Metallic Iron
Elemental iron exhibits distinct physical and chemical properties. Pure iron is a soft, lustrous, gray-white metal with high tensile strength. It is malleable, meaning it can be hammered into sheets without breaking, and ductile, allowing it to be drawn into thin wires. Iron is ferromagnetic, meaning it can be strongly attracted to magnets or even form magnets.
Chemically, pure iron is reactive, readily dissolving in dilute mineral acids. Its most noticeable reaction is oxidation, commonly known as rusting, which occurs when iron is exposed to oxygen and moisture. This reaction forms reddish-brown iron oxides, such as hydrated iron(III) oxide. Due to its strength and affordability, iron is widely used in construction, manufacturing, and the automotive industry, often alloyed with other elements to produce materials like steel and cast iron.
Iron’s Essential Functions in the Body
Within the human body, iron exists not as a solid metal but primarily in ionic forms, specifically ferrous (Fe2+) and ferric (Fe3+), bound within complex biological molecules. Approximately 70% of the body’s iron is found in hemoglobin, a protein in red blood cells that transports oxygen from the lungs to tissues throughout the body. Iron is also present in myoglobin, a protein responsible for storing oxygen in muscle cells, making it available for muscle activity.
Beyond oxygen transport and storage, iron is crucial for many enzyme systems. These iron-dependent enzymes are involved in metabolic processes, including cellular respiration, which produces energy for cells, and DNA synthesis. Iron’s ability to easily switch between its ionic states (Fe2+ and Fe3+) enables it to participate in electron transfer reactions, fundamental for these biological functions.
Transforming Dietary Iron for Bodily Use
The human body cannot produce iron and must obtain it from dietary sources. Dietary iron comes in two primary forms: heme iron, found mainly in animal products like meat, poultry, and seafood, and non-heme iron, present in both plant foods and animal sources. Heme iron is more readily absorbed by the body, with absorption rates ranging from 15% to 35%. Non-heme iron absorption is lower, between 2% and 20%, and can be influenced by other food components.
Upon consumption, iron is primarily absorbed in the duodenum and upper jejunum of the small intestine. Non-heme iron, often in the ferric (Fe3+) state, must first be converted to the ferrous (Fe2+) state by enzymes on the intestinal cell surface to be absorbed. Once absorbed into intestinal cells, iron can be stored as ferritin, a protein that holds iron within cells, or released into the bloodstream. In the blood, iron binds to transferrin, a transport protein that delivers it to various tissues and cells throughout the body.
Distinguishing Body Iron from Metal Iron
The fundamental difference between metallic iron and iron in the body lies in their chemical state and physical presentation. Metallic iron is elemental iron (Fe) in its solid, unbonded form, characterized by its bulk properties like strength, malleability, and electrical conductivity. It exists as a structural material, used in large quantities for industrial applications. In contrast, iron in the body is present as dissolved ions (Fe2+ or Fe3+) or is tightly bound within complex organic molecules like proteins. It is never found as a free, solid metal in living tissues due to its potential toxicity in that form.
The functions of these forms are distinct. Metallic iron serves industrial and structural purposes, while biological iron performs metabolic and physiological roles, such as oxygen transport and enzymatic reactions. Metallic iron is a bulk material, whereas iron in the body is a trace element, essential in small amounts (an adult human body contains 4 to 5 grams of iron). The iron in a bridge and the iron in your blood, though derived from the same element, behave as differently as carbon in a diamond does from carbon in sugar, due to their unique chemical arrangements and environments.