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I'm writing a report on the spectrophotometric determination of iron. In my introduction I want to briefly explain the importance of iron in the human body.

Wikipedia states that "Iron is essential to life due to its unusual flexibility to serve as both an electron donor and acceptor"

1.) So is iron the only element that can do that? If not, what is it about iron that makes it the only element found in hemoglobin?

2.) What is it about being able to gain and lose electrons make it essential to life? I think it has something to do with oxygen being able to bind with iron. But there are so many elements that can bind with iron! What is special about iron?

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1.) So is iron the only element that can do that? If not, what is it about iron that makes it the only element found in hemoglobin?

No, iron is not the only metal that will fit inside the porphyrin structure. But it's ability to be an electron acceptor and donor makes it very useful for oxygen and carbon dioxide to bind to the hemoglobin. Since iron is also very abundant, it was easy to incorporate it and utilize it's properties.

Other macrocyclic structures include chlorophyll with magnesium in the center and vitamin B12 with cobalt in the center.

2.) What is it about being able to gain and lose electrons make it essential to life? I think it has something to do with oxygen being able to bind with iron. ... What is special about iron?

Iron is abundant. The iron in the hemoglobin can bind oxygen and carbon dioxide weakly.

You can find more information on the Wikipedia page.

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  • $\begingroup$ Not sure carbon dioxide binds to hemoglobin. Crummy ligand for that. Carbon _mon_oxide binds to hemoglobin (big problem) through its exposed carbon atom, but AFAIK carbon _di_oxide just ends up in the carbonic acid/bicarbonate buffer. $\endgroup$ – Oscar Lanzi Jan 29 '17 at 13:56
  • $\begingroup$ @OscarLanzi IMO I believe that the lungs expel more carbon dioxide than what would defuse from a carbonate buffer. $\endgroup$ – LDC3 Jan 30 '17 at 7:15
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I would like to clarify this Q & A.

Both Fe(II) and Fe(III) are found in metalloproteins that are essential to life. Fe(II) can serve as an electron donor, and Fe(III) an electron acceptor in life.

  1. "So is iron the only element that can do that?" No, for electron transfer, many biological transition metals (Co, Ni, Cu...) can also participate in redox reactions in life (but not Mg(II) in chlorophyll)

  2. However, Fe(II) in hemoglobin is not a good example for "electrons donor/acceptor". Although the oxidative state of Fe(II) in heme changes slightly when binding to O2 or CO, it never oxidizes to Fe(III). (If it does, it loses its ability to bind O2). The ferrous ion in hemoglobin is important in oxygen transport.

  3. "So is iron the only element that can do that?" No, for oxygen transport, copper ion in haemocyanin can also transport oxygen in the blood of spiders, crabs and octopuses. When oxygenated, they are blue.

  4. "What is it about being able to gain and lose electrons make it essential to life?" Because biological redox reactions (e.g. cellular respiration) are essential to life. Each biological metal is important in its own specific biological function. Their roles are the product of evolution. For oxygen transport, neither heme nor iron is indispensable for that function in other life forms.

  5. Iron is abundant in earth crust mainly as iron(III) oxides. However, iron is NOT abundant in bioavailability. Ferric and Ferrous hydroxides are so poorly soluble in the environment that many species have developed sophisticated iron acquisition systems such as siderophore to fight for this "scarce" element as an essential nutrient.

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Great answers, but I would like to add one additional use of iron at the cellular level.

In the electron-transport-chain (ETC), within the mitochondria of each cell, are Iron-Sulfur clusters that are responsible for the oxidation of carbohydrates in aerobic respiration.

If you were to use search-terms, "ETC and citric acid cycle respiratory complexes", you should be able to find lots of info on this.

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