Why is iron used in haemoglobin molecules?

Iron is used in haemoglobin molecules because it binds with oxygen, enabling the transport of oxygen throughout the body.

Haemoglobin is a protein molecule found in red blood cells, which plays a crucial role in oxygen transport. The structure of haemoglobin is complex, consisting of four protein chains, each of which contains a heme group. The heme group is the component that contains an iron atom, and it is this iron atom that is capable of binding with oxygen.

The reason why iron is used in haemoglobin is due to its unique chemical properties. Iron is a transition metal, which means it can exist in multiple oxidation states. In the case of haemoglobin, iron is present in the Fe2+ state. This allows it to form a coordinate bond with oxygen, effectively 'grabbing' the oxygen molecule and holding it in place. This is a reversible process, meaning that when the red blood cell reaches a part of the body that requires oxygen, the iron can release the oxygen molecule, allowing it to be used by the body's cells.

Furthermore, the iron atom in the heme group is coordinated to a nitrogen atom in a histidine residue of the protein chain. This histidine residue plays a crucial role in facilitating the binding and release of oxygen. When oxygen binds to the iron atom, it causes a slight change in the shape of the haemoglobin molecule. This change is transmitted to the histidine residue, which then triggers the release of oxygen.

In summary, the use of iron in haemoglobin is essential for the transport of oxygen in the body. Its ability to form a coordinate bond with oxygen, combined with its interaction with the histidine residue, allows it to effectively pick up oxygen in the lungs and deliver it to the body's cells where it is needed. Without iron, this vital process would not be possible.