Which of the following statements is not correct? (a) In oxyhaemoglobin, \(\mathrm{Fe}^{2+}\) is paramagnetic. (b) During respiration, the size of \(\mathrm{Fe}^{2+}\) increases when it changes from diamagnetic to paramagnetic state. (c) Four heme groups are present in haemoglobin. (d) Heme is the prosthetic group and it is non protein part of haemoglobin.

Understanding oxyhaemoglobin is crucial for grasping how our blood cells transport oxygen throughout our body. Oxyhaemoglobin is formed when hemoglobin within red blood cells binds to oxygen. This robust complex is essential for oxygen delivery to various tissues and organs. The iron (Fe) in the heme group remains in a Fe^{2+} state and is pivotal for oxygen attachment.

When we inhale, oxygen molecules enter the lungs and encounter hemoglobin in the pulmonary capillaries. Each hemoglobin molecule can bind four oxygen molecules, one per heme group, resulting in the formation of oxyhaemoglobin. Through this efficient process, oxygen is then transported via the bloodstream until it reaches tissues, where it is released to facilitate cellular respiration.

Exploring the concept of paramagnetism in hemoglobin helps clarify common misconceptions. Paramagnetic materials are attracted by a magnetic field due to the presence of unpaired electrons, which create a magnetic moment. In the context of hemoglobin, the iron ion Fe^{2+} is often thought to exhibit paramagnetism.

However, in oxyhaemoglobin, the oxygen molecule forms a stable complex with the Fe^{2+} ion, effectively pairing up any unpaired electrons. This interaction does not lead to a paramagnetic state; instead, oxyhaemoglobin is diamagnetic, meaning it weakens an external magnetic field, due to the absence of unpaired electrons. The transition from a deoxy-state, which can slightly be paramagnetic, to an oxy-state does not involve a change in the size of the Fe^{2+} ion, debunking another myth.

The term prosthetic groups refers to non-protein components bound to proteins, crucial for their biological activity. These components can be organic or metal ions and are permanently attached to proteins during the post-translational modification. In hemoglobin, the heme group is the prosthetic group responsible for oxygen binding.

Proteins usually have complex functions that simple amino acid chains cannot accomplish on their own. Thus, prosthetic groups are like keys to activating these proteins and enabling them to carry out their functions. In hemoglobin's case, without the heme group, the protein would not be able to bind and release oxygen, which is pivotal for cellular respiration. Each heme in hemoglobin magnifies its ability to transport oxygen, making it exceptionally efficient.

The heme group is a cornerstone in the functionality of hemoglobin, as each heme is capable of binding one molecule of oxygen. Composed of an iron ion at its center, called the iron protoporphyrin complex, the heme is nestled into the hemoglobin's protein structure. The iron's precise coordination with nitrogen atoms from the porphyrin ring allows it to readily engage with oxygen.

The heme group plays a direct role in the ability of red blood cells to pick up oxygen in the lungs and then release it where it's most needed in the body. Although the heme is the non-protein part, it's integral to the protein's overall function, proving that proteins often rely on such groups to accomplish their diverse and vital roles.