Problem 58 - A Complete Resource Book in Chemistry for JEE Main

Question

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.

Step-by-Step Solution

Answer

Statement (a) is not correct.

Step 1: Analyze Statement (a)

The statement claims that in oxyhaemoglobin, \(\mathrm{Fe}^{2+}\) is paramagnetic. Remember that oxyhaemoglobin forms when oxygen binds to deoxyhaemoglobin. In this state, iron is in a low-spin state, which is diamagnetic. Therefore, statement (a) is incorrect because it contradicts the fact that the \(\mathrm{Fe}^{2+}\) is not paramagnetic but rather diamagnetic in oxyhaemoglobin.

Step 2: Analyze Statement (b)

This statement mentions that during respiration, the size of \(\mathrm{Fe}^{2+}\) increases when it changes from diamagnetic to paramagnetic. Understand that when \(\mathrm{Fe}^{2+}\) becomes paramagnetic, it enters a high-spin state where the ionic radius indeed increases. This happens when the oxygen is released from haemoglobin, suggesting that the statement is correct.

Step 3: Analyze Statement (c)

The statement states that there are four heme groups present in haemoglobin. Recall that haemoglobin is a tetramer composed of two alpha and two beta chains, and each chain contains one heme group. So, there are indeed four heme groups in a single haemoglobin molecule, making this statement correct.

Step 4: Analyze Statement (d)

According to the statement, heme is the prosthetic group and is the non-protein part of haemoglobin. Heme indeed acts as a prosthetic group, meaning it is permanently bound to the protein and not composed of amino acids. It serves as the site for oxygen binding, so the statement is correct.

Key Concepts

Oxyhaemoglobin

Oxyhaemoglobin is a special form of haemoglobin that forms when oxygen molecules attach to the iron ions in haemoglobin. In this form, the iron ion, which is ext{Fe}^{2+}, transitions into a low-spin state. This is important because low-spin states are associated with being diamagnetic.
This means that oxyhaemoglobin does not have unpaired electrons and is not attracted to a magnetic field.
It's crucial in the oxygen transport process because it allows haemoglobin to efficiently pick up oxygen in the lungs and transport it throughout the body.

Paramagnetic and Diamagnetic States

These two states relate to how different substances respond to magnetic fields, and they are based on the electron configuration of the atoms or ions.

• Paramagnetic: In this state, the substance has unpaired electrons, which cause it to be attracted to magnetic fields. Examples include ext{Fe}^{2+} in its high-spin state.
• Diamagnetic: Here, all the electrons are paired, resulting in no attraction to a magnetic field. Oxyhaemoglobin with its low-spin ext{Fe}^{2+} being a good example.

The switch between these states during hematological processes, such as when oxygen binds or is released, results in a slight change in the iron ion size within haemoglobin.

Haemoglobin Structure

Haemoglobin is a complex protein critical to transporting oxygen in the blood from the lungs to other parts of the body. Its structure is made up of four subunits: two alpha and two beta chains.
Each subunit carries a vital component called the heme group, which holds the iron ion. The total of four heme groups allows each haemoglobin molecule to bind up to four oxygen molecules.
This multi-subunit structure allows haemoglobin to pick up oxygen efficiently and deliver it to tissues needing oxygen for metabolism. This flexibility is key to the protein’s function.

Heme Group

The heme group is a critical component of haemoglobin and is responsible for its oxygen-carrying capacity. It consists of an iron ion ( ext{Fe}^{2+} ) centrally located within a large heterocyclic organic ring known as porphyrin.
The heme is what allows oxygen to bind to haemoglobin in red blood cells. Notably, the heme is a prosthetic group, meaning it is permanently associated with the protein but is not composed of amino acids.
It distinguishes haemoglobin from many other proteins, providing a site where the molecule can bind to oxygen effectively.