Problem 96 - Objective Biology for NEET

Question

The transport of \(\mathrm{CO}_{2}\) by the blood is primarily dependent on the (a) Solubility of \(\mathrm{CO}_{2}\) in blood. (b) Presence of carbonic anhydrase in RBCs. (c) Ability of haemoglobin to bind and transport \(\mathrm{CO}_{2}\). (d) Ability of other blood proteins.

Step-by-Step Solution

Answer

The primary factor for the transport of \(\mathrm{CO}_{2}\) by blood is the presence of carbonic anhydrase in RBCs (Option b).

Step 1: Understanding the role of \(\mathrm{CO}_{2}\) solubility in blood

While it's true that the solubility of \(\mathrm{CO}_{2}\) in blood allows \(\mathrm{CO}_{2}\) to be carried in the bloodstream, it isn't the primary factor. Therefore, option (a) can be ruled out.

Step 2: Examining the role of carbonic anhydrase in RBCs

Carbonic anhydrase accelerates the conversion of \(\mathrm{CO}_{2}\) and water to bicarbonate (\(\mathrm{HCO}_{3}^{-}\)) and hydrogen ions (\(\mathrm{H}^{+}\)). This process facilitates the transport of \(\mathrm{CO}_{2}\) in the form of bicarbonate ions. Thus, option (b) is a potentially correct answer.

Step 3: Considering the ability of haemoglobin to bind and transport \(\mathrm{CO}_{2}\)

Haemoglobin can indeed bind \(\mathrm{CO}_{2}\) and transport it in the blood. However, this method of transport accounts for only about 20% of the \(\mathrm{CO}_{2}\) carried in the blood, so it's not the primary factor. As a result, option (c) can be discarded.

Step 4: Assessing the ability of other blood proteins

Although other proteins in the blood can bind \(\mathrm{CO}_{2}\), their role in transporting \(\mathrm{CO}_{2}\) isn't significant enough to be considered the primary factor. Hence, option (d) can be ruled out.

Key Concepts

Carbonic Anhydrase

Carbonic anhydrase is an enzyme found in red blood cells ( RBCs ). It plays a crucial role in the process of transporting carbon dioxide ( CO_2 ) in the blood. This enzyme speeds up the conversion of CO_2 into bicarbonate ions, a major mechanism of CO_2 transport.

Inside the RBCs, carbonic anhydrase catalyzes the reaction between CO_2 and water, producing bicarbonate ions ( HCO_3^- ) and hydrogen ions ( H^+ ). This reaction happens very fast, allowing a substantial amount of CO_2 to be transformed into bicarbonate. Since bicarbonate is a soluble form easily carried by plasma, this enzymatic activity is essential for effective CO_2 transport.

• Rapid conversion: The enzyme allows quicker conversion of waste CO_2 into bicarbonate.
• Important in buffering: Helps maintain pH balance in the blood as bicarbonate can also act as a buffer.

Without carbonic anhydrase, the efficiency of CO_2 transport in the blood would drastically reduce, highlighting its importance in respiratory physiology.

Bicarbonate Ions

Bicarbonate ions play a critical role in the transport of carbon dioxide in the blood. Once carbonic anhydrase catalyzes the formation of bicarbonate ions and hydrogen ions from CO_2 and water, bicarbonate ions primarily circulate in the plasma.

The majority of CO_2 in the blood is carried in the form of bicarbonate ions. They are more soluble in blood compared to gaseous CO_2 , which allows for efficient transport from body tissues to the lungs.

• Exchange with chloride ions: This happens via the "chloride shift" or the "Hamburger effect". Bicarbonate ions are exchanged with chloride ions in the red blood cells to maintain electric neutrality.
• Paper in buffering: Bicarbonate acts as a major buffer, helping maintain pH balance, resisting acidic changes in the bloodstream.

This buffering role is vital for preventing acidosis during transportation, making bicarbonate ions indispensable in respiratory and systemic physiological processes.

Haemoglobin Binding

Haemoglobin, well-known for carrying oxygen in blood, also assists in carbon dioxide transport. It binds to CO_2 , forming what is known as carbaminohaemoglobin, although this accounts for only about 20% of total CO_2 transport.

While this is a secondary method of transport compared to bicarbonate formation, it is still significant.

• Binding sites: CO_2 binds to the amino groups on the globin chains of haemoglobin.
• Bohr Effect: Helps release oxygen to tissues; as haemoglobin binds CO_2 , it releases some of the oxygen it's carrying, facilitating oxygen delivery where it's needed most.

This characteristic underlines haemoglobin's dynamic role in not only oxygen transport but also CO_2 transport, aiding in maintaining efficient respiration.