Question

Carbon dioxide is transported in blood mainly as:

• A. Carbaminohaemoglobin
• B. Dissolved \(CO_2\)
• C. Bicarbonate ions
• D. Carbonic acid

Hint:

The enzyme Carbonic Anhydrase is essential for the rapid conversion of \(CO_2\) into bicarbonate. It is found in extremely high concentrations within Red Blood Cells.

Answer 1

The Correct Option is C

Step 1: Understanding the Concept:
Carbon dioxide (\(CO_2\)) is a waste product of cellular respiration. It is transported from the tissues to the lungs through the bloodstream in three different chemical forms.

Step 2: Detailed Explanation:
The distribution of \(CO_2\) transport is as follows:
1. As Bicarbonate ions (\(HCO_3^-\)): This is the major form, accounting for about \(70\%\) of \(CO_2\) transport. Within the RBCs, \(CO_2\) reacts with water to form carbonic acid (\(H_2CO_3\)) via the enzyme carbonic anhydrase. This acid then dissociates into bicarbonate and hydrogen ions.
2. As Carbaminohaemoglobin: About \(20-25\%\) of \(CO_2\) binds directly to the amino groups of the haemoglobin protein.
3. In Dissolved Form: About \(7\%\) of \(CO_2\) is transported simply dissolved in the plasma.

Step 3: Final Answer:
Carbon dioxide is transported mainly in the form of bicarbonate ions.

Answer 2

The dominant transport form of carbon dioxide can be worked out by following the chemical reaction it undergoes once it enters the blood, using the enzyme carbonic anhydrase found in red blood cells, and checking each option against that reaction.

1. Carbaminohaemoglobin: A portion of carbon dioxide does bind directly to the amino groups of haemoglobin without any chemical conversion, but this direct-binding route only accounts for a smaller share of the total, roughly one fifth to a quarter.
2. Dissolved CO2: Some carbon dioxide simply dissolves in plasma without reacting further, but even though carbon dioxide is far more soluble in water than oxygen is, this physically dissolved fraction remains the smallest of the three routes.
3. Bicarbonate ions: Inside red blood cells, carbonic anhydrase rapidly converts carbon dioxide and water into carbonic acid, which then dissociates into bicarbonate ions and hydrogen ions: \( CO_2 + H_2O \rightarrow H_2CO_3 \rightarrow HCO_3^- + H^+ \). The bicarbonate ion then diffuses out into the plasma to be carried to the lungs, and because this enzyme-driven reaction proceeds fast and continuously, it processes the largest share of carbon dioxide, around seventy percent.
4. Carbonic acid: This is only a fleeting intermediate in the reaction above, it does not accumulate or persist as a stable transport form because it dissociates almost immediately into bicarbonate and hydrogen ions.

Tracing the reaction pathway shows that the enzyme-catalysed conversion into bicarbonate ions handles the bulk of carbon dioxide transport, far more than direct binding or simple dissolving.

So the correct answer is Bicarbonate ions.