The correct option is As bicarbonate.
Explanation
Carbon dioxide ($CO_2$) produced by cellular metabolism is transported by the blood to the lungs for elimination. This transport occurs through three primary mechanisms: in a dissolved state, bound to haemoglobin, and as bicarbonate ions. The distribution among these methods is determined by the chemical properties of $CO_2$ and enzymatic activity within red blood cells.
Detailed Analysis:
- As Bicarbonate (Correct): This is the dominant mode of transport, accounting for approximately 70% of total $CO_2$. Inside red blood cells (RBCs), the enzyme carbonic anhydrase catalyzes the reaction between $CO_2$ and water to form carbonic acid ($H_2CO_3$). This acid is unstable and rapidly dissociates into hydrogen ions ($H^+$) and bicarbonate ions ($HCO_3^-$). The bicarbonate ions then diffuse from the RBCs into the plasma.
- As Carbamino-haemoglobin: Approximately 20-25% of $CO_2$ is transported bound to the amine group of the globin protein in haemoglobin. This complex is chemically distinct from carboxyhaemoglobin (which involves carbon monoxide).
- Dissolved in Plasma: Carbon dioxide is more soluble in water than oxygen. Approximately 7% of $CO_2$ is transported simply dissolved in the blood plasma.
- As Carbonic Acid: While carbonic acid is an intermediate product in the formation of bicarbonate, it dissociates almost immediately at physiological pH. Therefore, it does not act as a significant reservoir for transport.
Key Takeaway:
The majority of carbon dioxide in the blood is transported as bicarbonate ions ($HCO_3^-$) due to the catalytic action of carbonic anhydrase in red blood cells.