A Level Biology Practice Exam

In what form is the majority of carbon dioxide transported in the blood?

1. As dissolved CO2 in plasma

2. As bicarbonate ions

3. As carbamino hemoglobin

4. It is not transported; it is solely exhaled

The correct answer is: As bicarbonate ions

The majority of carbon dioxide transported in the blood is in the form of bicarbonate ions. When carbon dioxide enters the blood from the tissues, it interacts with water, facilitated by the enzyme carbonic anhydrase, to form carbonic acid (H2CO3). This acid quickly dissociates into bicarbonate ions (HCO3-) and hydrogen ions (H+). This bicarbonate is then transported in the plasma, playing a crucial role in maintaining the acid-base balance in the body. This method of transport is efficient and allows for a high concentration of CO2 to be carried without significantly affecting the pH of the blood. While some carbon dioxide is indeed dissolved in plasma and a smaller portion binds to hemoglobin as carbamino compounds, the predominant form in which carbon dioxide travels through the bloodstream is as bicarbonate ions. The incorrect answer option suggesting that CO2 is not transported at all fails to acknowledge that a significant portion is indeed transported in various forms, predominantly as bicarbonate.