Questions: Which of the following is true regarding the transport of carbon dioxide? Most CO2 is transported dissolved in the blood plasma. Most CO2 is transported attached to hemoglobin. Most CO2 is transported in plasma as bicarbonate ions. Most CO2 is transported as chloride ions. Most CO2 is transported attached to the heme portion of hemoglobin.

The answer is the third one: Most $\mathrm{CO}_{2}$ is transported in plasma as bicarbonate ions.

Explanation for each option:

1. Most $\mathrm{CO}_{2}$ is transported dissolved in the blood plasma.

   • This is incorrect. While some $\mathrm{CO}_{2}$ is dissolved in the blood plasma, it is not the primary method of transport.

2. Most $\mathrm{CO}_{2}$ is transported attached to hemoglobin.

   • This is incorrect. Although some $\mathrm{CO}_{2}$ binds to hemoglobin to form carbaminohemoglobin, it is not the main form of transport.

3. Most $\mathrm{CO}_{2}$ is transported in plasma as bicarbonate ions.

   • This is correct. The majority of $\mathrm{CO}_{2}$ in the blood is converted to bicarbonate ions ($\mathrm{HCO}_3^-$) for transport in the plasma.

4. Most $\mathrm{CO}_{2}$ is transported as chloride ions.

   • This is incorrect. $\mathrm{CO}_{2}$ is not transported as chloride ions. The chloride shift is a process that helps maintain ionic balance during the transport of $\mathrm{CO}_{2}$, but $\mathrm{CO}_{2}$ itself is not transported as chloride ions.

5. Most $\mathrm{CO}_{2}$ is transported attached to the heme portion of hemoglobin.

   • This is incorrect. $\mathrm{CO}_{2}$ does not bind to the heme portion of hemoglobin; it binds to the globin part of the molecule.

Summary: Most $\mathrm{CO}_{2}$ is transported in plasma as bicarbonate ions.