How is carbon dioxide transported by hemoglobin?

• A. CO2 binds to the heme iron to form carbaminohemoglobin
• B. CO2 binds to the plasma only
• C. CO2 binds to the amino groups of hemoglobin to form carbaminohemoglobin
• D. CO2 is transported only as bicarbonate in plasma

Answer: (C)

The key idea is that carbon dioxide can hitch a ride on the hemoglobin protein itself, not on the iron of the heme. CO2 diffuses into red blood cells and binds to the amino terminal groups of the globin chains of hemoglobin, forming carbaminohemoglobin. This binding occurs on the protein portion and is reversible, allowing CO2 to be released in the lungs when it’s exhaled.

This is different from how CO binds to heme iron (which is a different molecule, carbon monoxide). Also, CO2 isn’t limited to plasma transport; a portion binds to hemoglobin as carbaminohemoglobin, while a larger portion is carried as bicarbonate in plasma after conversion by carbonic anhydrase inside red blood cells. So binding to amino groups on hemoglobin to form carbaminohemoglobin is the correct mechanism for this transport pathway.