Questions: How is most (70%) of the carbon dioxide transported in blood? - bound to the protein portion of hemoglobin - bound to the iron/heme of hemoglobin - as bicarbonate ion (HCO3-) in plasma - carbon dioxide (CO2) gas dissolved in plasma

The answer is the third one: as bicarbonate ion (HCO₃⁻) in plasma.

Explanation for each option:

• Bound to the protein portion of hemoglobin: This option is incorrect. While some carbon dioxide is transported in the blood bound to hemoglobin, it is not the majority. This form is known as carbaminohemoglobin, and it accounts for a smaller portion of CO₂ transport.

• Bound to the iron/heme of hemoglobin: This option is incorrect. Carbon dioxide does not bind to the iron/heme portion of hemoglobin. Instead, oxygen binds to the iron/heme group.

• As bicarbonate ion (HCO₃⁻) in plasma: This option is correct. The majority of carbon dioxide in the blood is transported in the form of bicarbonate ions. CO₂ diffuses into red blood cells, where it is converted to carbonic acid by the enzyme carbonic anhydrase, and then dissociates into bicarbonate ions and hydrogen ions. The bicarbonate ions are then transported in the plasma.

• Carbon dioxide (CO₂) gas dissolved in plasma: This option is incorrect. While a small amount of carbon dioxide is dissolved directly in the plasma, it is not the primary method of transport.

In summary, most carbon dioxide is transported in the blood as bicarbonate ions in the plasma.