Which enzyme catalyzes the hydration of CO2 to bicarbonate in red blood cells?

• A. Lungs
• B. Red blood cells
• C. Plasma
• D. Kidney

Answer: (B)

The key idea is that a specific enzyme, carbonic anhydrase, rapidly converts carbon dioxide and water into carbonic acid, which quickly becomes bicarbonate and a proton. This speeding up happens inside red blood cells, where carbonic anhydrase sits in high concentration. Because of this, CO2 produced by tissues is efficiently transformed into bicarbonate for transport in the plasma, with chloride exchange maintaining charge balance. The lungs, plasma, and kidneys don’t perform this rapid hydration step; the lungs handle gas exchange and the reverse reaction to expel CO2, while plasma and kidney don’t provide the same enzyme activity to drive the hydration process. Hence, the correct choice points to red blood cells as the site where this enzyme catalyzes CO2 hydration.