Final answer:
The enzyme carbonic anhydrase in red blood cells catalyzes the conversion of carbon dioxide to carbonic acid, which then forms bicarbonate ions as a part of the bicarbonate buffer system.
Step-by-step explanation:
The enzyme within red blood cells that catalyzes the conversion of unbound carbon dioxide (CO₂) to carbonic acid (H₂CO₃) is carbonic anhydrase (CA). This enzyme is crucial in the bicarbonate buffer system for transporting CO₂ from tissues to the lungs. Carbonic anhydrase accelerates the reaction CO₂ + H₂O ⇒ HCO₃- + H+, where carbonic acid spontaneously dissociates into a bicarbonate ion (HCO₃-) and a hydrogen ion (H+). The bicarbonate ion is then exchanged for a chloride ion in a process called the chloride shift, assisting in maintaining the acid-base balance in blood.
The reaction occurs quickly enough to allow continuous uptake of CO₂ into the blood and to keep blood pH stable, as hemoglobin binds free H+ ions. When blood reaches the lungs, carbonic anhydrase facilitates the reconversion of bicarbonate into carbon dioxide, which is then exhaled.