So one negative charge enters, and one negative charge leaves. Now, once our bicarbonate is inside our red blood cell, only then can it basically interact with the H ion and form our carbonic acid. And only then can our carbonic anhydrase catalyze the formation of CO2 and water by breaking this carbonic acid down. So remember, the carbonic anhydrase enzyme is only found within our red blood cell. It is not found inside the blood plasma. And that's why this anion must first cross the membrane and enter the red blood cell before we can actually form our carbon dioxide.