Q.Name the enzyme that accelerate the following reaction :
CO2 + H2O ⇌ H2CO3 ⇌ HCO3⁻ + H⁺

The Problem: How Does Blood Carry CO₂?

Imagine you are a CO₂ molecule produced in a muscle cell. You need to get to the lungs to be breathed out. Blood plasma can dissolve some of you directly — but only about 5–10% of all CO₂ travels that way. The rest needs a chemical disguise.

The trick: turn CO₂ into bicarbonate ($HCO_3^-$), which dissolves beautifully in blood. But this transformation happens inside red blood cells (RBCs), not in plasma. And that creates a traffic problem — because bicarbonate needs to get out of the RBC and into the plasma to be carried to the lungs. That's where the chloride shift and carbonic anhydrase come in.

The Enzyme: Carbonic Anhydrase

Inside every RBC is an enzyme called carbonic anhydrase. It speeds up the reaction between CO₂ and water by a factor of about a million.

The first product is carbonic acid ($H_2CO_3$), which instantly dissociates into a hydrogen ion and bicarbonate. Without carbonic anhydrase, this reaction would be far too slow to handle the CO₂ your body produces every second.

The Problem of Bicarbonate Buildup

Now bicarbonate ($HCO_3^-$) is inside the RBC. It needs to enter the plasma. But the RBC membrane is not freely permeable to charged ions. So how does bicarbonate get out?

It swaps places with chloride ($Cl^-$). This exchange is called the chloride shift (or Hamburger phenomenon).

The Full Picture: Step by Step

1. CO₂ diffuses from tissues into RBCs.
2. Carbonic anhydrase converts CO₂ + H₂O into $H^+$ and $HCO_3^-$.
3. Bicarbonate concentration inside the RBC rises.
4. Bicarbonate exits the RBC via a membrane transport protein (band 3, or anion exchanger 1).
5. To keep the cell electrically neutral, one chloride ion enters the RBC for every bicarbonate that leaves.
6. The $H^+$ produced is buffered by hemoglobin (which also releases O₂ — the Bohr effect).

What Happens in the Lungs?

At the lungs, everything reverses:

• Bicarbonate re-enters the RBC in exchange for chloride (reverse chloride shift).
• Carbonic anhydrase drives the reverse reaction: $HCO_3^- + H^+ \rightarrow H_2CO_3 \rightarrow CO_2 + H_2O$. …