Amino acids contain an acidic (carboxyl group) and a basic (amino) group within the same molecule. In aqueous solution, they neutralize each other. The carboxyl group loses a proton while the amino group accepts it. S a result, A dipolar or zwitter-ion is formed.

In zwitter ionic form, the amino acid show amphoteric behaviour as they react with both acids and bases. In the acidic medium, COO^- ion of the zwitter-ion accepts a proton to form the cation first, while in the basic medium,⁺NH₃ ion loses a proton to form the anion, as shown below:-

Thus, amino acids show amphoteric behaviour.