How do you explain amphoteric nature of amino acids?

Explanation

Amino acids show amphoteric nature i.e., they behave both as an acid and as a base. This can be explained as follows:
Amino acids contain an acidic (carboxyl group)a and basic (amino group) group in the same molecule. In aqueous solution, they neutralize each other. The carboxyl group loses a proton while the amino group accepts it. As a result, a dipolar or zwitter ion is formed.
`underset(alpha-"Amino acid")(H_2 N - underset(R)underset(|)CN-COON) hArr underset("Zwitter ion")(H_3 overset(+)N - underset(R)underset(|)CH-COO^-)`
In zwitter ionic form, `alpha`-amino acids show amphoteric behaviour. In the acidic medium, `-COO^-` ion of the zwitter ion accepts a proton to form the cation (I) while in the basic medium, `overset(+)NH_3` ion-loses a proton to form the anion (II).

Thus, in dipolar form `-overset(+)NH_3` group acts as the acid while `-COO^-` group acts as the base.