Amino acids show amphoteric behaviour. Why?
In aqueous solutions, the carboxyl groups of amino acids lose a proton. Amino groups of amino acids, on the other hand, accept a proton, giving rise to a dipolar ion known as zwitter ion.
This is neutral, but contains both positive and negative charges. In the zwitter ionic form, amino acids show amphoteric behaviour as they react with both acids and bases.
In aqueous solution, the carboxyl group of an amino acid can lose a proton and the amino group can accept a proton to give a dipolar ion known as zwitter ion.
Therefore, in zwitter ionic form, the amino acid can act both as an acid and as a base.
Thus, amino acids show amphoteric behaviour.