Amino acids contain an acidic (carboxyl group) and a basic (amino group) group in the same molecule. In aqueous solution, they neutralise each other. The carboxyl group loses a proton while the amino group accepts it. As a result, a dipolar or zwitterion is formed.
`H_(2)N-underset(R )underset(|)(CH)-COOHrarrH_(3)overset(o+)(N)-underset("Zwitterion")underset(R )underset(|)(CH)-COO^(Theta)`
In zwitterionic form, `alpha-`amino acids show amphoteric behaviour as they react with both acids and bases. In the acidic medium, the `COoverset(Theta)(O)` ion of the zwitterion accdepts a proton to form the cation `(I)` while in the basic medium, the `overset(o+)(N)H_(3)` ion loses a proton to form the anion `(II)`.
Thus, the `overset(N)H_(3)` group acts as the acid while the `COoverset(Theta)(O)` group acts as the base.
