ZWITTER ION 

(i)    Amino acids contain both acidic carboxyl group -(COOH) and basic amino group in the same molecules. 
(ii)    In aqueous solution, the acidic carboxyl group can lose a proton and basic amino group can gain a proton in a kind of internal acid – base reaction.

(iii)    The product of this internal reaction is called a Dipolar or a Zwitter ion. 
(iv)    The Zwitter ion is dipolar, changed but overall electrically neutral and contain both a positive and negative charge. 
(v)    Amino acid in the dipolar ion form are amphoteric in nature. 
(vi)    Depending upon the pH of the solution, the amino acid can donate or accept proton.

Solved Example.     In acidic medium, amino acids are present as

Solution: