Because amino acids contain both carboxy and amino groups in their molecules, they are amphoteric in nature, i.e., they behave both as acids and bases. Amino acids actually exist as inner salts, called zwitterions. A zwitterionic structure is possible for amino acids because the amino group is basic in nature and can accept a proton from the acidic carboxy group. A zwitterion can be represented as shown below.

The highly polar nature of zwitterion allows the formation of strong crystal lattices similar to the ionic compounds. Amino acids, therefore, resist conversion from solid to liquid state and do not melt but decompose on heating.
The zwitterionic nature is also reflected in their higher solubility in water and low solubility in nonpolar solvents. In addition to the above observations, large dipole moments also indicate the zwitterionic nature of amino acids.
Let us now study the zwitterionic form of amino acids in more detail. You can see in the zwitterion shown above that the amino group is protonated and the carboxy group exists as carboxylate anion. Thus, the acidic group is a substituted ammonium ion and the basic group is the carboxylate anion. As a result in strongly acidic medium i.e., at low pH, the carboxylate group will be protonated to yield the following species.

Let us next consider the species present in strongly basic medium, i.e., at higher pH of the solution. Under these conditions, the proton will be removed from the NH₃⁺ group to yield the following species.

Thus, we can write a combined equation for the acid-base behaviour of the amino acids as shown below:

You can see that a low pH, species I has a net positive charge and has two acidic sites (NH₃⁺ and COOH). On the other hand, at high pH, species II has a net negative charge and has two basic sites (NH₂ and COO–). It is clear from the above equation that the amino group will be first protonated and then the carboxylate anion. Also, at some intermediate pH, the amino acid exists as a zwitterion with no net charge. The pH at which this occurs is known as isoelectric point, pHi of the amino acid. At this pH, the amino acid is stationary in an electric field i.e., it migrates neither to the negative pole nor to the positive pole because the charges on it are balanced.