Afunctional group on an amino acid that is polar and can become positively charged:

Amino group (NH3)

Explanation:

The amino acids have a central carbon atom to which four functional groups are bonded. These are namely a carboxyl group (COOH), an amino group (NH3), a hydrogen atom and an R group (it varies for different amino acids).

Amino group (NH3) is common to all the standard 20 amino acids. NH3 is a polar group since higher electronegativity of nitrogen atom allows it to pull the shared electrons of the covalent bonds towards itself. It makes the nitrogen atom of amino group partially negative and the hydrogen atoms carry a partial positive charge (a dipole is present).

Amino group (NH3) accepts one proton and becomes positively charged (NH4+).