The correct option is
A They act as both acids as well as bases
Amino acids are said to be amphoteric in nature because they act as both acids as well as bases as they contain acidic COOH group and basic
NH2 group. When an amino acid is dissolved in water, at a particular pH, it exists in solution as dipolar ion or zwitterion. They can react with both acids and bases to give salts. The ionisable nature of
NH2 and COOH groups can cause them to form
NH+3 and
COO− respectively.
At low pH, the carboxylic group in a zwitterion accepts a proton and becomes uncharged, so that the overall charge on the molecule is positive (cation). At high pH, a proton is released from
NH+3 group of the zwitterion and becomes uncharged, so that the overall charge on the molecule is negative (anion).