Final answer:
Amino acids can exist as zwitterions or ions depending on the pH value of the solution. At a certain pH value, most amino acid molecules exist as zwitterions, which have both a positive and a negative charge. The addition of acid or base can change the charge of the amino acid.
Step-by-step explanation:
An amino acid can exist in two states: as a zwitterion or as an ion. A zwitterion is a species that has both a positively charged group and a negatively charged group. In solution, at a certain pH value, most amino acid molecules exist as zwitterions. When an acid is added, the carboxylate group captures a hydrogen ion, resulting in a positively charged amino acid. When a base is added, the amino group removes a hydrogen ion, producing a negatively charged amino acid.