Final answer:
Amino acids can act as both an acid and a base due to their structure, which contains both an acidic group (-COOH) and a basic group (-NH2). At a certain pH value, most amino acids exist as zwitterions, with both positive and negative charges. Adding acid or base to a solution containing zwitterions causes the amino acid to become positively or negatively charged, helping to maintain the pH of the system.
Step-by-step explanation:
Amino acids are able to show both acidic and basic properties because of their structure. They have an acidic group (-COOH) and a basic group (-NH2) that allows them to act as both an acid and a base. At a certain pH value, most amino acid molecules exist as zwitterions, which are compounds that have both positive and negative charges. When acid is added, the carboxylate group captures a hydrogen ion, giving the amino acid a positive charge. When base is added, the H* ion is removed from the amino group, resulting in a negatively charged amino acid. In both cases, the amino acid acts to maintain the pH of the system by removing the added acid or base from solution.