Final answer:
Amino acids exist as zwitterions due to their amphoteric nature, containing both acidic and basic functional groups, which allows them to maintain equilibrium by capturing or releasing protons to buffer changes in pH.
Step-by-step explanation:
Amino acids exist as zwitterions in the solid state and in aqueous solution due to the presence of both a basic amino group (-NH3+) and an acidic carboxyl group (-COO−) within the same molecule. This dual nature makes amino acids amphoteric molecules, capable of acting as either an acid or a base. At the isoelectric point (pI), the amino acid has no net charge, because the positive and negative charges balance each other out. The zwitterionic form is stabilized by ionic interactions, similar to inorganic salts, which accounts for the high melting and decomposition points of amino acids.
When an acid is added to a zwitterion, the carboxylate group captures a hydrogen ion, becoming positively charged. Conversely, when a base is added, a hydrogen ion is removed from the amino group, resulting in a negative charge. In both scenarios, amino acids help maintain the pH by capturing or releasing protons.