How can amino acids act as both acids and bases?

Amino acids are amphoteric, meaning they have both acidic and basic properties. An animo acid has at least one primary amine group, NH2, and at least one carboxyl group, COOH. In an acidic environment where the concentration of protons in solution is high, protons from the solution are donated to the amine group. A dative covalent bond is formed between the nitrogen atom and the proton, forming a cation with a both +1 charge and an NH3 group. As such, the amino acid has accepted a proton, acting as a base. In an alkaline environment the concentration of protons in the solution is low. The carboxyl group donates its hydrogen atom to form a positive H+ ion, or proton. This leaves an anion with a -1 charge and a COO group. Here the amino acid has donated a proton, acting as an acid. It is possible for both processes to happen at the same time. The PH value at which this occurs is called the isoelectric point. The species formed has both a positive and negative charge and is called a zwitterion.