Amino acids are organic molecules that contain both an amine group (–NH₂) and a carboxyl group (–COOH), along with a variable side chain known as the R group. Each amino acid is primarily composed of carbon, hydrogen, oxygen, and nitrogen, while some also contain additional elements in their side chains. Although around 500 amino acids have been identified, only 20 are encoded by the genetic code. Amino acids can be classified using different criteria based on their structure and properties.