Amino acids are divided into 5 based on their side chains: aliphatic non-polar, neutral polar, negatively charged, positively charged, and aromatic. based on the body's ability to synthesize, there are essential amino acids (the body cannot produce) and non-essential (the body can make). examples of essential amino acids: phenylalanine, valine, methionine, lysine. examples of non-essential amino acids: alanine, arginine, serine, glycine, proline, thyrozine, glutamic acid.