These symbols indicate either the presence of certain chemical properties in the amino acid side chain or whether humans must obtain the amino acid in the diet (essential) or not (nonessential). Symbol Meaning   Side Chain Polarity The side chain is nonpolar and hydrophobic The side chain is polar and hydrophilic The side chain contains a full positive charge The side chain contains a full negative charge   Side Chain Functional Groups The side chain is aliphatic, consisting of saturated hydrocarbon The side chain contains a hydroxyl group The side chain contains sulfur The side chain contains an amide group   Side Chain Ring Systems The side chain is straight chain, containing no ring systems The side chain contains an aromatic ring system The residue is a secondary amino acid   Dietary Requirement The amino acid is nonessential, and need not be included in the human diet The amino acid is essential, and must be included in the human diet