A class of chemicals containing both an amino and an acidic group in the molecule. They exhibit properties of amines and acids depending on the pH of the solution they are in. Amino acids are present in the uncombined form in tissues, and when combined with each other constitute the building blocks of proteins and peptides. Amino acids and peptides contribute to the flavour of fish flesh. Some have a sweet taste, others a bitter taste. Glutamic acid imparts a meaty flavour in the presence of other amino acids.