Protein makes up the structure of all cells and tissues in the body including muscle tissue internal organs tendons skin hair and nails the 20 amino acids are the building blocks of protein 12 of these amino acids can be made in the body so are referred to as non essential amino acids whilst the other 8 are known as essential amino acids as they must be supplied in the diet phenylalanine is an essential amino acid protein makes up the structure of all cells and tissues in the body inc