Tryptophan, polarised light micrograph. Tryptophan is an essential amino acid used in the body in the biosynthesis of proteins. Tryptophan cannot be manufactured by the body but must be supplied through the diet. Tryptophan residues help to anchor membrane proteins within the cell membrane. It is a biochemical precursor for serotonin, melatonin, niacin and auxin. Magnification: x70 when printed 10cm wide.