Keratin composes all parts of a feather and sometimes forms a thin flat strip that is similar to a barb, but termed a biot. A tough, fibrous protein molecule, keratin contains significant amounts of sulfur-containing amino acids, especially cysteine. Disulfide bridges formed between sulfur atoms in filaments of keratin, which are helical in shape, are largely responsible for the relative rigidity of the protein, which is insoluble in water and resistant to enzymes that are able to cleave most other protein molecules. Keratin does, however, breakdown when exposed to sulfuric acid, yielding the nitrogenous substances leucin and tyrosin. In addition to feathers, keratin is the primary structural component of hair, claws, horns, reptile scales, and baleens, the sieve-like structures some whales use to filter food particles from surrounding water.