What substance forms the tough cell walls of most fungi?

The substance forming the tough cell walls of most fungi is chitin.

Chitin is a tough, nitrogen-containing polysaccharide built from repeating units of N-acetylglucosamine. In fungal walls it works with glucans and structural proteins to provide strength, shape and protection. This wall surrounds the plasma membrane and helps fungal cells resist mechanical stress and changes in water pressure.

Chitin is also found in the exoskeletons of arthropods such as insects, crabs and lobsters, making it a useful comparison: fungi and arthropods are not closely related, but both use this durable polymer for structural support. Fungal cells also differ from plant cells because their walls do not primarily use cellulose.

The wording “most fungi” matters. True fungi generally have chitin-rich walls, but fungus-like organisms such as oomycetes are not true fungi and typically have cellulose- and glucan-based walls instead. Lignin is associated mainly with woody plant tissues, keratin is a structural protein in animals, and cellulose is the characteristic wall polymer of plants.