Cell wall

A cell wall is a tough, fairly rigid covering that lies outside the plasma membrane. Plants, fungi, algae, many protists and most bacteria and archaea have one. Animal cells do not; they depend instead on an internal cytoskeleton and a flexible extracellular matrix of proteins and sugars.

Its main job is mechanical. Water tends to flow into cells by osmosis, and in dilute surroundings a cell without a wall can swell until it bursts. The wall resists that pressure, which makes plant cells firm, and it also sets the cell's shape and shields it from drying out and physical damage. What the wall is made of depends on the organism: cellulose in plants, chitin in fungi, silica in diatoms, and in bacteria peptidoglycan, a mesh of sugar chains cross-linked by short peptides. Archaea build their walls from other materials and never use peptidoglycan.

The bacterial wall matters in the lab and the clinic. The Gram stain sorts bacteria by wall structure: gram-positive species have a thick layer of peptidoglycan, while gram-negative species have a thin one covered by an outer membrane. Human cells contain no peptidoglycan, so antibiotics such as penicillin that block its assembly can weaken bacteria while leaving our own cells unharmed. Bacteria that lack a wall, such as Mycoplasma, are naturally resistant to these drugs.