Cell Wall

A cell wall is a rigid layer outside the plasma membrane that gives a cell its shape and protects it from bursting. It is found in plants, fungi, algae, bacteria and most archaea, but not in animal cells, and its chemistry differs between these groups.

The primary wall of a plant cell is built from fibers of cellulose embedded in a matrix of hemicellulose and pectin. Cells that need extra support, such as those of wood, add a thick secondary wall strengthened with lignin. A sticky layer, the middle lamella, glues neighboring cells together. The wall resists the turgor pressure of the cell, and the first cells ever seen, in cork by Robert Hooke in 1665, were actually empty walls.

Fungal walls contain chitin and glucans, and some antifungal drugs target them. Bacterial walls are made of peptidoglycan, a mesh of sugars and short peptides that the antibiotic penicillin attacks, and Gram-negative bacteria add an outer membrane. Archaea have no peptidoglycan, and many have a protein surface layer or a different polymer, one of the differences between archaea and bacteria.

The wall controls the rate and direction of growth, because cells expand when it is loosened. It also acts as a filter for molecules, as a defense against pathogens, and as the source of fibers for paper and textiles. Animal cells instead rely on the extracellular matrix, and the plasma membrane lies directly beneath the wall in the cell membrane.

• Cellulose — the main plant wall fiber
• Peptidoglycan — the bacterial wall polymer
• Turgor Pressure — the force the wall resists