Cell Wall
The cell wall is a rigid outer layer outside the cell membrane in plants, fungi, and bacteria that gives the cell shape and support.
Definition
A cell wall is a stiff outer layer around some cells that gives them shape and protection; plant cells have one, animal cells do not. It is a rigid, porous layer outside the cell membrane in plants, fungi, algae, and bacteria, made mainly of cellulose in plants, chitin in fungi, and peptidoglycan in bacteria, and it provides support, guards against mechanical damage, and keeps the cell from bursting when water enters by osmosis. The plant wall is a composite of cellulose microfibrils (linear chains of beta-1,4-linked glucose) embedded in a matrix of hemicellulose and pectin, with lignin added to secondary walls for compressive strength and water resistance, and plasmodesmata cross it to connect neighboring cytoplasms. The wall is freely permeable to water and small solutes, so it does not regulate transport, but it resists turgor pressure and sets the direction of cell expansion. Bacterial peptidoglycan walls are the target of penicillin, which blocks the cross-linking enzymes.
Example
A stalk of celery stays crisp because each cell has a firm wall; when celery goes limp the cells have lost water, but the walls keep them from bursting when water returns. A plant cell placed in pure water swells until its wall pushes back, producing turgor pressure that can reach $0.5$ to $1.0\ \text{MPa}$, several times atmospheric pressure, while an animal cell in the same water has no wall and bursts. Cellulose is the most abundant organic polymer on Earth: a single molecule may contain $10{,}000$ glucose units, and bundles of these chains have a tensile strength comparable to steel by weight, which is why cotton and paper are durable.
Key Insight
The cell wall sits outside the membrane like a box around a balloon; it supports the cell but does not decide what gets in, since the membrane does that. The wall is why plants can stand tall without a skeleton, and wood is mostly the thickened cellulose walls of dead cells. Humans cannot digest cellulose because we lack the enzyme cellulase, so plant cell walls pass through us as dietary fiber, while cows and termites rely on symbiotic microbes to break the beta linkage our enzymes cannot.