A cell wall is a relatively rigid extracellular structure surrounding the plasma membrane of many cells. It provides mechanical support, helps maintain cell shape and can protect cells against physical stresses and excessive water uptake. Cell walls occur in plants, fungi, many algae and most bacteria, although their chemical composition differs substantially among these groups. In plants, the principal structural polysaccharide is cellulose, embedded within a matrix containing hemicelluloses and pectins.
Fungal cell walls contain substantial amounts of chitin and other polysaccharides, while bacterial cell walls commonly contain peptidoglycan. Because these structures have different compositions and evolutionary origins, cell wall is a functional term rather than the name of one universal material. Cell walls are external to the plasma membrane and are distinct from the cell membrane itself, which is a selectively permeable lipid bilayer.
The plant cell wall is organised into several layers and specialised regions. A young plant cell commonly produces a primary cell wall containing cellulose microfibrils embedded in a matrix of hemicellulose and pectin. Adjacent plant cells are connected through a middle lamella rich in pectic substances, helping cement neighbouring cells together. Some mature plant cells develop a secondary cell wall inside the primary wall.
Secondary walls can contain additional cellulose and hemicellulose and may become impregnated with lignin, producing substantial strength and resistance to compression. The relative composition and arrangement of these materials vary among tissues. Wood, for example, derives much of its mechanical strength from thick secondary cell walls containing cellulose, hemicellulose and lignin.
Cellulose provides much of the tensile strength of plant cell walls. It consists of long chains of glucose linked through beta one four glycosidic bonds. These chains associate through extensive hydrogen bonding to form cellulose microfibrils. Hemicelluloses interact with cellulose and help organise the wall matrix, while pectins contribute to hydration, adhesion and mechanical properties. Lignin is a complex aromatic polymer deposited particularly in secondary walls and is important for rigidity, water transport and resistance to degradation.
The resulting material is a composite rather than a simple layer of cellulose. The organisation of polymers at microscopic and molecular scales determines the mechanical properties of different tissues. Plants can therefore construct walls that are flexible in growing tissues, highly reinforced in supporting tissues or specialised for transport and protection.
One major function of the cell wall is to resist internal pressure. Water entering a plant cell by osmosis increases the pressure of the cell contents against the wall. This internal pressure, called turgor pressure, contributes to the firmness of nonwoody plant tissues. Without a sufficiently strong wall, water uptake could cause excessive expansion or rupture of the plasma membrane. The wall therefore works together with the plasma membrane and vacuole to regulate the physical state of the cell. When plant cells lose water, turgor pressure decreases and tissues can become limp. Cell walls do not actively pump water or determine osmotic balance by themselves, but they provide the mechanical framework that allows cells to tolerate the pressure generated by water movement.
Cell walls also contribute to communication and transport between plant cells. Plant cells are connected by microscopic channels called plasmodesmata, which cross the cell wall and allow controlled movement of certain molecules and signals between neighbouring cells. The wall itself is porous at the molecular scale and can permit movement of water and small solutes through the apoplast, although the precise permeability depends on wall composition and tissue. Plants can modify cell wall properties during growth, defence and development. Enzymes can alter polysaccharide cross linking and wall structure, allowing expansion in growing cells. Other modifications can strengthen walls after pathogen attack or mechanical injury. Cell walls are therefore dynamic structures rather than completely inert shells.
Bacterial cell walls have a different organisation. Most bacteria possess peptidoglycan, a polymer consisting of sugar chains cross linked by short peptides. Gram positive bacteria generally have a thick peptidoglycan layer, while Gram negative bacteria have a thinner peptidoglycan layer located within a more complex envelope that includes an outer membrane containing lipopolysaccharide. These structural differences are important in bacterial classification, antibiotic susceptibility and immune recognition. Some antibiotics interfere with peptidoglycan synthesis, exploiting the fact that bacterial cell wall construction differs from animal cell biology. Fungal cell walls instead contain chitin and glucans, making them chemically distinct from both plant and bacterial walls.
Cell walls provide protection against mechanical damage and environmental stresses, but they also create constraints. A rigid wall can limit rapid changes in cell shape and movement, so organisms with cell walls have evolved mechanisms for controlled growth and remodelling. Plant cells expand through coordinated loosening and rebuilding of their walls, often involving enzymes and changes in turgor pressure. Secondary walls can become extremely strong but generally reduce the capacity for further expansion. In multicellular organisms, different cell wall architectures allow tissues to perform different functions, from flexible leaf tissues to highly reinforced vascular and support tissues. The wall therefore contributes both strength and developmental organisation.
A cell wall should not be confused with a cell membrane. The plasma membrane is a thin, flexible lipid bilayer containing proteins and is responsible for selective transport, signalling and maintenance of internal conditions. The cell wall is an external structural matrix that provides mechanical support and protection. Animal cells normally lack cell walls, which contributes to their ability to change shape and participate in specialised forms of movement.
Plants, fungi and bacteria instead use chemically different wall structures suited to their biological needs. Understanding this distinction is important because the two structures cooperate but perform fundamentally different functions. Cell walls are therefore major determinants of cell shape, environmental resistance, tissue architecture and, in microorganisms, susceptibility to particular antimicrobial treatments.
