A cell wall is a more or less solid layer surrounding a cell. They are found in bacteria, archea, fungi, plants, and algae. Animals and most other protists have cell membranes without surrounding cell walls. When a cell wall is removed using cell wall degrading enzymes, the plasma membrane surrounded cell is called a protoplast.

Plant cell walls

Plant cell walls have a number of functions: they provide rigidity to the cell for structural and mechanical support, maintaining cell shape, the direction of cell growth and ultimately the architecture of the plant. The cell wall also prevents expansion when water enters the cell. The term turgor is used to describe this pressure that is induced by excess water inside the plant cell. Cell walls protect agains pathogens and the environment and are a store of carbohydrate for the plant.

The primary cell wall, built by the plant first, is composed of cellulose microfibrils aligned at all angles. Microfibrils are held together by hydrogen bonds to provide a high tensile strength. Cell walls of neighbouring cells are held together by a shared gelatinous membrane called the middle lamella, which contains magnesium and calcium pectates (salts of pectic acid). Cells interact though plasmodesma(ta) which are cytoplasmic channels lined with plasma membrane that connect the protoplasts of adjacent cells across the cell wall.

In some plants, and cell types after a maximum size or point in development has been reached, a secondary wall is constructed below the primary wall. Unlike the primary wall, the microfibrils are aligned mostly in the same direction, and with each additional layer the orientation changes slightly. Cells with secondary cell walls are rigid. Cell to cell communication is possible though pits in the secondary cell wall which allow plamodesma to connect cells though the secondary cell walls.

Composition of plant cell walls

The major carbohydrates making up the primary cell wall are cellulose, pectin and hemicellulose. Insoluble cellulose fibres are meshed in to a matrix of called of pectin and hemicelluloses, they give the plant strength and support.

Plant cells walls also incorporate a number of proteins, the most abundant include hydroxyproline-rich glycoproteins (HRGP) also called the extensins, the arabinogalactan proteins (AGP), the glycine-rich proteins (GRPs), and the proline-rich proteins (PRPs). With the exception of glycine-rich proteins, all the previously mentioned proteins are glycosylated and contain hydroxyproline (Hyp) and they also contain highly repetitive sequences that can be shared between them, and chimeric proteins incorporating one or more domains of the exist. Most cell wall proteins are cross-linked to the cell wall and may have structural functions.

Secondary cell walls may contain lignin and suberin, making walls rigid.

The relative composition of carbohydrates, secondary compounds and protein varies between plants and between the cell type and age.

Prokaryotic cell walls

Cell walls of bacteria are primarily used for protection against hostile environments or, in the case of pathogenic bacteria, against the immune system of the host. They contain peptidoglycan, which can be made visible in Gram_positive bacteria by Gram staining. The cell walls of bacteria are also vital for containing the high osmotic pressure inside bacterial cells caused by the high concentration of solutes in the cytoplasm, which can often be as high as 15 atmospheres. Many antibiotics, including penicillin and its derivatives, target the cell wall of bacteria.

The cell walls of archaea are not made of peptidoglycan, but some archaea may contain pseudopeptidoglycan, which is composed of N-acetyltalosaminuronic acid, instead of N_acetyl muramic acid in peptidoglycan.

Fungal cell walls

Not all species of fungi have cell walls but in those that do, the cell walls are composed of cellulose and chitin, the same carbohydrate that gives strength to the exoskeletons of insects. They serve a similar purpose to those of plant cells, giving fungal cells rigidity and strength to hold their shape and preventing osmotic lysis. It also limits the entry of molecules that may be toxic to the fungus, like plant produced and synthetic fungicides.

The composition, properties, and form of the fungal cell wall change during the cell cycle and depending on growth conditions.