<Dd> t . Cytoskeleton </Dd> <P> A cell wall is a structural layer surrounding some types of cells, just outside the cell membrane . It can be tough, flexible, and sometimes rigid . It provides the cell with both structural support and protection, and also acts as a filtering mechanism . Cell walls are present in most prokaryotes (except mycoplasma bacteria), in algae, plants and fungi but rarely in other eukaryotes including animals . A major function is to act as pressure vessels, preventing over-expansion of the cell when water enters . </P> <P> The composition of cell walls varies between species and may depend on cell type and developmental stage . The primary cell wall of land plants is composed of the polysaccharides cellulose, hemicelluloses and pectin . Often, other polymers such as lignin, suberin or cutin are anchored to or embedded in plant cell walls . Algae possess cell walls made of glycoproteins and polysaccharides such as carrageenan and agar that are absent from land plants . In bacteria, the cell wall is composed of peptidoglycan . The cell walls of archaea have various compositions, and may be formed of glycoprotein S - layers, pseudopeptidoglycan, or polysaccharides . Fungi possess cell walls made of the N - acetylglucosamine polymer chitin . Unusually, diatoms have a cell wall composed of biogenic silica . </P> <P> A plant cell wall was first observed and named (simply as a "wall") by Robert Hooke in 1665 . However, "the dead excrusion product of the living protoplast" was forgotten, for almost three centuries, being the subject of scientific interest mainly as a resource for industrial processing or in relation to animal or human health . </P>

The main component of the cell wall is
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