= Cell wall
{wiki}
A <cell wall> is a supporting structure outside a <cell membrane>. It helps preserve shape and bears the load when <osmosis> raises internal <pressure>. The <cell membrane> supplies the <selective permeability> described in its own article; the <cell wall> supplies mechanical reinforcement. These functions cooperate: a porous supporting layer can restrain expansion without being the boundary that retains dissolved <solutes>. <Turgor pressure> makes this cooperation visible in a firm leaf; <plasmolysis> and a <lysozyme wall-removal experiment> separate the two functions experimentally.<Biology 2e by Mary Ann Clark and others/5.2 Passive Transport>[Biology 2e, §5.2, Tonicity in Living Systems, Figure 5.13: wall support and plasmolysis]{ref}
The material is not universal. Most <bacteria> use <peptidoglycan>, whose linked <sugar> chains and <amino acid> cross-links are explained below. A <plant cell wall> uses <cellulose>; a <fungal cell wall> contains <chitin> and <glucans>. <Animals> lack a <cell wall>, some <bacteria> are naturally wall-free, and <archaea> have different wall chemistry. Thus a <cell wall> is neither a universal cellular component nor one chemical recipe.<Biology 2e by Mary Ann Clark and others/22.2 Structure of Prokaryotes: Bacteria and Archaea>[Biology 2e, §22.2, Figure 22.13 and The Cell Wall of Prokaryotes: wall-free bacteria and archaeal differences]{ref}<Biology 2e by Mary Ann Clark and others/4.3 Eukaryotic Cells>[Biology 2e, §4.3, Animal Cells versus Plant Cells and The Cell Wall: cellulose and animal comparison]{ref}<Biology 2e by Mary Ann Clark and others/24.1 Characteristics of Fungi>[Biology 2e, §24.1, Cell Structure and Function: chitin and glucans]{ref}
\Image[/cell-envelope-comparison.png]
{title=Representative <cell envelopes>: different supporting layers outside the <cell membrane>}
{description=Original layer-order schematic comparing <Gram-positive bacteria>, <Gram-negative bacteria>, a <plant cell wall>, a <fungal cell wall>, a member of <archaea> with a protein wall and an <animal> cell. Colors distinguish a <cell wall> from a <cell membrane> and a <bacterial outer membrane>. Thicknesses and spacing are illustrative, not measurements. These examples do not exhaust variation within any group.}
The <bacterial outer membrane> in typical <Gram-negative bacteria> is an additional membrane outside the <peptidoglycan>, not another name for that supporting mesh. The <cell envelope> includes the boundary layers together. An extra barrier can change access to the <peptidoglycan>, so response to <lysozyme> cannot be inferred from the mere presence of a <cell wall>.<Biology 2e by Mary Ann Clark and others/22.2 Structure of Prokaryotes: Bacteria and Archaea>[Biology 2e, §22.2, Figure 22.16: inner membrane, peptidoglycan and outer membrane]{ref}
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