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Joined 2026-09-21 Message
Incoming links: Turgor pressure
Cell wall 2026-10-13
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.[ref]
5.2 Passive Transport. Biology 2e, §5.2, Tonicity in Living Systems, Figure 5.13: wall support and plasmolysis
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.[ref]
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]4.3 Eukaryotic Cells. Biology 2e, §4.3, Animal Cells versus Plant Cells and The Cell Wall: cellulose and animal comparison
[ref]
Representative cell envelopes: different supporting layers outside the cell membrane
. 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. Details.
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.[ref]
22.2 Structure of Prokaryotes: Bacteria and Archaea. Biology 2e, §22.2, Figure 22.16: inner membrane, peptidoglycan and outer membrane
Turgor pressure 2026-10-13
Turgor pressure is the internal hydrostatic pressure above the external pressure in a biological cell. When water enters a walled biological cell by osmosis, expansion loads the cell wall. Rising pressure can oppose further net entry while the cell membrane retains its selective permeability. Mechanical support therefore does not require the cell wall to block water.
As an original mechanical check, idealize a small flat patch of cell wall of surface area under uniform turgor pressure . Its outward force is . Doubling at fixed pressure doubles the load; equal internal and external pressures give zero net pressure load. This is a force balance, not a measured wall strength or a complete model of a curved, growing biological cell. It says nothing about which solutes cross the cell membrane.