Cell wall

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The cell wall is a very tough, flexible and sometimes fairly rigid layer that surrounds some types of cells. It surrounds the cell membrane and provides these cells with structural support and protection. In addition the cell wall is acting as a filtering mechanism. A major function of the cell wall is to act as a pressure vessel, preventing over-expansion when water enters the cell. Cell walls are found in plants, fungi and prokaryotic cells but not in mycoplasmas.

The material in the cell wall varies between species, and can also differ depending on cell type and developmental stage. In bacteria, peptidoglycan forms the cell wall. Archaean cell walls have various compositions, and may be formed of glycoprotein S-layers, pseudopeptidoglycan, or polysaccharides. Fungi possess cell walls made of the glucosamine polymer chitin, and algae typically possess walls made of glycoproteins and polysaccharides. Unusually, diatoms have a cell wall composed of biogenic silica.[citation needed] Often, other accessory molecules are found anchored to the cell wall.


Diagram of the plant cell, with the cell wall in green.


Rigidity of cell walls[edit]

In most cells, the cell wall is flexible, meaning that it will bend rather than holding a fixed shape, but has considerable tensile strength. The apparent rigidity of primary plant tissues is enabled by cell walls, but not due to the walls' stiffness. Hydraulic turgor pressure creates this rigidity, along with the wall structure. The flexibility of the cell walls is seen when plants wilt, so that the stems and leaves begin to droop, or in seaweeds that bend in water currents. As John Howland states it:

Think of the cell wall as a wicker basket in which a balloon has been inflated so that it exerts pressure from the inside. Such a basket is very rigid and resistant to mechanical damage. Thus does the prokaryote cell (and eukaryotic cell that possesses a cell wall) gain strength from a flexible plasma membrane pressing against a rigid cell wall.[1]




The primary cell wall of most plant cells is freely permeable and permits the passage of small molecules and small proteins, with size exclusion estimated to be 30-60 kDa. Key nutrients, especially water and carbon dioxide, are distributed throughout the plant from cell wall to cell wall in apoplastic flow. The pH is an important factor governing the transport of molecules through cell walls.[2]

Cell wall evolution[edit]

Cell walls evolved independently in many groups, even in the photosynthetic eukaryotes. In these lineages, the cell wall is involved in the evolution of the multicellularity, terrestrialization and vascularization.[3]

Plant cell walls[edit]

The walls of plant cells must have sufficient tensile strength to withstand internal osmotic pressures of several times atmospheric pressure that result from the difference in solute concentration between the cell interior and external water.[4] Plant cell walls vary from 0.1 to several µm in thickness.[5]


Molecular structure of the primary cell wall in plants.

Up to three strata or layers may be found in plant cell walls:[6]

  • The middle lamella, a layer rich in pectins. This outermost layer forms the interface between adjacent plant cells and glues them together.
  • The secondary cell wall, a thick layer formed inside the primary cell wall after the cell is fully grown. It is not found in all cell types. Some cells, such as the conducting cells in xylem, possess a secondary wall containing lignin, which strengthens and waterproofs the wall.


In the primary (growing) plant cell wall, the major carbohydrates are cellulose, hemicellulose and pectin. The cellulose microfibrils are linked via hemicellulosic tethers to form the cellulose-hemicellulose network, which is embedded in the pectin matrix. The most common hemicellulose in the primary cell wall is xyloglucan. In grass cell walls, xyloglucan and pectin are reduced in abundance and partially replaced by glucuronarabinoxylan, another type of hemicellulose. Primary cell walls characteristically extend (grow) by a mechanism called acid growth, which involves turgor-driven movement of the strong cellulose microfibrils within the weaker hemicellulose/pectin matrix, catalyzed by expansin proteins. The outer part of the primary cell wall of the plant epidermis is usually impregnated with cutin and wax, forming a permeability barrier known as the plant cuticle.


  • cellulose, 35-50%
  • xylan, 20-35%, a type of hemicellulose
  • lignin, 10-25%, a complex phenolic polymer that penetrates the spaces in the cell wall between cellulose, hemicellulose and pectin components, driving out water and strengthening the wall.

Additionally, structural proteins (1-5%) are found in most plant cell walls; they are classified as hydroxyproline-rich glycoproteins (HRGP), arabinogalactan proteins (AGP), glycine-rich proteins (GRPs), and proline-rich proteins (PRPs). Each class of glycoprotein is defined by a characteristic, highly repetitive protein sequence. Most are glycosylated, contain hydroxyproline (Hyp) and become cross-linked in the cell wall. These proteins are often concentrated in specialized cells and in cell corners. Cell walls of the epidermis and endodermis may also contain suberin or cutin, two polyester polymers that function as permeability barriers.[7] The relative composition of carbohydrates, secondary compounds and protein varies between plants and between the cell type and age.






In some plants and cell types, after a maximum size or point in development has been reached, a secondary wall is constructed between the plasma membrane and primary wall.[8] 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 through pits in the secondary cell wall that allow plasmodesma to connect cells through the secondary cell walls.

Algal cell walls[edit]

Scanning electron micrographs of diatoms showing the external appearance of the cell wall

Like plants, algae have cell walls.[9] Algal cell walls contain either polysaccharides (such as cellulose (a glucan)) or a variety of glycoproteins (Volvocales) or both. The inclusion of additional polysaccharides in algal cells walls is used as a feature for algal taxonomy.

  • Mannans: They form microfibrils in the cell walls of a number of marine green algae including those from the genera, Codium, Dasycladus, and Acetabularia as well as in the walls of some red algae, like Porphyra and Bangia.
  • Xylans:
  • Alginic acid: It is a common polysaccharide in the cell walls of brown algae.
  • Sulfonated polysaccharides: They occur in the cell walls of most algae; those common in red algae include agarose, carrageenan, porphyran, furcelleran and funoran.



In brown algae, phlorotannins may be a constituent of the cell walls.[12]

Fungal cell walls[edit]

Chemical structure of a unit from a chitin polymer chain.

There are several groups of organisms that may be called "fungi".[vague] Some of these groups have been transferred out of the Kingdom Fungi, in part because of fundamental biochemical differences in the composition of the cell wall. Most true fungi have a cell wall consisting largely of chitin and other polysaccharides.[13] True fungi do not have cellulose in their cell walls.[14]

True fungi[edit]

In fungi, the cell wall is the outer-most layer, external to the plasma membrane. The fungal cell wall is a matrix of three main components:[15]

Cell walls of water and slime molds[edit]

The group Oomycetes, also known as water molds, are saprotrophic plant pathogens like fungi. Until recently they were widely believed to be fungi, but structural and molecular evidence[16] has led to their reclassification as heterokonts, related to autotrophic brown algae and diatoms. Unlike fungi, oomycetes typically possess cell walls of cellulose and glucans rather than chitin, although some genera (such as Achlya and Saprolegnia) do have chitin in their walls.[17] The fraction of cellulose in the walls is no more than 4 to 20%, far less than the fraction of glucans.[17] Oomycete cell walls also contain the amino acid hydroxyproline, which is not found in fungal cell walls.

The dictyostelids are another group formerly classified among the fungi. They are slime molds that feed as unicellular amoebae, but aggregate into a reproductive stalk and sporangium under certain conditions. Cells of the reproductive stalk, as well as the spores formed at the apex, possess a cellulose wall.[18] The spore wall has been shown to possess three layers, the middle of which is composed primarily of cellulose, and the innermost is sensitive to cellulase and pronase.[18]

Prokaryotic cell walls[edit]

Bacterial cell walls[edit]

Diagram of a typical Gram-positive bacterium. The cell envelope comprises a plasma membrane, seen here in green, and a thick peptidoglycan-containing cell wall (the yellow layer). No outer lipid membrane is present, as would be the case in Gram-negative bacteria. The red layer, known as the capsule, is distinct from the cell envelope.
Further information: Cell envelope

Around the outside of the cell membrane is the bacterial cell wall. Bacterial cell walls are made of peptidoglycan (also called murein), which is made from polysaccharide chains cross-linked by unusual peptides containing D-amino acids.[19] Bacterial cell walls are different from the cell walls of plants and fungi which are made of cellulose and chitin, respectively.[20] The cell wall of bacteria is also distinct from that of Archaea, which do not contain peptidoglycan. The cell wall is essential to the survival of many bacteria, although L-form bacteria can be produced in the laboratory that lack a cell wall.[21] The antibiotic penicillin is able to kill bacteria by preventing the cross-linking of peptidoglycan and this causes the cell wall to weaken and lyse.[20] The lysozyme enzyme can also damage bacterial cell walls.



Archaeal cell walls[edit]

Although not truly unique, the cell walls of Archaea are unusual. Whereas peptidoglycan is a standard component of all bacterial cell walls, all archaeal cell walls lack peptidoglycan,[25] with the exception of one group of methanogens.[1] In that group, the peptidoglycan is a modified form very different from the kind found in bacteria.[25] There are four types of cell wall currently known among the Archaea.

One type of archaeal cell wall is that composed of pseudopeptidoglycan (also called pseudomurein). This type of wall is found in some methanogens, such as Methanobacterium and Methanothermus.[26] While the overall structure of archaeal pseudopeptidoglycan superficially resembles that of bacterial peptidoglycan, there are a number of significant chemical differences. Like the peptidoglycan found in bacterial cell walls, pseudopeptidoglycan consists of polymer chains of glycan cross-linked by short peptide connections. However, unlike peptidoglycan, the sugar N-acetylmuramic acid is replaced by N-acetyltalosaminuronic acid,[25] and the two sugars are bonded with a β,1-3 glycosidic linkage instead of β,1-4. Additionally, the cross-linking peptides are L-amino acids rather than D-amino acids as they are in bacteria.[26]

A second type of archaeal cell wall is found in Methanosarcina and Halococcus. This type of cell wall is composed entirely of a thick layer of polysaccharides, which may be sulfated in the case of Halococcus.[26] Structure in this type of wall is complex and as yet is not fully investigated.


In other Archaea, such as Methanomicrobium and Desulfurococcus, the wall may be composed only of surface-layer proteins,[1] known as an S-layer. S-layers are common in bacteria, where they serve as either the sole cell-wall component or an outer layer in conjunction with polysaccharides. Most Archaea are Gram-negative, though at least one Gram-positive member is known.[1]

Other cell coverings[edit]

Many protists and bacteria produce other cell surface structures apart from cell walls, external (extracellular matrix) or internal.[27][28][29] Many algae have a sheath or envelope of mucilage outside the cell made of exopolysaccharides. Diatoms build a frustule from silica extracted from the surrounding water; radiolarians, foraminiferans, testate amoebae and silicoflagellates also produce a skeleton from minerals, called test in some groups. Many green algae, such as Halimeda and the Dasycladales, and some red algae, the Corallinales, encase their cells in a secreted skeleton of calcium carbonate. In each case, the wall is rigid and essentially inorganic. It is the non-living component of cell. Some golden algae, ciliates and choanoflagellates produces a shell-like protective outer covering called lorica. Some dinoflagellates have a theca of cellulose plates, and coccolithophoridss have coccoliths.

An extracellular matrix is also present in metazoans, and its composition varies between cells.

See also[edit]


  1. ^ a b c d Howland, John L. (2000). The Surprising Archaea: Discovering Another Domain of Life. Oxford: Oxford University Press. pp. 69–71. ISBN 0-19-511183-4. 
  3. ^ http://public.wsu.edu/~lange-m/Documnets/Teaching2011/Popper2011.pdf
  4. ^ http://www.madsci.org/posts/archives/2006-11/1164842041.Cb.r.html
  5. ^ Campbell, Neil A.; Reece, Jane B.; Urry, Lisa A.; Cain, Michael L.; Wasserman, Steven A.; Minorsky, Peter V.; Jackson, Robert B. (2008). Biology (8th ed.). p. 118. ISBN 978-0-8053-6844-4. 
  7. ^ Laurence Moire, Alain Schmutz, Antony Buchala, Bin Yan, Ruth E. Stark, and Ulrich Ryser (1999). "Glycerol Is a Suberin Monomer. New Experimental Evidence for an Old Hypothesis". Plant Physiol 119 (3): 1137–1146. doi:10.1104/pp.119.3.1137. PMC 32096. PMID 10069853. 
  8. ^ Campbell, Neil A.; Reece, Jane B.; Urry, Lisa A.; Cain, Michael L.; Wasserman, Steven A.; Minorsky, Peter V.; Jackson, Robert B. (2008). Biology (8th ed.). p. 119. ISBN 978-0-8053-6844-4. 
  9. ^ Sendbusch, Peter V. (2003-07-31). "Cell Walls of Algae". Botany Online. Retrieved on 2007-10-29.
  10. ^ Raven, J. A. (1983). "The transport and function of silicon in plants". Biol. Rev. 58 (2): 179–207. doi:10.1111/j.1469-185X.1983.tb00385.x. 
  11. ^ Furnas, M. J. (1990). "In situ growth rates of marine phytoplankton : Approaches to measurement, community and species growth rates". J. Plankton Res. 12 (6): 1117–1151. doi:10.1093/plankt/12.6.1117. 
  12. ^ Koivikko, Riitta; Loponen, Jyrki; Honkanen, Tuija; Jormalainen, Veijo (2005). "Contents of soluble, cell-wall-bound and exuded phlorotannins in the brown alga Fucus vesiculosus, with implications on their ecological functions". Journal of Chemical Ecology 31 (1): 195–212. doi:10.1007/s10886-005-0984-2. PMID 15839490. 
  13. ^ Hudler, George W. (1998). Magical Mushrooms, Mischievous Molds. Princeton, NJ: Princeton University Press, 7. ISBN 0-691-02873-7.
  14. ^ Webster, John & Weber, Roland W.S. (2007) "Introduction to Fungi" New York, NY: Cambridge University Press, 6."
  15. ^ a b c d Webster, John & Weber, Roland W.S. (2007) "Introduction to Fungi" New York, NY: Cambridge University Press, 5-7."
  16. ^ Sengbusch, Peter V. (2003-07-31). "Interactions between Plants and Fungi: the Evolution of their Parasitic and Symbiotic Relations". biologie.uni-hamburg.de. Retrieved on 2007-10-29.
  17. ^ a b Alexopoulos, C. J., C. W. Mims, & M. Blackwell (1996). Introductory Mycology 4. New York: John Wiley & Sons, 687-688. ISBN 0-471-52229-5.
  18. ^ a b Raper, Kenneth B. (1984). The Dictyostelids. Princeton, NJ: Princeton University Press, 99-100. ISBN 0-691-08345-2.
  21. ^ Joseleau-Petit D, Liébart JC, Ayala JA, D'Ari R (September 2007). "Unstable Escherichia coli L forms revisited: growth requires peptidoglycan synthesis". J. Bacteriol. 189 (18): 6512–20. doi:10.1128/JB.00273-07. PMC 2045188. PMID 17586646. 
  23. ^ Hugenholtz P; Rogozin, Igor B; Grishin, Nick V; Tatusov, Roman L; Koonin, Eugene V (2002). "Exploring prokaryotic diversity in the genomic era". Genome Biol 3 (2): reviews0003.1–reviews0003.8. doi:10.1186/gb-2002-3-2-reviews0003. PMC 139013. PMID 11864374. 
  24. ^ Walsh F, Amyes S (2004). "Microbiology and drug resistance mechanisms of fully resistant pathogens.". Curr Opin Microbiol 7 (5): 439–44. doi:10.1016/j.mib.2004.08.007. PMID 15451497. 
  25. ^ a b c White, David. (1995) The Physiology and Biochemistry of Prokaryotes, pages 6, 12-21. (Oxford: Oxford University Press). ISBN 0-19-508439-X.
  26. ^ a b c d Brock, Thomas D., Michael T. Madigan, John M. Martinko, & Jack Parker. (1994) Biology of Microorganisms, 7th ed., pages 818-819, 824 (Englewood Cliffs, NJ: Prentice Hall). ISBN 0-13-042169-3.
  27. ^ Preisig, H.R. Terminology and nomenclature of protist cell surface structures. In: The Protistan Cell Surface. Protoplasma special edition, 1994.
  28. ^ Becker, B. The cell surface of flagellates. In: The Flagellates. Unity, diversity and evolution. Ed.: Barry S. C. Leadbeater and J. C. Green Taylor and Francis, London 2000.

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