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Chapter 2

The Cell Wall

 
Cell walls are composed of polysaccharides, proteins, and aromatic substances. The primary wall of the cell is extensible but constrains the final size and shape of every cell. Facing walls of adjacent cells adhere to each other at the middle lamella. In some cells, secondary walls are deposited on the inner surface of the primary wall after growth has stopped. Cell walls become specialized for the function of the approximately 40 cell types that plants comprise.
      The cellulose microfibrils form the scaffold of all cell walls and are tethered together by cross-linking glycans; this framework is embedded in a gel of pectic substances. There are at least two types of primary walls. The Type I walls of dicots and non-commelinoid monocots have xyloglucan– cellulose networks embedded in a pectin-rich matrix and can be further cross-linked with a network of structural proteins. The Type II walls of commelinoid monocots have glucuronoarabinoxylan–cellulose networks in a relatively pectin-poor matrix. Ferulate esters and other hydroxycinnamic acids and aromatic substances cross-link the Type II walls.
      The cell wall is born at the cell plate. Cellulose microfibrils are synthesized at the surface of the plasma membrane at terminal complexes called particle rosettes, whereas all noncellulosic cross-linking glycans and pectic substances are made at the Golgi apparatus and secreted. All cell wall sugars are synthesized de novo from interconversion of nucleotide sugars, which are the substrates for polysaccharide synthases and glycosyl-transferases.
      Cell enlargement depends on the activities of endoglycosidase, endotransglycosy-lase, or expansin, or some combination of these, but cell shape is largely governed by the pattern of cellulose deposition. Cell enlargement also is accompanied by numerous changes in the structure of the wall’s cross-linking glycans and pectin matrix. Termination of cell growth is accompanied by cross-linking reactions involving proteins and aromatic substances.
      In addition to their use in wood, paper, and textile products, cell walls are the major textural component in fresh fruits and vegetables and constitute important dietary fibers in human nutrition. Transgenic plants with altered cell wall structures will become an important factor in crop and biomass improvement.

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