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Chapter
2 |
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The
Cell Wall
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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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