CELL WALL
Cell
wall is complex in nature .It remains differentiated in the following layers:
(i) Middle Lamella
(ii)Primary cell wall
(iii)Secondary cell
wall
(iv)Tertiary cell wall
(i) Middle Lamella-The cells of plant tissue generally remain
cemented together by inter cellular matrix known as middle lamella. It is
composed of pectin, lignin & some other proteins. They are dissolved in
strong acids.
(ii) Primary Cell Wall- It first formed cell wall. Its thickness is
about 100-200 NM. It is the outer most layer of the cell & it forms the only
cell wall in the immature meristamatic & parenchymatous cells. It is
comparatively thin & permeable. It is composed of loosely organised network
of cellulose microfibrils associated with hemicellulose pectins &
glycoproteins. Pectins are important in imparting flexibility & makes
primary cell wall to expand during cell growth. The cellulose synthesizing
enzyme that synthesize cellulose microfibrils are localized within plasma
membrane. These are called rosettes; add glucose molecules to growing
microfibrils. It is composed of polysaccharide cellulose but other substance
may be incorporated in it. Lignin or suberin may also present in it &
epidermal cells of leaves & stem also posses cutin & cuter waxes &
are impermeable to water. Primary cell wall of yeast and fungi is composed of
chitn.
(iii) Secondary Cell Wall- It lies after primary cell wall. It is
thick, permeable & lies close to tertiary cell wall or lies close to plasma
membrane .It consist of bulk of many layers and at maturity cell wall consist It gives shape & mechanical strength to
the cell .It is chemically composed of compactly arranged cellulose &
lignins.
CHEMICAL NATURE
Plant's cell wall are
composed of carbohydrates known as cellulose. Besides cellulose various
chemical substances as hemicelluloses, pectin, lignin, cutin & chitn. The
cell wall also contains certain minerals like calcium & magnesium in the
form of carbonates & silicates. The cellulose is a polysaccharide & it
is the most abundantly occurring chemical substance of most plant cell.
Chemically cellulose contains long chains of glucose molecules. Glucose is the
structural unit of cellulose & about 3000 glucose molecule are linked
together to form long chain of cellulose molecule. A bundle of 100 chain
molecules of cellulose forces the elementary fibril known as micelle. The 20
micelle when get arranged parallel form the fibrils of 25nm thick known as
microfibrils (5-15nm in diametre).These microfibrils form large sized bundles
0.5micrometre thick of cellulose fibre to form the macrofibrils. The
macrofibrils consisting of many cellulose fibrils. The hemicellulose is
composed of monosaccharide units such as arabinose, xylose, mannos&
galactose. It occurs sometimes in between the macrofibrils of cellulose.
HEMICELLULOSE
They are heterogeneous
groups of carbohydrates polymers constructed from various 5&6 carbons
sugars including xylose, arabinose, mannose & galactose. The hemicellulose
xylan which utilizes the pentose xylose as its main building block, accounts as
much as 50% of the cell wall in woody tissues. Hemicellulose molecule binds to
the cellulose microfibrils &to each other creating a coating that helps to
bond these microfibrils together into a rigid interconnected network of
cellulose & hemicellulose.
PECTINS
These
are the polymers of carbohydrates D-galactouronic acid & D-glucouronic acid
units .Like the glycosoaminoglycane of animal cell, they readily form hydrated
gels. This property of pectin molecules is responsible for gelatin that occurs
during the process of making jams and jellies .Pectin molecules are involved in
binding adjacent cell wall together & in forming the matrix in which
cellulose microfibrils are embedded. (FIG II)
LIGNINS
They are group of
polymerized aromatic phenols. They occurs mainly in woody tissues, as cross
linked network that contributes to the hardening of the cell wall. Lignin
molecules are localized between the cellulose fibrils, where they function to
rest compression forces .
GLYCOPROTEINS
They are important
constituent of plant cell wall, accounting as much as 10% of the total mass.
Among the wall glycoprotein a group of common glycoprotein are called
extensins. Extensins & other glycoprotein forms cross linked networks with
each other as well as with cellulose microfibrils
OTHER COMPONENT
In addition to the preceding components a small
percent of the total mass of the cell wall is accounted for the lipids,
including waxes & other complex polymers .Minerals such as Ca & K also
occurs in the plant cell wall in the form of inorganic salts.
The cell wall originates
in the developing cell plate. During telophase of the mitotic cell cycle, the
phragmosome, a flattened membranous vesicle containing cell wall
components,froms across the cell within a cytoskeletal array called the
phragmoplast.
The noncellulosic cell wall polysaccharides synthesized in the golgi
apparatus and packaged in the vesicles fuse with the growing cell plate. The
plate grows outward until the edges of the membranous vesicle fuse with the
plasma membrane, creating two cells. Finally the new cell wall fuses with the
existing primary cell wall,
The plant golgi apparatus is a factory for the
synthesis, processing and targeting of glycoproteins. Thus except for cellulose
the polysaccharides, the structural proteins, and a broad spectrum of enzymes
are coordinately secreted in golgi derived vesicles and targeted to the cell
wall.
Cell enlargement depends
on the activities of endoglycosidase, endotransglycosylase or expansin or some
combination of these. But cell shape is largely governed by the pattern of
cellulose deposition. Termination of cell growth is accompanied by cross
linking reaction involving proteins and aromatic substances.
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