CELL WALL
INTRODUCTION:A cell wall is a structural layer just surrounded by the same types
of cells, just outside the plasma membrane. It is a rigid, semi- elastic, semi-
transparent, supportive and protective covering of a cell in plants, fungi, prokaryotes
and some protists. It can be tough, flexible and sometimes may be rigid. It was first
observed by Robert Hooke in 1665, when he saw some dead empty cork cells under
the microscope.
IMPORTANT/SALIENT FEATURES:
➔ Cell wall lies outside the plasmalemma.
➔ It is made or built up of the materials sent out by the plasma membrane and
inside the cell wall.
➔ Cell walls are not completely dead or non living structures.
➔ It is metabolically active.
➔ The composition of cell walls varies in different groups.
➔ Prokaryotic cell wall consists of peptidoglycans that contain polymers of
modified sugars ( N-acetylglucosamine and N-acetylmuramic acid )
crosslinked by short peptides.
CHEMICAL COMPOSITION OF CELL WALL:
In the cell wall of higher plants, tensile fibres are made up of polysaccharides -
cellulose. Certain other compounds such as hemicellulose , pectins and proteins are
also present in the cell wall.
1.CELLULOSE:
A cellulose molecule is a long, unbranched chain of glucose molecules. There are
about 6,000 glucose units in each chain. About 30 to 60 cellulose molecules form a
bundle called microfibrils, about 5-10 nm thick. It has BetA1- 4 linkage. The cellulose
, molecules in a microfibril are oriented parallel to one another and hold together by
hydrogen bonds. Cellulose molecules are polymerised at the cell surface.
2.MATRIX OF THE CELL WALL:
The matrix is an amorphous gel-like material. Materials of matrix are synthesized
within the cytoplasm and they are carried to the cell surface in the secretory vesicles.
The matrix is composed of three types of macromolecules:
● Hemicellulose (5-15%)
● Pectins. (2-8%)
● Proteins. (1-2%)
● Water. (60-70%)
1.HEMICELLULOSE:
Hemicellulose is a branched polysaccharides, having a backbone composed of one
sugar, such as glucose and a side chain of other sugar such as xylose. It also
consists of arabinose, galactose, mannose and sometimes uronic acids.
Hemicellulose is attached to the surface of microfibrils and tightly binds with them.
2.PECTINS(PECTIC ACIDS);
Pectins are a heterogeneous class of negatively charged polysaccharides rich in
galacturonic acid. Pectins hold water and form a gel that fills the space between the
fibrous filaments. When a plant is attacked by pathogens, fragments of pectins get
released from the wall, triggering a defensive response by the plant cell. The primary
cell wall of plants, a matrix composed of pectins only contains - Galacturonic acids.
In secondary wall of plants, different types of pectins groups are present such as:
● Homogalacturonan
● Arabinose
● Galactose
● Galacturonic acid.
● Rhaminose
INTRODUCTION:A cell wall is a structural layer just surrounded by the same types
of cells, just outside the plasma membrane. It is a rigid, semi- elastic, semi-
transparent, supportive and protective covering of a cell in plants, fungi, prokaryotes
and some protists. It can be tough, flexible and sometimes may be rigid. It was first
observed by Robert Hooke in 1665, when he saw some dead empty cork cells under
the microscope.
IMPORTANT/SALIENT FEATURES:
➔ Cell wall lies outside the plasmalemma.
➔ It is made or built up of the materials sent out by the plasma membrane and
inside the cell wall.
➔ Cell walls are not completely dead or non living structures.
➔ It is metabolically active.
➔ The composition of cell walls varies in different groups.
➔ Prokaryotic cell wall consists of peptidoglycans that contain polymers of
modified sugars ( N-acetylglucosamine and N-acetylmuramic acid )
crosslinked by short peptides.
CHEMICAL COMPOSITION OF CELL WALL:
In the cell wall of higher plants, tensile fibres are made up of polysaccharides -
cellulose. Certain other compounds such as hemicellulose , pectins and proteins are
also present in the cell wall.
1.CELLULOSE:
A cellulose molecule is a long, unbranched chain of glucose molecules. There are
about 6,000 glucose units in each chain. About 30 to 60 cellulose molecules form a
bundle called microfibrils, about 5-10 nm thick. It has BetA1- 4 linkage. The cellulose
, molecules in a microfibril are oriented parallel to one another and hold together by
hydrogen bonds. Cellulose molecules are polymerised at the cell surface.
2.MATRIX OF THE CELL WALL:
The matrix is an amorphous gel-like material. Materials of matrix are synthesized
within the cytoplasm and they are carried to the cell surface in the secretory vesicles.
The matrix is composed of three types of macromolecules:
● Hemicellulose (5-15%)
● Pectins. (2-8%)
● Proteins. (1-2%)
● Water. (60-70%)
1.HEMICELLULOSE:
Hemicellulose is a branched polysaccharides, having a backbone composed of one
sugar, such as glucose and a side chain of other sugar such as xylose. It also
consists of arabinose, galactose, mannose and sometimes uronic acids.
Hemicellulose is attached to the surface of microfibrils and tightly binds with them.
2.PECTINS(PECTIC ACIDS);
Pectins are a heterogeneous class of negatively charged polysaccharides rich in
galacturonic acid. Pectins hold water and form a gel that fills the space between the
fibrous filaments. When a plant is attacked by pathogens, fragments of pectins get
released from the wall, triggering a defensive response by the plant cell. The primary
cell wall of plants, a matrix composed of pectins only contains - Galacturonic acids.
In secondary wall of plants, different types of pectins groups are present such as:
● Homogalacturonan
● Arabinose
● Galactose
● Galacturonic acid.
● Rhaminose
