A cell wall is an outer layer surrounding certain cells that is outside of the cell membrane. All cells have cell membranes, but generally only plants, fungi, algae, most bacteria, and archaea have cells with cell walls. The cell wall provides strength and structural support to the cell, and can control to some extent what types and concentrations of molecules enter and leave the cell. The materials that make up the cell wall differ depending on the type of organism. The cell wall has evolved many different times among different groups of organisms.
It is flexible, but provides strength to the cell, which helps protect the cell against physical damage. It also gives the cell its shape and allows the organism to maintain a certain shape overall. The cell wall can also provide protection from pathogens such as bacteria that are trying to invade the cell. The structure of the cell wall allows many small molecules to pass through it, but not larger molecules that could harm the cell.
The main component of the plant cell wall is cellulose, a carbohydrate
that forms long fibers and gives the cell wall its rigidity. Cellulose fibers
group together to form bundles called microfibrils. Other important
carbohydrates include hemicellulose, pectin, and liginin. These carbohydrates
form a network along with structural proteins to form the cell wall. Plant
cells that are in the process of growing have primary cell walls, which are
thin. Once the cells are fully grown, they develop secondary cell walls. The
secondary cell wall is a thick layer that is formed on the inside of the
primary cell wall. This layer is what is usually meant when referring to a
plant’s cell wall. There is also another layer in between plant cells called
the middle lamella; it is pectin-rich and helps plant cells stick together.
The cell walls of plant cells help them maintain turgor pressure,
which is the pressure of the cell membrane pressing against the cell wall.
Ideally, plants cells should have lots of water within them, leading to high
turgidity. Whereas a cell without a cell wall, such as an animal cell, can
swell and burst of too much water diffuses into it, plants need to be in
hypotonic solutions (more water inside than outside, leading to lots of water
entering the cell) to maintain turgor pressure and their structural shape. The
cell wall efficiently holds water in so that the cell does not burst. When
turgor pressure is lost, a plant will begin to wilt. Turgor pressure is what
gives plant cells their characteristic square shape; the cells are full of
water, so they fill up the space available and press against each other.
Algae Cell Walls
Algae are a diverse group, and the diversity in their cell walls
reflects this. Some algae, such as green algae, have cell walls that are
similar in structure to those of plants. Other algae, such as brown algae and
red algae, have cellulose along with other polysaccharides or fibrils. Diatoms
have cell walls that are made from silicic acid. Other important molecules in
algal cell walls include mannans, xylans, and alginic acid.
Fungi Cell Walls
The cell walls of fungi contain chitin, which is a glucose derivative
that is similar in structure to cellulose. Layers of chitin are very tough;
chitin is the same molecule found in the rigid exoskeletons of animals such as
insects and crustaceans. Glucans, which are other glucose polymers, are also
found in the fungal cell wall along with lipids and proteins. Fungi have
proteins called hydrophobins in their cell walls. Found only in fungi,
hydrophobins give the cells strength, help them adhere to surfaces, and help
control the movement of water into the cells. In fungi, the cell wall is the
most external layer, and surrounds the cell membrane.
Bacteria and Archaea Cell Walls
The cell walls of bacteria usually contain the polysaccharide
peptidoglycan, which is porous and lets small molecules through. Together, the
cell membrane and cell wall are referred to as the cell envelope. The cell wall
is an essential part of survival for many bacteria. It provides mechanical
structure to bacteria, which are single-celled, and it also protects them from
internal turgor pressure. Bacteria have higher concentration of molecules such
as proteins within themselves as compared to their environment, so the cell
wall stops water from rushing into the cell. Differences in cell wall thickness
also make Gram staining possible. Gram staining is used for the general
identification of bacteria; bacteria with thick cell walls are gram-positive,
while bacteria with thinner cell walls are gram-negative.
While archaea are similar in many ways to bacteria, hardly any
archaeal walls contain peptidoglycan. There are several different types of cell
walls in archaea. Some are composed of pseudopeptidoglycan, some have
polysaccharides, some have glycoproteins, and others have surface-layer
proteins (called an S-layer, which can also be found in bacteria).
Related Biology Terms
Cell membrane – A membrane found on the outside of all cells that
separates them from the outside environment.
Turgor pressure – Water pressure inside cells.
Chitin – A polysaccharide that is a main component of fungal cell
walls and also of the exoskeletons of certain animals like insects.
Points to remember
A cell wall is an outer rigid semi-elastic supportive and protective
layer.
It is present around the plasma membrane.
It provides mechanical support and helps in maintaining the shape of
the plant cell.
The cell wall is present in the plant cell and absent in the animal
cell which distinguishes them from each other.
The cell wall is formed by the protoplast. Any plant cell which is
devoid of the cell wall is called the protoplast.
The plant cell is mostly made up of the following components:
Cellulose
Hemicellulose
Pectin
Protein
Plant Cell Wall
In both the primary and secondary cell walls of the plant, cellulose
is present.
Cellulose is an insoluble carbohydrate.
The fibrous structure present in the cell wall maintains the integrity
of the structure.
In the primary cell wall, Pectin is present predominantly.
It plays the important role in:
Expansion
Strength
Porosity
Adhesion
Intercellular signaling
Other non-cellulosic polysaccharides include xyloglucan, glucan,
xylan, mannan, and callose.
Based on the sugar substitutes and side chains, pectic and
non-cellulosic polysaccharides can be distinguished further too.
During biosynthesis, these components are attached to the
polysaccharides.
These substituents are important in determining the solubility and
viscosity within the cell wall.
They are also responsible for determining the interaction between
polysaccharides and proteins.
The cell wall of fungi is made of chitin.
The cell wall of bacteria is made of the protein, lipid, and
polysaccharides complex.
Structure of Plant Cell wall
It is derived from the living protoplast.
It consists of the middle lamella, primary cell wall, plasmodesmata,
secondary cell wall, and pits.
Middle lamella
After the cytokinesis, it is the first-formed layer.
It is present in between the two adjacent cells.
It is made up of calcium and magnesium pectate.
It helps to join the two adjacent cells.
Primary cell wall
It is the first formed cell wall.
It is present in the inner side of the middle lamella.
It is the thin and permeable layer that can be expanded.
Cutin and cutin waxes are present in some epidermal cells of the leaf
and stem. It makes the primary cell wall impermeable.
It is formed before the growth and development of the cell.
It is made up of matrix and microfibrils.
Matrix is made up of water, hemicelluloses, pectin, lipids, and
proteins.
Microfibrils are embedded in the gel-like matrix.
The primary cell wall of the plant is made of cellulose.
In the fungi, chitin makes the primary cell wall, and in bacteria
murein makes it.
Primary cell wall forms the only cell wall in the immature meristematic
and parenchymatous cells.
Plasmodesma (plural: plasmodesmata)
Plasmodesmata are cytoplasmic or protoplasmic bridges present in the
primary cell wall of adjacent cells.
They form a protoplasmic continuum called symplast.
They transfer cytoplasmic materials among adjacent cells.
Secondary cell wall
The secondary cell wall is situated inner to the primary cell wall.
This is the thick layer, permeable, and cannot be expanded.
It forms after the growth and development of the cell.
It is present in the cells of the thick-walled dead tissue of the
plant. Eg: Cells of sclerenchyma, tracheids, and vessels.
It is differentiated into the outer layer (S1), middle layer (S2), and
inner layer (S3).
Each layer is made up of a matrix and microfibrils.
The chemical composition of the matrix is almost similar to the matrix
of the primary cell.
Microfibrils of the secondary cell wall is made up of cellulose and
lignin.
Some chemicals like suberin, silica, wax, resins, oils, etc. are also
deposited in the secondary cell wall.
Pits
In the secondary cell wall, pits are the unthickened areas or
depressed areas.
A pit consists of a pit cavity or pit chamber and pit membrane.
The pit membrane consists of the primary cell wall and middle lamella.
The pit membrane is permeable.
So pit helps in rapid translocation of materials between two adjacent
cells.
Tertiary cell wall
In some plant cells, there is the presence of another cell wall
beneath the secondary cell wall. It is known as the tertiary cell wall.
The morphology, chemistry, and staining properties of the tertiary
cell wall are different from the primary and secondary cell walls.
In the tertiary cell wall, xylan is also present in it.
Functions of plant cell wall
It provides mechanical support as the skeletal framework in the plant.
It protects the inner components of the cell from mechanical injuries.
It is permeable to the water and solutes. It is the presence of the
water-filled channels which allows the free diffusion of water and
water-soluble substances. Eg: gas, salt, sugar, hormones.
It prevents entry of the pathogenic agents inside the cell acting as
the first line of defense.
When the cell is kept in the hypotonic solution, it prevents the
osmotic bursting of the cell.
In the cell wall, cutin, wax, silica, and suberin is present which
reduces the rate of transpiration.
The cell wall of root hairs helps in the absorption of sap from the
soil.
Walls of tracheids and vessels help in the conduction of sap.
Middle lamella helps to join the adjacent cells.
Plasmodesmata help in the transfer of cytoplasmic materials among
adjacent cells.
Cell wall in the defense mechanism
During the infection, oligosaccharides elicitors can be released.
These substances can be released from the host plant’s cell wall i.e
DAMPs (Damage-associated Molecular patterns) or they can be from the pathogen
cell wall i.e PAMPs ( Pathogen-associated Molecular patterns).
It occurs during the process of degradation.
In the plasma membrane, immune receptors are present which receive
these elicitors.
It then activates the defense responses of DAMP or PAMP-triggered
immunity.
References
Verma, P. S., & Agrawal, V. K. (2006). Cell Biology, Genetics,
Molecular Biology, Evolution & Ecology (First edition). S . Chand and
company Ltd.
Houston, K., Tucker, M. R., Chowdhury, J., Shirley, N., & Little,
A. (2016). The plant cell wall: A complex and dynamic structure as revealed by
the responses of genes under stress conditions. Frontiers in Plant Science,
7(AUG2016), 1–18. https://doi.org/10.3389/fpls.2016.00984
Shakya, M., Mehta, D. K. R., Gautam, M., Pokharel, K. R., &
Khanal, K. (2077). Principles of Biology (First edition). Asmita Books
Publisher and Distributors Ltd.
Resources
Cell Wall Structure and Function
Cell Wall Composition and Function
Primary and Secondary Cell Wall
No comments:
Post a Comment