Dominic Rowney
Cell Walls
A cell wall is a rigid structure deposited outside of the cell membrane.
Cell walls consist of a network of fibres, which give strength and support but
are freely permeable to solutes, unlike membranes, which are partially
permeable. Many types of organisms have cell walls, but animals do not. The
most commonly known about cell wall is the plant cell wall, which usually
consists of cellulose. Cellulose is a polysaccharide composed of beta-glucose
monomers. These form long straight chains joined by hydrogen bonds:
Hydrogen
Bond
Cellulose
Chitin
These long straight chains are called microfibrils these are wound into
macrofibrils and have a high tensile strength. So a plant cell that exerts a high
pressure on the wall due to it being full of water makes it turgid, it provides
strength and support. Plant cell walls are mainly made of cellulose, but can
also contain other polysaccharides as a secondary wall such as
hemicellulose, pectin, lignin that gives xylem support and suberin that
waterproofs epidermal cells. There are often links through plant cell walls
called plasmodesmata, which join to adjacent cells. Cellulose is also found in
the cell walls of some protista; such as dinoflagellate; and green algae.
Alginate is another polysaccharide that makes up cell walls; specifically
of many red and brown algae. Alginate consists of an alternating sequence of
alpha-L-guluronic acid and beta-D-mannuronic acid monosaccharides:
Na
Na
Are able to form gels in the presence of polyvalent ions, of which calcium is
the most important. These ions form links between the alginate molecules and
create a three-dimensional network.
The cell walls of fungi are made up of another polysaccharide, called
chitin. It is used in a similar way to cellulose, to give fungi cell strength and
structure, it is also found in the exo-skeletons of insects and crustaceans. Its
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Dominic Rowney
monomer; N-acetyl glucosamine; has a similar structure to glucose except
that it replaces hydroxyl group on carbon 2 with an acetylamino group. Chitin
makes a good cell wall because of
its tightly bonded structure it is highly
N-Acetyl
crystalline, insoluble and unreactive,
Glucosamine
making it a suitable material for cell
walls.
Acetylamino
Groups
Bacterial cell walls come in two types gram positive and gram negative. Any
bacteria’s cell wall is made of murien, which is a polysaccharide cross-linked
with polypeptide chains or a glycoprotein as it is known. An example of this is
peptidoglycan which is found in the cell walls of gram positive bacteria, gram
positive have a thick cell wall with additional protein and carbohydrate
deposits. A gram negative bacterial cell wall has a thin wall with a lipid coating
that can provide the cell with protection. The rigidity of the cell wall confers the
shape of the cell it can be:
Coccus
(Spherical)
Bacillus
(Rod)
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Spirilium
(Helical)