Third lecture
3- Cell wall of bacteria
Cell wall of bacteria
The cell envelope is composed of the plasma membrane and cell wall. As
in other organisms, the bacterial cell wall provides structural integrity to
the cell. In prokaryotes, the primary function of the cell wall is to protect
the cell from internal turgor pressure caused by the much higher
concentrations of proteins and other molecules inside the cell compared
to its external environment. The bacterial cell wall differs from that of all
other organisms by the presence of peptidoglycan which is located
immediately outside of the cytoplasmic membrane. Peptidoglycan is
made up of a polysaccharide backbone consisting of alternating NAcetylmuramic acid (NAM) and N-acetylglucosamine (NAG) residues in
equal amounts. Peptidoglycan is responsible for the rigidity of the
bacterial cell wall and for the determination of cell shape. It is relatively
porous and is not considered to be a permeability barrier for small
substrates. While all bacterial cell walls (with a few exceptions e.g.
extracellular parasites such as Mycoplasma) contain peptidoglycan, not
all cell walls have the same overall structures. Since the cell wall is
required for bacterial survival, but is absent in eukaryotes,
several antibiotics notably the (penicillins andcephalosporins) stop
bacterial infections by interfering with cell wall synthesis, while having
no effects on human cells which have no cell wall only a cell
membrane.There are two main types of bacterial cell walls, those
of Gram-positive bacteria and those of Gram-negative bacteria, which are
differentiated by their Gram staining characteristics. For both these types
of bacteria, particles of approximately 2 nm can pass through the
peptidoglycan.[3] If the bacterial cell wall is entirely removed, it is called
a protoplast while if it's partially removed, it is called a spheroplast. βLactam antibiotics such as penicillin inhibit the formation of
peptidoglycan cross-links in the bacterial cell wall. The
enzyme lysozyme, found in human tears, also digests the cell wall of
bacteria and is the body's main defense against eye infections.
The Gram-positive cell wall[edit]
Gram-positive cell walls are thick and the peptidoglycan ( also known
as murein) layer constitutes almost 95% of the cell wall in some Gram-
positive bacteria and as little as 5-10% of the cell wall in Gram-negative
bacteria. The cell wall of some Gram-positive bacteria can be completely
dissolved by lysozyme, as this enzyme attacks the bonds between GA and
MA. In other Gram-positive bacteria, such asStaphylococcus aureus, the
walls are resistant to the action of lysozyme. They have O-acetyl groups
on carbon-6 of some MA residues. The matrix substances in the walls of
Gram-positive bacteria may be polysaccharides or teichoic acids. The
latter are very widespread, but have been found only in Gram-positive
bacteria. There are two main types of teichoic acid: ribitol teichoic acids
and glycerol teichoicacids. The latter one is more widespread. These
acids are polymers of ribitol phosphate and glycerol phosphate,
respectively, and only located on the surface of many Gram-positive
bacteria. However, the exact function of teichoic acid is debated and not
fully understood. A major component of the gram-positive cell wall
is lipoteichoic acid. One of its purposes is providing an antigenic
function. The lipid element is to be found in the membrane where its
adhesive properties assist in its anchoring to the membrane.
The Gram-negative cell wall[edit]
Gram-negative cell walls are thin and unlike the Gram-positive cell walls,
they contain a thin peptidoglycan layer adjacent to the cytoplasmic
membrane. The chemical structure of the outer membrane's
lipopolysaccharides is often unique to specific bacterial sub-species and
is responsible for many of the antigenic properties of these strains.
Lipopolysaccharides, also called endotoxins, are composed of
polysaccharides and lipid A which are responsible for much of the
toxicity of Gram-negative bacteria. It consists of characteristic
lipopolysaccarides embedded in the membrane.
Plasma membrane[edit]
The plasma membrane or bacterial cytoplasmic membrane is composed of
a phospholipid bilayer and thus has all of the general functions of a cell
membrane such as acting as a permeability barrier for most molecules and
serving as the location for the transport of molecules into the cell. In
addition to these functions, prokaryotic membranes also function in energy
conservation as the location about which aproton motive force is generated.
Unlike eukaryotes, bacterial membranes (with some exceptions
e.g. Mycoplasma and methanotrophs) generally do not contain sterols.
However, many microbes do contain structurally related compounds
called hopanoids which likely fulfill the same function.
Unlike eukaryotes, bacteria can have a wide variety of fatty acids within
their membranes. Along with typical saturated and unsaturated fatty acids,
bacteria can contain fatty acids with additional methyl, hydroxy or even
cyclic groups. The relative proportions of these fatty acids can be modulated
by the bacterium to maintain the optimum fluidity of the membrane (e.g.
following temperature change).
As a phospholipid bilayer, the lipid portion of the outer membrane is
impermeable to charged molecules. However, channels called porins are
present in the outer membrane that allow for passive transport of
many ions, sugars and amino acids across the outer membrane. These
molecules are therefore present in the periplasm, the region between the
cytoplasmic and outer membranes. The periplasm contains the
peptidoglycan layer and many proteins responsible for substrate binding
or hydrolysis and reception of extracellular signals. The periplasm is thought
to exist in a gel-like state rather than a liquid due to the high concentration
of proteins and peptidoglycan found within it. Because of its location
between the cytoplasmic and outer membranes, signals received and
substrates bound are available to be transported across the cytoplasmic
membrane using transport and signalling proteins imbedded there.
………………..
Bacterial Cell Wall
Bacterial cell wall is made up of peptidoglycans also known as murein. The cell wall of
bacteria is essential for the survival of bacteria.
Cell wall of bacteria is broadly classified into two types: gram positive and gram
negative. The names are given to the reaction of the cells to gram staining. This experiment
is employed for the classification of bacterial species.
The gram positive bacteria have a thick cell wall and is made up of many layers of
peptidoglycan and teichoic acids.
The gram negative bacteria have thinner cell walls, and is made up of few layers of
peptidoglycans and is surrounded by a lipid membrane containing lipopolysacccharides and
lipoproteins.
Fungi Cell Wall
Fungi cell wall consists of chitin and other polysaccharides. They do not have cellulose in
their cell walls. Species of fungi that possess a cell wall have a plasma membrane and three
layers of cell wall material surrounding it. These layers are made up of chitin, glucans and a
layer a of mannoproteins (mannose containing glycoproteins).