Prokaryotic Cell Wall | Biology Ideas | Diagram and Structure

PROKARYOTIC CELL WALL

Complex, semirigid structure responsible for the shape of the cell Wall shape and strength is primarily due to peptidoglycan.

The cell wall surrounds the underlying, fragile plasma ( membrane and protects it and the interior of the cell from adverse changes in the outside environment.

Almost all prokaryotes have cell walls.

The major function of the cell wall is to prevent bacterial cells from rupturing when the water pressure inside the cell is greater than that outside the cell.

It also helps maintain the shape of a bacterium and serves as a point of anchorage for flagella.

It contributes to the ability of some species to cause disease and is the site of action of some antibiotics.

The chemical composition of the cell wall is used to differentiate major types of bacteria.

Species of Bacteria can be divided into two major groups, called Gram positive and Gram negative.

The distinction between Gram positive and Gram negative bacteria is based on the Gram stain reaction.

Prokaryotic Cell Wall

Periplasmic space

A space is seen between the plasma membrane and the outer membrane of Gram negative bacteria, and a similar but smaller gap between the plasma membrane and wall in Gram positive bacteria.

Filled with a loose network of peptidoglycan.

The substance that occupies the periplasmic space is the periplasm.

Gram positive cells may have periplasm even if they lack a discrete, obvious periplasmic space.

Periplasmic space in Gram negative bacteria range from 1 nm to as great as 71 nm.

It may constitute about 20 to 40% of the total cell volume (around 30 to 70 nm).

It is filled with enzymes and proteins.

The periplasmic space of Gram negative bacteria contains many proteins that participate in nutrient acquisition for example, hydrolytic enzymes attacking nucleic acids and phosphorylated molecules, and binding proteins involved in transport of materials into the cell.

Denitrifying and chemolithoautotrophic bacteria have electron transport proteins in their periplasm.

The periplasmic space also contains enzymes involved in peptidoglycan synthesis and the modification of toxic compounds that could harm the cell.

Gram positive bacteria, they secrete several enzymes that ordinarily would be periplasmic in Gram negative bacteria.

Such secreted enzymes are often called exo-enzymes.

Some enzymes remain in the periplasm and are attached to the plasma membrane.

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Peptidoglycan Structure

Peptidoglycan or murein is an enormous polymer composed of many identical subunits.

The polymer contains two sugar derivatives, N acetyl glucosamine and N acetylmuramic acid (the actyl ether of N acetylglucosamine), and several different amino acids, three of which D glutamic acid, D alanine, and meso diaminopimelic acid are not found in proteins.

The presence of D amino acids protects against attack by most peptidases.

The backbone of this polymer is composed of alternating N acetylglucosamine and N acetylmuramic acid residues.

A peptide chain of four alternating D and L amino acids is connected to the carboxyl group of N acetylmuramic acid.

Many bacteria substitute another di amino acid usually L lysine, in the third position for meso diaminopimelic acid.

Chains of linked peptidoglycan subunits are joined by crosslinks between the peptides.

Often the carboxyl group of the terminal D alanine is connected directly to the amino group of diaminopimelic acid, but a peptide inter bridge is present in Gram positive bacteria.

Most Gram negative cell wall peptidoglycan lacks the peptide inter bridge.

This cross linking results in an enormous peptidoglycan sac that is actually one dense, interconnected network.

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Gram positive cell wall

In most Gram positive bacteria, the cell wall consists of many layers of peptidoglycan, forming athick, rigid structure.

The cell walls of Gram positive bacteria contain teichoic acids, which consist primarily of an alcohol(such as glycerol and phosphate.

There are two classes of teichoic acids.
1.lipoteichoic acid which spans the peptidoglycan layer and is linked to the plasma membrane
2.Wall teichoic acid which is linked to the peptidoglycan layer.

Because of their negative charge (from the phosphate groups), teichoic acids may bind and regulate the movement of cations (positive ions)in to and out of the cell.

Give the Gram positive cell wall its negative charge.

A role in cell growth, preventing extensive wall breakdown and possible cell lysis.

Provide much of the wall’s antigenic specificity.

The cell walls of Gram positive streptococci are covered with various polysaccharides that allow them to be grouped into medically significant types.

The segment of a teichoic acid made of phosphate, glycerol, and a side chain, R R may represent D alanine, glucose, or other molecules Prescott.

Gram negative cell envelope

The cell walls of Gram negative bacteria consist of one or a very few layers of peptidoglycan and an outer membrane.

Peptidoglycan may constitute not more than 5 to10 of the wall weight.

Contains only one or two layers or sheets of peptidoglycan.

They are more susceptible to mechanical breakage.

Gram negative cell walls do not contain teichoic acids.

The outer membrane lies outside the thin peptidoglycan layer.

Braun’s lipoprotein

The most abundant membrane protein

A small lipoprotein covalently joined to the under lying peptidoglycan and embedded in the outer membrane by its hydrophobic end.

The outer membrane and peptidoglycan are so firmly linked by this lipoprotein that they can be isolated as one unit.

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Adhesion site

Structure that may strengthen the Gram negative wall and hold the outer membrane in place is the adhesion site.

The outer membrane and plasma membrane appear to be in direct contact at many locations in the Gram negative wall with these adhesion sites.

Adhesion sites may be regions of direct contact or possibly true membrane fusions.

Substances can move into the cell through these adhesion sites rather than traveling through the periplasm.

 

Porins

Part of the permeability of the outer membrane is due to proteins in the membrane, called porins that form channels.

Porins permit the passage of molecules such as nucleotides, disaccharides, peptides, amino acids, vitamin B 12 and iron.

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