Prokaryotic Cell Wall

Prokaryotic Cell Wall

  • Complex, the semi-rigid structure responsible for the shape of the cell Wall shape and strength is primarily due to peptidoglycan.
  • The cell wall surrounds the lower and delicate plasma membrane and protects it and the inside of the cell from adverse changes in the external environment.
  • Almost all prokaryotes have cell walls.
  • The main function of the cell wall is to protect the virus cells from being broken down when the water pressure inside the cell is greater than that outside the cell.
  • It also helps maintain the structure of the virus and acts as a flagella reinforcement.
  • It contributes to the ability of other pathogens and is a source of antimicrobial activity.
  • The chemical composition of the cell wall is used to differentiate between major types of bacteria.
  • Species of bacteria can be divided into two main groups, called Gram-positive and Gram-negative.
  • The distinction between Gram-positive and Gram-negative bacteria is based on the Gram stain reaction.

Periplasmic space

  • 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 can have periplasm even if they do not have a distinct, visible periplasmic area.
  • Periplasmic space in Gram-negative bacteria ranges 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 the 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 conversion of toxic chemicals that can damage the cell.
  • Gram-positive bacteria, 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.
  • Read More about prokaryotic cell wall…

Peptidoglycan Structure

  • Peptidoglycan or murein is a large polymer with many similar units.
  • The polymer contains two sugar derivatives, N acetyl glucosamine and N acetylmuramic acids (the acetyl 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 the invasion of many peptidases.
  • The backbone of this polymer is made up of alternatives to N acetylglucosamine and N acetylmuramic acid.
  • A peptide chain of four D and L amino acid exchanges are linked 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 connected peptidoglycan subunits are connected by crosslinks between 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 does not have a peptide inter bridge.
  • This link results in a large peptidoglycan sac that is actually a single, dense, interconnected network.

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Gram positive cell wall

In most Gram-positive bacteria, the cell wall contains many layers of peptidoglycan, which form a thick, solid structure.

The cell walls of Gram-containing bacteria contain teichoic acids, which are high in alcohol (such as glycerol and phosphate).

There are two classes of teichoic acids.

  1. lipoteichoic acid which opens the peptidoglycan layer and binds to the plasma membrane.
  2. Wall teichoic acid is linked to the peptidoglycan layer.

Due to their negative charge (from phosphate groups), teichoic acids can bind and regulate the flow of cations (positive ions) inside and outside the cell

Give the Gram-positive cell wall its negative charge.

It plays a role important in cell growth, preventing the collapse of the broad wall and the occurrence of 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 collected in medicinal important species.

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 be over 5 to 10 times the weight of a wall.

Contains 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

The lower lipoprotein joins the lower peptidoglycan and concentrates on the outer layer by its hydrophobic end.

The outer layer and peptidoglycan are so closely linked to this lipoprotein that it can be divided into a single unit.

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Adhesion site

The 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.

The adhesive areas can be direct contact regions or perhaps real membrane fusions.

Things can get into the cell through these adhesive sites rather than going through the periplasm.


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

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