- A bacterial cell shows various parts and these parts have specific structure and functions.
- These parts are separated by a rigid cell wall some parts are on the internal side of the cell wall and some parts are on the external side of the cell wall.
- The parts present on the internal side of the cell wall are cell membrane, cytoplasm, nuclear region, reserve food material etc.
- The cellular parts present on external to the cell wall are flagella, capsule and pilia.
Structure and function of Cell wall
- The cell wall is the outer covering of bacterial cell in absence of capsule.
- This cell wall is rigid in nature which imparts a proper shape to bacteria.
- Cell wall encloses all internal parts of the cell.
- On the basis of typical structure, composition and Gram staining they are classified
A) Gram-positive bacteria.
Types of bacterial cell
A) The cell wall of Gram-positive bacteria.
- It is thick monolayered and accounts about 10 to 50% dry weight of cell.
- Gram-positive cell wall is mainly composed of two important components.
2. Teichoic acid.
Components of peptidoglycan of Gram-positive cell.
- Peptidoglycan is an important component of the cell wall.
- It is also called as glycopeptide and mucopeptide.
- Peptidoglycan is present almost in all bacteria except Halococcus and Halobacter.
- The three main components of peptidoglycan are
a ) Glycan backbone strand.
b ) Tetrapeptide side chain.
c ) Cross bridges.
Structure of peptidoglycan of the gram positive cell wall.
a ) Glycan backbone strand.
- Glycan backbone strand contains a long chain of amino sugars these sugars are N-acetylmuramic acid and N-acetyl glucosamine.
- These sugars are linked to each other by Beta-1,4 linkage.
- It is the unbranched chain.
b ) Tetrapeptide side chain
- Tetrapeptide side chain consists of four amino acids in a particular sequence.
- The sequence is L-alanine, D-glutamic acid, L-lysin, D-alanine.
- Each N-acetylmuramic acid of glycan strand is linked to four amino acid by the peptide bond.
- It is called as tetrapeptide side chain.
c ) Cross-linkage
- When two adjacent tetrapeptide side chain is linked to each other by a glycan molecule called as cross-linkage.
- The carboxyl group of fourth amino aid D-alanine of the side chain is linked to amino group third amino acid L-lysin of second tetrapeptide side chain.
- The linkage of carboxyl group and the amino group is formed by five glycan molecules so called as a cross bridge.
2. Teichoic acid
- Teichoic acid is the second most important component of the Gram-positive bacterial cell wall.
- Teichoic acid is a water-soluble polymer.
- It has repeating sugars.
- Depending on types of repeating sugars teichoic acid is of two types.
- They are Glycerol teichoic acid and Ribitol teichoic acid.
1.Glycerol teichoic acid
- It is a long chain of glycerol 3 phosphate molecule.
- These glycerol 3 phosphate are linked by phosphodiester bonds.
2. Ribitol teichoic acid.
- It is a long chain of ribitol 5 phosphate molecule.
- These are also linked by phosphodiester bonds.
Functions of the cell wall.
- Cell wall gives rigidity, specific size and shape to the cell.
- Cell wall protects the cell from osmotic pressure.
- In high salt concentration cell wall protects the cell from the explosion.
- It plays important role in growth and cell division.
- Cell wall components such as lipopolysaccharides and teichoic acid provide an attachment site for bacteriophages.
- The cell wall is a permeable membrane and because of its permeability, it provides transport of salts, water and other nutrients in the cell.