explain cell envelope

Cell envelope consists of a cell wall and plasma membrane. Cell envelope in some bacteria also contains an outer covering known as capsule in addition to cell wall and plasma membrane. Cell envelope provides rigidity to the cell and prevents the cell from bursting in hypotonic environment. It also protects the interior components of the cell by separating them from the outside environment.

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Cell Envelope

The cell envelope may be defined as the cell membrane and cell wall plus an outer membrane if one is present. The cell wall consists of the peptidoglycan layer and attached structures. Most bacterial cell envelopes fall into two major categories (Figure 1): Gram positive and Gram negative. This is based on Gram staining characteristics that reflect major structural differences between the two groups. Other types of cell wall are found in a few bacterial species (neither Gram positive nor Gram negative).

The peptidoglycan is a single bag-shaped, highly cross-linked macromolecule that surrounds the bacterial cell membrane and provides rigidity. It is huge (billions in molecular weight; compare proteins which are thousands in molecular weight). Peptidoglycan consists of a glycan (polysaccharide) backbone consisting of N-acetyl muramic acid and N-acetyl glucosamine with peptide side chains containing D- and L- amino acids and in some instances diaminopimelic acid. The side chains are cross-linked by peptide bridges. These peptide bridges vary in structure among bacterial species. Muramic acid, D-amino acids and diaminopimelic acid are not synthesized by mammals. PG is found in all eubacteria except Chlamydia andMycoplasma.

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CELL ENVELOPE

The cell envelope is the cell membrane and cell wall plus an outer membrane, if one is present.

Most bacterial cell envelopes fall into two major categories: Gram positive and Gram negative. These are differentiated by their Gram staining characteristics.

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 (poly-N-acetylglucosamine and N-acetylmuramic acid), which is located immediately outside of the cytoplasmic membrane. 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. intracellular parasites such as Mycoplasma) contain peptidoglycan, not all cell walls have the same overall structures. This is notably expressed through the classification into Gram positive and Gram negative bacteria.

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