Cell Envelope of Bacteria

Cell Envelope of Bacteria

Cell envelope is an external covering that lies outside the cytoplasm. It is composed of two or three basic layers: the cell wall, the cell membrane and in some bacteria the outer membrane.

Structure of Prokaryotic Cell Wall

Prokaryotic cells almost always are bounded by a chemically complex cell wall. Cell wall lies beneath the external structures (capsules, sheaths and flagella). Cell wall lies external to the plasma membrane (cell membrane). Cell wall of eubacteria is made up of peptidoglycan or murein, whereas that of Archaeobacteria is composed of proteins, glycoproteins or polysaccharides. A few genera such as Methanobacterium, have cell walls composed of pseudomurein, a polymer whose structure superficially resemble eubacteria peptidoglycan of eubacteria but differs markedly in chemical composition.(Note: Ordinary or typical bacteria are sometimes called eubacteria to distinguish them from the phylogenetically distinct group known as archaeobacteria). Peptidoglycan is a cross linked polymer of enormous strength and rigidity. It is a polymer composed of many identical subunits (Figure 7.11). Peptidoglycan differs somewhat in composition and structure from one species to another, but it is basically a polymer of N-acetylglucosamine(NAG), N-acetylmuramic acid(NAM), L-alanine, D-alanine, D-glutamate, and a diamino acid (LL- or meso-diaminopimelic acid, L-lysine, L-ornithine, or L- diaminobutyric acid).

Cell wall may contain other substances in addition to peptidoglycan. For instance, Staphylococcus aureus and Streptococcus fecalis contain teichoic acids (polymer of acidic polysaccharides) covalently linked to peptidoglycan. Cell wall of Gram positive bacteria contain very little lipid but Mycobacterium and Corynebacterium cell walls are rich in mycolic acid (or Cord factor) which make them acid fast. When stained, the cells cannot be decolorized easily despite treatment with dilute acids. Mycoplasma lack cell wall.

Protoplast is a bacterial cell consisting of cell material bound by a cytoplasmic membrane.

Spheroplast is a bacterial cell with two membranes namely the cytoplasmic membrane and the outer membrane but no cell wall.

Functions of cell wall

·        It gives shape to bacteria like a bicycle tyre that maintains the necessary shape and prevents the more delicate inner tube (the cytoplasmic membrane) from bursting when it is expanded.

·        It protects bacteria from osmotic lysis in dilute solutions (hypotonic environment).

·        It protects cell from toxic substances.