4) Cell wall:  It is a rigid structure that lies beneath external structures( Pili, capsule, flagella) & external to cytoplasmic membrane. It gives shape & rigidity  to cell. Cell wall consists network of peptidoglycan layer( murein or mucopeptide).
Structure Of Peptidoglycan layer:  
                   A peptidoglycan is only found in prokaryotic cell surrounding cytoplasm & differs  from species to species. Basically, it is composed alternating unit of NAM(N-acteylmuramic acid) & NAG(N-acteylglucosamine). It also consists of peptide of four or five amino acids ( L-alanine, D-alanine , D-glutamate,  and either lysine or Diaminopimelic acid(DAP).  

                 So, peptidoglycan simply  consists of polysaccharides backbone composed of alternating units of NAG & NAM  with short peptide chain projecting from NAM units. There peptide chains are cross linked with each other & this cross linkage can be different  in Gram positive & gram negative bacterial cells.

In Gram Negative, the  cross-linkage occurs by peptide linkage of amino group of  DAP to carboxyl group of D-alanine. Eg. E.coli  peptidoglycan with direct cross linkage, a typical gram negative bacteria.

In Gram positive, the cross linkage occurs by peptide inter bridge.  Eg. The inter bridge between Staphylococcus aureus  consists of five glycine molecules.   

Fig: Peptidoglycan Composition

                                Fig: A Model Of Constitution of Bacterial cell wall



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