. Explain the term Cell wall
1. It is present outside the membrane and is a rigid structure.
2. The cell walls of almost all the eubacteria (true bacteria) are made up of peptidoglycan, also called murein or mucopeptide. As the name suggests, the peptidoglycan consists of two components; peptide portion and a glycan or sugar portion.
3. The glycan portion is composed of alternating units of amino sugar, N-acetyl glucosamine (NAG) and N-¬acetyl muramic acid (NAM) joined together by , 1-4 glycosidic linkage. This forms the backbone structure of cell wall. The mucopeptide chains are laterally joined by amino acids for stuctural rigidity. In many Gram (+) bacteria and actinomycetes there are four amino acids – L-alanine, D-glutamic, L-lysine and D-alanine. On the other hand, in Gram (–) bacteria, myxobacteria and blue green algae, diaminopimelic acid (DAP) is present in place of lysine.
4. In Gram (+) bacteria, the cell wall has a thick peptidoglycan layer (90%) and also contains teichoic acids, formed of glucose, phosphate and alcohol. Teichoic acid has several functions such as binding metals, acting as receptor sites for some viruses, and maintaining cells at low pH to prevent degradation of cell wall by self-produced enzymes.
5. The cell wall of Gram –ve bacteria is much more complex. The peptidoglycan layer is very thin making up only 10% or less. However, the most interesting feature is the presence of an outer membrane that covers a thin underlying layer of peptidoglycan. The outer face of outer membrane contains lipopolysaccharides, a part of which is integrated into the membrane lipids. The inner face has a number of proteins, which are anchored into peptidoglycan. The outer membrane of Gram –ve bacteria contains proteins called porin, and these proteins function as channels for the entry and exit of hydrophilic low molecular weight substances.
6. In Mycobacterium, Corynebacterium and Noccardia, the wall is that of Gram (+) type but a part of their cell wall is made up of a very long chain of the fatty acid called mycolic acid.
7. Due to the presence of the outer membrane, Gram-negative bacteria are rich in lipids that make about 11¬-12% of the dry weight of the wall. Teichoic acid is absent in this case.