Pokok Bahasan Mata Kuliah Mikrobiologi Dasar Program Studi Biologi (S1) Fak. Biologi UNSOED

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Dinding Sel Prokariotik

The cell wall is a rigid structure that lies just outside the plasma membrane; it provides the characteristic shapes of the various procaryotes and protects them from osmotic lysis
1 The cell walls of most bacteria contain peptidoglycan; the cell walls of archaea lack peptidoglycan and instead are composed of proteins, glycoptoteins, or polysaccharides
2 The cell walls of gram-positive bacteria and gram-negative bacteria differ greatly, but both have a periplasmic space, which usually contains a variety of proteins; these proteins can be involved in nutrient acquisition, electron transport, peptidoglycan synthesis or in modification of toxic compounds
Peptidoglycan (murein) is a polysaccharide polymer found in bacterial cell walls; it consists of polysaccharide chains cross-linked by peptide bridges
Gram-positive cell walls-consist of a thick layer of peptidoglycan and large amounts of teichoic acids
Gram-negative cell walls
1 They consist of a thin layer of peptidoglycan surrounded by an outer membrane composed of lipids, lipoproteins, and a large molecule known as lipopolysaccharide (LPS). LPS can play a protective role and can also act as an endotoxin, causing some of the symptoms characteristic of gram-negative bacterial infections; there are no teichoic acids in gram-negative cell walls
2 The outer membrane is more permeable than the plasma membrane because of porin proteins that form channels through which small molecules (600-700 daltons) can pass
The mechanism of Gram staining-involves constricting the thick peptidoglycan layer of gram-positive cells, thereby preventing the loss of the crystal violet stain during the brief decolorization step; the thinner, less cross-linked peptidoglycan layer of gram-negative bacteria cannot retain the stain as well, and these bacteria are thus more readily decolorized when treated with alcohol
The cell wall and osmotic protection-the cell wall prevents swelling and lysis of bacteria in hypotonic solutions. However, in hypertonic habitats, the plasma membrane shrinks away from the cell wall in a process known as plasmolysis

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