Structure of Outer Membrane
Eubacteria and Archaeobacteria (Gram positive and Gram negative) differ with respect to their cell walls. Gram negative cell walls are more complex. An outer membrane surrounds a thin underlying layer of peptidoglycan (Table 7.4). Outer membrane is bilayered, consisting mainly of phospholipids, proteins and lipopolysaccharide (LPS).
LPS is composed of three parts which are covalently linked to each other. They are
1. Lipid A which is firmly embedded in the membrane,
2. Core polysaccharide that is located at the membrane surface and
3. Polysaccharide O antigens that extend like whiskers from the membrane surface into the surrounding medium
Special protein channels called porins span the membrane. The points of contact between outermembrane and cytoplasmic membrane are known as adhesions. Outer membrane is anchored to peptidoglycan layer by means of Braun’s lipoprotein. Periplasmic space between the cell membrane and the outer membrane.
· It serves as an impermeable barrier to prevent the escape of important enzymes (such as those involved in cell wall growth) from the periplasmic space.
· It serves as a barrier to various external chemicals and enzymes that could damage the cell. For example, the walls of many Gram positive bacteria can be easily destroyed by treatment with an enzyme called lysozyme, which selectively dissolves peptidoglycan. However, Gram negative bacteria are refractory to this enzyme because large protein molecules of enzyme cannot penetrate the outer membrane. Only when outer membrane is damaged the enzyme can penetrate.
· Porins allow the smaller molecules, such as amino acids, monosaccharides to pass across.
· Adhesions are export sites for newly synthesised LPS and porins, and are sites at which pili and flagella are made.
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