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The Bacterial Cell Wall
The bacterial cell wall is a unique structure which surrounds the cell
membrane. Although not present in every bacterial species, the cell wall is very
important as a cellular component. Structuraly, the wall is necessary for:
- Maintaining the cell's characteristic
shape- the rigid wall compensates for the flexibility of the
phospholipid membrane and keeps the cell from assuming a spherical
shape
- Countering the effects of osmotic
pressure- the strength of the wall is responsible for keeping the cell
from bursting when the intracellular osmolarity is much greater than the
extracellular teichoic osmolarity
- Providing attachment sites for
bacteriophages-teichoic acids attached to the outer surface of
the wall are like landing pads for viruses that infect bacteria
- Providing a rigid platform for surface
appendages- flagella, fimbriae, and pili all
emanate from the wall and extend beyond it
The cell walls of all bacteria are not identical. In fact, cell wall
composition is one of the most important factors in bacterial species analysis
and differentiation. There are two major types of walls: Gram-positive and
Gram-negative. The cell wall of Gram-positive
bacteria consists of many polymer layers of peptidoglycan
connected by amino acid bridges. The peptidoglycan polymer is
composed of an alternating sequence of N-acetylglucosamine and
N-acetyl-muraminic acid. It's a lot easier to just remember NAG and NAMA. Each
peptidoglycan layer is connected, or crosslinked, to the other by a bridge made
of amino acids and amino acid derivatives. The particular amino acids vary among
different species, however. The crosslinked peptidoglycan molecules form a
network which covers the cell like a grid. Also, 90% of the Gram-positive cell
wall is comprised of peptidoglycan.
The cell wall of Gram-negative bacteria is much thinner, being comprised of
only 20% peptidoglycan. Gram-negative
bacteria also have two unique regions which surround the outer
plasma membrane: the periplasmic space and the lipopolysaccharide layer. The
periplasmic space separates the outer plasma membrane from the
peptidoglycan layer. It contains proteins which destroy potentially dangerous
foreign matter present in this space. The lipopolysaccharide layer is
located adjacent to the exterior peptidoglycan layer. It is a phospholipid
bilayer construction similar to that in the cell membrane and is attached to the
peptidoglycan by lipoproteins. The lipid portion of the LPS contains a toxic
substance, called Lipid A, which is responsible for most of the pathogenic
affects associated with harmful Gram-negative bacteria. Polysaccharides which
extend out from the bilayer also contibute to the toxicity of the LPS. The LPS,
lipoproteins, and the associated polysaccharides together form what is known as
the outer membrane.
Keep in mind that the cell wall is not a regulatory structure like the cell
membrane. Although it is porous, it is not selectively permeable and will let
anything pass that can fit through its gaps.
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