The cell envelope is composed
of the plasma membrane and cell wall. As in other organisms, the bacterial cell
wall provides structural integrity to the cell. In prokaryotes, the primary
function of the cell wall is to protect the cell from internal turgor pressure
caused by the much higher concentrations of proteins and other molecules inside
the cell compared to its external environment. The bacterial cell wall differs
from that of all other organisms by the presence of peptidoglycan which is
located immediately outside of the cytoplasmic membrane. Peptidoglycan is made
up of a polysaccharide backbone consisting of alternating N-Acetylmuramic acid
(NAM) and N-acetylglucosamine (NAG) residues in equal amounts. Peptidoglycan is
responsible for the rigidity of the bacterial cell wall and for the
determination of cell shape. It is relatively porous and is not considered to
be a permeability barrier for small substrates. While all bacterial cell walls
(with a few exceptions e.g. extracellular parasites such as Mycoplasma) contain
peptidoglycan, not all cell walls have the same overall structures. Since the
cell wall is required for bacterial survival, but is absent in some eukaryotes,
several antibiotics (notably the penicillins and cephalosporins) stop bacterial
infections by interfering with cell wall synthesis, while having no effects on
human cells which have no cell wall only a cell membrane. There are two main
types of bacterial cell walls, those of gram-positive bacteria and those of
gram-negative bacteria, which are differentiated by their Gram staining
characteristics. For both these types of bacteria, particles of approximately 2
nm can pass through the peptidoglycan. If the bacterial cell wall is entirely
removed, it is called a protoplast while if it's partially removed, it is
called a spheroplast. β-Lactam antibiotics such as penicillin inhibit the
formation of peptidoglycan cross-links in the bacterial cell wall. The enzyme
lysozyme, found in human tears, also digests the cell wall of bacteria and is
the body's main defense against eye infections.
Sunday, December 11, 2016
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Cellular wall
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