Download lec1

Lec:1
Microbiology
Ass.lecturer: Israa Al- Yasiri
Eukaryotic and Prokaryote cell structure:
Prokaryote In comparison to eukaryotes, the intracellular
features of the bacterial cell are extremely simplistic(figure1).
Bacteria do not contain organelles in the same sense as
eukaryotes as shown in table (1).
Table (1):Characteristic of Prokaryotic bacterial cell and
Eukaryotic cell.
characteristic
Prokaryotic
Eukaryotic cell.
bacterial cell.
DNA within a nuclear membrane .
No
yes
Mitotic division
No
yes
No
yes
Chromosome number.
one
More than one
Membrane–bound organelles
No
yes
Size of ribosome.
70s
80s
Cell wall containing peptidoglycan.
yes
No
DNA
associated with histon
Such as mitochondria and lysosomes.
Lec:1
Microbiology
Ass.lecturer: Israa Al- Yasiri
Figure (1): Prokaryote cell structure.
Cell morphology:
Bacteria come in a wide variety of shapes
Perhaps the most elemental structural property of bacteria is
cell morphology (shape). Typical examples include:

coccus (spherical)

bacillus (rod-like)

spirillum (spiral)

filamentous
Lec:1
Microbiology
Ass.lecturer: Israa Al- Yasiri
Figure (1) :the bacterial shape.
Bacterial cell size:
Bacteria range in size fro m about 0.2 to 5 µm the smallest
bacteria (mycoplasma)are about the same size as the largest
viruses ( poxviruses) and are the smallest organisms capable of
existing outside a host .the longest bacteria rods approach the
size of some yeasts and human red blood cell.
Lec:1
Microbiology
Ass.lecturer: Israa Al- Yasiri
The bacterial structure:
 bacterial cell wall:
The structure of peptidoglycan.
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
(poly-N-
acetylglucosamine and N-acetylmuramic acid), which is located
immediately
outside
of
the
cytoplasmic
membrane.
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. intracellular parasites such as Mycoplasma)
contain peptidoglycan, not all cell walls have the same overall
structures. There are two main types of bacterial cell walls,
Gram positive and Gram negative, which are differentiated by
their Gram staining characteristics. For both Gram-positive and
Gram-negative bacteria, particles of approximately 2 nm can
pass through the peptidoglycan.
Lec:1
Microbiology
Ass.lecturer: Israa Al- Yasiri
The Gram positive cell wall is characterized by the presence
of a very thick peptidoglycan layer, which is responsible for the
retention of the crystal violet dyes during the Gram staining
procedure. in the Gram positive cell wall are polyalcohols called
teichoic acids, some of which are lipid-linked to form
lipoteichoic acids. Because lipoteichoic acids are covalently
linked to lipids within the cytoplasmic membrane they are
responsible for linking the peptidoglycan to the cytoplasmic
membrane. Teichoic acids give the Gram positive cell wall an
overall negative charge due to the presence of phosphodiester
bonds between teichoic acid monomers.
Unlike the Gram positive cell wall, the Gram negative cell wall
contains a thin peptidoglycan layer adjacent to the cytoplasmic
membrane, which is responsible for the cell wall's inability to
retain the crystal violet stain upon decolourisation with ethanol
during Gram staining. In addition to the peptidoglycan layer, the
Gram negative cell wall also contains an additional outer
membrane composed by phospholipids and lipopolysaccharides
which
face
into
the
external
environment.
As
the
lipopolysaccharides are highly-charged, the Gram negative cell
wall has an overall negative charge. The chemical structure of
the outer membrane lipopolysaccharides is often unique to
specific bacterial strains (i.e. sub-species) and is responsible for
many of the antigenic properties of these strains.
Lec:1
Microbiology
Ass.lecturer: Israa Al- Yasiri
 The bacterial cytoplasmic membrane:
The bacterial cytoplasmic membrane is composed of a
phospholipid bilayer and thus has all of the general functions of
a cell membrane such as acting as a permeability barrier for
most molecules and serving as the location for the transport of
molecules into the cell. In addition to these functions,
prokaryotic membranes also function in energy conservation as
the location about which a proton motive force is generated.
Unlike eukaryotes, bacterial membranes (with some exceptions
e.g. Mycoplasma generally do not contain sterols. However,
many microbes do contain structurally related compounds called
hopanoids which likely fulfill the same function. Unlike
eukaryotes, bacteria can have a wide variety of fatty acids
within their membranes. Along with typical saturated and
unsaturated fatty acids, bacteria can contain fatty acids with
additional methyl, hydroxy or even cyclic groups. The relative
proportions of these fatty acids can be modulated by the
bacterium to maintain the optimum fluidity of the membrane
(e.g. following temperature change).
As a phospholipid bilayer, the lipid portion of the outer
membrane is impermeable to charged molecules. However,
channels called porins are present in the outer membrane that
allow for passive transport of many ions, sugars and amino acids
across the outer membrane. These molecules are therefore
Lec:1
Microbiology
Ass.lecturer: Israa Al- Yasiri
present in the periplasm, the region between the cytoplasmic and
outer membranes. The periplasm contains the peptidoglycan
layer and many proteins responsible for substrate binding or
hydrolysis and reception of extracellular signals. The periplasm
it is thought to exist as a gel-like state rather than a liquid due to
the high concentration of proteins and peptidoglycan found
within it. Because of its location between the cytoplasmic and
outer membranes, signals received and substrates bound are
available to be transported across the cytoplasmic membrane
using transport and signalling proteins imbedded there.