4. CELL ENVELOPE
The bacteria cell envelope is a complex multilayered structure
that serves to protect these organisms from their
unpredictable and often hostile environment.
There are three principal layers in the cell envelope
1. Cytoplasmic or inner membrane
2. The peptidoglycan cell wall
3. The outer membrane
5. CYTOPLASMIC MEMBRANE
A cell is the basic unit of life, and all organisms
are made up of one or many cells. One of the
things that all cells have in common is a cell
membrane. It is a barrier that separates a cell
from its surrounding environment. This outer
boundary of the cell is also called the plasma
membrane. It is composed of four different
types of molecules:
Phospholipids
Cholesterol
Proteins
Carbohydrates
6. Phospholipids
Phospholipids make up the basic structure of a cell
membrane. A single phospholipid molecule has two
different ends: a head and a tail. The head end
contains a phosphate group and is hydrophilic. This
means that it likes or is attracted to water molecules.
The tail end is made up of two strings of hydrogen
and carbon atoms called fatty acid chains. These
chains are hydrophobic, or do not like to mingle with
water molecules.
7. Cholesterol, Proteins and
Carbohydrates
Cholestrol molecules are made up of four rings of hydrogen and
carbon atoms. They are hydrophobic and are found among the
hydrophobic tails in the lipid bilayer.
Cholesterol molecules are important for maintaining the
consistency of the cell membrane. They strengthen the
membrane by preventing some small molecules from crossing it.
Some plasma membrane proteins are located in the lipid bilayer
and are called integral proteins. Other proteins, called peripheral
proteins, are outside of the lipid bilayer.
Carbohydrates, or sugars, are sometimes found attached to
proteins or lipids on the outside of a cell membrane.
Together, these carbohydrates form the glycocalyx.
The glycocalyx of a cell has many functions. It provides
cushioning and protection for the plasma membrane, and it is
also important in cell recognition.
10. The cell envelope of Gram-negative bacteria
consists of two membranes, the outer
membrane and the cytoplasmic membrane.
The cytoplasmic membrane is composed of a
phospholipid bilayer, whereas the outer
membrane comprises an interior leaflet of
phospholipids and an exterior leaflet of
lipopolysaccharide (LPS); LPS is composed
of lipid A, the core oligosaccharide and O
antigen. In between the two membranes is
the periplasmic space, which contains the
peptidoglycan (PG) layer and periplasmic
proteins. The PG layer comprises long
polymers of the repeating disaccharide N-
acetylglucosamine–N-acetylmuramic acid
(NAG–NAM) that are linked via peptide
bridges.
11.
12. PEPTIDOGLYCANE -
LIPOPROTEIN COMPLEX
The covalently linked peptidoglycan-
lipoprotein complex of the gram-negative
cell wall can be thought of as the
inelastic foundation of the whole-cell
envelope and is, therefore, somewhat
analogous to the gram-positive cell wall,
the plant cell wall, and the animal cell's
glycocalyx.
The peptidoglycan of gram negative
bacteria has a relatively open molecular
structure due to a deficiency of cross-
linking .
13. PEPTIDOGLYCANE -
LIPOPROTEIN COMPLEX
Peptidoglycan is present in the gram-
negative cell envelope in highly variable
amounts.
the peptidoglycan layer exerts morphological
control over the cytoplasmic elements of the
cell.
Isolated peptidoglycan structures also
maintain the shape of the cell.
peptidoglycan layer is assembled from
components produced in the cytoplasmic
membrane
The turgor pressure of the living cell would
force the cytoplasmic membrane outward
against the inelastic peptidoglycan layer .
14. We would expect the peptidoglycan
layer to bind specific enzymes, in the
same way that penicillinase is bound by
the cell walls of Bacillus licheniformis
and alkaline phosphatase (APase) is
bound by certain gram-positive bacterial
and plant cell walls, and to concentrate
certain ions, much as grampositive
walls concentrate Al3" and plant cell
walls concentrate Ca2".
15.
16. PERIPLASMIC ZONE
The periplasm is a concentrated gel-
like matrix in the space between the
inner cytoplasmic membrane and the
bacterial outer membrane called
the periplasmic space in gram-
negative bacteria. Using cryo-electron
microscopy it has been found that a
much smaller periplasmic space is
also present in gram-positive bacteria
18. OUTER LAYER
Outside the periplasm most
important layer called outer
membrane.
This lipid-protein bilayer is
usually considered to be the
outermost layer of the gram-
negative wall.
“It is a membrane which
possesses proteins,
phospholipids, and
lipopolysaccharides (LPSs) and
which separates the external
environment from the
periplasm.”
19. cont..
Outer membrane is
similar to cell
membrane bilayer
structure.
One of the unusual
features of the outer
membrane is its
asymmetric
distribution of lipids
over its inner and
outer faces. The outer
face contains
(virtually) all of the
LPS, whereas the
inner face has most
of the phospholipid.
21. cont..
Lipids attach with multiple sugar moities called LPS
(endotoxin).
Lipid A portion made of 2 Glucoseamine derivatives and
each of which attached one phosphate group also called
pyrophosphate, also 3 fatty acid chains are attached.
This lipid A portion is endotoxic.
Normally when the bacteria is living, Lipid A is embadded not
coming out, only this Lipid A portion come outside when the
bacteria is death, if bacteria is kill lipid A is exposed to body then
body will react.
Core polysaccharide
Made of 10 sugars.
Linked the region of lipid A and O polysaccharide.
22. cont...
O polysaccharide (O-Antigen)
Structure is highly variable
Also made of sugars.
Immunogenic property
Properties
LPS prevents harmful substances and allowing
useful substances.
LPS also helps them to detect bacteriophage.
23. cont...
PORIN PROTEINS:
Porins are found in the outer membranes of Gram-negative bacteria,
mitochondria, and chloroplasts.
Low molecular weight .
Transmembrane protein.
Porin proteins form channels through which small molecules (600-
700) dalton can pass.
Porins protein helps in the entry of substances useful into the
bacteria.
25. THREE STAGES
1- Cytoplasmic Stage:
Synthesis of precursors (NAG,NAM)
2- Membrane Stage : (Elongation and
Transfer)
Transfer of the precursors from cytosol to
membrane and incorporation into the growing
peptidoglycan
3- Extracellular Stage: (Crosslinking)
Crosslinking of linear chains of peptidoglycan by
membrane bound transpeptidases (beta-lactams)
27. Stage 2: Elongatin And
Transfer
BACTOPRENO
L:
carrier molecule
that plays major
role in insertion of
peptidoglycan
precursors.
Bonds to
NAM/NAG/
pentapeptide
precursors.
28. Stage 3 : Crosslinking
Neighboring
peptidoglycan
chain are
crosslinked
through their
peptide side
chains
This reaction is
catalysed by
transpeptidase
Transpeptidase is
a penicillin
binding protein
Beta-lactam
inhibit
tranpeptidase
30. Cont..
Cell wall also contain variety of degradative
enzymes and these have been localized
within the periplasmic space and at cell
surface.
Wall associated degradative enzymes
include:
• Deoxyribonuclease
• APase
• Ribonuclease
• Uridine 5’- diphosphatase
31.
32. Cont..
• 3’-nucleotidase
• 5’-nucleotidase
• ADP
Both 3’ nucleotidase and 5’ nucleotidase
localized in the outer cell wall layer of Ecoli and
APase localized in the periplasmic space and
also in the periplasmic space of Ecoli.
It is clear that a wall associated enzymes
may occur in more than one zone of gram
negative cell wall.
33. Function Of Cell Wall
Associated Enzymes:
Cell wall associated enzymes of gram
negative bacteria are very similar to the
degradative enzymes.
Digestive function, multilayered structure.
Degradative enzymes which are located in
the periplasmic area act on substrate which
diffuse into the zone of the cell wall.
Products are available to binding.
