Microbiology lecture pdf

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Microbiology and Bioprocess Technology (BTC 302)
- Microbial Cell Structures -
CELL MEMBRANE, RIBOSOME
Most of the texts and images of this presentation are from Prescott, Harley, and Klein’s Microbiology book
(7th Ed). Some are from Brock Biology of Microorganisms (14th Ed)
The study materials/presentations are solely meant for academic purposes and they can be reused, reproduced, modified, and
distributed by others for academic purposes only with proper acknowledgements
Presentation prepared by Prof Sufia K Kazy, NIT Durgapur

2. Presentation prepared by Prof Sufia K Kazy, NIT Durgapur 2
CELL MEMBRANE FUNCTIONS
Membranes are an absolute requirement for all living organisms.
The plasma membrane encompasses the cytoplasm. It is the chief
point of contact with the cell’s internal environment.
The plasma membrane serves as a selectively permeable barrier -
it allows particular ions and molecules to pass, either into or out
of the cell, while preventing the movement of other molecules.
Many substances can only cross the plasma membrane with
assistance of transport proteins/carriers. Membrane transport
systems are used for nutrient uptake, waste excretion, and protein
secretion.

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The procaryotic plasma membrane is the location of a
variety of crucial metabolic processes - respiration,
photosynthesis, synthesis of lipids and cell wall constituents.
The membrane contains special receptor molecules that
help procaryotes detect and respond to chemicals (signals)
in their surroundings.
The plasma membrane is essential for the survival of
organisms. The membrane prevents the loss of essential
cellular components through leakage.

4. CELL MEMBRANE STRUCTURE
All membranes apparently have a common, basic design. Prokaryotic
membranes can differ in terms of the lipids they contain. Membrane
chemistry can be used to identify particular bacterial species.
The most widely accepted model for membrane structure is the fluid
mosaic model of Singer and Nicholson - membranes are lipid bilayers
within which proteins float.
Cell membranes are very thin structures, about 5 to 10 nm thick. The
membrane lipids are organized in two sheets (layers) of molecules arranged
end-to-end. Within the lipid bilayer, membrane proteins are present.
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Bacterial plasma membrane is highly organized and
asymmetric system that also is flexible and
dynamic.
Lipids are not homogeneously distributed in the
plasma membrane. There are domains in which
particular lipids are concentrated.
Lipid composition of bacterial membranes varies
with environmental temperature.

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Most membrane-associated lipids are structurally
asymmetric - with polar and nonpolar ends –
therefore called amphipathic.
The polar ends interact with water – hydrophilic; the
nonpolar - hydrophobic ends are insoluble in water
and tend to associate with one another.
In aqueous environments, amphipathic lipids can
form a bilayer.

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The outer surfaces of the bilayer membrane are
hydrophilic, whereas hydrophobic ends (fatty acid
chains) are buried in the interior of bilayer - away from
the surrounding water.
Many of the amphipathic membrane lipids are
phospholipids (like eucaryotes).
Membrane lipids can rotate, move laterally or
transversely (flip-flop - from top layer to bottom layer or
vice-versa).

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Bacteria lack sterols (steroid-containing lipids of
eucaryotes, such as cholesterol). But, many
bacterial membranes contain sterol-like
molecules called hopanoids.
Hopanoids probably stabilize the membrane.
There is evidence that hopanoids have
contributed significantly to the formation of
petroleum.

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Two types of membrane proteins have been identified
Peripheral proteins – they are loosely connected to
the membrane inner surface and can be easily
removed. They are soluble in aqueous solutions and
make up about 20 to 30% of total membrane proteins.
Integral proteins – embedded within the membrane
bilayer; about 70 to 80% of membrane proteins are
integral proteins.

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Integral proteins are not easily extracted from membranes and
are insoluble in aqueous solutions.
Integral proteins are also amphipathic; their hydrophobic regions
are buried in the lipid, while the hydrophilic portions project
from the membrane surface.
Integral proteins can diffuse laterally in the membrane to new
locations, but they do not flip-flop or rotate through the lipid
layer.
Carbohydrates often are attached to the outer surface of plasma
membrane proteins (glycoproteins) or lipids (glycolipids).

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Plasma membrane
infoldings within the cell
are present in many
bacteria.
This can become
extensive and complex in
photosynthetic bacteria
(cyanobacteria and
purple bacteria) or in
bacteria with very high
respiratory activity
(nitrifying bacteria).

14. Internal membranous structures can be observed in some bacteria
in the form of aggregates of spherical vesicles, flattened vesicles,
or tubular membranes. Their function may be to provide a larger
membrane surface for greater metabolic activity.
One membranous structure – mesosome - sometimes reported in
bacteria. Mesosomes appear to be invaginations of the plasma
membrane within cytoplasm. Variety of functions have been
ascribed to mesosomes.
But many bacteriologists believe that they are artifacts generated
during the chemical fixation of bacteria for electron microscopy.
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15. ARCHAEAL MEMBRANES
One of the most distinctive features of the Archaea is
the nature of their membrane lipids.
They differ from both Bacteria and Eucarya in having
branched chain hydrocarbons attached to glycerol by
ether links rather than fatty acids connected by ester
links (as in bacteria, eucarya).
Sometimes two glycerol groups at opposite ends are
linked to form an extremely long tetraether chain. 15

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Usually the diether hydrocarbon chains are 20 carbons in
length, and the tetraether chains are 40 carbons in length.
Cells can adjust the overall length of the tetraethers by
cyclizing the chains to form pentacyclic rings.
Phosphate-, sulfur- and sugar-containing groups can be
attached to the third carbons of the diethers and
tetraethers, making them polar lipids.
Polar lipids predominate in the membrane, and 70 to 93% of
the membrane lipids are polar. The remaining lipids are
nonpolar and are usually derivatives of squalene.

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The basic design of archaeal membranes is
similar to that of Bacteria and eucaryotes —
there are two hydrophilic surfaces and a
hydrophobic core.
When C20 diethers are used, a regular bilayer
membrane is formed. When the membrane is
constructed of C40 tetraethers, a monolayer
membrane with much more rigidity is formed.

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The membranes of extreme thermophiles such
as Thermoplasma and Sulfolobus, which grow
best at temperatures over 85°C, are almost
completely tetraether monolayers.
Archaea that live in moderately hot
environments have a mixed membrane
containing some regions with monolayers and
some regions with bilayers.

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RIBOSOMES
The cytoplasmic matrix often is packed with ribosomes -
they also may be loosely attached to the plasma membrane.
Ribosomes are very complex structures made of both
protein and ribonucleic acid (RNA). They are the site of
protein synthesis.
Cytoplasmic ribosomes synthesize proteins destined to
remain within the cell, whereas plasma membrane
ribosomes make proteins for transport to the outside.

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Procaryotic ribosomes are smaller than the
ribosomes of eucaryotic cells.
Procaryotic ribosomes are 70S ribosomes (as
opposed to 80S in eucaryotes), have
dimensions of about 14 to 15 nm by 20 nm,
molecular weight of approximately 2.7 million,
and are constructed of a 50S and a 30S
subunit.

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The S stands for Svedberg unit - the unit of the
sedimentation coefficient - a measure of the
sedimentation velocity in a centrifuge; the faster a
particle travels when centrifuged, the greater its
Svedberg value or sedimentation coefficient.
The sedimentation coefficient is a function of a
particle’s molecular weight, volume, and shape.
Heavier and more compact particles normally have
larger Svedberg numbers and sediment faster.

24. The bacterial ribosome. The 50S and 30S bacterial subunits, split apart to visualize the surfaces that
interact in the active ribosome. The structure on the left is the 50S subunit with tRNAs (displayed as
green backbone structures) bound to sites E, P, and A,; Proteins appear as blue wormlike structures. The
rRNA is represented as a gray. The structure on the right is the 30S subunit. Protein backbones are brown
wormlike structures and the rRNA is a lighter tan surface.

25. Summary of the composition
and mass of ribosomes in
bacteria and eukaryotes
Ribosomal subunits are
identified by their S (Svedberg
unit) values - sedimentation
coefficients that refer to their
rate of sedimentation in a
centrifuge.
The S values are not
necessarily additive when
subunits are combined,
because rates of
sedimentation are affected by
shape as well as mass.

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Carl Woese realized in the
1970s that the sequence of
rRNA molecules and their
genes could be used to
infer evolutionary
relationships between
different organisms.
DNA sequences provide a
record of past evolutionary
events and can be used to
determine phylogeny,
which is the evolutionary
history and relationship of
organisms.

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Ribosomal RNA genes are excellent candidates for phylogenetic
analysis because they are - (1) universally distributed, (2)
functionally constant, (3) highly conserved, and (4) of adequate
length to provide a deep view of evolutionary relationships.
The first effective tool for the evolutionary classification of
microorganisms. The utility of SSU rRNAs is extended by the
presence of certain sequences that are variable among organisms
and other regions that are quite stable.
The variable regions enable comparison between closely related
microbes, while the stable sequences allow the comparison of
distantly related microorganisms.

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Since 1977 more than 2.3 million SSU rRNA sequences
have been generated and used to characterize the
vast diversity of the microbial world.
The Ribosomal Database Project (RDP;
http://rdp.cme.msu.edu) contains an ever-growing
collection of these sequences and provides
computational programs for their analysis and for the
construction of phylogenetic relationship trees.