Download Prokaryotic cells A prokaryote is a simple, unicellular organism that

Prokaryotic cells
A prokaryote is a simple, unicellular organism that lacks an organized nucleus
or other membrane-bound organelle.
CELL SHAPES
Procaryotes come in a variety of shapes including spheres (cocci), rods
(bacilli), ovals (coccobacilli), curved rods (vibrios), rigid helices
(spirilla), and flexible helices (spirochetes)
Some lack a single, characteristic form and are called pleiomorphic
cocci (s., coccus) – spheres
diplococci (s., diplococcus) – pairs
streptococci – chains
staphylococci – grape-like clusters
tetrads – 4 cocci in a square
sarcinae – cubic configuration of 8 cocci
CELL ARRANGEMENT.
During the reproductive process, some cells remain attached to each other to
form chains, clusters, square planar configurations (tetrads)
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SIZE OF BACTERIAL CELLS
Procaryotic cells vary in size although they are generally smaller than most
eucaryotic cells (smaller than 0.2µ in diameter to those more than 50 µm in
diameter).
A few very large prokaryotes, such as surgeonfish symbiont Epulopiscium
fishelsoni (80 µm in diameter and can be more than 0.6 mm in length).
The bacterium Escherichia coli, are about 1x3 µm and are typical for the
vast majority of prokaryotes.
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Reasons most prokaryotes are very small
1.Nutrients and waste products pass more readily into and out of a small cell
than a large cell, thus accelerating cellular metabolism and growth.
This is because relative to cell volume, small cells contain more surface area
than do large cells. The surface to-volume (S/V) ratio .
A cell with a smaller radius (r) value therefore has a higher S/V ratio than a
cell with a larger r value – .
2.Because growth rate depends to some extent on the rate of nutrient
exchange, the higher S/V of small cells typically supports more rapid growth
than for larger cells.
3.Small cells will typically develop larger populations than will large cells .
4.The small size of prokaryotes may ultimately explain why these organisms
tend to adapt rapidly to changing environmental conditions and easily exploit
new habitats (conditions that allow beneficial mutations to be immediately
expressed
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A.THE PLASMA MEMBRANE
The plasma membrane consists of a phospholipid bilayer with
hydrophilic surfaces (interact with water) and a hydrophobic interior
(insoluble in water); such asymmetric molecules are said to be
AMPHIPATHIC
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Archaeobacterial membranes have a monolayer instead of a bilayer
structure
The lipids of Archaea have ETHER linkages between glycerol and their
hydrophobic side chains.
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MEMBRANE PROTEINS
Proteins are associated with the membrane and may be either peripheral
(loosely associated and easily removed) or integral (embedded within
the membrane and not easily removed)
The membrane is highly organized, asymmetric, flexible, and dynamic
Chemical Composition of Membranes
The general structure of biological membranes is a phospholipid bilayer.
Phospholipids contain both hydrophobic (fatty acid) and hydrophilic
(glycerol-phosphate) components and can exist in many different
chemical form as a result of variation in the groups attached to the
glycerol backbone.
In a phospholipid, the fatty acids point inward toward each other to form
a hydrophobic environment, while the hydrophilic portions remain
exposed to the aqueous external environment.
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Thin sections of the cytoplasmic membrane can be seen with the
electron microscope. The cytoplasmic membrane appears as two lightcolored lines separated by a darker area. This unit membrane, as it is
called (because each phospholipid leaf forms half of the “unit”),
consists of a phospholipid bilayer with proteins embedded in it.
The major proteins of the cytoplasmic membrane typically have very
hydrophobic external surfaces in the regions of the protein that span the
membrane, and hydrophilic surfaces that make contact with the
environment and the cytoplasm.
The overall structure of the cytoplasmic membrane is stabilized by
hydrogen bonds and hydrophobic interactions.
In addition, cations such as Mg2+ and Ca2+ help stabilize the membrane
by combining ionically with negative charges of the phospholipids.
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THE PLASMA MEMBRANE SERVES SEVERAL FUNCTIONS FOR THE
CELL
a. Forms a boundary between cytoplasm and cell external, It retains the
cytoplasm and separates the cell from its environment
b. It serves as a selectively permeable barrier, allowing some molecules
to pass into or out of the cell while preventing passage of other
molecules
c. It is the location of a variety of crucial metabolic processes including
respiration, photosynthesis, lipid synthesis, and cell wall synthesis
d. It may contain special receptor molecules that enable bacterial
detection of and response to chemicals in the surroundings
e. The genetic material is bound to the cell membrane during cell
division which appears to have some role in the redistribution of this
material between the two new cells
B. INTERNAL MEMBRANE SYSTEMS
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1.Mesosomes are structures formed by invaginations of the plasma
membrane that may play a role in cell wall formation during division, in
chromosome replication and distribution, and in secretory processes;
however, mesosomes may be artifacts generated during chemical fixation for
electron microscopy
2.Photosynthetic bacteria may have complex infoldings of the plasma
membrane that increase the surface area available for photosynthesis
3.Bacteria with high respiratory activity may also have extensive infoldings
that provide a large surface area for greater metabolic activity
4.These internal membranes may be aggregates of spherical vesicles,
flattened vesicles, or tubular membranes
C. THE CYTOPLASMIC MATRIX
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The cytoplasmic matrix is the substance between the membrane and
the nucleoid
It is featureless in electron micrographs but is often packed with
ribosomes and inclusion bodies
Despite the homogenous appearance, the matrix is highly organized
with respect to protein location
Cytoskeletal elements
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Cell division Cell shape
Cell shape
INCLUSION BODIES

o
o
o
o
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
o
o
Inclusion bodies are granules of organic or inorganic material
that are stored by the cell for future use.
 ORGANIC:
Glycogen
Poly-β-hydroxybutyrate(PHB)
Cyanophycin proteins (N storage)
Carboxysomes (reserve Rubisco, site of CO2 fixation)
Some are not bounded by a membrane
Others are enclosed by a single-layered membrane
 INORGANIC:
Polyphosphate granules (=Volutin) (=metachromic)
Magnetosome: Contain iron in the form of magnetite
FUNCTION OF INCLUSION BODIES ???????
Gas vacuoles
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Aggregates of gas vesicles with a membrane made of protein
A type of inclusion body found in cyanobacteria and some other
aquatic forms
They provide buoyancy for these organisms and keep them at or
near the surface of their aqueous habitat
RIBOSOMES
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Ribosomes are complex structures consisting of protein and RNA
Sites for the synthesis of cellular proteins
Procaryotic ribosomes are similar in structure to, but smaller and less
complex than, eucaryotic ribosomes
70S (sveldberg unit): 50S+30S
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Nucleoid
The chromosome (CMS)
•
Closed circular, double-stranded DNA molecule
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Looped and coiled extensively
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DNA+RNA+proteins
•
Nucleoid proteins probably aid in folding
Nucleoid proteins differ from histones
The Plasmids
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usually small, closed circular DNA molecules
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exist and replicate independently of chromosome
•
not required for growth and reproduction
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may carry genes that confer selective advantage (e.g., drug resistance)
•
Virulence and Metabolic
What are EPISOMES?????
The loss of plasmid is called ………………..
The cell envelope
The cell wall in addition to cell membrane (together making
Periplasm
Gap between plasma membrane and cell wall (gram-positive bacteria), or
between plasma membrane and outer membrane (gram-negative bacteria).
Periplasmic enzymes:
•
Found in periplasm of gram-negative bacteria
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Function:
–
nutrient acquisition
–
electron transport
–
peptidoglycan synthesis
–
modification of toxic compounds
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Exoenzyme
•
secreted by gram-positive bacteria
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perform many of the same functions that periplasmic enzymes do for
gram-negative bacteria
CELL WALL
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•
Bacteria are divided into two major groups based on the response to
Gram-stain procedure.
–
gram-positive bacteria stain purple
–
gram-negative bacteria stain pink
staining reaction due to cell wall structure
Cell Wall Components
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Major component (~50%) is peptidoglycan
•
Important component of both gram-positive and gram-negative bacteria
•
Glycan chain cross-linked with peptides
•
Glycan: Two alternating sugars form backbone (sugar derivatives)
–
N-acetylglucosamine (composed of ?????)
–
N-acetylmuramic acid (composed of ?????)
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Some amino acids are not observed in proteins
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Importance of D-form of A.A. ?????
Peptidoglycan cross-links
Gram Positive

Cross links between the peptides= Peptide interbridge (in Gram
positive walls only)
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Composed primarily of peptidoglycan
•
May also contain large amounts of teichoic acids
•
Some gram-positive bacteria have layer of proteins on surface of
peptidoglycan
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Polymers of Glycerol or Ribitol joined by phosphate groups
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GRAM NEGATIVE
•
Consist of a thin layer of peptidoglycan surrounded by an outer
membrane
•
Outer membrane composed of lipids, lipoproteins, and
lipopolysaccharide (LPS)(lipids+carbohydrates)
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No teichoic acids
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more permeable than plasma membrane due to presence of porin
proteins and transporter proteins
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porin proteins form channels through which small molecules (600-700
daltons) can pass
•
LPS Consist of three parts:
 lipid A
 core polysaccharide
 O side chain (O antigen)
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Functions of LPS
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protection from host defenses (O antigen)
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contributes to negative charge on cell surface (core polysaccharide)
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helps stabilize outer membrane structure (lipid A)
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can act as an exotoxin (lipid A)
Gram Staining
Mechanism of Gram staining?????
Osmotic protection and Cell Wall
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Cell wall protects against osmotic lysis
•
osmotic lysis
 can occur when cells are in hypotonic solutions
 movement of water into cell causes swelling and lysis due to
osmotic pressure
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•
Cell walls do not protect against plasmolysis
Plasmolysis
o occurs when cells are in hypertonic solutions
[solute]outside cell > [solute]inside cell
o water moves out of cell causing cytoplasm to shrivel and pull
away from cell wall
o useful in food preservation e.g., dried foods and jellies
 osmotic lysis
o basis of lysozyme and penicillin action
•
•
protoplast – cell completely lacking cell wall
spheroplast – cell with some cell wall remaining
Archeaobacteria cell walls lack peptidoglycan
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Composed of proteins, glycoproteins, or polysaccharides
Paracrystalline suface layer (S-layer): S layers are regularly structured
layers of protein or glycoprotein
Pseudopeptidoglycan
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What are the differences between cell wall in eubacteria and archeae???
Components External to the Cell Wall
Glycocalyx
Capsules and slime layers
o Layers of polysaccharides lying outside the cell wall (tight matrix)
o Capsules are well organized and tightly bound
o Protect the bacteria from phagocytosis, viral infection, pH fluctuations,
osmotic stress, hydrolytic enzymes, or the predacious bacterium
Bdellovibrio
o Slime layers are diffused and unorganized
o Biofilms ??????
Pili and fimbriae
Fimbriae: Short, thin, hairlike protein appendages
o Bacterial attachment to surfaces, short, thin, hairlike,
proteinaceous appendages
o mediate attachment to surfaces
o some (type IV fimbriae) required for twitching motility or gliding
motility that occurs in some bacteria
Pili: attachment to other bacteria during sexual mating (Specialized “sex” pilus
– conjugation)
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Flagella and motility
Flagella are threadlike locomotor appendages extending outward from the
plasma membrane and cell wall
Arrangement of flagella:
a. Monotrichous- single flagellum
b. Lophotrichous- cluster (tuft) of
flagella at one, or both ends
c. Amphitrichous-single flagellum at each pole
d. Peritrichous- distribution over the entire surface of the bacterium
Flagellar ultrastructure:
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The flagellum consists of a hollow filament composed of a single protein
known as flagellin.
Filament- Hook- Basal body
The hook is a short curved wider segment that links the filament to the basal
body (a series of rings that drives flagellar rotation).The motor is anchored in
the cytoplasmic membrane and cell wall.
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The motor consists of a small central rod that passes through a series of
rings.
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In G-negative bacterial cells an outer ring called the L-Ring is anchored
in the lipopolysaccharide layer.
o A second ring called the P-Ring, is anchored in the peptidoglycan
layer of the cell wall.
o A third set of rings, called the MS and C-Rings are located within
the cytoplamic membrane and the cytoplasm respectively.
In G-positive bacterial cells “which lack the outer membrane” only the
inner pair of rings is present.
Surrounding the inner ring and anchored in the cytoplasmic membrane
are a series of proteins called “Mot Proteins”.
A final set of proteins, called the “Fli Proteins” function as the motor
switch, reversing the direction of rotation of the flagella in response to
intracellular signals.
The energy required for rotation of the flagellum comes from the proton
motive force (proton movement across the cytoplasmic membrane through
the Mot complex drives rotation of the flagellum). It requires 1000 protons to
be translocated per single rotation of the flagellum.
Synthesis of flagella:
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Involves many genes for the hook and basal body, as well as the gene
for flagellin.
New molecules of flagellin are transported through the hollow filament
so that the growth of the flagellum is from the tip, not from the base.
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The mechanism of flagellar movement appears to be rotation; the hook and
helical structure of the flagellum causes the flagellum to act as a propeller,
thus driving the bacterium through its watery environment
a. Counterclockwise rotation causes forward motion (called a run(
b. Clockwise rotation disrupts forward motion (resulting in a tumble(
Axial filaments cause flexing and spinning movements that allow spirochetes
to move
Gliding motility is a mechanism used by some procaryotes by which they
coast along solid surfaces; no visible structure is associated with this form of
motility
Chemotaxis: directed movement of bacteria either towards a chemical
attractant or away from a chemical repellent
A. The concentrations of these materials is detected by
chemoreceptors in the surfaces of the bacteria
B. Directional travel toward a chemo-attractant is caused by lowering
the frequency of tumbles (twiddles), thereby lengthening the runs when
traveling up the gradient, but allowing tumbling to occur at normal
frequency when traveling down the gradient
C. Directional travel away from a chemorepellent involves similar but
opposite responses
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•
D. The mechanism of control of tumbles and runs is complex with
several protein intermediates
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involves conformational changes in proteins
•
also involves methylation or phosphorylation of proteins
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Fast with responses occurring in as little as 200 meters/second
Endospore
Special, resistant, dormant structure formed by some bacteria, which enables
them to resist harsh environmental conditions
–
heat
–
radiation
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chemicals
–
desiccation
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Spore formation (sporulation)
Normally commences when growth ceases because of lack of nutrients, and
is a complex, multistage process
Transformation of dormant spores into active vegetative cells is also a
complex, multistage process that includes activation (preparation) of the
spore, germination (breaking of the spores dormant state), and outgrowth
(emergence of the new vegetative cell(
•
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activation
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prepares spores for germination
–
often results from treatments like heating
germination
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spore swelling
–
rupture of absorption of spore coat
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•
–
loss of resistance
–
increased metabolic activity
outgrowth
–
emergence of vegetative cell
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What makes apores resistant???
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calcium (complexed with dipicolinic acid)
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acid-soluble, DNA-binding proteins
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dehydrated core
•
spore coat
•
DNA repair enzymes
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