Download Lecture 2 - Microscopy and Cell Structure S11 2 slides per page

1/21/2011
Cell Structure
Chapter 3
Morphology of Prokaryotic Cell
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Cytoplasmic Membrane
• Cytoplasmic membrane
– Delicate thin fluid structure
– Surrounds cytoplasm of cell
– Defines boundary
– Serves as a selectively permeable barrier
• Barrier between cell and external environment
• Permits passage of only certain molecules, such
as water, small hydrophobic molecules and gases
Cytoplasmic Membrane
• Membrane structure is
a lipid bilayer with
embedded proteins
– Bilayer consists of two
opposing leaflets
• Leaflets composed of
phospholipids
– Each contains a
hydrophilic phosphate
head and hydrophobic
fatty acid tail
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Cytoplasmic Membrane
• Membrane is embedded with numerous
proteins
– More that 200 different proteins
– Proteins function as receptors and transport
gates
– Provides mechanism to sense surroundings
– Proteins are not stationary,
stationary but constantly
changing position “The fluid mosaic model”
Cytoplasmic Membrane
• Molecules pass through the membrane via:
– simple diffusion
OR
– transport mechanisms that may require carrier
proteins and energy
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Cytoplasmic Membrane
• Molecules pass through the membrane via:
– simple diffusion the process by which
molecules move freely across the
membrane
Cytoplasmic Membrane
• An example of simple
diffusion – OSMOSIS
– The ability of water to
flow freely across the
cytoplasmic membrane
– Water flows to equalize
solute concentrations
inside and outside the
cell
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Cytoplasmic Membrane
• Membrane also the site of
gy p
production
energy
• Energy produced through
series of embedded
proteins
– Electron transport chain
– Proteins are used in the
formation of proton motive
force
– Energy produced in proton
motive force is used to
drive other transport
mechanisms
Cytoplasmic Membrane
• Molecules pass through the membrane via:
– simple diffusion
OR
– transport mechanisms that may require carrier
proteins and energy
• facilitated
f ilit t d diff
diffusion
i
• active transport
• group translocation
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Cytoplasmic Membrane
Cytoplasmic Membrane
• Facilitated diffusion
– Moves compounds across membrane by
exploiting a concentration gradient
• Flow from area of greater concentration to area of
lesser concentration
– Molecules are transported until equilibrium is reached
• System can only eliminate concentration gradient;
it cannot create one
• No energy is required for facilitated diffusion
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Cytoplasmic Membrane
• Active transport
– Moves compounds against a concentration
gradient
– Requires an expenditure of energy
– Two primary mechanisms:
• Proton motive force
• ATP Binding Cassette system
Cytoplasmic Membrane
• Proton motive force
– Transporters allow protons
i t cellll
into
• Protons either bring in or
expel other substances
– Example: efflux pumps
used in antimicrobial
resistance
• ATP Binding Cassette
system (ABC transport)
– Use binding proteins to
scavenge and deliver
molecules to transport
complex
– Example: maltose transport
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Cytoplasmic Membrane
• Group transport
– Transport mechanism that
chemically alters molecule
during passage
• Phosphotransferase system
example of group transport
mechanism
– Phosphorylates sugar
molecule during transport
» Phosphorylation changes
molecule and therefore
does not change sugar
balance across the
membrane
Cytoplasmic Membrane
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Cell Wall
• Bacterial cell wall
– Rigid structure
– Surrounds
S
d
cytoplasmic
membrane
– Determines shape
of bacteria
– Holds cell together
– Prevents cell from
bursting
– Unique chemical
structure
Cell Wall
• Rigidity of cell wall is due to
peptidoglycan (PTG)
– Compound found only in
bacteria
• Basic structure of peptidoglycan
– Alternating series of two subunits
• N-acetylglucosamine(NAG)
• N-acetylmuramic acid (NAM)
– Joined subunits form glycan chain
• Glycan chains held together by string
of four amino acids
– Tetrapeptide chain
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Cell Wall
• Gram-positive cell wall
– Relatively thick layer of
PTG
• As many as 30
– Regardless of
thickness, PTG is
permeable to
numerous substances
–T
Teichoic
i h i acid
id
component of PTG
• Gives cell negative
charge
Cell Wall
• Gram-negative cell wall
– More complex than G+
– Only contains thin layer of
PTG
• PTG sandwiched between
outer membrane and
cytoplasmic membrane
• Region between outer
membrane and cytoplasmic
membrane is called periplasm
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Cell Wall
• Outer membrane of Gram-negative bacteria
– Constructed of lipid bilayer
• Much like cytoplasmic membrane but outer leaflet made of
lipopolysaccharides not phospholipids
• Outer membrane also called the lipopolysaccharide layer or
LPS layer
– LPS severs as barrier to a large number of molecules
• Small molecules or ions pass through channels called porins
– Portions of LPS medically significant
• O-specific polysaccharide side chain
• Lipid A
Cell Wall
• O-specific polysaccharide side chain
– Directed away from membrane
– Used to identify certain species or strains
• E. coli O157:H7 refers to specific O-side chain
• Lipid A
– Portion that anchors LPS molecule in lipid
p bilayer
y
– Plays role in recognition of infection
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Cell Wall
• PTG as a target
– Many antimicrobials interfere with the
synthesis of PTG
– Examples include
• Penicillin
• Lysozyme
Cell Wall
• PTG as a target
– Many antimicrobials interfere with the
synthesis of PTG
– Examples include
• Penicillin: binds proteins involved in PTG
synthesis
• Lysozyme:
L
produced
d
d iin many b
body
d fl
fluids
id and
d
breaks bonds between NAG and NAM
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Layers External to Cell Wall
• Capsules and Slime Layer
– General function
• Protection
– P
Protects
t t b
bacteria
t i from
f
host
h t
defenses
• Attachment
– Enables bacteria to adhere
– Capsule is a distinct gelatinous
layer
– Slime layer is irregular diffuse
layer
– Chemical
Ch i l composition
iti off
capsules and slime layers varies
depending on bacterial species
• Most are made of
polysaccharide
– Referred to as glycocalyx
» Glyco = sugar calyx =
shell
Flagella and Pili
• Some bacteria have protein appendages
– Not essential for life
• Aid in survival in certain environments
– They include
• Flagella
• Pili
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Flagella and Pili
• Flagella
– Long protein structure
– Responsible for motility
• Use propeller-like
movements to push
bacteria
• Can rotate more than
100,00 revolutions/minute
– 82 mile/hour
– Some important in bacterial
pathogenesis
• H. pylori penetration
through mucous coat
Flagella and Pili
• Flagella structure has
three basic parts
– Filament
• Extends to exterior
• Made of proteins called
flagellin
– Hook
• Connects
C
t fil
filamentt to
t cellll
– Basal body
• Anchors flagellum into cell
wall
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Flagella and Pili
• Bacteria use flagella for
motility
– Motile through sensing
chemicals
• Chemotaxis
– If chemical compound is
nutrient
• Acts as attractant
– If compound is toxic
• Acts as repellent
• Fl
Flagella
ll rotation
t ti
responsible for run and
tumble movement of
bacteria
Flagella and Pili
• Pili
– Considerably shorter and
thinner than flagella
– Similar in structure
• Protein subunits
– Function
• Attachment
– These pili called fimbre
• Movement
• Conjugation
– Mechanism of DNA
transfer
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Internal Structures
• Bacterial cells have variety of internal structures
• Some
S
structures
t t
are essential
ti l for
f life
lif
– Chromosome
– Ribosome
• Others are optional and can confer selective
advantage
– Pl
Plasmid
id
– Storage granules
– Endospores
Internal Structures
• Chromosome
– Resides in cytoplasm
y p
• In nucleoid space
– Typically single chromosome
– Circular double-stranded molecule
– Contains all essential genetic information
• Plasmid
– Circular DNA molecule
• Generally 0
0.1%
1% to 10% size of
chromosome
– Extrachromosomal
• Independently replicating
– Encode characteristic
• Potentially enhances survival
– Antimicrobial resistance
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Internal Structure
• Ribosome
– Involved in protein
synthesis
th i
– Composed of large and
small subunits
• Units made of riboprotein
and ribosomal RNA
– Prokaryotic ribosomal
subunits
• Large = 30S
• Small = 50S
• Total = 70S
– Larger than eukaryotic
ribosomes
• 40S, 60S, 80S
• Difference often used as
target for antimicrobials
Internal Structures
• Endospores
–D
Dormant cellll types
– Resistant to damaging
conditions, such as heat,
desiccation, chemicals and
UV light
– Vegetative cell produced
through germination
Common bacteria that produce
endospores include Clostridium
and Bacillus
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Internal Structures
• Bacteria sense starvation and begin
sporulation
– Growth stops
– DNA duplicated
– Cell splits
• Cell splits unevenly
– Larger component engulfs small component,
produces forespore within mother cell
» Forespore enclosed by two membranes
– Forespore becomes core
– PTG between membranes forms core wall
and cortex
– Mother cell proteins produce spore coat
– Mother cell degrades and releases
endospore
Eukaryotic Plasma Membrane
• Similar in chemical structure and function of cytoplasmic
membrane of prokaryote
• Proteins in bilayer perform specific functions
– Transport
– Maintain cell integrity
– Receptors for cell signaling
• Membrane contains sterols for strength
– A
Animal
i l cells
ll contain
t i cholesterol
h l t l
– Fungal cells contain ergosterol
• Difference in sterols target for antifungal medications
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Protein Structures of
Eukaryotic Cell
• Eukaryotic cells have unique structures
th t distinguish
that
di ti
i h them
th
from
f
prokaryotic
k
ti
– Cytoskeleton
– Flagella
– Cilia
– 80s ribosome
Eukaryotic Plasma Membrane
• Endocytosis
– Pinocytosis
– Phagocytosis
• Important to bodies
defenses
• Breaks down microbial
material
• Exocytosis
– Reverse of phagocytosis
– Releases contents into
environment
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Eukaryotic Plasma Membrane
• Endocytosis
– Phagocytosis
• Specific type of endocytosis
• Important in body defenses
• Phagocyte sends out pseudopods to surround microbes
– Phagocyte brings microbe into vacuole
» Vacuole = phagosome
• Phagosome fuses with a sack of enzymes and toxins
– Sack = lysosome
– Fusion of phagosome and lysosome creates phagolysosome
» Microbe dies in phagolysosome
• Phagosome breaks down microbial material
Eukaryotic Plasma Membrane
• Exocytosis
– Reverse of endocytosis
– Vesicles inside cell fuse with plasma
membrane
– Releases contents into external environment
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