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Chapter 3A:
Prokaryotic Cells
1. Overview of Prokaryotic Cells
2. The Plasma Membrane & Membrane Transport
3. External Prokaryotic Cell Structures
4. Internal Prokaryotic Cell Structures
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3.1 – Overview of Prokaryotic Cells
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Prokaryotic
Cell Structure
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Prokaryotic Morphologies
Morphology refers to the shape
of individual cells:
coccus = round or spherical
bacillus = rod-shaped
vibrio = curved rod
coccobacillus = short rod, oval
spirillum = spiral shape
spirochete = long, coiled/helical
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Arrangements
Arrangement refers to the
characteristic grouping of
multiple prokaryotic cells
following cell division:
diplo- = pairs
tetrad = 4 cells in a square
strepto- = linear chain
staphylo- = irregular
clusters
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3.2 – The Plasma Membrane &
Membrane Transport
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The Plasma (Cell) Membrane
barrier between inside & outside of cells
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Diffusion & Osmosis
OSMOSIS = diffusion of water across semi-permeable membrane
overall movement from high to low concentration
passive, no energy required
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Tonicity in Cells with Cell Walls
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Tonicity in Cells without Cell Walls
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Facilitated Diffusion
Diffusion (high to low conc.)
across a membrane with
the help of a membrane
protein:
• Protein channels for small
molecules or ions (e.g.,
sodium channels)
• Carrier proteins for larger
molecules (e.g., glucose
carrier)
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Active Transport
ACTIVE TRANSPORT = movement from low to high concentration via
protein pumps – requires energy (e.g., ATP)
Sodium-PotassiumPump
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3.3 – External Structures
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Bacterial Cell Walls
2 general cell wall structures – Gram-positive and Gram-negative
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PEPTIDOGLYCAN LPS
Peptidoglycan & Lipopolysaccharide (LPS)
These cell wall materials are unique to bacteria.
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Bacterial Glycocalyx (“sugar coat”)
Outermost layer that surrounds the bacterium
• called a slime layer if loosely attached, water soluble
• called a capsule if compact, tightly attached to cell wall
usually, but not always, made of polysaccharides
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Bacterial Flagellum
*
*
*
rotates, used for motility
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Flagellar Arrangements
Peritrichous bacteria
can “run” in a specific
direction by rotating
flagella in one direction,
and “tumble” by
reversing the rotation
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Motility with Peritrichous Flagella
Motile bacteria can
undergo taxis or
movement in response
to something it senses.
• e.g., chemotaxis
(movement in
response to a
chemical
substance)
Somewhat indirect,
based on longer runs,
less tumbles in direction
of “good stuff”
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Axial Filaments
Bundle of endoflagella
surrounded by an outer sheath
found in spirochetes
• anchored at one end of cell
and rotate in unison
• rotates cell like a “corkscrew”
to propel it forward
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Fimbriae/Pili Non-motile appendages that are chemically and
functionally different than flagella.
• typically small, multiple structures
involved primarily in adhesion,
invasion of tissue
• one or more relatively large pili
can be involved in gene transfer
between bacteria during conjugation
(sex pilus or conjugation pilus)
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3.4 – Internal Structures
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Prokaryotic Ribosomes
Ribosomes consist of 1 large and 1 small subunit.
Carry out protein synthesis (i.e., translation of mRNA).
• both subunits are made of rRNA & ribosomal proteins
• smaller, somewhat structurally different from eukaryotic ribosomes
• are specifically targeted by some antibiotics
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Endospores When conditions are bad, a few types of Gram+ bacteria can form
endospores:
• inactive, dormant cells
enclosed in a highly
resistant spore coat
• remain dormant until
conditions are good
(even for many years!)
• very resistant to heating,
freezing, desiccation,
radiation, etcendospore position is characteristic of the species
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Endospore Formation
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The Genetic MaterialA region called the nucleoid contains the circular, single bacterial
chromosome (DNA + non-histone proteins):
• usually several million base pairs (bp) in
size
• the E. coli genome, for example, is
~4 mega-bp’s (4 Mbp)
• contains all bacterial genes plus an
origin of replication (Ori)
• Ori is where DNA replication starts,
is essential to copy the chromosome
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Plasmids
Some bacteria have extrachromosomal,
non-essential circular DNA molecules
called plasmids:
• much smaller than bacterial
chromosome
• have own Ori so they are copied when
cell divides
• several kilo-base pairs (usually 3-6 kb)
plasmid
map
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What’s the Role of Plasmids?
Plasmids generally contain genes that confer some sort of
survival advantage:
1) genes providing protection from toxic substances
2) genes enabling the metabolism of additional sources of energy
3) genes for toxins to kill microbial competitors, enhance pathogenicity
4) genes involved in gene transfer by conjugation
• e.g., antibiotic resistance
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Inclusions are deposits of various
materials (metals, gas, nutrients, etc)
found in certain types of bacteria
(e.g., magnetosomes).
Chromatophores are pigment-containing
infoldings of the plasma membrane in some
photosynthetic bacteria.
Inclusions & Chromatophores
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Key Terms for Chapter 3A
• bacillus, coccus, vibrio, spirillum, spirochete
• glycocalyx, capsule, fimbriae/pili
• peptidoglycan, teichoic acid, LPS, endotoxin
• endospores, vegetative
• plasmids, nucleoid
• inclusions, chromatophores
• diplo-, strepto-, staphylo-, tetrad
• flagellum, motility; mono-, amphi-, lopho-, peritrichous