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Chapter 3
•  Structures of cells
Microscopy Reveals Two Cell Types
•  Microscopy reveals two cell types
•  Prokaryotic cells (Bacteria, Archaea)
–  Smaller size gives high surface area to low volume
•  Facilitates rapid uptake of nutrients, excretion of
wastes
•  Allows rapid growth
–  Disadvantages include vulnerability to threats
including predators, parasites, and competitors
•  Eukaryotic cells (Eukarya)
–  Larger, more complex, many cellular
processes take place in compartments
3.1. Microscopic Techniques: The Instruments
•  Light microscope can magnify 1,000x
–  Common, important tool in microbiology
•  Electron microscope (1931) can magnify
more than 100,000x
•  Atomic force microscope (1980s) can
produce images of individual atoms on a
surface
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The Eukaryotic Cell
•  Eukaryotic cells larger than prokaryotic cells
–  Internal structures far more complex
–  Have abundance of membrane-enclosed compartments
termed organelles
–  Animal, plant cells share similarities, have differences
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Rough
Nucleus
endoplasmic
Nuclear envelope
reticulum
Nucleolus
with ribosomes
Nucleus
Nuclear envelope
Nucleolus
Cytoplasm
Rough
endoplasmic
reticulum
with ribosomes
Plasma membrane
Centriole
Smooth
endoplasmic
reticulum
Mitochondrion
Cytoskeleton
Actin filament
Microtubule
Ribosomes
Golgi
apparatus
Intermediate
filament
Peroxisome
(a)
Cytoskeleton
Intermediate
filament
Smooth
endoplasmic
reticulum
Microtubule
Actin
filament
Ribosomes
Golgi
apparatus
Central
vacuole
Peroxisome
Mitochondrion
Chloroplast
(opened to
show thylakoids)
Adjacent cell wall
Lysosome
Cell wall
Plasma membrane
Cytoplasm
(b)
Figure 2.11b
Cytoplasmic
membrane
Endoplasmic
reticulum
Ribosomes
Nucleus
Nucleolus
Nuclear
membrane
Golgi
complex
Cytoplasm
Mitochondrion
Chloroplast
Eukaryote
© 2012 Pearson Education, Inc.
3.13. Membrane-Bound Organelles
•  Mitochondria generate ATP
–  Bounded by two lipid bilayers
–  Mitochondrial matrix contains DNA, 70S
ribosomes
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•  Endosymbiotic theory: evolved from bacterial cells
Ribosome
Matrix
DNA
Crista
Intermembrane space
Inner membrane
Outer membrane
(a)
(b)
(b): © Keith Porter/Photo Researchers, Inc.
0.1 µm
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3.13. Membrane-Bound Organelles
•  Chloroplasts are site of photosynthesis
–  Found only in plants, algae
–  Harvest sunlight to generate ATP
•  ATP used to convert CO2 to sugar and starch
–  Contain DNA and 70S ribosomes, two lipid
bilayers
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
•  Endosymbiotic theory: evolved from cyanobacteria
Ribosome
DNA
Thylakoids
Stroma
Thylakoid membrane
Outer membrane
Inner membrane
Thylakoid disc
© George Chapman/Visuals Unlimited/Getty
The Prokaryotic Cell
Pilus
Ribosomes
Cytoplasm
Chromosome
(DNA)
Nucleoid
Cell wall
Flagellum
(b)
Capsule
Cell wall
0.5 µm
Cytoplasmic
membrane
(a)
(b): Courtesy of L. Santo, H. Hohl, and H. Frank, "Ultrastructure of Putrefactive Anaerobe 3679h During Sporulation,
Journal of Bacteriology 99:824, 1969. American Society for Microbiology
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Capsule or Glycocalyx
•  Outermost layer
•  Polysaccharide or
polypeptide
•  Allows cells to adhere to a
surface
•  Contributes to bacterial
virulence-avoid
phagocytosis
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Capsules and Slime Layers
•  Gel-like layer outside cell wall that protects
or allows attachment to surface
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•  Capsule: distinct, gelatinous
•  Slime layer: diffuse, irregular
•  Most are glycocalyx (sugar shell)
although some are polypeptides
•  Allow bacteria to adhere to surfaces
•  Once attached, cells can grow as biofilm
Cell in intestine
Capsule
(a)
2 µm
•  Polysaccharide encased community
•  Example: dental plaque
•  Some capsules allow bacteria to evade
host immune system
(b)
1 µm
(a): Courtesy of K.J. Cheng and J. W. Costerton; (b): Courtesy of A. Progulske and
S.C. Holt, Journal of Bacteriology, 143:1003-1018, 1980
Filamentous Protein Appendages
Filamentous Protein Appendages
•  Flagella
•  Three parts
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•  Filament
•  Hook
•  Basal body
Flagellin
Filament
Hook
Flagellum
E. coli
Basal
body
Harvests the energy
of the proton motive force
to rotate the flagellum.
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Flagella - motility
Rotate like a propeller
Proton motive force used for energy
Presence/arrangement can be
used as an identifying marker
Peritrichous
Polar
Other (ex. tuft on both ends)
Flagella - motility
Chemotaxis - Directed movement towards/away from a chemical
• Cell movement is due to a series of runs and tumbles
•  Runs are longer when
cell is going in the right
direction
Filamentous Protein Appendages
•  Pili are shorter than
flagella
•  Types that allow surface
attachment termed
fimbriae
•  Twitching motility, gliding
motility involve pili
•  Sex pilus used to join
bacteria for DNA transfer
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Sex pilus
Flagellum
Other pili
(a)
1 µm
Epithelial cell
Bacterium
Bacterium
with pili
(b)
5 µm
(a): Courtesy of Dr. Charles Brinton, Jr.; (b): U.S. Department of Agriculture/Harley W. Moon
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3.8. Filamentous Protein Appendages
•  Chemotaxis
–  Bacteria sense chemicals and move accordingly
•  Nutrients may attract, toxins may repel
–  Movement is series of runs and tumbles
–  Other responses observed
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A cell moves via a series of runs and tumbles.
Tumble (T)
•  Aerotaxis
•  Magnetotaxis
•  Thermotaxis
•  Phototaxis
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Tumble (T)
Run (R)
The cell moves randomly
when there is no
concentration gradient of
attractant or repellent.
T
Flagellum
T
When a cell senses it is moving toward
an attractant, it tumbles (T) less frequently,
resulting in longer runs (R).
Gradient of attractant concentration
T
T
R
R
Magnetite particles
0.4 mm
© D. Blackwill and D. Maratea/Visuals Unlimited
Cell Wall
Provides rigidity to the cell
(prevents it from bursting)
Cell Wall
Provides rigidity to the cell
(prevents it from bursting)
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Cell Wall
• Peptidoglycan - rigid molecule; unique to
bacteria
• Alternating subunits of NAG and NAM
form glycan chains
• Glycan chains are connected to each other
via peptide chains on NAM molecules
N-acetylmuramic acid (NAM)
N-acetylglucosamine (NAG)
Gr+ cells have extra protein linkages
Cell Wall
What kind of cell wall is this?
Cell Wall
• Peptidoglycan - rigid molecule; unique to
bacteria
• Alternating subunits of NAG and NAM
form glycan chains
• Glycan chains are connected to each other
via peptide chains on NAM molecules
Medical significance of peptidoglycan
• Target for selective toxicity; synthesis is
targeted by certain antimicrobial
medications (penicillins, cephalosporins)
• Recognized by innate immune system
• Target of lysozyme (in egg whites, tears)
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Cell Wall Gram-positive
Thick layer of peptidoglycan
Teichoic acids
The Gram-Negative Cell Wall
•  Outer membrane
–  Bilayer made from lipopolysaccharide (LPS)
–  Important medically: signals immune system of
invasion by Gram-negative bacteria
•  Small levels elicit appropriate response to eliminate
•  Large amounts accumulating in bloodstream can yield
deadly response
•  LPS is called endotoxin
•  Includes Lipid A (immune system recognizes) and O
antigen (can be used to identify species or strains)
The Gram-Negative Cell Wall
•  Gram-negative
cell wall has
thin peptidoglycan layer
•  Outside is
unique outer
membrane
O antigen
(varies in length and
composition)
Porin protein
Core polysaccharide
Lipid A
Lipopolysaccharide
(LPS)
(b)
Outer
membrane
(lipid bilayer)
Outer
membrane
Peptidoglycan
Lipoprotein
Periplasm
Cytoplasmic
membrane
Peptidoglycan
Periplasm
(c)
Cytoplasmic
membrane
(inner membrane;
lipid bilayer)
Outer
Cytoplasmic
Peptidoglycan membrane Periplasm membrane
(a)
(d)
0.15 µm
(d): © Terry Beveridge, University of Guelph
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Cell Wall
Gram-negative
Thin layer of peptidoglycan
Outer membrane - additional
membrane barrier; porins permit passage
lipopolysaccharide (LPS)
- ex. E. coli O157:H7
endotoxin
- recognized by innate immune system
Cytoplasmic membrane
Phospholipid bilayer, embedded with proteins
• Defines the boundary of the cell
• Semi-permeable; excludes all
but water, gases, and some
small hydrophobic molecules
• Transport proteins function as
selective gates (selectively permeable)
• Control entrance/expulsion of
antimicrobial drugs
• Receptors provide a sensor system
Cytoplasmic membrane
• Defines the boundary of the cell
• Semi-permeable; excludes all
but water, gases, and some
small hydrophobic molecules
• Transport proteins function as
selective gates (selectively permeable)
• Control entrance/expulsion of
antimicrobial drugs
• Receptors provide a sensor system
• Phospholipid bilayer, embedded with proteins
• Fluid mosaic model
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Cytoplasmic membrane
Electron transport chain - Series of proteins that eject protons from the cell,
creating an electrochemical gradient
Proton motive force is used to fuel:
• Synthesis of ATP (the cell s energy currency)
• Rotation of flagella (motility)
• One form of transport
If a function of the cell
membrane is transport…..
•  How is material transported in/out of the
cell?
–  Passive transport
•  No ATP
•  Along concentration gradient
–  Active transport
•  Requires ATP
•  Against concentration gradient
Types of transport
•  Passive transport
•  Simple diffusion
•  Facilitated diffusion
•  Osmosis
•  Active transport
•  System that uses proton motive force
•  System that uses ATP
•  Group translocation
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Permeability of the membrane
Osmosis
Facilitated Diffusion
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3.5. Directed Movement of Molecules Across
Cytoplasmic Membrane
•  Most molecules must pass through proteins
functioning as selective gates
–  Termed transport systems
•  Proteins may be called permeases, carriers
–  Membrane-spanning
–  Highly specific: carriers transport certain molecule type
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Small molecule
1 Transport protein recognizes
a specific molecule.
2 Binding of that molecule changes
the shape of the transport protein.
3 The molecule is released on the
other side of the membrane.
Active Transport
Directed Movement of Molecules Across Cytoplasmic
Membrane
•  Protein secretion: active movement out of cell
Examples: extracellular enzymes, external structures
–  Proteins tagged for secretion via signal sequence of
amino acids
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Macromolecule
Extracellular
enzyme
Subunit of
macromolecule
Signal
sequence
Preprotein
P P P
ATP
P P
ADP
+ Pi
a The signal sequence on the preprotein targets
it for secretion and is removed during the
secretion process. Once outside the cell, the
protein folds into its functional shape.
b Extracellular enzymes degrade macromolecules
so that the subunits can then be transported
into the cell using the mechanisms shown in
figure 3.29.
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Internal Structures
•  Chromosome forms gel-like region: the nucleoid
–  Single circular double-stranded DNA
•  Packed tightly via binding proteins and supercoiling
•  Plasmids are circular, supercoiled, dsDNA
–  Usually much smaller; few to several hundred genes
•  May share with other bacteria; antibiotic resistance
can spread this way
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DNA
(a)
0.5 µm
(b)
1.3 µm
(a): © CNRI/SPL/Photo Researchers, Inc.; (b): © Dr. Gopal Murti/SPL/Photo Researchers
Internal structures: Ribosomes
Protein complexes responsible for
protein synthesis.
Contain a molecule of RNA
termed ribosomal RNA. These RNA molecules are very
highly conserved among bacteria
and archaea: meaning the
sequence of the gene does not
change much even between
different species.
Internal structures:Storage
Granules
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•  Sporulation triggered by carbon,
nitrogen limitation
–  Starvation conditions begin 8hour process
–  Endospore layers prevent
damage
•  Exclude molecules (e.g.,
lysozyme)
–  Cortex maintains core in
dehydrated state, protects
from heat
–  Core has small proteins that
bind and protect DNA
–  Calcium dipicolinate seems to
play important protective role
•  Germination triggered by heat,
chemical exposure
1
Vegetative growth stops;
DNA is duplicated.
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2
A septum forms, dividing
the cell asymmetrically.
3
4
The larger compartment
engulfs the smaller
compartment, forming a
forespore within a
mother cell.
Forespore
Peptidoglycan-containing
material is laid down between
the two membranes that now
surround the forespore.
Peptidoglycan-containing
material
5
Mother cell
Core wall
The mother cell is degraded
and the endospore released.
Cortex
Spore coat
Internal
Structures:
Endospores
Bacteria
That
Lack
a Cell
•  Some
bacteria
lack a
cell wall
Wall
–  Mycoplasma species have extremely variable
shape
–  Penicillin, lysozyme do not affect
–  Cytoplasmic membrane contains sterols that
increase strength
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Courtesy of Dr. Edwin S. Boatman
2 µm
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Cell Walls of the Domain
•  Members of Archaea have variety of cell
Archaea
walls
–  Probably due to wide range of environments
•  Includes extreme environments
–  However, Archaea less well studied than
Bacteria
–  No peptidoglycan
–  Some have similar molecule
pseudopeptidoglycan
–  Many have S-layers that self-assemble
•  Built from sheets of flat protein or glycoprotein
subunits
Archaeal cell membranes
archaeal phospholipid: 1,
isoprene chains; 2, ether
linkages; 3, L-glycerol
moieties; 4, phosphate group. bacterial or eukaryotic
phospholipid: 5, fatty acid
chains; 6, ester linkages; 7, Dglycerol moiety; 8, phosphate
group. lipid bilayer of
bacteria and
eukaryotes
lipid monolayer of
some archaea.
Lab Exercise 5: Simple Staining
and
Exercise 1: Ubiquity of
organisms
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3.2. Microscopic Techniques: Dyes and Staining
•  Samples can be immobilized, stained to visualize
•  Basic dyes (positive charge)
–  Attracted to negatively charged cellular components
•  Acidic dyes (negative charge)
–  Negative staining: cells repel, so colors background
–  Can be done as wet mount
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Spread thin film of
specimen over slide.
Allow to air dry.
Pass slide
through
flame to
heat-fix
specimen.
Flood the smear with
stain, rinse, and dry.
Examine with microscope.
3.2. Microscopic Techniques: Dyes and Staining
•  Simple staining involves one dye
•  Differential staining used to distinguish different
types of bacteria
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