Chapter 3: Cell Structure and Function in Bacteria and Archea Flashcards
Morphology is
Cell shape
Major cell morphologies
• Coccus (pl. cocci): spherical or
ovoid
• Rod/bacillus: cylindrical shape
• Spirillum: loose spiral shape
Cells with unusual shapes
Spirochetes, appendaged
bacteria, and filamentous
bacteria
Cell Morphology
Morphology typically does not predict physiology, ecology,
phylogeny, etc. of a prokaryotic cell
Selective forces may be involved in setting the morphology
• Optimization for nutrient uptake (small cells and those with
high surface-to-volume ratio)
• Swimming motility in viscous environments or near surfaces
(helical or spiral-shaped cells)
• Gliding motility (filamentous bacteria)
Size range for prokaryotes:
0.2 µm to > 700
µm in diameter
Examples of very large prokaryotes
• Epulopiscium fishelsoni
• Thiomargarita namibiensis
Size range for eukaryotic cells:
10 to >200 µm in diameter
Most cultured rod-shaped bacteria are
between
0.5 and 4.0 µm wide and <15 µm long
Advantages to being small
Small cells have more surface area relative to cell
volume than large cells (i.e., higher S/V)
– support greater nutrient exchange per
unit cell volume
– tend to grow faster than larger cells
Lower Limits of Cell Size
• Cellular organisms <0.15 µm in diameter are unlikely
• Open oceans tend to contain small cells (0.2–0.4 µm in diameter)
Cytoplasmic membrane:
• Thin structure that surrounds the cell
• 6–8 nm thick
• Vital barrier that separates cytoplasm from
environment
• Highly selective permeable barrier; enables
concentration of specific metabolites and excretion of
waste products
Composition of Membranes
• General structure is phospholipid bilayer (Contain both hydrophobic and hydrophilic
components)
• Can exist in many different chemical
forms as a result of variation in the
groups attached to the glycerol backbone
• Fatty acids point inward to form
hydrophobic environment; hydrophilic
portions remain exposed to external
environment or the cytoplasm
Cytoplasmic Membrane
• 6–8 nm wide
• Embedded proteins
• Stabilized by hydrogen bonds and
hydrophobic interactions
• Mg2+ and Ca2+ help stabilize membrane by
forming ionic bonds with negative
charges on the phospholipids
• Somewhat fluid
Membrane Proteins
• Outer surface of cytoplasmic membrane can interact with a variety of
proteins that bind substrates or process large molecules for transport
• Inner surface of cytoplasmic membrane interacts with proteins
involved in energy-yielding reactions and other important cellular
functions
Integral membrane proteins
• Firmly embedded in the membrane
Peripheral membrane proteins
• One portion anchored in the membrane
Membrane-Strengthening Agents
Sterols
• Rigid, planar lipids found in eukaryotic membranes Strengthen and stabilize
membranes
Hopanoids
• Structurally similar to sterols
• Present in membranes of many Bacteria
Archaeal Membranes
• Ether linkages in phospholipids of
Archaea (Figure 3.6)
• Bacteria and Eukarya that have ester
linkages in phospholipids
• Archaeal lipids lack fatty acids, have
isoprenes instead
• Major lipids are glycerol diethers and
tetraethers (Figure 3.7a and b)
• Some archaeal lipids form monolayers
while others form bilayers, whereas
all bacterial lipids form bilayers.
Functions of the Cytoplasmic Membrane
- Permeability Barrier
• Polar and charged molecules must be transported
• Transport proteins accumulate solutes against the concentration gradient - Protein Anchor
• Holds transport proteins in place - Energy Conservation
Carrier-Mediated Transport Systems
• Show saturation effect
• Highly specific
Three major classes of transport
systems in prokaryotes
• Simple transport
Driven by the energy in the proton motive force
• Group translocation
Chemical modification of the transported substance driven by phosphoenolpyruvate
• ABC system
Periplasmic binding proteins are involved and energy comes from ATP
Three transport events are possible:
• Uniporters transport in one direction
across the membrane
• Symporters function as cotransporters
• Antiporters transport a molecule
across the membrane while
simultaneously transporting another
molecule in the opposite direction
Simple Transport: Lac Permease of Escherichia coli
• Lactose is transported into E. coli by the simple transporter lac permease, a symporter
• Activity of lac permease is energy driven
• Other symporters, uniporters, and antiporters
The Phosphotransferase System in E. coli
• Type of group translocation: substance transported is chemically modified during transport
across the membrane
• Best-studied system
• Moves glucose, fructose, and mannose
• Five proteins required
• Energy derived from phosphoenolpyruvate
ABC (ATP-Binding Cassette) Systems
• >200 different systems identified in
prokaryotes
• Often involved in uptake of organic
compounds (e.g., sugars, amino acids),
inorganic nutrients (e.g., sulfate,
phosphate), and trace metals
• Typically display high substrate specificity
• Contain periplasmic binding proteins
Protein Export: Translocases
responsible for exporting proteins through and inserting into prokaryotic membranes
Sec translocase system
exports proteins and inserts integral
membrane proteins into the membrane
Type III secretion system
common in pathogenic bacteria; secreted
protein translocated directly into host
Peptidoglycan
Rigid layer that provides
strength to cell wall
Polysaccharide composed of
• N-acetylglucosamine and Nacetylmuramic acid
• Amino acids
• Lysine or diaminopimelic acid
(DAP)
• Cross-linked differently in
gram-negative bacteria and
gram-positive bacteria (Figure
3.17)
Gram-Positive Cell Walls
Can contain up to 90% peptidoglycan
Common to have teichoic acids (acidic
substances) embedded in the cell wall
Lipoteichoic acids: teichoic acids covalently
bound to membrane lipids
Prokaryotes That Lack Cell Walls
Mycoplasmas
• Group of pathogenic bacteria
Thermoplasma
• Species of Archaea
Lipopolysaccharide (LPS) layer
• LPS consists of core polysaccharide and
O-polysaccharide
• LPS replaces most of phospholipids in outer half of outer membrane
• Endotoxin: the toxic component of LPS
Porins
channels for movement of
hydrophilic low-molecular weight
substances
Periplasm
space located between
cytoplasmic and outer membranes
~15 nm wide
Contents have gel-like consistency
Houses many proteins
Cell Walls of Archaea
No peptidoglycan
Typically no outer membrane
Pseudomurein
• Polysaccharide similar to peptidoglycan
(Figure 3.21)
• Composed of N-acetylglucosamine and
N-acetyltalosaminuronic acid
• Found in cell walls of certain
methanogenic Archaea
Cell walls of some Archaea lack
pseudomurein
S-Layers
• Most common cell wall type among
Archaea
• Consist of protein or glycoprotein
• Paracrystalline structure
Ribosomes
Complex structures, sites of protein synthesis.
• Consisting of protein/RNA.
Entire ribosome.
• Bacterial/archaeal ribosome = 70S.
• Eukaryotic (80S) S = Svedburg unit.
Bacterial and archaeal ribosomal RNA.
• 16S small subunit.
• 23S and 5S in large subunit.
• At least one archaeon have additional 5.8S rRNA (also seen in eukaryotic large subunit).
Proteins in ribosomes vary.
• Archaea more similar to eukarya than to bacteria, but there are some that are unique to archaea.
The Nucleoid
Irregularly shaped region in bacteria and archaea.
Usually not membrane bound (few exceptions).
Location of single circular chromosome and
associated proteins.
Some evidence for polyploidy in some archaeons.
Supercoiling and nucleoid-associated proteins (NAPs, including histones in some cases) aid in folding and chromosome condensation.
Capsules and Slime Layers
Polysaccharide layers (Figure 3.23)
• May be thick or thin, rigid or flexible
Assist in attachment to surfaces
Protect against phagocytosis
Resist desiccation
Fimbriae
• Filamentous protein structures
• Enable organisms to stick to surfaces
or form pellicles
Pili
• Filamentous protein structures
• Typically longer than fimbriae
• Assist in surface attachment
• Facilitate genetic exchange between
cells (conjugation)
• Type IV pili involved in twitching
motility
Cannulae
• Hollow, tubelike structures
on surface of thermophilic
archae in genus Pyrodictium.
• Function unknown.
• May be involved in
formation of networks of
multiple daughter cells.
Hami
• Archaeal external structure still not well understood
• ‘Grappling hook’ appearance.
• Involvement in cell adhesion mechanisms
Cell Inclusions
Carbon storage polymers
• Poly-b-hydroxybutyric acid
(PHB): lipid (Figure 3.26)
• Glycogen: glucose polymer
Polyphosphates:
accumulations of inorganic phosphate
Sulfur globules: composed of elemental sulfur
Magnetosomes: magnetic storage inclusions
Gas Vesicles
Confer buoyancy in planktonic cells
Spindle-shaped, gas-filled structures
made of protein
Gas vesicle impermeable to water
Molecular Structure of Gas Vesicles
• Gas vesicles are composed of two
proteins: GvpA and GvpC
• Function by decreasing cell density
Endospores
Highly differentiated cells resistant to heat, harsh
chemicals, and radiation (Figure 3.32)
“Dormant” stage of bacterial life cycle (Figure 3.33)
Ideal for dispersal via wind, water, or animal gut
Only present in some gram-positive bacteria
Endospore Structure
Structurally complex
Contains dipicolinic acid
Enriched in Ca2+
Core contains small acid-soluble
proteins (SASPs)
The Sporulation Process
• Complex series of events
• Genetically directed
Flagellum (pl. flagella):
structure that assists in swimming
different arrangements: peritrichous, polar, lophotrichous
helical in shape
Flagellar Structure
Consists of several components
Filament composed of flagellin
Move by rotation
Differences of Archaeal Flagella
• Flagella thinner
• More than one type of flagellin
protein
• Filament is not
hollow
• Hook and basal body
difficult to distinguish
• More related to type IV
bacterial pili
• Growth occurs at the base, not
the end
Flagellar Synthesis
• Several genes are required for flagellar synthesis and motility
• MS ring made first
• Other proteins and hook made next
• Filament grows from tip
Flagella increase or decrease
rotational speed in relation to strength
of the ______________
proton motive force
Differences in swimming motions
• Peritrichously flagellated cells move
slowly in a straight line
• Polarly flagellated cells move more
rapidly and typically spin around
Gliding Motility
• Flagella-independent motility
• Slower and smoother than
swimming
• Movement typically occurs along
long axis of cell
• Requires surface contact
Mechanisms
• Excretion of polysaccharide slime
• Type IV pili
• Gliding-specific proteins
Taxis: directed movement in response to chemical or physical gradients
• Chemotaxis: response to chemicals
• Phototaxis: response to light
• Aerotaxis: response to oxygen
• Osmotaxis: response to ionic strength
• Hydrotaxis: response to water
Chemotaxis
• Best studied in E. coli
• Bacteria respond to temporal, not spatial, difference in chemical concentration
• “Run and tumble” behavior (Figure 3.47)
• Attractants and receptors sensed by chemoreceptors
Measuring Chemotaxis
• Measured by inserting a
capillary tube containing
an attractant or a repellent
in a medium of motile
bacteria
• Can also be seen under a
microscope
