Download 1/25/12 Cell Structure 1

I. Cell Shape and Size
• 3.1 Cell Morphology
• 3.2 Cell Size and the Significance of
Smallness
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Figure 3.1
Coccus
Spirochete
Stalk
Rod
Hypha
Budding and
appendaged bacteria
Spirillum
Filamentous bacteria
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3.1 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)
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3.2 Cell Size and the Significance of
Smallness
• Size range for prokaryotes: 0.2 µm to >700 µm in
diameter
– Most cultured rod-shaped bacteria are between
0.5 and 4.0 µm wide and <15 µm long
– Examples of very large prokaryotes
• Epulopiscium fishelsoni (Figure 3.2a)
• Thiomargarita namibiensis (Figure 3.2b)
• Size range for eukaryotic cells: 10 to >200 µm in
diameter
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3.2 Cell Size and the Significance of
Smallness
• Surface-to-Volume Ratios, Growth Rates, and
Evolution
– Advantages to being small (Figure 3.3)
• 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
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3.3 The Cytoplasmic Membrane in
Bacteria and Archaea
• 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
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3.3 The Cytoplasmic Membrane
• Composition of Membranes
– General structure is phospholipid bilayer
(Figure 3.4)
• 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
Animation: Membrane Structure
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Figure 3.4
Glycerol
Fatty acids
Phosphate
Ethanolamine
Hydrophilic
region
Fatty acids
Hydrophobic
region
Hydrophilic
region
Glycerophosphates
Fatty acids
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3.3 The Cytoplasmic Membrane
• Cytoplasmic Membrane (Figure 3.5)
– 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
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Figure 3.5
Out
Phospholipids
Hydrophilic
groups
6–8 nm
Hydrophobic
groups
In
Integral
membrane
proteins
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Phospholipid
molecule
Figure 3.6
Ester
Bacteria
Eukarya
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Ether
Archaea
Figure 3.7d
Out
Glycerophosphates
Phytanyl
Membrane protein
In
Lipid bilayer
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Figure 3.7e
Out
Biphytanyl
In
Lipid monolayer
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3.4 Functions of the Cytoplasmic
Membrane
• Permeability Barrier (Figure 3.8)
– Polar and charged molecules must be
transported
– Transport proteins accumulate solutes against
the concentration gradient
• Protein Anchor
– Holds transport proteins in place
• Energy Conservation
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III. Cell Walls of Prokaryotes
• 3.6 The Cell Wall of Bacteria: Peptidoglycan
• 3.7 The Outer Membrane
• 3.8 Cell Walls of Archaea
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3.6 The Cell Wall of Bacteria:
Peptidoglycan
Peptidoglycan (Figure 3.16)
– Rigid layer that provides strength to cell wall
– Polysaccharide composed of
•
•
•
•
N-acetylglucosamine and N-acetylmuramic acid
Amino acids
Lysine or diaminopimelic acid (DAP)
Cross-linked differently in gram-negative bacteria
and gram-positive bacteria (Figure 3.17)
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Figure 3.17
Polysaccharide
backbone
Interbridge
Peptides
Escherichia coli
(gram-negative)
Staphylococcus aureus
(gram-positive)
Peptide bonds
Y
X
Glycosidic bonds
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3.6 The Cell Wall of Bacteria:
Peptidoglycan
• Gram-Positive Cell Walls (Figure 3.18)
– 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
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Figure 3.18
Peptidoglycan
cable
Ribitol
Wall-associated
protein
Teichoic acid
Cytoplasmic membrane
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Peptidoglycan
Lipoteichoic
acid
3.6 The Cell Wall of Bacteria:
Peptidoglycan
• Prokaryotes That Lack Cell Walls
– Mycoplasmas
• Group of pathogenic bacteria
– Thermoplasma
• Species of Archaea
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3.7 The Outer Membrane
• Total cell wall contains ~10% peptidoglycan
(Figure 3.20a)
• Most of cell wall composed of outer membrane
(aka 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
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Figure 3.20a
O-polysaccharide
Core polysaccharide
Lipid A
Protein
Out
Lipopolysaccharide
(LPS)
Porin
Cell
wall
8 nm
Outer
membrane
Periplasm
Peptidoglycan
Phospholipid
Lipoprotein
Cytoplasmic
membrane
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In
3.7 The Outer Membrane
• Structural differences between cell walls of
gram-positive and gram-negative Bacteria are
responsible for differences in the Gram stain
reaction
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3.8 Cell Walls of Archaea
• No peptidoglycan
• Typically no outer membrane
• Pseudomurein
– Polysaccharide similar to peptidoglycan
(Figure 3.21)
– Composed of N-acetylglucosamine and Nacetyltalosaminuronic acid
– Found in cell walls of certain methanogenic
Archaea
• Cell walls of some Archaea lack pseudomurein
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3.8 Cell Walls of Archaea
• S-Layers
– Most common cell wall type among Archaea
– Consist of protein or glycoprotein
– Paracrystalline structure (Figure 3.22)
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Figure 3.22
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3.9 Cell Surface Structures
• 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
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Figure 3.23
Cell Capsule
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3.9 Cell Surface Structures
• Fimbriae
– Filamentous protein structures (Figure 3.24)
– Enable organisms to stick to surfaces or form
pellicles
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Figure 3.24
Flagella
Fimbriae
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3.9 Cell Surface Structures
• Pili
–
–
–
–
Filamentous protein structures (Figure 3.25)
Typically longer than fimbriae
Assist in surface attachment
Facilitate genetic exchange between cells
(conjugation)
– Type IV pili involved in twitching motility
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Figure 3.25
Viruscovered
pilus
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