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
Rod
Spirillum
Spirochete
Stalk Hypha
Budding andappendaged bacteria
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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• 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
3.3 The Cytoplasmic Membrane
Animation: Membrane StructureAnimation: Membrane Structure
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Figure 3.4
Fatty acids
Glycerol
Phosphate
Ethanolamine
Fatty acids
Glycerophosphates
Hydrophilicregion
region
region
Hydrophobic
Hydrophilic
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
Phospholipids
Integralmembraneproteins
6–8 nm
Hydrophilicgroups
groupsHydrophobic
Out
In
Phospholipidmolecule
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Figure 3.6
EsterEther
ArchaeaBacteriaEukarya
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Figure 3.7d
Lipid bilayer
In
Out
Membrane protein
Glycerophosphates
Phytanyl
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Figure 3.7e
Out
In
Lipid monolayer
Biphytanyl
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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 Polysaccharidebackbone
Peptides
Escherichia coli(gram-negative)
Staphylococcus aureus(gram-positive)
Interbridge
Pe
pti
de
bo
nd
s
Glycosidic bondsX
Y
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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
Peptidoglycancable
Ribitol
Wall-associatedprotein
Teichoic acid Peptidoglycan
Lipoteichoicacid
Cytoplasmic membrane
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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
Peptidoglycan
Core polysaccharide
Lipid A Protein
Porin
Out
8 nm
Phospholipid
Lipopolysaccharide(LPS)
Lipoprotein
In
Outermembrane
Periplasm
Cytoplasmic membrane
Cellwall
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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 N-acetyltalosaminuronic 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
Virus-coveredpilus
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