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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