Chapter 3 Lecture Outline Cell Structure and Function

Chapter 3Lecture Outline

Cell Structure and Function

Microbiology: An Evolving Science © 2009 W. W. Norton & Company, Inc. 2

Basic Structure of a Prokarytic Cell

Cell membrane (+)

Nucleoid (+)

Plasmid (+/-)

Ribosomes (+)

Inclusion body (+/-)

Flagella (+/-)

Pili (+/-)

Fimbria (+/-)

Additional layer (+/-)

Cell wall (mostly +)


The Bacterial Cell Cytoplasm surrounded by envelope

Cytoplasm contains DNA in form of nucleoid Envelope has lipid membrane boundary

Plus structural cell wall



Cross Section through a Gram-Negative Bacterium


Made of lipid bilayerDouble layer of

phospholipids Separate cytoplasm

from outside world Proteins embedded in membrane

Anchor membranes to envelopeSense the outside worldTransport materials into cell

Cell Membranes


Functions of the Cell Membrane

Permeability barrier Prevents leakage of cytoplasmic metabolites into the

environment Selective permeability also prevents diffusion of most solutes Active transport

Structural support Anchor for membrane proteins Anchor for cell appendages

Pili Filaments Flagella

Detection of environmental signals Secretion of virulence factors and communication signals Site of energy conversion (ATP generation)


Differences between Prokaryotic and Eukaryotic Membranes Phospholipid composition Absence of sterols in prokaryotic

membranes In many bacteria hopanoids instead


Differences betweenBacteria and Archaea Unlike Bacteria (and Eukarya), in which

ester linkages bond fatty acids to glycerol, Archaea contain ether-linked lipids

No fatty acids but isoprene (5 carbon hydrocarbon) in Archaea

Some Archaea have lipid monolayers

H2C-O-C-RH2C-O-C-R

OEster Ether

Bacterial Cell Envelope Bacterial cell wall

Peptidoglycan Two major classes Originally differentiated based on staining characteristic

Gram positive Thick peptidoglycanl ayer, retains the dye crystal violet

Gram negative

Thin peptidoglycan layer, does not retain the dye crystal violet

Additional layers S-layer in gram positive bacteria only Outer membrane in gram negative bacteria only Slime layers or capsule

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Peptidoglycan Strands of alternating repeats of N-acetylglucosamine

and N-acetylmuramic acid cross-linked between strands by short peptides Via N-acetylmuramic acid 4 amino acids

L-alanine, D-alanine, D-glutamic acid, and either L-lysine or meso-diaminopimelic acid

Lipoteichoic acids(embedded in

cytoplasmic membrane)

(some species)

Periplasm


S-Layer Composed of symmetrical

hexagonal protein subunits Protection Molecular sieve First described in archaea Recently discovered in

gram-positive bacteria Why only recently?


Capsule (not all species) Polysaccharide mainly

Outer Membrane Lipopolysaccharide (LPS)

In outer leaflet only

Outer membrane proteins Lipoprotein

Connects peptidoglycan with outer membrane

Thin cell wall 4-amino acid crosslinks

in peptidoglycan Thick periplasm

Plasma membrane

The Gram-Negative Envelope

OM


Gram-Negative Envelope


Lipopolysaccharide (LPS)

Highly variable O-polysaccharide side chains

Core sugars Conserved lipid A

(toxic) Endotoxin Fever inducing

Capsule Optional Most exterior layer Predominantly made from

organized carbohydrate Sometimes made from protein

(e.g. Bacillus anthracis)

Protective Antiphagocytic

Related structures which serve more for attachment Slime (loose net work) Glycocalix (very fine network)

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Galactan Arabinan Outer bilayer with

phenolic glycolipids Capsule with loosely

associated phospholipids and phenolic lipids

Mycobacterial Envelope Structure


Cytoplasmic contents

Cytoplasmic skeleton Nucleoid Plasmids (in some organisms) Ribosomes Specialized structures


Determines cell shape

CreS “Crescentin” Curves inner side of crescent-shaped

bacteria

FtsZ Forms a “Z ring” essential for cell

septation Analog to tubulin

The Bacterial Cytoskeleton


Single loop of double-stranded DNASingle molecule of DNA~4x106 bp in many bacteriaCompacted via supercoiling

Attached to cell envelopeNo membrane separates

DNA from cytoplasm

Replicates once for each cell division

The Bacterial Nucleoid


Gene Expression RNA Polymerase transcribes DNA to mRNA Ribosome translates RNA to Protein

Processes occur simultaneously


Bacterial Ribosomes


Intracellular Structures Related to Photosynthesis Cyanobacteria have thylakoids

Extensively folded inner membrane Contain chlorophyll Ancestors of chloroplasts

Carboxysomes fix carbon Use energy to make sugar

Other bacterial photosynthetic pigments Purple membranes in some Phycobilisome proteins collect light energy


Storage Granules Intracellular deposits of material

Glycogen (sugar) for energyPHB (fatty acid polymer) for energyPolyphosphate to store materialSulfur for disposal

Iridescent sulfur granules

Carboxysomes,lipid energy-storage granules


Cell Appendages

Proteins Composed of identical subunit that differ among

species Pili

pilin Fimbria Flagella

flagellin

Attachment

Motility

Attachment, specialized pili for plasmid transfer


Cell Attachment Essential for bacterial pathogenicity Fimbriae, pili

Stalks attach cells to surfacesExtension of cell cytoplasm

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Cell Motility Flagella

Long, helical protein filaments Attached at ends, or over whole cell Rotate to propel cell Proton passage drives rotation

Clockwise or counterclockwise


Concept QuizWhich one of these membranes is NOT found in gram-negative bacteria?

a. plasma membrane

b. thylakoid membrane

c. nuclear membrane

d. outer membrane


Concept QuizPeptidoglycan is composed primarily of

a. sugars and amino acids

b. sugars and nucleic acids

c. nucleic acids and lipids


Concept QuizAn extension of the cytoplasm that attaches bacteria to a surface is called a

a. pilus

b. flagellum

c. fimbrium

d. stalk

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Chapter 3 Lecture Outline Cell Structure and Function