chapter overview

MicrobiologyAn Evolving Science

Third Edition

Joan L. Slonczewski and John W. Foster

Copyright © 2014 W. W. Norton & Company, Inc. Permission required for reproduction or display

PowerPoint® Lecture Outlines Prepared by Johnny El-Rady, University of South Florida

3 Cell Structure and Function

2

Chapter Overview

A synopsis of the bacterial cell

How cell parts are studied

The plasma membrane and transport

The cell wall and other outer layers

The nucleoid: structure and expression

How bacterial cells divide

Specialized structures, including pili and stalks

Bacterial flagella and chemotaxis

3

Introduction

Most bacteria share fundamental traits.

- Thick, complex outer envelope

- Compact genome

- Tightly coordinated cell functions

Archaea, like bacteria, are prokaryotes

Have unique membrane and envelope structures

Eukaryotic cells have a nucleus and extensive membranous organelles

4

3.1 The Bacterial Cell: An Overview

In the early twentieth century, the cell was envisioned as a bag of “soup” full of floating ribosomes and enzymes.

Modern research shows that the cell’s parts fit together in a structure that is ordered, though flexible.

5

Cytoplasm = Consists of a gel-like network

Cell membrane = Encloses the cytoplasm

Cell wall = Covers the cell membrane

Nucleoid = Non-membrane-bound area of the cytoplasm that contains the chromosome in the form of looped coils

Flagellum = External helical filament whose rotary motor propels the cell

Model of a Bacterial Cell

6

Figure 3.1

7

Cell study requires isolation and analysis of cell parts.

- Cell fractionation

- Cells must be broken up by techniques that allow subcellular parts to remain intact.

- Examples of such techniques include:

- Mild detergent analysis

- Sonication

- Enzymes

- Mechanical disruption

Observing Cell Parts

8

A key tool of subcellular fractionation is the ultracentrifuge.

- The high rotation rate produces centrifugal forces strong enough to separate particles by size.

- Parts are then subjected for structural and biochemical analysis.

Figure 3.2

9

An approach that is complementary to cell fractionation is genetic analysis

- Different types of strains can be used:

- Mutant strains which are selected for loss of a given function

- Strains that are intentionally mutated as to lose or alter a gene

- Strains that are constructed with “reporter genes” fused to a gene encoding a protein of interest

- The phenotype of the mutant cell may yield clues about the function of the altered part

10

All cells share common chemical components.

- Water

- Essential ions

- Small organic molecules

- Macromolecules

Cell composition varies with species, growth phase, and environmental conditions.

Biochemical Composition of Bacteria

12

3.2 The Cell Membrane and Transport

The structure that defines the existence of a cell is the cell membrane.

Figure 3.4

13

Membranes have approximately equal parts of phospholipids and proteins.

A phospholipid consists of glycerol with ester links to two fatty acids and a phosphoryl head group

- May have side chain

The two layers of phospholipids in the bilayer are called leaflets.

Membrane Lipids

Figure 3.5

14

Membrane proteins serve numerous functions, including:

- Structural support

- Detection of environmental signals

- Secretion of virulence factors and communication signals

- Ion transport and energy storage

Have hydrophilic and hydrophobic regions that lock the protein in the membrane

Membrane Proteins

15

The cell membrane acts as a semipermeable barrier.

Selective transport is essential for survival.

- Small uncharged molecules, such as O2 and CO2, easily permeate the membrane by diffusion.

- Water tends to diffuse across the membrane in a process called osmosis.

Transport across the Cell Membrane

16

Weak acids and weak bases exist partly in an uncharged form that can diffuse across the membrane and change the pH of the cell.

Figure 3.6

17

Polar molecules and charged molecules require transport through specific protein transporters.

- Passive transport = Molecules move alongtheir concentration gradient

- Active transport = Molecules move against their concentration gradient

- Requires energyFigure 3.7

18

Phospholipids vary with respect to their phosphoryl head groups and their fatty acid side chains.

- Cardiolipin or diphosphatidylglycerol

- A double phospholipid linked by a glycerol

- Concentration increases in bacteria grown to starvation

- Localizes to the cell poles

- Fatty acid chains may be unsaturated

- And may also contain cyclic structures

Membrane Lipid Diversity

19

Figure 3.9Figure 3.8

20

Membranes also include planar molecules that fill gaps between hydrocarbon chains.

- In eukaryotic membranes, the reinforcing agents are sterols, such as cholesterol.

- In bacteria, the same function is filled by hopanoids, or hopanes.

22

Archaea have the most extreme variations in phospholipid side-chain structures.

- Ether links between glycerol and fatty acids

- Hydrocarbon chains are branched terpenoids.

23

3.3 The Cell Wall and Outer Layers

How do prokaryotes protect their cell membrane?

- For most species, the cell envelope includes at least one structural supporting layer

- The most common structural support is the cell wall

- Nevertheless, a few prokaryotes, such as the mycoplasmas, have a cell membrane with no outer layers

24

The cell wall confers shape and rigidity to the cell, and helps it withstand turgor pressure.

The bacterial cell wall, or the sacculus, consists of a single interlinked molecule.

The Cell Wall Is a Single Molecule

Figure 3.13

25

Most bacterial cell walls are made up of peptidoglycan (or murein).

The molecule consists of:

- Long polymers of two disaccharides called N-acetylglucosamine and N-acetylmuramic acid, bound to a peptide of 4-6 amino acids

- The peptides can form cross-bridges connecting the parallel glycan strands.

Peptidoglycan Structure

26

Figure 3.14

27

Peptidoglycan is unique to bacteria

- Thus, the enzymes responsible for its biosynthesis make excellent targets for antibiotics

- Penicillin inhibits the transpeptidase that cross-links the peptides

- Vancomycin prevents cross-bridge formation by binding to the terminal D-Ala-D-Ala dipeptide

- Unfortunately, the widespread use of such antibiotics selects for evolution of resistant strains

Peptidoglycan Structure

28

Most bacteria have additional envelope layers that provide structural support and protection.

Envelope composition defines:

- Gram-positive bacteria (thick cell wall)

- Example: The phylum Firmicutes

- Gram-negative bacteria (thin cell wall)

- Example: The phylum Proteobacteria

Gram-Positive & Gram-Negative Bacteria

29

Figure 3.15

30

Has multiple layers of peptidoglycan

- Threaded by teichoic acids

The capsule

- Made of polysaccharide and glycoprotein

- Protects cells from phagocytosis

- Found also in Gram-negative cells

Gram-Positive Cell Envelope

Figure 3.16

31

S-layer

- An additional protective layer commonly found in free-living bacteria and archaea

- Crystalline layer of thick subunits consisting of protein or glycoprotein

- May contribute to cell shape and help protect the cellfrom osmotic stress

Gram-Positive Cell Envelope

Figure 3.17

32

Mycobacterial Cell Envelopes

Figure 3.18

Mycobacterium tuberculosis and M. leprae have very complex cell envelopes

- Include unusual membrane lipids (mycolic acids) and unusual sugars (arabinogalactans)

33

The thin peptidoglycan layer consists of one or two sheets

- Covered by an outer membrane, which confers defensive abilities and toxigenic properties on many pathogens

- Inward facing leaflet includes lipoprotein

- Outward facing leaflet contains:

- Lipopolysaccharides

- Porins

The Gram-Negative Outer Membrane

34

Figure 3.19

35

Eukaryotic microbes possess their own structures to avoid osmotic shock.

- Algae form cell walls of cellulose.

- Fungi form cell walls of chitin.

- Diatoms form exoskeletons of silicate.

- Paramecia possess a contractile vacuole to pump water out of the cell

Eukaryotic Microbes

36

Figure 3.22

37

Shape-determining proteins

- FtsZ = Forms a “Z ring” in spherical cells

- MreB = Forms a coil inside rod-shaped cells

- CreS “Crescentin” = Forms a polymer along the inner side of crescent-shaped bacteria

Bacterial Cytoskeleton

Figure 3.23

38

3.4 The Nucleoid, RNA and Protein Synthesis

Eukaryotic cells have a well defined nucleus delimited by a nuclear membrane

In contrast, prokaryotic cells have a nucleoid region that extends throughout the cytoplasm

Figure 3.24

39

The E. coli nucleoid appears as clear regions that exclude the ribosome and contain the DNA strands.

DNA Is Organized in the Nucleoid

Figure 3.26

The nucleoid forms about 50 loops or domains. Within each domain, the DNA is supercoiled by DNA-binding proteins.

Figure 3.25

40

RNA polymerase transcribes DNA into a single strand of RNA.

- For most genes, it is messenger RNA.

mRNA immediately binds to a ribosome for translation into a polypeptide.

This is aided by transfer RNA (tRNA), which brings the amino acids to the ribosome.

In prokaryotes, translation is tightly coupled to transcription.

Transcription and Translation

41

Figure 3.27

42

In prokaryotes, membrane proteins and secreted proteins are synthesized in association with the cell membrane.

- This is aided by the signal recognition particle (SRP), which binds to the growing peptide.

Protein Synthesis and Secretion

43

3.5 Cell Division

Cell division, or cell fission, requires highly coordinated growth and expansion of all the cell’s parts.

Unlike eukaryotes, prokaryotes synthesize RNA and proteins continually while the cell’s DNA undergoes replication.

Bacterial DNA replication is coordinated with the cell wall expansion and ultimately the separation of the two daughter cells.

44

In prokaryotes, a circular chromosome begins to replicate at its origin, or ori site.

Two replications forks are generated, which proceed outward in both directions.

- At each fork, DNA is synthesized by DNA polymerase with the help of accessory proteins (the replisome).

- As the termination site is replicated, the two forks separate from the DNA.

DNA Is Replicated Bidirectionally

45

Figure 3.30

46

DNA Replication

47

Replication of the termination site triggers growth of the dividing partition, or septum.

The septum grows inward, at last constricting and sealing off the two daughter cells.

Septation Completes Cell Division

Figure 3.31

48

The spatial orientation of septation has a key role in determining the shape and arrangement of cocci.

- Parallel planes

- Streptococci

- Random planes

- Staphylococci

- Perpendicular planes

- Tetrads

- Sarcinae Figure 3.32

49

3.6 Cell Polarity and Aging

Bacterial cell poles differ in their origin and “age”

- This phenomenon is called polar aging

In bacteria that appear superficially symmetrical, polar differences may appear at cell division

- Bacillus species can undergo an asymmetrical cell division to form an endospore at one end.

- Other bacteria expand their cells by extending one pole only

50

Some bacteria generate two kinds of daughter cells: one stationary and the other mobile.

Example: The flagellum-to-stalk transition of the bacterium Caulobacter crescentus

Figure 3.34

51

Turns out that not only is a bacterial cell asymmetrical, but the actual process of cell division itself determines that the poles of each daughter cell differ chemically from each other

Figure 3.37

52

3.7 Specialized Structures Thylakoids = Extensively folded intracellular

membranes

Carboxysomes = Polyhedral bodies packed with the enzyme Rubisco for CO2 fixation

Gas vesicles = To increase buoyancy

Figure 3.38

53

3.7 Specialized Structures Storage granules =

- Glycogen, PHB, and PHA, for energy

- Sulfur, for oxidation

Magnetosomes

- Membrane-embedded crystals of magnetite, Fe3O4

- Orient the swimming of magnetotactic bacteria

Figure 3.39

54

Pili or fimbriae are straight filaments of pilin protein

- Used in attachment

Sex pili are used in conjugation.

Stalks are membrane-embedded extensions of the cytoplasm.

- Tips secrete adhesion factors called holdfasts

Nanotubes are intercellular connections that pass materialfrom one cell to the next.

Figure 3.40

Figure 3.42

55

Prokaryotes that are motile generally swim by means of rotary flagella

Peritrichous cells have flagella randomly distributed around cell

- The flagella rotate together in a bundle behind the swimming cell

Lophotrichous cells have flagella at the end(s)

Monotrichous cells have a single flagellum

Rotary Flagella

Figure 3.43

56

Each flagellum is a spiral filament of protein monomers called flagellin.

The filament is rotated by a motor driven by the proton motive force.

- Note: Flagella rotate either clockwise (CW) or counterclockwise (CCW) relative to the cell.

Figure 3.44

57

Chemotaxis is the movement of a bacterium in response to chemical gradients.

Attractants cause CCW rotation.- Flagella bundle together.- Push cell forward- “Run”

Repellents cause CW rotation.- Flagellar bundle falls apart. - “Tumble” = Bacterium briefly stops, then changes direction

Chemotaxis

Figure 3.43

58

The alternating runs and tumbles cause a “random walk.”

- Receptors detect attractant concentrations.

- Sugars, amino acids

- Attractant concentration increases and prolongs run.

- This is termed a “biased random walk.”

- Causes a net movement of bacteria toward attractants (or away from repellents)

Chemotaxis

59

Figure 3.45

60

Chemotaxis

61

Chapter Summary While prokaryotes are diverse, they share certain

fundamental traits and biochemistry.

The study of cells employs various methods including subcellular fractionation, structural analysis, and genetic analysis.

The cell membrane consists of a phospholipid bilayer containing proteins.

Bacterial phospholipids are ester-linked, while those of Archaea contain ether linkages.

The Gram-negative cell envelope is much more complex than that of Gram-positive cells.

62

Chapter Summary The DNA of prokaryotes is organized into loops in

the nucleoid.

- Transcription and translation are coupled.

Most bacteria divide by binary fission.

- Cell growth and DNA replication are coordinated.

Bacteria may have specialized structures, including thylakoids, storage granules, and magnetosomes.

Pili and stalks are used for attachment.

Flagella are rotary appendages used for movement and chemotaxis.

63

Concept Check – Section 3.1

All of the following are used to lyse cells except

a) mild detergents

b) enzymes

c) sonication

d) electrophoresis

e) mechanical disruption

64


The structure in prokaryotes that performs the same function as mitochondria in eukaryotes is the

a) cell membrane.

b) chloroplast.

c) outer membrane.

d) cell wall.

65


Which of the following statements about membrane lipids is TRUE?

a) Lipids of bacteria have ether linkages, while those of archaea have ester linkages

b) Lipids of bacteria have ester linkages, while those of archaea have ether linkages

c) Lipids of bacteria and archaea have ester linkages

d) Lipids of bacteria and archaea have ether linkages

66


Which of the following is NOT a component of peptidoglycan?

a) N-acetylmuramic acid

b) N-acetylglucosamine

c) Lipopolysaccharide

d) Amino acids

e) Peptide cross-links

67


All of the following are true about the prokaryotic outer membrane except

a) it is composed of a phospholipid bilayer.

b) it is found only in Gram-negative bacteria.

c) it contains porins.

d) it contains a toxic component.

e) it contains a lipopolysaccharide layer.

68


Which of the following terms is NOT defined correctly?

a) Supercoiling – Twisting of DNA upon itself, resulting in compaction

b) Translation – Conversion of DNA into RNA

c) Transertion – Coupling of protein synthesis to membrane insertion

d) Chaperones – Enzyme complexes that help fold the polypeptide into a functional structure

69

Concept Check – Section 3.5 Which of the following statements about the

prokaryotic chromosome is TRUE?

a) It has a single origin of replication and the DNA is replicated unidirectionally

b) It has a single origin of replication and the DNA is replicated bidirectionally

c) It has multiple origins of replication and the DNA is replicated unidirectionally

d) It has multiple origins of replication and the DNA is replicated bidirectionally

70


In Caulobacter crescentus, a stalked cell divides to produce

a) two stalked cells

b) two flagellated cells

c) a stalked cell and a flagellated cell

d) a stalked cell and one with no appendages

e) a flagellated cell and one with no appendages

71


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

a) pilus.

b) flagellum.

c) fimbrium.

d) stalk.

e) nanotube.

72


All of the following statements about prokaryotic flagella are correct except

a) they are driven by the proton motive force.

b) they are found in both Gram-positive and Gram-negative bacteria.

c) their motor is embedded in the cell envelope.

d) they are used for chemotaxis.

e) they move with a whiplike motion.

chapter overview

Documents