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Presentation3 - Microbio Presentation3 - Microbio Presentation Transcript

  • 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 & stalks● Bacterial flagella and chemotaxis 1
  • The Bacterial Cell: An OverviewMost prokaryotes share fundamental traits. - Thick, complex outer envelope - Compact genome - Tightly coordinated cell functionsModern research shows that the cell’s parts fit together in a structure that is ordered, though flexible. 2
  • Figure 3.1 3
  • The Bacterial Cell: An OverviewCytoplasm = Consists of a gel-like networkCell membrane = Encloses the cytoplasmCell wall = Covers the cell membraneNucleoid = Non-membrane-bound area of the cytoplasm that contains the chromosome in the form of looped coilsFlagellum = External helical filament whose rotary motor propels the cell 4
  • Biochemical Composition of BacteriaAll cells share common chemical components. - Water - Essential ions - Small organic molecules - MacromoleculesCell composition varies with species, growth phase, and environmental conditions. 5
  • How We Study Cell PartsCell study requires isolation of cell parts. - Cell disruption- Subcellular fractionation- Structural analysis- Genetic analysis 6
  • Isolating Parts of CellsCells 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 7
  • Subcellular Fractionation A key tool ofsubcellularfractionation is theultracentrifuge.-The high rotation rateproduces centrifugalforces strong enoughto separate particlesby size. Figure 3.4 8
  • The Cell MembraneThe structure that defines the existence of a cell is the cell membrane.Figure 3.7 9
  • Membrane ConstituentsMembranes have approximately equal parts Figure 3.8 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 10
  • Membrane ConstituentsMembrane proteins serve numerous functions, including: - Structural support - Detection of environmental signals - Secretion of virulence factors and communication signals - Ion transport and energy storageHave hydrophilic and hydrophobic regions that lock the protein in the membrane 11
  • Transport across the Cell MembraneThe cell membrane acts as a semipermeable barrier.Selective transport is essential for survival. - Small uncharged molecules, such as O 2 and CO2, easily permeate the membrane by diffusion. - Water tends to diffuse across the membrane in a process called osmosis. 12
  • 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 3.9 13
  • Polar molecules and charged molecules require transport through specific protein transporters. Figure 3.10 - Passive transport = Molecules move along their concentration gradient - Active transport = Molecules move against their concentration gradient - Requires energy 14
  • Membrane LipidsPhospholipids vary with respect to their phosphoryl head groups & their fatty acid side chains. Figure 3.11Figure 3.12 15
  • 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. Figure 3.13 16
  • Archaea have the most extreme variations in phospholipid side-chain structures. - Ether links between glycerol and fatty acids - Hydrocarbon chains are branched terpenoids. Figure 3.14 17
  • The Cell WallThe 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. Figure 3.16 18
  • Peptidoglycan StructureMost 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. 19
  • Figure3.17 20
  • Gram-Positive and Gram-Negative BacteriaMost 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 Figure 1.1 21
  • Gram-Positive Cell EnvelopeCapsule (not all species) - Made of polysaccharidesS-Layer (not all species) - Made of proteinThick cell wall - Amino acid cross-links in peptidoglycan - Teichoic acids for strengthPlasma membrane Figure 3.18a 22
  • Mycobacterial Cell EnvelopesMycobacterium Figure 3.21 tuberculosis and M. leprae have very complex cell envelopes. - Include unusual membrane lipids (mycolic acids) and unusual sugars (arabinogalactans) 23
  • Gram-Negative Outer MembraneThe thin peptidoglycan layer consists of one or two sheets.-Covered by an outer membrane, which confers defensive abilities and toxigenic properties on many pathogens Figure 3.18b 24
  • Figure3.22 25
  • Table 3-2 Subcellular location of proteins in Gram-negative bacteria. 26
  • Figure 3-23 Outer membrane analysis by centrifugation. 27
  • Figure 3-24 Sucrose porin. 28
  • The NucleoidAn important function of the cell envelope is to contain and protect the cell’s genome.Eukaryotes have a membrane-bound nucleus.Prokaryotes have a nucleoid region that extends throughout the cytoplasm. Figure 29 3.26
  • The E. coli nucleoid The nucleoid forms about appears as clear 50 loops or domains. regions that exclude Within each domain, the the ribosome and contain the DNA DNA is supercoiled by strands. DNA-binding proteins. Figure Figure 3.27 3.28 30
  • Cell DivisionCell 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. 31
  • DNA ReplicationIn 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. 32
  • Figure3.32 33
  • DNA ReplicationAnimation: Replisome Movement in a Dividing Cell Click box to launch animation 34
  • Septation Completes Cell Division 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. Figure 3.33 35
  • Septation Completes Cell Division The spatial orientation of septation has a key role in determining the shape and arrangement of cocci. Figure 3.34 36
  • The Bacterial CytoskeletonShape-determining proteins Special Topic 3.2 Figure 2 - 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 37
  • Cell AttachmentPili or fimbriae are straight filaments of protein monomers called pilin.Sex pili are used in conjugation. FigureStalks are membrane- 3.40 embedded extensions of the cytoplasm. - Tips secrete adhesion factors called holdfasts. Figure 3.41 38
  • Rotary FlagellaProkaryotes that are motile generally swim by means of rotary flagella.Peritrichous cells have flagella randomly distributed around the cell. Figure 3.42aLophotrichous cells have flagella at the end(s).Monotrichous cells have a single flagellum. 39
  • 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 counterclock wise (CCW) relative to the cell. Figure 40 3.43
  • ChemotaxisChemotaxis 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 Figure 41 3.42
  • ChemotaxisThe 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) 42
  • Figure3.44 43
  • ChemotaxisAnimation: Chemotaxis: Molecular Events Click box to launch animation 44
  • 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. 45
  • 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. 46
  • Concept QuizWhich one of these membranes is notfound in Gram-negative bacteria?a) Plasma membraneb) Inner membranec) Nuclear membraned) Outer membrane 47
  • Concept QuizPeptidoglycan is composed primarily ofa) sugars and amino acids.b) sugars and nucleic acids.c) nucleic acids and lipids.d) amino acids and lipids. 48
  • Concept QuizAn extension of the cytoplasm that attachesbacteria to a surface is called aa) pilus.b) flagellum.c) fimbrium.d) stalk. 49
  • Concept QuizThe structure in prokaryotes that performsthe same function as mitochondria ineukaryotes is thea) cell membrane.b) chloroplast.c) outer membrane.d) cell wall. 50
  • Concept QuizIn archaeal membranes, the glycerol islinked to the hydrocarbon chains by _____bonds.a) etherb) esterc) glycand) peptide 51
  • Concept QuizAll of the following statements about prokaryotic flagella are correct excepta)they are driven by the proton motive force.b)they are found in both Gram-positive and Gram-negative bacteria.c) they move with a whiplike motion.d)they are used for chemotaxis. 52