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Transcript
1/21/2011 Cell Structure Chapter 3 Morphology of Prokaryotic Cell 1 1/21/2011 Cytoplasmic Membrane • Cytoplasmic membrane – Delicate thin fluid structure – Surrounds cytoplasm of cell – Defines boundary – Serves as a selectively permeable barrier • Barrier between cell and external environment • Permits passage of only certain molecules, such as water, small hydrophobic molecules and gases Cytoplasmic Membrane • Membrane structure is a lipid bilayer with embedded proteins – Bilayer consists of two opposing leaflets • Leaflets composed of phospholipids – Each contains a hydrophilic phosphate head and hydrophobic fatty acid tail 2 1/21/2011 Cytoplasmic Membrane • Membrane is embedded with numerous proteins – More that 200 different proteins – Proteins function as receptors and transport gates – Provides mechanism to sense surroundings – Proteins are not stationary, stationary but constantly changing position “The fluid mosaic model” Cytoplasmic Membrane • Molecules pass through the membrane via: – simple diffusion OR – transport mechanisms that may require carrier proteins and energy 3 1/21/2011 Cytoplasmic Membrane • Molecules pass through the membrane via: – simple diffusion the process by which molecules move freely across the membrane Cytoplasmic Membrane • An example of simple diffusion – OSMOSIS – The ability of water to flow freely across the cytoplasmic membrane – Water flows to equalize solute concentrations inside and outside the cell 4 1/21/2011 Cytoplasmic Membrane • Membrane also the site of gy p production energy • Energy produced through series of embedded proteins – Electron transport chain – Proteins are used in the formation of proton motive force – Energy produced in proton motive force is used to drive other transport mechanisms Cytoplasmic Membrane • Molecules pass through the membrane via: – simple diffusion OR – transport mechanisms that may require carrier proteins and energy • facilitated f ilit t d diff diffusion i • active transport • group translocation 5 1/21/2011 Cytoplasmic Membrane Cytoplasmic Membrane • Facilitated diffusion – Moves compounds across membrane by exploiting a concentration gradient • Flow from area of greater concentration to area of lesser concentration – Molecules are transported until equilibrium is reached • System can only eliminate concentration gradient; it cannot create one • No energy is required for facilitated diffusion 6 1/21/2011 Cytoplasmic Membrane • Active transport – Moves compounds against a concentration gradient – Requires an expenditure of energy – Two primary mechanisms: • Proton motive force • ATP Binding Cassette system Cytoplasmic Membrane • Proton motive force – Transporters allow protons i t cellll into • Protons either bring in or expel other substances – Example: efflux pumps used in antimicrobial resistance • ATP Binding Cassette system (ABC transport) – Use binding proteins to scavenge and deliver molecules to transport complex – Example: maltose transport 7 1/21/2011 Cytoplasmic Membrane • Group transport – Transport mechanism that chemically alters molecule during passage • Phosphotransferase system example of group transport mechanism – Phosphorylates sugar molecule during transport » Phosphorylation changes molecule and therefore does not change sugar balance across the membrane Cytoplasmic Membrane 8 1/21/2011 Cell Wall • Bacterial cell wall – Rigid structure – Surrounds S d cytoplasmic membrane – Determines shape of bacteria – Holds cell together – Prevents cell from bursting – Unique chemical structure Cell Wall • Rigidity of cell wall is due to peptidoglycan (PTG) – Compound found only in bacteria • Basic structure of peptidoglycan – Alternating series of two subunits • N-acetylglucosamine(NAG) • N-acetylmuramic acid (NAM) – Joined subunits form glycan chain • Glycan chains held together by string of four amino acids – Tetrapeptide chain 9 1/21/2011 Cell Wall • Gram-positive cell wall – Relatively thick layer of PTG • As many as 30 – Regardless of thickness, PTG is permeable to numerous substances –T Teichoic i h i acid id component of PTG • Gives cell negative charge Cell Wall • Gram-negative cell wall – More complex than G+ – Only contains thin layer of PTG • PTG sandwiched between outer membrane and cytoplasmic membrane • Region between outer membrane and cytoplasmic membrane is called periplasm 10 1/21/2011 Cell Wall • Outer membrane of Gram-negative bacteria – Constructed of lipid bilayer • Much like cytoplasmic membrane but outer leaflet made of lipopolysaccharides not phospholipids • Outer membrane also called the lipopolysaccharide layer or LPS layer – LPS severs as barrier to a large number of molecules • Small molecules or ions pass through channels called porins – Portions of LPS medically significant • O-specific polysaccharide side chain • Lipid A Cell Wall • O-specific polysaccharide side chain – Directed away from membrane – Used to identify certain species or strains • E. coli O157:H7 refers to specific O-side chain • Lipid A – Portion that anchors LPS molecule in lipid p bilayer y – Plays role in recognition of infection 11 1/21/2011 Cell Wall • PTG as a target – Many antimicrobials interfere with the synthesis of PTG – Examples include • Penicillin • Lysozyme Cell Wall • PTG as a target – Many antimicrobials interfere with the synthesis of PTG – Examples include • Penicillin: binds proteins involved in PTG synthesis • Lysozyme: L produced d d iin many b body d fl fluids id and d breaks bonds between NAG and NAM 12 1/21/2011 Layers External to Cell Wall • Capsules and Slime Layer – General function • Protection – P Protects t t b bacteria t i from f host h t defenses • Attachment – Enables bacteria to adhere – Capsule is a distinct gelatinous layer – Slime layer is irregular diffuse layer – Chemical Ch i l composition iti off capsules and slime layers varies depending on bacterial species • Most are made of polysaccharide – Referred to as glycocalyx » Glyco = sugar calyx = shell Flagella and Pili • Some bacteria have protein appendages – Not essential for life • Aid in survival in certain environments – They include • Flagella • Pili 13 1/21/2011 Flagella and Pili • Flagella – Long protein structure – Responsible for motility • Use propeller-like movements to push bacteria • Can rotate more than 100,00 revolutions/minute – 82 mile/hour – Some important in bacterial pathogenesis • H. pylori penetration through mucous coat Flagella and Pili • Flagella structure has three basic parts – Filament • Extends to exterior • Made of proteins called flagellin – Hook • Connects C t fil filamentt to t cellll – Basal body • Anchors flagellum into cell wall 14 1/21/2011 Flagella and Pili • Bacteria use flagella for motility – Motile through sensing chemicals • Chemotaxis – If chemical compound is nutrient • Acts as attractant – If compound is toxic • Acts as repellent • Fl Flagella ll rotation t ti responsible for run and tumble movement of bacteria Flagella and Pili • Pili – Considerably shorter and thinner than flagella – Similar in structure • Protein subunits – Function • Attachment – These pili called fimbre • Movement • Conjugation – Mechanism of DNA transfer 15 1/21/2011 Internal Structures • Bacterial cells have variety of internal structures • Some S structures t t are essential ti l for f life lif – Chromosome – Ribosome • Others are optional and can confer selective advantage – Pl Plasmid id – Storage granules – Endospores Internal Structures • Chromosome – Resides in cytoplasm y p • In nucleoid space – Typically single chromosome – Circular double-stranded molecule – Contains all essential genetic information • Plasmid – Circular DNA molecule • Generally 0 0.1% 1% to 10% size of chromosome – Extrachromosomal • Independently replicating – Encode characteristic • Potentially enhances survival – Antimicrobial resistance 16 1/21/2011 Internal Structure • Ribosome – Involved in protein synthesis th i – Composed of large and small subunits • Units made of riboprotein and ribosomal RNA – Prokaryotic ribosomal subunits • Large = 30S • Small = 50S • Total = 70S – Larger than eukaryotic ribosomes • 40S, 60S, 80S • Difference often used as target for antimicrobials Internal Structures • Endospores –D Dormant cellll types – Resistant to damaging conditions, such as heat, desiccation, chemicals and UV light – Vegetative cell produced through germination Common bacteria that produce endospores include Clostridium and Bacillus 17 1/21/2011 Internal Structures • Bacteria sense starvation and begin sporulation – Growth stops – DNA duplicated – Cell splits • Cell splits unevenly – Larger component engulfs small component, produces forespore within mother cell » Forespore enclosed by two membranes – Forespore becomes core – PTG between membranes forms core wall and cortex – Mother cell proteins produce spore coat – Mother cell degrades and releases endospore Eukaryotic Plasma Membrane • Similar in chemical structure and function of cytoplasmic membrane of prokaryote • Proteins in bilayer perform specific functions – Transport – Maintain cell integrity – Receptors for cell signaling • Membrane contains sterols for strength – A Animal i l cells ll contain t i cholesterol h l t l – Fungal cells contain ergosterol • Difference in sterols target for antifungal medications 18 1/21/2011 Protein Structures of Eukaryotic Cell • Eukaryotic cells have unique structures th t distinguish that di ti i h them th from f prokaryotic k ti – Cytoskeleton – Flagella – Cilia – 80s ribosome Eukaryotic Plasma Membrane • Endocytosis – Pinocytosis – Phagocytosis • Important to bodies defenses • Breaks down microbial material • Exocytosis – Reverse of phagocytosis – Releases contents into environment 19 1/21/2011 Eukaryotic Plasma Membrane • Endocytosis – Phagocytosis • Specific type of endocytosis • Important in body defenses • Phagocyte sends out pseudopods to surround microbes – Phagocyte brings microbe into vacuole » Vacuole = phagosome • Phagosome fuses with a sack of enzymes and toxins – Sack = lysosome – Fusion of phagosome and lysosome creates phagolysosome » Microbe dies in phagolysosome • Phagosome breaks down microbial material Eukaryotic Plasma Membrane • Exocytosis – Reverse of endocytosis – Vesicles inside cell fuse with plasma membrane – Releases contents into external environment 20