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Transcript
Chapter 3 • Structures of cells Microscopy Reveals Two Cell Types • Microscopy reveals two cell types • Prokaryotic cells (Bacteria, Archaea) – Smaller size gives high surface area to low volume • Facilitates rapid uptake of nutrients, excretion of wastes • Allows rapid growth – Disadvantages include vulnerability to threats including predators, parasites, and competitors • Eukaryotic cells (Eukarya) – Larger, more complex, many cellular processes take place in compartments 3.1. Microscopic Techniques: The Instruments • Light microscope can magnify 1,000x – Common, important tool in microbiology • Electron microscope (1931) can magnify more than 100,000x • Atomic force microscope (1980s) can produce images of individual atoms on a surface 1 The Eukaryotic Cell • Eukaryotic cells larger than prokaryotic cells – Internal structures far more complex – Have abundance of membrane-enclosed compartments termed organelles – Animal, plant cells share similarities, have differences Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Rough Nucleus endoplasmic Nuclear envelope reticulum Nucleolus with ribosomes Nucleus Nuclear envelope Nucleolus Cytoplasm Rough endoplasmic reticulum with ribosomes Plasma membrane Centriole Smooth endoplasmic reticulum Mitochondrion Cytoskeleton Actin filament Microtubule Ribosomes Golgi apparatus Intermediate filament Peroxisome (a) Cytoskeleton Intermediate filament Smooth endoplasmic reticulum Microtubule Actin filament Ribosomes Golgi apparatus Central vacuole Peroxisome Mitochondrion Chloroplast (opened to show thylakoids) Adjacent cell wall Lysosome Cell wall Plasma membrane Cytoplasm (b) Figure 2.11b Cytoplasmic membrane Endoplasmic reticulum Ribosomes Nucleus Nucleolus Nuclear membrane Golgi complex Cytoplasm Mitochondrion Chloroplast Eukaryote © 2012 Pearson Education, Inc. 3.13. Membrane-Bound Organelles • Mitochondria generate ATP – Bounded by two lipid bilayers – Mitochondrial matrix contains DNA, 70S ribosomes Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. • Endosymbiotic theory: evolved from bacterial cells Ribosome Matrix DNA Crista Intermembrane space Inner membrane Outer membrane (a) (b) (b): © Keith Porter/Photo Researchers, Inc. 0.1 µm 2 3.13. Membrane-Bound Organelles • Chloroplasts are site of photosynthesis – Found only in plants, algae – Harvest sunlight to generate ATP • ATP used to convert CO2 to sugar and starch – Contain DNA and 70S ribosomes, two lipid bilayers Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. • Endosymbiotic theory: evolved from cyanobacteria Ribosome DNA Thylakoids Stroma Thylakoid membrane Outer membrane Inner membrane Thylakoid disc © George Chapman/Visuals Unlimited/Getty The Prokaryotic Cell Pilus Ribosomes Cytoplasm Chromosome (DNA) Nucleoid Cell wall Flagellum (b) Capsule Cell wall 0.5 µm Cytoplasmic membrane (a) (b): Courtesy of L. Santo, H. Hohl, and H. Frank, "Ultrastructure of Putrefactive Anaerobe 3679h During Sporulation, Journal of Bacteriology 99:824, 1969. American Society for Microbiology Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Capsule or Glycocalyx • Outermost layer • Polysaccharide or polypeptide • Allows cells to adhere to a surface • Contributes to bacterial virulence-avoid phagocytosis 3 Capsules and Slime Layers • Gel-like layer outside cell wall that protects or allows attachment to surface Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. • Capsule: distinct, gelatinous • Slime layer: diffuse, irregular • Most are glycocalyx (sugar shell) although some are polypeptides • Allow bacteria to adhere to surfaces • Once attached, cells can grow as biofilm Cell in intestine Capsule (a) 2 µm • Polysaccharide encased community • Example: dental plaque • Some capsules allow bacteria to evade host immune system (b) 1 µm (a): Courtesy of K.J. Cheng and J. W. Costerton; (b): Courtesy of A. Progulske and S.C. Holt, Journal of Bacteriology, 143:1003-1018, 1980 Filamentous Protein Appendages Filamentous Protein Appendages • Flagella • Three parts Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. • Filament • Hook • Basal body Flagellin Filament Hook Flagellum E. coli Basal body Harvests the energy of the proton motive force to rotate the flagellum. 4 Flagella - motility Rotate like a propeller Proton motive force used for energy Presence/arrangement can be used as an identifying marker Peritrichous Polar Other (ex. tuft on both ends) Flagella - motility Chemotaxis - Directed movement towards/away from a chemical • Cell movement is due to a series of runs and tumbles • Runs are longer when cell is going in the right direction Filamentous Protein Appendages • Pili are shorter than flagella • Types that allow surface attachment termed fimbriae • Twitching motility, gliding motility involve pili • Sex pilus used to join bacteria for DNA transfer Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Sex pilus Flagellum Other pili (a) 1 µm Epithelial cell Bacterium Bacterium with pili (b) 5 µm (a): Courtesy of Dr. Charles Brinton, Jr.; (b): U.S. Department of Agriculture/Harley W. Moon 5 3.8. Filamentous Protein Appendages • Chemotaxis – Bacteria sense chemicals and move accordingly • Nutrients may attract, toxins may repel – Movement is series of runs and tumbles – Other responses observed Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. A cell moves via a series of runs and tumbles. Tumble (T) • Aerotaxis • Magnetotaxis • Thermotaxis • Phototaxis Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Tumble (T) Run (R) The cell moves randomly when there is no concentration gradient of attractant or repellent. T Flagellum T When a cell senses it is moving toward an attractant, it tumbles (T) less frequently, resulting in longer runs (R). Gradient of attractant concentration T T R R Magnetite particles 0.4 mm © D. Blackwill and D. Maratea/Visuals Unlimited Cell Wall Provides rigidity to the cell (prevents it from bursting) Cell Wall Provides rigidity to the cell (prevents it from bursting) 6 Cell Wall • Peptidoglycan - rigid molecule; unique to bacteria • Alternating subunits of NAG and NAM form glycan chains • Glycan chains are connected to each other via peptide chains on NAM molecules N-acetylmuramic acid (NAM) N-acetylglucosamine (NAG) Gr+ cells have extra protein linkages Cell Wall What kind of cell wall is this? Cell Wall • Peptidoglycan - rigid molecule; unique to bacteria • Alternating subunits of NAG and NAM form glycan chains • Glycan chains are connected to each other via peptide chains on NAM molecules Medical significance of peptidoglycan • Target for selective toxicity; synthesis is targeted by certain antimicrobial medications (penicillins, cephalosporins) • Recognized by innate immune system • Target of lysozyme (in egg whites, tears) 7 Cell Wall Gram-positive Thick layer of peptidoglycan Teichoic acids The Gram-Negative Cell Wall • Outer membrane – Bilayer made from lipopolysaccharide (LPS) – Important medically: signals immune system of invasion by Gram-negative bacteria • Small levels elicit appropriate response to eliminate • Large amounts accumulating in bloodstream can yield deadly response • LPS is called endotoxin • Includes Lipid A (immune system recognizes) and O antigen (can be used to identify species or strains) The Gram-Negative Cell Wall • Gram-negative cell wall has thin peptidoglycan layer • Outside is unique outer membrane O antigen (varies in length and composition) Porin protein Core polysaccharide Lipid A Lipopolysaccharide (LPS) (b) Outer membrane (lipid bilayer) Outer membrane Peptidoglycan Lipoprotein Periplasm Cytoplasmic membrane Peptidoglycan Periplasm (c) Cytoplasmic membrane (inner membrane; lipid bilayer) Outer Cytoplasmic Peptidoglycan membrane Periplasm membrane (a) (d) 0.15 µm (d): © Terry Beveridge, University of Guelph 8 Cell Wall Gram-negative Thin layer of peptidoglycan Outer membrane - additional membrane barrier; porins permit passage lipopolysaccharide (LPS) - ex. E. coli O157:H7 endotoxin - recognized by innate immune system Cytoplasmic membrane Phospholipid bilayer, embedded with proteins • Defines the boundary of the cell • Semi-permeable; excludes all but water, gases, and some small hydrophobic molecules • Transport proteins function as selective gates (selectively permeable) • Control entrance/expulsion of antimicrobial drugs • Receptors provide a sensor system Cytoplasmic membrane • Defines the boundary of the cell • Semi-permeable; excludes all but water, gases, and some small hydrophobic molecules • Transport proteins function as selective gates (selectively permeable) • Control entrance/expulsion of antimicrobial drugs • Receptors provide a sensor system • Phospholipid bilayer, embedded with proteins • Fluid mosaic model 9 Cytoplasmic membrane Electron transport chain - Series of proteins that eject protons from the cell, creating an electrochemical gradient Proton motive force is used to fuel: • Synthesis of ATP (the cell s energy currency) • Rotation of flagella (motility) • One form of transport If a function of the cell membrane is transport….. • How is material transported in/out of the cell? – Passive transport • No ATP • Along concentration gradient – Active transport • Requires ATP • Against concentration gradient Types of transport • Passive transport • Simple diffusion • Facilitated diffusion • Osmosis • Active transport • System that uses proton motive force • System that uses ATP • Group translocation 10 Permeability of the membrane Osmosis Facilitated Diffusion 11 3.5. Directed Movement of Molecules Across Cytoplasmic Membrane • Most molecules must pass through proteins functioning as selective gates – Termed transport systems • Proteins may be called permeases, carriers – Membrane-spanning – Highly specific: carriers transport certain molecule type Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Small molecule 1 Transport protein recognizes a specific molecule. 2 Binding of that molecule changes the shape of the transport protein. 3 The molecule is released on the other side of the membrane. Active Transport Directed Movement of Molecules Across Cytoplasmic Membrane • Protein secretion: active movement out of cell Examples: extracellular enzymes, external structures – Proteins tagged for secretion via signal sequence of amino acids Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Macromolecule Extracellular enzyme Subunit of macromolecule Signal sequence Preprotein P P P ATP P P ADP + Pi a The signal sequence on the preprotein targets it for secretion and is removed during the secretion process. Once outside the cell, the protein folds into its functional shape. b Extracellular enzymes degrade macromolecules so that the subunits can then be transported into the cell using the mechanisms shown in figure 3.29. 12 Internal Structures • Chromosome forms gel-like region: the nucleoid – Single circular double-stranded DNA • Packed tightly via binding proteins and supercoiling • Plasmids are circular, supercoiled, dsDNA – Usually much smaller; few to several hundred genes • May share with other bacteria; antibiotic resistance can spread this way Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. DNA (a) 0.5 µm (b) 1.3 µm (a): © CNRI/SPL/Photo Researchers, Inc.; (b): © Dr. Gopal Murti/SPL/Photo Researchers Internal structures: Ribosomes Protein complexes responsible for protein synthesis. Contain a molecule of RNA termed ribosomal RNA. These RNA molecules are very highly conserved among bacteria and archaea: meaning the sequence of the gene does not change much even between different species. Internal structures:Storage Granules 13 • Sporulation triggered by carbon, nitrogen limitation – Starvation conditions begin 8hour process – Endospore layers prevent damage • Exclude molecules (e.g., lysozyme) – Cortex maintains core in dehydrated state, protects from heat – Core has small proteins that bind and protect DNA – Calcium dipicolinate seems to play important protective role • Germination triggered by heat, chemical exposure 1 Vegetative growth stops; DNA is duplicated. Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 2 A septum forms, dividing the cell asymmetrically. 3 4 The larger compartment engulfs the smaller compartment, forming a forespore within a mother cell. Forespore Peptidoglycan-containing material is laid down between the two membranes that now surround the forespore. Peptidoglycan-containing material 5 Mother cell Core wall The mother cell is degraded and the endospore released. Cortex Spore coat Internal Structures: Endospores Bacteria That Lack a Cell • Some bacteria lack a cell wall Wall – Mycoplasma species have extremely variable shape – Penicillin, lysozyme do not affect – Cytoplasmic membrane contains sterols that increase strength Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Courtesy of Dr. Edwin S. Boatman 2 µm 14 Cell Walls of the Domain • Members of Archaea have variety of cell Archaea walls – Probably due to wide range of environments • Includes extreme environments – However, Archaea less well studied than Bacteria – No peptidoglycan – Some have similar molecule pseudopeptidoglycan – Many have S-layers that self-assemble • Built from sheets of flat protein or glycoprotein subunits Archaeal cell membranes archaeal phospholipid: 1, isoprene chains; 2, ether linkages; 3, L-glycerol moieties; 4, phosphate group. bacterial or eukaryotic phospholipid: 5, fatty acid chains; 6, ester linkages; 7, Dglycerol moiety; 8, phosphate group. lipid bilayer of bacteria and eukaryotes lipid monolayer of some archaea. Lab Exercise 5: Simple Staining and Exercise 1: Ubiquity of organisms 15 3.2. Microscopic Techniques: Dyes and Staining • Samples can be immobilized, stained to visualize • Basic dyes (positive charge) – Attracted to negatively charged cellular components • Acidic dyes (negative charge) – Negative staining: cells repel, so colors background – Can be done as wet mount Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Spread thin film of specimen over slide. Allow to air dry. Pass slide through flame to heat-fix specimen. Flood the smear with stain, rinse, and dry. Examine with microscope. 3.2. Microscopic Techniques: Dyes and Staining • Simple staining involves one dye • Differential staining used to distinguish different types of bacteria 16