Cell Structure. Morphology of Prokaryotic Cell. Cytoplasmic Membrane 4/6/2011. Chapter 3. Cytoplasmic membrane

Transcription

1 Cell Structure Chapter 3 Morphology of Prokaryotic Cell Cytoplasmic membrane Delicate thin fluid structure Surrounds cytoplasm of cell Defines boundary 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 1

2 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 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, but constantly changing position The fluid mosaic model Molecules pass through the membrane via: simple diffusion OR transport mechanisms that may require carrier proteins and energy 2

3 Molecules pass through the membrane via: simple diffusion the process by which molecules move freely across the 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 Membrane also the site of energy production 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 3

4 Molecules pass through the membrane via: simple diffusion OR transport mechanisms that may require carrier proteins and energy facilitated diffusion active transport group translocation 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 4

5 Active transport Moves compounds against a concentration gradient Requires an expenditure of energy Two primary mechanisms: Proton motive force ATP Binding Cassette system Proton motive force Transporters allow protons into cell 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 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 5

6 Bacterial cell wall Rigid structure Surrounds cytoplasmic membrane Determines shape of bacteria Holds cell together Prevents cell from bursting Unique chemical structure 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 6

7 Gram-positive cell wall Relatively thick layer of PTG As many as 30 Regardless of thickness, PTG is permeable to numerous substances Teichoic acid component of PTG Gives cell negative charge 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 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 7

8 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 bilayer Plays role in recognition of infection PTG as a target Many antimicrobials interfere with the synthesis of PTG Examples include Penicillin Lysozyme PTG as a target Many antimicrobials interfere with the synthesis of PTG Examples include Penicillin: binds proteins involved in PTG synthesis Lysozyme: produced in many body fluids and breaks bonds between NAG and NAM 8

9 Layers External to Capsules and Slime Layer General function Protection Protects bacteria from host defenses Attachment Enables bacteria to adhere Capsule is a distinct gelatinous layer Slime layer is irregular diffuse layer Chemical composition of 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 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 9

10 Flagella and Pili Flagella structure has three basic parts Filament Extends to exterior Made of proteins called flagellin Hook Connects filament to cell Basal body Anchors flagellum into cell wall 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 Flagella rotation responsible for run and tumble movement of bacteria Flagella and Pili 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 10

11 Internal Structures Bacterial cells have variety of internal structures Some structures are essential for life Chromosome Ribosome Others are optional and can confer selective advantage Plasmid Storage granules Endospores Internal Structures Chromosome Resides in cytoplasm In nucleoid space Typically single chromosome Circular double-stranded molecule Contains all essential genetic information Plasmid Circular DNA molecule Generally 0.1% to 10% size of chromosome Extrachromosomal Independently replicating Encode characteristic Potentially enhances survival Antimicrobial resistance Ribosome Involved in protein synthesis 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 Structure 11

12 Internal Structures Endospores Dormant cell 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 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 Animal cells contain cholesterol Fungal cells contain ergosterol Difference in sterols target for antifungal medications 12

13 Protein Structures of Eukaryotic Cell Eukaryotic cells have unique structures that distinguish them from prokaryotic 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 Eukaryotic Plasma Membrane Endocytosis Phagocytosis Specific type of endocytosis Important in body defenses Phagocyte sends out pseudopods to surround microbes 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 13

14 Eukaryotic Plasma Membrane Exocytosis Reverse of endocytosis Vesicles inside cell fuse with plasma membrane Releases contents into external environment 14