Microbiology: Cell Structure and Function PDF

CHAPTER 3 Cell Structure and Function Copyright © 2023 by W. W. Norton & Company, Inc. Chapter Objectives (Ch 3.1 - 3.4, 3.6) 1. Compare and contrast cell envelopes of Gram positive and negative bacteria 2. Describe the structure, function, and importance of the microbial cell membrane 3. Understand...

CHAPTER 3 Cell Structure and Function Copyright © 2023 by W. W. Norton & Company, Inc. Chapter Objectives (Ch 3.1 - 3.4, 3.6) 1. Compare and contrast cell envelopes of Gram positive and negative bacteria 2. Describe the structure, function, and importance of the microbial cell membrane 3. Understand why Gram-positive bacteria are more susceptible to penicillin than Gram-negative bacteria 4. Describe how specialized structures contribute to survival in specific environments 2 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. 3 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. 4 Model of a Bacterial Cell Cytoplasm: consists of a gel-like network Cell membrane: encloses the cytoplasm Cell wall: rigid structure external to 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 5 Biochemical Composition of Bacteria § All cells share common chemical components: Water Essential ions Small organic molecules Macromolecules § Cell composition varies with species, growth phase, and environmental conditions. 6 3.2 The Cell Membrane and Transport § The cell membrane is a selectively permeable barrier that separates the internal cellular components from the external environment. 7 Membrane Lipids § A phospholipid consists of glycerol with ester links to two fatty acids and a phosphoryl head group. Hydrophilic Head Hydrophobic Tail Extracellular Space § The two layers of phospholipids are called the phospholipid bilayer Intracellular Space 8 Membrane Proteins Serve Numerous Functions Structural support Detection of environmental signals Secretion of virulence factors and communication signals Ion transport and energy storage 9 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 passive diffusion. Water diffuses across the membrane in a process called osmosis. 10 Transport across the Cell Membrane § Polar molecules and charged molecules require transport through specific protein transporters. Passive transport: molecules move from an area of high concentration to an area of lower concentration. 11 Transport across the Cell Membrane Active transport: molecules move from an area of low concentration to an area of higher concentration. – REQUIRES ENERGY ATP 12 3.3 The Envelope and Cytoskeleton § How do prokaryotes protect the cell membrane? For most species, the cell envelope includes at least one structural supporting layer. – Most common support is the cell wall. 13 3.3 The Envelope and Cytoskeleton Nevertheless, a few prokaryotes, such as the mycoplasmas, have a cell membrane with no outer layers. 14 The Cell Wall Is a Single Molecule § The cell wall confers shape and rigidity to the cell and helps it withstand turgor pressure. 15 Peptidoglycan Structure § Most bacterial cell walls are made up of peptidoglycan § The molecule consists of: § Long polymers of repeating disaccharides bound to a short peptide § The peptides can form crossbridges connecting the parallel glycan strands. 16 Peptidoglycan Structure § Peptidoglycan is unique to bacteria. The enzymes responsible for its biosynthesis make excellent targets for antibiotic (penicillin-binding proteins). – Penicillin and Vancomycin inhibits cross-bridge formation 17 Cell Envelope of Bacteria § Most bacteria have additional envelope layers that provide structural support and protection. § Envelope composition defines: Gram-positive bacteria (thick cell wall) Gram-negative bacteria (thin cell wall) Mycobacteria (complex, multilayered cell wall) 18 Cell Envelope: Gram-positive and Gram-negative Which is more susceptible to penicillin? 19 Gram-Negative Cell Envelope LPS is a potent activator of the immune system. 20 Capsule Made of polysaccharide and glycoprotein Protects cells from phagocytosis Found in some Gram-positive and Gram-negative cells 21 S-layer An additional protective layer commonly found in free-living bacteria and archaea Crystalline layer of thick subunits consisting of protein or glycoprotein Contributes to cell shape and protect the cell from osmotic stress 22 Mycobacterial Cell Envelope § Very complex cell envelopes. Unusual membrane lipids (mycolic acids) and sugars (arabinogalactans) § Grow slowly—but they effectively resist host defenses. § Cultures can take weeks to grow instead of hours! 23 3.6 Specialized Structures § Thylakoids: extensively folded intracellular membranes found in photosynthetic bacteria § Carboxysomes: polyhedral bodies packed with the enzyme for CO2 fixation § Gas vesicles: protein-bound gas-filled structures that increase buoyancy 24 3.6 Specialized Structures § Pili are straight filaments of pilin protein. Used in attachment § Gram-negative enteric bacteria use sex pili for DNA exchange. 25 Rotary Flagella § Prokaryotes that are motile generally swim by means of rotary flagella. 26 Rotary Flagella § Each flagellum is a spiral filament of protein monomers called flagellin. Flagella rotate either clockwise (CW) or counterclockwise (CCW) relative to the cell. https://www.youtube.com/watch?v=dYt5135_0bs 27 Chemotaxis § Chemotaxis is the movement of a bacterium in response to chemical gradients. § Attractants cause CCW rotation. Flagella bundle together Push cell forward “Run” § Repellents (or absence of attractants) cause CW rotation. Flagellar bundle falls apart. “Tumble” – Bacterium briefly stops, then changes direction. 28 Chemotaxis § 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.” 29 3.4 Bacterial Cell Division § Bacterial cell division, or fission, requires highly coordinated growth and formation of all the cell’s parts. 30 Cell Division by Septation § As DNA synthesis terminates, the cell divides by a process called septation, the formation of the septum. § The septum grows inward from the sides of the cell, at last constricting and sealing off the two daughter cells. 31 Cell Division by Septation § Septation requires rapid biosynthesis of all envelope components, including membranes and cell wall. § The overall process of septation is managed by a protein complex called the divisome. 32