Prokaryotic Cell Structure and function (Part I) BIO3124 Lecture #3 (I)

Plasma Membrane Properties and Functions defines the existence of a cell Made of lipid bilayer Double layer of phospholipids Surrounds the cell approx. 5-10 nm in thickness Separates exterior environment from interior Dynamic selective barrier Concentrates certain components intracellulary Allows excretion of waste Sense the outside world Several metabolic processes ex. Respiration, photosynthesis

Fluid Mosaic Model of Membrane Structure Lipid bilayer in which proteins float (Singer and Nicholson model)

Membrane proteins Membrane proteins serve numerous functions, including: - Structural support - Detection of environmental signals - Secretion of virulence factors and communication signals - Ion transport and energy storage Have hydrophilic and hydrophobic regions that lock the protein in the membrane 4

Phosphatidylethanolamine Membrane lipids Amphipathic phospholipids polar ends (hydrophilic) Glycerol, negative charge (outer leaflet) Ethanolamine, positive charge (inner leaflet) Phosphatidylethanolamine nonpolar ends (hydrophobic) Tails of fatty acids Palmitic acid Oleic acid (kinking) increase fluidity Cyclopropane conversion aging cells

Bacterial Membranes differ from eukaryotes in lacking sterols do contain hopanoids, sterol-like molecules synthesized from similar precursors Stabilize bacterial membranes total mass on earth ~1012 tons a highly organized, asymmetric structure, flexible and dynamic

Archaeal membranes Etherglycerol, not ester bond Terpene derived lipids some have a monolayer membranes Tetra-ether glycerol Cyclopentane: isoprene cyclization Increased stability

Archaeal membranes Moderately thermophilic - Bilayer or mixed Extreme thermophiles eg. Solfolobus and Theromoplasma

Role of cell membrane in energy metabolism bacterial cell membranes involved in ETC Gradual energy release forming proton gradient across membrane 9

Animation: A bacterial electron transfer system

The Proton Motive Force The transfer of H+ through a proton pump generates an electrochemical gradient of protons, called a proton motive force. - It drives the conversion of ADP to ATP through ATP synthase. - This process is known as the chemiosmotic theory. 11

PMF Drives Many Cell Functions Besides ATP synthesis, Dp drives many cell processes including: rotation of flagella, uptake of nutrients, and efflux of toxic drugs 12

ATP synthase mechanism Note: pump also works in reverse to create H+ gradient

Cell Transport Transporters pass material in/out of cell Passive transport follows gradient of material Pumps use energy ATP or PMF Move material against their gradient Passive diffusion lets small molecules into cell

The Bacterial Cell Wall rigid structure that lies just outside the plasma membrane

Functions of cell wall provides characteristic shape to cell protects the cell from osmotic lysis may also contribute to pathogenicity very few procaryotes lack cell walls, ie Mycoplasmas

Evidence of protective nature of the cell wall Lysozyme treatment Penicillin inhibits peptidoglycan synthesis

Gram negative cell wall few PG layers, defined Periplasmic space unique outer membrane, LPS, Braun’s lipoprotein Braun’s

Gram positive cell walls Multiple PG layers, periplasmic space exposed Teichoic acid

Peptidoglycan (Murein) Structure Mesh-like polymer composed of identical subunits contains N-acetyl glucosamine and N-acetylmuramic acid and several different amino acids chains of linked peptidoglycan subunits are cross linked by peptides

Cell wall unit structures

G- G+ Bacterial cell wall Top: E. coli peptidoglycan; direct cross-linking, typical of many gram-negative bacteria Bottom: Staphylococcus aureus peptidoglycan NAM= N-acetylglucosoamine; NAG= N-acetylmuramic acid

Animation: Bacterial peptidoglycan cell walls

Wall Assembly Cleavage by autolysin Pre-formed subunits added. Bridges created (transpeptidation)

Archaeal cell walls lack peptidoglycan Resemble G+ thick wall cell wall varies from species to species but usually consists of complex hetero-polysaccharides and glycoproteins eg. Methanosarcina, and Halcoccus have complex polysacharides resembling those of eukaryotic connective tissue extracellular matrix Methanogens have walls containing pseudomurein

Archaeal cell walls: Pseudomurein NAG NAT NAT instead of NAM; links to NAG by β(1→3) glycosidic linkage instead of β(1→4) no lactic acid between NAT and peptides NAT connects to tetra-peptide through C6 instead of NAM C3 in eubacteria in some tetra-peptide consists of L-amino acids instead of D-amino acids

The Gram-Positive Envelope Capsule (not all species) Polysaccharide S-Layer (not all species) Made of protein Thick cell wall Teichoic acids for strength Thin periplasm Plasma membrane

Gram-Positive Cell Walls CW composed primarily of peptidoglycan contain large amounts of teichoic acids polymers of glycerol or ribitol joined by phosphate groups some gram-positive bacteria have layer of proteins on the surface of peptidoglycan Isolated cell wall from Bacillus megaterium; latex spheres are 0.25 micrometer in diameter

The Gram-Negative Envelope Capsule (not all species) Polysaccharide Outer Membrane LPS (lipopolysaccharide) In outer leaflet only Braun lipoprotein Thin cell wall one or two layers of peptidoglycan Thick periplasm Plasma membrane Peptidoglycan cell wall

Braun (Murein) lipoprotein Braun lipoprotein Bridges inner leaflet of outer membrane to peptidoglycan 67 aa protein with N-terminal Cyc-triglyceride C-terminal lysine connected to mDAP by peptide bond

Porins more permeable than plasma membrane due to presence of porin proteins and transporter proteins porin proteins form channels through which small molecules (600-700 daltons) can pass

Lipopolysaccharides (LPSs) consists of three parts lipid A core polysaccharide O-side chain (O antigen)

Importance of LPS protection from host defenses (O antigen variation) contributes to negative charge on cell surface (core polysaccharide) helps stabilize outer membrane structure (lipid A) can act as an endotoxin (lipid A)

Capsules, Slime Layers, and S-Layers layers of material lying outside the cell wall capsules usually composed of polysaccharides, some have proteins well organized and not easily removed from cell eg. Klebsiella and Pneumococcus slime layers similar to capsules except diffuse, unorganized and easily removed a capsule or slime layer composed of organized, thick polysaccharides can also be referred to as a glycocalyx

Capsules, Slime Layers, and S-Layers regularly structured layers of proteins or glycoproteins In bacteria the S- layer is external to the cell wall common among Archaea, act as molecular sieve letting passage of small molecules S-layer of Thermoproteus tenax

Functions of capsules, slime layers, and S-layers protection from host defenses (e.g., phagocytosis) protection from harsh environmental conditions (e.g., desiccation) attachment to surfaces protection from viral infection or predation by bacteria protection from chemicals in environment (e.g., detergents) facilitate motility of gliding bacteria protection against osmotic stress

Pili and Fimbriae Fimbriae (s., fimbria) Sex pili (s., pilus) short, thin, hairlike, proteinaceous appendages up to 1,000/cell mediate attachment to surfaces some (type IV fimbriae) required for twitching motility or gliding motility that occurs in some bacteria Sex pili (s., pilus) similar to fimbriae except longer, thicker, and less numerous (1-10/cell) required for mating (conjugation) Produced by F+ strains The fimbriae of P. vulgaris