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BIOL 3340. Chapter 3. Chapter 3. Microbial Cell Structure. Types of Cells. Two major classes: eukaryotes & prokaryotes.
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BIOL 3340 Chapter 3
Chapter 3 Microbial Cell Structure
Types of Cells • Two major classes: eukaryotes &prokaryotes. • Differences: the materials making up the nucleus of eukaryotic cells are separated from the rest of the cell by the nuclear membrane, whereas in prokaryotic cells these materials are not separated. • All animals and plant cells are eukaryotic including fungi. Bacteria, cyanobacteria and the mycoplasmas are prokaryotic.
Size, Shape, andArrangement of Bacterial cells Cocci (s., coccus) – spheres • diplococci (s., diplococcus) – pairs • streptococci – chains • staphylococci – grape-like clusters • tetrads – 4 cocci in a square • sarcinae – cubic configuration of 8 cocci
……Size, Shape, andArrangement Bacilli (s., bacillus): – rods • coccobacilli – very short rods • vibrios – resemble rods, comma shaped • spirilla (s., spirillum) – rigid helices • spirochetes – flexible helices • mycelium – network of long, multinucleate filaments Check on line lab Manual for Bacterial shapes)
……Size, Shape, andArrangement • Sizes: • Typically ~ 0.1 - 20 m (with some exceptions) • Typical coccus: ~ 1 m (e.g. Staphylococcus) • Typical short rod: ~ 1 x 5 m (e.g. E. coli) • Barely within the best resolution of a good compound light microscope
Cell Structureof Procaryotes Prokaryotic cells The constituents of a typical bacterium are as follows: • Bacterial Cell Wall and Capsule– bacteria are surrounded by a cell wall, which not only contains polysaccharide but also contains protein and lipid. • In some bacteria, the cell wall is surrounded by the capsule. • The cell wall and capsule provide shape and form to the bacterium and also acts as a physical barrier between the bacterium and its environment. • Nucleoids – in bacteria the nuclear material is concentrated in a region called the nucleoid within the cytoplasm.
….Cell Structure • There is no membrane-bound nucleus in prokaryotes. • Instead the DNA is located within a specialized region of the cytoplasm of the cell called the nucleoid region. • There is no nuclear membrane surrounding the nucleoid. • Bacterial flagella – many bacteria possess one or more flagella for locomotion.
Gram-negative Cell Walls and Acid Fast Fast cell wall in Chapter 3
Procaryotic Cell Membranes Cell Membranes: • membranes are an absolute requirement for all living organisms. • plasma membrane encompasses the cytoplasm • some procaryotes also have internal membrane systems
Functions of the Plasma Membrane • separation of cell from its environment • selectively permeable barrier • some molecules are allowed to pass into or out of the cell • transport systems aid in movement of molecules • detection of and response to chemicals in surroundings with the aid of special receptor molecules in the membrane
…Phospholipid layer • polar ends • interact with water • hydrophillic • nonpolar ends • insoluble in water • hydrophobic
Membrane Proteins Peripheral proteins: • loosely associated with the membrane and easily removed Integral proteins • embedded within the membrane and not easily removed
Procaryotic Cytoplasm Cytoplamic Matrix: • Cytoplasm contains the nucleoid,ribosomes and inclusion bodies • lacks organelles bound by unit membranes • composed largely of water • is a major part of the protoplasm (the plasma membrane and everything within)
..Cytoplasmic Matrix • Viscous aqueous suspension of proteins, nucleic acid, dissolved organic compounds, mineral salts • Network of protein fibers similar to the eukaryotic cytoskeleton. Cytoplasmic Inclusion Bodies: • granules of organic or inorganic material that are stockpiled by the cell for future use. • some are enclosed by a single-layered membrane
….Cytoplasmic inclusions Cytoplasmic inclusions: • Glycogen Granules • Poly--hydroxybutyrate granules • Lipid droplets • Gas vacuoles • Metachromatic granules(Phosphate crystals or volutin granules) • Sulfur Granules
Ribosomes Ribosomes: • complex structures consisting of protein and RNA • sites of protein synthesis • smaller than eucaryotic ribosomes • procaryotic ribosomes 70S • eucaryotic ribosomes 80S
The Nucleoid Nucleoid: • irregularly shaped region • location of chromosome • usually 1/cell • not membrane-bound
The Procaryotic Chromosome The Chromosomes: • usually a closed circular, double-stranded DNA molecule • looped and coiled extensively
Plasmids Plasmids: • usually small, closed circular DNA molecules • exist and replicate independently of chromosome • have relatively few genes present
Procaryotic Cell Walls Prokaryotic Cell Wall: • rigid structure that lies just outside the plasma membrane (detail to continue)
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
Cell Walls of Bacteria Gram Staining developed by Gram in 1888: • bacteria are divided into two major groups based on the response to gram-stain procedure • gram-positive bacteria stain purple • gram-negative bacteria stain pink • staining reaction due to cell wall structure
Gram-Positive Cell Walls • Gram positive bacteria composed primarily of peptidoglycan • Peptidoglycan are polymers which contains N-acetylglucosamine and N-acetylmuramic acid and several different amino acids • Walls contain teichoic acid ( polymers of glycerol or ribitol joined by phosphate groups)
..Gram-Positive Cell Walls • The periplasmicspace lies between plasma membrane and cell wall and is smaller than that of gram-negative bacteria • periplasmhas relatively few proteins • enzymes secreted by gram-positive bacteria are called exoenzymes
Gram-Negative Cell Walls • consist of a thin layer of peptidoglycan surrounded by an outer membrane • outer membrane composed of lipids, lipoproteins, and lipopolysaccharide (LPS) • no teichoic acids
…..Gram Negative • more complex than gram-positive walls • periplasmic space differs from that in gram-positive cells • may constitute 20-40% of cell volume • many enzymes present in periplasm
Assignments • Features of a prokaryotic cell • List the differences between a gram positive and gram negative cell wall.
Variations on Cell Wall Architecture Acid-fast Cell Walls: • Many genera in the “High GC gram-positive” bacterial group contain mycolic acids, embedded in the peptidoglycan . • Mycolic acids are a class of waxy, extremely hydrophobic lipids. • Certain genera contain very large amounts of this lipid, and are difficult to gram stain. • These genera may be identified by the “acid-fast” staining technique. • Includes Mycobacterium and Nocardia.
..Variations on Cell Wall Architecture Mycoplasmas: • Bacteria that are naturally have no cell walls • Includes Mycoplasma and Ureaplasma Archaea : • Have archaea cell walls with no peptidoglycan • Many have cell walls containing pseudomurein, a polysaccharide similar to peptidoglycan but containing N-acetylglucosamine and N-acetyltalosaminuronic acid .
Capsules, Slime Layers, and S-Layers Layers of material lying outside the cell wall • capsules • usually composed of polysaccharides • well organized and not easily removed from cell • slime layers • similar to capsules except diffuse, unorganized and easily removed. • a capsule or slime layer composed of polysaccharides can also be referred to as a glycocalyx
S-layers: S-layers: • regularly structured layers of protein or glycoprotein. • in bacteria the S-layer is external to the cell wall. • Regular “floor tile” pattern. • Function not clear -- Stability?
Functions of Capsules, Slime Layers, and S-layers • protection from host defenses (e.g., phagocytosis) • protection from harsh environmental conditions (e.g., desiccation) , chemicals or osmotic stress • attachment to surfaces • facilitate motility • nutrient Storage
Pili and Fimbriae Fimbriae (s., fimbria) • short, thin, hairlike, proteinaceous appendages up to 1,000/cell • mediate attachment to surfaces sex pili (s., pilus): • similar to fimbriae except longer, thicker, and less numerous (1-10/cell) • required for mating