An Introduction to Cells and Procaryotic Cell

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• Chapter 4
• An Introduction to Cells and Procaryotic Cell
• Structure and Function

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Characteristics of Cells and Life

• All living things (single and multicellular) are
  made of cells that share some common
  characteristics
• basic shape spherical, cubical, cylindrical
• internal content cytoplasm, surrounded by a
  membrane
• DNA chromosome(s), ribosomes, metabolic
  capabilities
• Two basic cell types eucaryotic and procaryotic

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Characteristics of Cells

• Eucaryotic cells animals, plants, fungi, and
  protists
• contain double-membrane bound nucleus with DNA
  chromosomes
• contain membrane-bound organelles that
  compartmentalize the cytoplasm and perform
  specific functions
• Procaryotic cells bacteria and archaea
• no nucleus or other membrane-bound organelles

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Characteristics of Life

• Growth and development
• Reproduction and heredity genome composed of
  DNA packed in chromosomes produce offspring
  sexually or asexually
• Metabolism chemical and physical life processes
• Movement and/or irritability respond to
  internal/external stimuli self-propulsion of
  many organisms
• Cell support, protection, and storage mechanisms
  cell walls, vacuoles, granules and inclusions
• Transport of nutrients and waste

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External Structures

• Appendages
• two major groups of appendages
• Motility flagella and axial filaments
  (periplasmic flagella)
• Attachment or channels fimbriae and pili
• Glycocalyx surface coating

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Flagella

• 3 parts
• filament long, thin, helical structure composed
  of protein flagellin
• hook- curved sheath
• basal body stack of rings firmly anchored in
  cell wall
• Number and arrangement of flagella varies
• monotrichous, lophotrichous, amphitrichous,
  peritrichous
• Functions in motility of cell through environment

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Flagellar Arrangements

1. Monotrichous single flagellum at one end
2. Lophotrichous small bunches arising from one
   end of cell
3. Amphitrichous flagella at both ends of cell
4. Peritrichous flagella dispersed over surface of
   cell slowest

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Flagellar Function

• Guide bacteria in a direction in response to
  external stimulus
• chemical stimuli chemotaxis positive and
  negative
• light stimuli phototaxis

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Fimbriae

• Fine, proteinaceous, hairlike bristles from the
  cell surface
• Function in adhesion to other cells and surfaces

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Pili

• Rigid tubular structure made of pilin protein
• Found only in Gram negative cells
• Function to join bacterial cells for partial DNA
  transfer called conjugation

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Glycocalyx

• Coating of molecules external to the cell wall,
  made of sugars and/or proteins
• Two types
• slime layer - loosely organized and attached
• capsule - highly organized, tightly attached
• Functions
• protect cells from dehydration and nutrient loss
• inhibit killing by white blood cells by
  phagocytosis contributing to pathogenicity
• attachment - formation of biofilms

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The Cell Envelope

• External covering outside the cytoplasm
• Composed of two basic layers
• cell wall and cell membrane
• Maintains cell integrity
• Two generally different groups of bacteria
  demonstrated by Gram stain
• Gram-positive bacteria thick cell wall composed
  primarily of peptidoglycan and cell membrane
• Gram-negative bacteria outer cell membrane, thin
  peptidoglycan layer, and cell membrane

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Insert figure 4.12 Comparative cell envelopes
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Structure of Cell Walls

• Determines cell shape, prevents lysis (bursting)
  or collapsing due to changing osmotic pressures
• Peptidoglycan is primary component
• unique macromolecule composed of a repeating
  framework of long glycan chains cross-linked by
  short peptide fragments

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Gram-positive Cell Wall

• Thick, homogeneous sheath of peptidoglycan
• 20-80 nm thick
• function in cell wall maintenance and enlargement
  during cell division move cations across the
  cell envelope stimulate a specific immune
  response

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Gram-negative Cell Wall

• Composed of an outer membrane and a thin
  peptidoglycan layer
• Outer membrane is similar to cell membrane
  bilayer structure
• outermost layer contains lipopolysaccharides and
  lipoproteins (LPS)
• endotoxin that may become toxic when released
  during infections
• may function as receptors and blocking immune
  response
• contains porin proteins in upper layer regulate
  molecules entering and leaving cell
• Bottom layer composed of phospholipids and
  lipoproteins

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Gram-negative Cell Wall

• Single, thin sheet of peptidoglycan
• Protective structure while providing some
  flexibility and sensitivity to lysis
• Periplasmic space surrounds peptidoglycan

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The Gram Stain

• Differential stain that distinguishes cells with
  a Gram-positive cell wall from those with a
  Gram-negative cell wall
• Gram-positive - retain crystal violet and stain
  purple
• Gram-negative - lose crystal violet and stain red
  from safranin counterstain
• Important basis of bacterial classification and
  identification
• Practical aid in diagnosing infection and guiding
  drug treatment

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Atypical Cell Walls

• Some bacterial groups lack typical cell wall
  structure i.e. Mycobacterium and Nocardia
• Gram-positive cell wall structure with lipid
  mycolic acid (cord factor)
• pathogenicity and high degree of resistance to
  certain chemicals and dyes
• basis for acid-fast stain used for diagnosis of
  infections caused by these microorganisms
• Some have no cell wall i.e. Mycoplasma
• cell wall is stabilized by sterols
• pleomorphic

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Cell Membrane Structure

• Phospholipid bilayer with embedded proteins
• Functions in
• providing site for energy reactions, nutrient
  processing, and synthesis
• transport into and out of the cell

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Bacterial Internal Structures

• Cell cytoplasm
• dense gelatinous solution of sugars, amino acids,
  and salts
• 70-80 water
• serves as solvent for materials used in all cell
  functions

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Bacterial Internal Structures

• Chromosome
• single, circular, double-stranded DNA molecule
  that contains all the genetic information
  required by a cell
• DNA is tightly coiled around a protein,
  aggregated in a dense area called the nucleoid.

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Bacterial Internal Structures

• Plasmids
• small circular, double-stranded DNA
• free or integrated into the chromosome
• duplicated and passed on to offspring
• not essential to bacterial growth and metabolism
• may encode antibiotic resistance, tolerance to
  toxic metals, enzymes and toxins
• used in genetic engineering- readily manipulated
  and transferred from cell to cell

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Bacterial Internal Structures

• Ribosomes
• made of 60 ribosomal RNA and 40 protein
• consist of two subunits large and small
• procaryotic differ from eucaryotic ribosomes in
  size and number of proteins
• site of protein synthesis
• present in all cells

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Bacterial Internal Structures

• Inclusions and granules
• intracellular storage bodies
• vary in size, number and content
• Bacterial cell can use them when environmental
  sources are depleted.
• examples glycogen, poly-b-hydroxybutyrate, gas
  vesicles for floating, sulfur and phosphate
  granules (metachromatic granules)

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Bacterial Internal Structures

• Endospores
• inert, resting, cells produced by some G genera
  Clostridium, Bacillus and Sporosarcina
• have a 2-phase life cycle
• vegetative cell metabolically active and
  growing
• endospore when exposed to adverse environmental
  conditions capable of high resistance and very
  long-term survival
• sporulation -formation of endospores
• hardiest of all life forms
• withstands extremes in heat, drying, freezing,
  radiation and chemicals
• not a means of reproduction
• germination- return to vegetative growth

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Endospores

• Resistance linked to high levels of calcium and
  dipicolinic acid
• Dehydrated, metabolically inactive
• thick coat
• Longevity verges on immortality - 25,250 million
  years.
• Resistant to ordinary cleaning methods and
  boiling
• Pressurized steam at 120oC for 20-30 minutes will
  destroy

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Bacterial Shapes, Arrangements, and Sizes

• Variety in shape, size, and arrangement but
  typically described by one of three basic shapes
• coccus - spherical
• bacillus rod
• coccobacillus very short and plump
• vibrio gently curved
• spirillum - helical, comma, twisted rod,
• spirochete spring-like

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Bacterial Shapes, Arrangements, and Sizes

• Arrangement of cells is dependent on pattern of
  division and how cells remain attached after
  division
• cocci
• singles
• diplococci in pairs
• tetrads groups of four
• irregular clusters
• chains
• cubical packets
• bacilli
• chains
• palisades

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Classification Systems in the Procaryotae

• Microscopic morphology
• Macroscopic morphology colony appearance
• Physiological / biochemical characteristics
• Chemical analysis
• Serological analysis
• Genetic and molecular analysis
• G C base composition
• DNA analysis using genetic probes
• Nucleic acid sequencing and rRNA analysis

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Bacterial Taxonomy Based on Bergeys Manual

• Bergeys Manual of Determinative Bacteriology
  five volume resource covering all known
  procaryotes
• classification based on genetic information
  phylogenetic
• two domains Archaea and Bacteria
• five major subgroups with 25 different phyla

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Major Taxonomic Groups of Bacteria

• Domain Archaea primitive, adapted to extreme
  habitats and modes of nutrition
• Domain Bacteria -
• Phylum Proteobacteria Gram-negative cell walls
• Phylum Firmicutes mainly Gram-positive with low
  G C content
• Phylum Actinobacteria Gram-positive with high G
  C content

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Diagnostic Scheme for Medical Use

• Uses phenotypic qualities in identification
• restricted to bacterial disease agents
• divides based on cell wall structure, shape,
  arrangement, and physiological traits

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Species and Subspecies

• Species a collection of bacterial cells which
  share an overall similar pattern of traits in
  contrast to other bacteria whose pattern differs
  significantly
• Strain or variety a culture derived from a
  single parent that differs in structure or
  metabolism from other cultures of that species
  (biovars, morphovars)
• Type a subspecies that can show differences in
  antigenic makeup (serotype or serovar),
  susceptibility to bacterial viruses (phage type)
  and in pathogenicity (pathotype)

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Procaryotes with Unusual Characteristics

• Free-living nonpathogenic bacteria
• Photosynthetic bacteria - use photosynthesis, can
  synthesize required nutrients from inorganic
  compounds
• Cyanobacteria (blue-green algae)
• Gram-negative cell walls
• extensive thylakoids with photosynthetic
  chlorophyll pigments and gas inclusions
• Green and purple sulfur bacteria
• contain photosynthetic pigment bacteriochlorophyll
  
• do not give off oxygen as a product of
  photosynthesis
• Gliding, fruiting bacteria
• Gram-negative
• Glide over moist surfaces

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Procaryotes with Unusual Characteristics

• Unusual forms of medically significant obligate
  intracellular parasites
• Rickettsias
• Very tiny, Gram-negative bacteria
• Most are pathogens that alternate between mammals
  and fleas, lice or ticks.
• Obligate intracellular pathogens
• Cannot survive or multiply outside of a host cell
• Cannot carry out metabolism on their own
• Rickettsia rickettisii Rocky Mountain spotted
  fever
• Rickettsia prowazekii epidemic typhus
• Coxiella burnetti Q fever

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Unusual Forms of Medically Significant

• Chlamydias
• Tiny
• Obligate intracellular parasites
• Not transmitted by arthropods
• Chlamydia trachomatis severe eye infection and
  one of the most common sexually transmitted
  diseases
• Chlamydia psittaci ornithosis, parrot fever
• Chlamydia pneumoniae lung infections

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Archaea The Other Procaryotes

• Constitute third Domain Archaea
• Seem more closely related to Domain Eukarya than
  to bacteria
• Live in the most extreme habitats in nature,
  extremophiles
• Adapted to heat, salt, acid pH, pressure and
  atmosphere
• Includes methane producers, hyperthermophiles,
  extreme halophiles, and sulfur reducers