Topic 3. The Prokaryotes

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Topic 3. The Prokaryotes

• Introduction, Structure Function,
• Classification, Examples

September 21, 2005 Biology 1001
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3.1 Introduction to the Prokaryotes

• First life to evolve about 3.5 BYA alone on
  Earth for 2 BYA
• Two groups diverged early in lifes history
  Archaea Bacteria
• Genetically diverse lineages due to almost 4
  billion years of evolution

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3.1 Introduction to the Prokaryotes

• Prokaryotes are microscopic, unicellular, and
  simple in form, but they dominate the biosphere
• Biomass 10X greater than all eukaryotes
• More bacteria in a handful of soil than people
  who have ever lived
• More than 4000 species described perhaps as many
  as 4 million exist

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3.1 Introduction to the Prokaryotes

• Inhabit diverse environments theyre almost
  everywhere
• Salty, acidic, hot, cold, anaerobicplaces where
  nothing else lives
• A wealth of metabolic diversity and other
  evolutionary adaptations

• Serve vital ecological roles
• Chemical recycling
• Mutually beneficial symbiotic relationships
• Most are not pathogenic!

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3.2 Structure and Function of Prokaryotes

• Small (usually 1-5 µM) and structurally simple
• Evolution and diversity at the chemical or
  metabolic level
• Three common shapes spheres, rods, and spirals
• Most are unicellular
• Some stick together and form clumps or chains

Thiomargarita namibiensis (750 µm)
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Prokaryotic Vs. Eukaryotic Cells

• All cells have the following components
• Plasma membrane membrane enclosing the
  cytoplasm
• Cytoplasm space between plasma membrane and
  nucleus, interior of cell in prokaryotes
• Cytosol semi-fluid substance in the cytoplasm
• Ribosomes organelles that synthesize proteins
• Chromosomes contain DNA and associated proteins
• Eukaryotic (eu true, karyonkernel) cells also
  have a membrane-bound nucleus that contains the
  chromosomes, are larger (10-100 µm), and contain
  other membranous organelles and structures

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Features of the Prokaryotic Cell

• In prokaryotic (probefore, karyonkernel) cells
  the single chromosome is concentrated in a
  non-membrane-bound region called the nucleoid
• In addition, prokaryotes may have smaller rings
  of DNA called plasmids that contain only a few
  genes (usually for antibiotic resistance or
  metabolism of rare nutrients) and replicate
  independently of the main chromosome

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Features of the Prokaryotic Cell

• Hairlike appendages called fimbriae (Sl. fimbria)
  or pili (Sl. pilus) allow prokaryotes to stick to
  their substrate or each other
• An external capsule (layer of polysaccharide or
  protein) also enables adherence, and provides
  protection for pathogens
• Nearly all prokaryotes have a cell wall, a rigid
  structure found outside the plasma membrane, that
  protects the cell helps maintains cell shape

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The Structure and Function of the Cell Wall
(Section 5.2 of Course Outline)

• Most Bacteria cell walls contain peptidoglycan
  a modified sugar polymer cross-linked by short
  polypeptides
• Archaea cell walls contain a variety of
  polysaccharides and proteins
• A technique called Gram stain is often used to
  classify Bacterial species on the basis of
  differences in cell wall composition
• Gram-positive bacteria have simpler walls with a
  large amount of peptidoglycan
• Gram-negative bacteria less peptidoglycan and are
  structurally more complex, with an outer membrane
  containing lipopolysaccharides

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Gram StainingFigure 27.3
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Motility

• About half of all prokaryotes are capable of
  directed movement or taxis, at speeds up to or
  exceeding 50 µm/sec
• The most common structures enabling prokaryotes
  to move are the flagella (Sl. flagellum)

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Internal Organization of Prokaryotes

• Lack complex organization but some do have
  specialized membranes that perform metabolic
  functions. These are usually infoldings of the
  plasma membrane.

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The Prokaryotic CellFigure 6.6