Topic 3. The Prokaryotes
Introduction,
Structure &
Function,
Classification,
Examples
September 21, 2005 Biology 1001
3.1 Introduction to the Prokaryotes
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First life to evolve about 3.5 BYA; alone on Earth for 2 BYA
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Two groups diverged early in life’s history – Archaea & Bacteria
Genetically diverse lineages due to almost 4 billion years of evolution
3.1 Introduction to the Prokaryotes
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Prokaryotes are microscopic, unicellular, and simple in form,
but they dominate the biosphere
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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
3.1 Introduction to the Prokaryotes
• Inhabit diverse environments – they’re almost everywhere
– Salty, acidic, hot, cold, anaerobic…places 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!
3.2 Structure and Function of Prokaryotes
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Small (usually 1-5 µM) and structurally simple
 Evolution and
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diversity at the chemical or metabolic level
Three common shapes – spheres, rods, and spirals
Thiomargarita namibiensis (750 µm)
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Most are unicellular
 Some
stick together and form clumps or chains
Prokaryotic Vs. Eukaryotic Cells
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All cells have the following components
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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, karyon=kernel) cells also have a
membrane-bound nucleus that contains the chromosomes,
are larger (10-100 µm), and contain other membranous
organelles and structures
Features of the Prokaryotic Cell
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In prokaryotic (pro=before, karyon=kernel) cells the single
chromosome is concentrated in a non-membrane-bound region
called the nucleoid
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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
Features of the Prokaryotic Cell
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Hairlike appendages called fimbriae (Sl. fimbria) or pili (Sl.
pilus) allow prokaryotes to stick to their substrate or each other
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An external capsule (layer of polysaccharide or protein) also
enables adherence, and provides protection for pathogens
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Nearly all prokaryotes have a cell wall, a rigid structure found
outside the plasma membrane, that protects the cell & helps
maintains cell shape
The Structure and Function of the Cell Wall
(Section 5.2 of Course Outline)
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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
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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
Gram Staining
Figure 27.3
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)
Internal Organization of Prokaryotes
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Lack complex organization but some do have specialized
membranes that perform metabolic functions. These are
usually infoldings of the plasma membrane.
The Prokaryotic Cell
Figure 6.6