Transcript Organismal Biology/27B1-StrctrFnctinReproProka

CHAPTER 27
PROKARYOTES AND THE ORIGINS OF
METABOLIC DIVERSITY
Section B1: The Structure, Function, and Reproduction
of Prokaryotes
1. Nearly all prokaryotes have a cell wall external to the plasma membrane
2. Many prokaryotes are motile
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Introduction
• Most prokaryotes are unicellular.
• Some species may aggregate transiently or form
true colonies, even extending to division of labor
between specialized cell types.
• The most common
shapes among
prokaryotes are
spheres (cocci),
rods (bacilli),
and helices.
Fig. 27.3
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• Most prokaryotes have diameters in the range of 15 μm, compared to 10-100 μm for most eukaryotic
cells.
• However, the largest prokaryote discovered so far has a
diameter of 0.75 mm.
• It is a sulfur-metabolizing
marine bacterium from
coastal sediments off
Namibia.
Fig. 26.4
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1. Nearly all prokaryotes have a cell wall
external to the plasma membrane
• In nearly all prokaryotes, a cell wall maintains the
shape of the cell, affords physical protection, and
prevents the cell from bursting in a hypotonic
environment.
• Most bacterial cell walls contain peptidoglycan, a
polymer of modified sugars cross-linked by short
polypeptides.
• The walls of archaea lack peptidoglycan.
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• The Gram stain is a valuable tool for identifying
specific bacteria, based on differences in their cell
walls.
• Gram-positive bacteria have simpler cell walls,
with large amounts of peptidoglycans.
Fig. 27.5a
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• Gram-negative bacteria have more complex cell
walls and less peptidoglycan.
• An outer membrane on the cell wall contains
lipopolysaccharides, carbohydrates bonded to lipids.
Fig. 27.5b
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• Among pathogenic bacteria, gram-negative
species are generally more threatening than grampositive species.
• The lipopolysaccharides on the walls are often toxic
and the outer membrane protects the pathogens from
the defenses of their hosts.
• Gram-negative bacteria are commonly more resistant
than gram-positive species to antibiotics because the
outer membrane impedes entry of antibiotics.
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• Many antibiotics, including penicillins, inhibit the
synthesis of cross-links in peptidoglycans,
preventing the formation of a functional wall,
particularly in gram-positive species.
• These drugs are a very selective treatment because
they cripple many species of bacteria without affecting
humans and other eukaryotes, which do not synthesize
peptidoglycans.
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• Many prokaryotes secrete another sticky
protective layer, the capsule, outside the cell wall.
• Capsules adhere the cells to their substratum.
• They may increase resistance to host defenses.
• They glue together the cells of those prokaryotes that
live as colonies.
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• Another way for prokaryotes to adhere to one
another or to the substratum is by surface
appendages called pili.
• Pili can fasten pathogenic bacteria to the mucous
membranes of its host.
• Some pili are
specialized for
holding two
prokaryote cells
together long
enough to transfer
DNA during
conjugation.
Fig. 27.6
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2. Many prokaryotes are motile
• About half of all prokaryotes are capable of
directional movement.
• The action of flagella, scattered over the entire
surface or concentrated at one or both ends, is the
most common method of movement.
• The flagella of prokaryotes differ in structure and
function from those of eukaryotes.
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• In a prokaryotic flagellum, chains of a globular protein
wound in a tight spiral from a filament which is attached to
another protein (the hook), and the basal apparatus.
Fig. 27.7
• Rotation of the filament is driven by the diffusion of protons into
the cell through the basal apparatus after the protons have been
actively transported by proton pumps in the plasma membrane.
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• A second motility mechanism is found in
spirochetes, helical bacteria.
• Two or more helical filaments under the cell wall are
attached to a basal motor attached to the cell.
• When the filaments rotate, the cell moves like a
corkscrew.
• A third mechanism occurs in cells that secrete a
jet of slimy threads that anchors the cells to the
substratum.
• The cell glides along at the growing end of threads.
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• In a relatively uniform environment, a flagellated
cell may wander randomly.
• In a heterogenous environment, many prokaryotes
are capable of taxis, movement toward or away
from a stimulus.
• With chemotaxis, binding between receptor cells on
the surface and specific substances results in
movement toward the source (positive chemotaxis) or
away (negative chemotaxis).
• Other prokaryotes can detect the presence of light
(phototaxis) or magnetic fields.
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