Chapter 27 Notes
Introduction
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Most prokaryotes are microscopic
Their collective biological mass is at least 10x more than eukaryotes
Reason for success of prokaryotes is a wealth of adaptations that enable various prokaryotes to inhabit diverse
environments
o Thrive almost everywhere
Biologists are discovering that prokaryotes have lots of genetic diversity
Prokaryotes classified into two domains:
o Archaea
o Bacteria
 Differ in structural, physiological, and biochemical ways
27.1
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Most prokaryotes are unicellular
o Some species group into colonies
o Diameter of cells 1-5 micrometers
Variety of shapes
o Spheres (cocci)
o Rods (bacilli)
o Spirals
Cell Surface Structures
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One of the most important features nearly all prokaryotic cells share is their cell wall
o Maintains shape
o Provides physical protection
o Prevents cell from bursting in a hypotonic environment
 In hypertonic, most lose water and shrink away from cell wall (plasmolyze)
o Cell walls differ in molecular composition and construction
 Contain peptidoglycan: network of modified-sugar polymers cross-linked by short polypeptides
 Encloses whole bacterium and anchors other molecules that extend from its surface
Prokaryotes can be classified by gram-staining
o Identifies two groups of bacteria based on differences in cell wall composition
 Gram positive: simpler walls with large amount of peptidoglycan
 Gram negative: less peptidoglycan and are more complex, with an outer membrane that
contains lipopolysaccharides (fats bonded to carbs0
o Valuable method in medicine
 Among pathogenic bacteria, gram-negative species are generally more threatening than grampositive species
 Gram-negative more resistant to antibiotics because outer membrane impedes entry of
drugs
Cell wall covered by capsule
o Capsule: sticky layer of polysaccharide or protein
 Enables prokaryotes to adhere to their substrate or to other individuals in colony
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 Shield pathogenic prokaryotes from attacks by host’s immune system
Stick to substrate or to one another
o Done so by fimbriae and pili
 Fimbriae: hairlike appendages; more numerous and shorter
 Pili: longer than fimbriae
 Specialized pili link prokaryotes during conjugation (process in which one cell transfers
DNA to another cell
Motility
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Half of all prokaryotes are capable of directional movement
One structure that helps them move is flagella
o Can be scattered over whole cell or concentrated at one or both ends
 Differ from eukaryotic flagella by structure and mechanism of propulsion; also smaller
Taxis: movement toward or away from stimulus
o Chemotaxis: respond to chemicals by changing their movement patterns
o Ex: solitary E. coli cells exhibit positive chemotaxis toward other members of their species to form
colonies
Internal and Genomic Organization
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Cell of prokaryotes simpler than those of eukaryotes
o Lack complex compartmentalization
Genome different than eukaryotes
o 1/1000 as much DNA
o Consist of a ring of DNA that have relatively few proteins associated with it
 Called chromosome
 Located in nucleoid region
 Region is part of the cytoplasm that appears lighter than surrounding cytoplasm
o Have smaller DNA rings called plasmids
 Consist of only a few genes
 Provide specific functions
 Ex: resistance to antibiotics, direct the metabolism of rarely encountered nutrients
 Can survive without its plasmids
 But plasmids increase its chance of survival
 Reproduce independently of main chromosome
 Readily transferred between partners when prokaryotes conjugate
Transcription, translation, and DNA replication fundamentally similar
o Prokaryotic ribosomes smaller and differ in protein and RNA content
Reproduction and Adaptation
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Can reproduce quickly in a favorable environment
Divide by binary fission
o Divide every 1-3 hours average
 Some in only 20 minutes
Prokaryotic reproduction limited, as the cells eventually exhaust their nutrient supply, poison themselves with
metabolic wastes, or are consumed by other organisms
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Face competition from other microorganisms
Ability to survive harsh conditions attribute to their success
o Some form endospores
 Endospores: resistant cells
 Cell produces copy of its chromosome and surrounds it with a tough wall; water is removed, and
metabolism stops
 To kill, must cook at 121 C
Prokaryotes can adapt quickly to changes in their environment through evolution by natural selection
o Mutations that confer greater fitness can swiftly become more common
Horizontal gene transfer also facilitates rapid evolution in prokaryotes
o Conjugation can permit the exchange of a plasmid containing a few genes or large groups of genes
 Once transferred, genes are incorporated into a prokaryote’s genome
o Major force in the long-term evolution of pathogenic bacteria