Chapter 27
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Transcript of Chapter 27
Chapter 27
Prokaryotes and the Origins of Metabolic
I. They’re almost everywhere
A. Outnumber all eukaryotic organisms combined
1. Flourish in all habitats
2. Many live in symbiotic relationships
B. Bacteria and archaea are the two main branches of prokaryotic evolution
Bacteria and archaea used to be grouped as Monerans
Now use a two Domain approach (taxonomic group above Kingdom )
3. Molecular studies indicate that Domain Archaea and Domain Bacteria diverged first and that Domain Archaea and Eukarya share a more recent common ancestor
Universal common Ancestor
BacteriaArchaea
Eukarya
A Comparison of the Three Domains of Life
Bacteria Archaea Eukarya
Nuclear membrane
Present Present Present
Bacteria Archaea Eukarya
Membrane-enclosed organelles
Absent Absent Present
Bacteria Archaea Eukarya
Peptidogylcan
in cell wall Present Absent Absent
Bacteria Archaea Eukarya
RNA polymerase
One kind Several kinds
Several kinds
Bacteria Archaea Eukarya
Initiator amino acid for start of protein synthesis
Formyl-methionine
Methionine Methionine
Bacteria Archaea Eukarya
Introns Absent Present in some genes
Present
Bacteria Archaea Eukarya
Response to antibiotics and streptomycin and chloramphenicol
Growth inhibited
Growth not inhibited
Growth not inhibited
4. Archaea are found in extreme habitats reminiscent of early earth and are referred to as:
Thermophiles—thrive in very hot temperatures
Halophiles—thrive in highly saline environments like the Dead Sea
Methanogens—release methane gas as a by-product of oxidizing hydrogen for energy
C. Nearly all prokaryotes have cell walls external to their plasma membranes
1. Cell walls of bacteria contain peptidoglycan, a matrix of polymers of sugars cross-linked by short polypeptides.
2. Gram stain is an important tool for identifying bacteria as “gram positive” (bacteria with walls containing a thick layer of peptidoglycan) or gram negative (bacteria with more complex walls including an outer liposaccharide membrane)
3. Gram negative bacteria are more pathogenic than gram positive because their liposaccharides may be toxic.
4. Outer membrane protects gram negative bacteria from hosts’ defenses and from antibiotics.
5. Many prokaryotes secrete a sticky capsule outside the cell wall that serves as protection and a glue for adhering to a substratum
6. Some may attach by appendages called pili which may be specialized to hold bacteria together during conjugation.
D. Most prokaryotes are motile
1. Many have flagella: either several scattered over the cell surface or concentrated at one or both ends
2. These flagella are different from eukaryotic flagella
E. The cellular and genomic organization of prokaryotes is fundamentally different from that of eukaryotes
1. No membrane bound organelles
2. DNA is circular, double-stranded and found in region called the nucleoid
3. Contains one thousandth of the DNA of a eukaryote
4. Smaller rings of DNA called plasmids may carry genes for antibiotic resistance or metabolism of unusual nutrients
plasmids reproduce independently
may be transferred between bacteria during conjugation
F. Populations grow and adapt rapidly
1. Divide asexually by binary fission (much less complicated than mitosis )
2. Methods of genetic recombination transformation --the uptake of genes
from the environment conjugation --the direct transfer of
genetic material from one bacterium to another
transduction --the transfer of genes by a virus
3. Mutations are the major source of genetic variation
Generation times may be as short as 20 minutes or as long as 3 hours.
5. Growth stops as a result of exhaustion of nutrients or the toxic accumulation of wastes
6. Some bacteria form endospores, which are tough-walled cells that can resist even boiling water
Cell shape
Can be spherical (coccus),
rods (bacillus),
Or helices (spirilla)
Cell size
1-5 micrometers in diameter; 1/10 the size of eukaryotic cells
Cell surface
Cell wall of peptidoglycan which maintains shape, provides physical protection and prevents the cell from bursting in a hypotonic environment
Motility
Flagella—cause bacteria to spiral and corkscrew
Internal Membranes
Infoldings of plasma membrane used in respiration; cyanobacteria have thylakoid membranes used in photosynthesis
Genome
1 Circular chromosome; may have both “F” and “R” plasmids
Reproduction and Growth
Binary fission which results in rapid geometric growth
Some bacterial genetic recombination is the result of transduction or conjugation in addition to mutation
II. Great diversity of Nutritional and metabolic adaptations: all organisms can be classified by how they obtain their energy and organic carbon compounds
A. Photoautotrophs—photosynthetic organisms that capture light from the sun but require carbon dioxide to make organic compounds. Cyanobacteria are prokaryotes that are phototrophs.
B. Chemoautotrophs—organisms that make organic carbon compounds from carbon dioxide, but get energy by oxidizing inorganic substances such as hydrogen sulfide, ammonia, or ferrous ions.
C. Photoheterotrophs—use light for energy but must obtain their carbon in organic form. Some marine prokaryotes use this mode of nutrition.
D. Chemoheterotrophs—must consume organic molecules for both energy and carbon. This mode of nutrition is found widely among prokaryotes as well as protists, fungi, and animals.
III. Oxygen requirements
A. Some prokaryotes cannot grow in the absence of oxygen for cellular respiration and are called obligate aerobes.
B. Some prokaryotes are poisoned by oxygen and are called obligate anaerobes. These prokaryotes normally convert energy by fermentation.
C. Some prokaryotes are flexible and are called faculative anaerobes and use oxygen if it is present, but can grow by fermentation in an anaerobic environment.
IV. Ecological role of Prokaryotes
A. Chemical recycling—primary decomposers of dead plant and animal bodies and waste products and thereby recycling matter.
B. Involved in symbiotic relationships
Mutualism +/+ Commensalism +/neutral Parasitism +/-
C. Agents of disease
Tuberculosis is caused by Mycobacterium tuberculosis
Lyme disease is caused by a spirochette bacterium called Borrelia burgdorferi
2. Usually cause illness by producing poisons called endotoxins
Example—Cholera is called by a bacterial exotoxin that causes the human gut to release chloride ions in to the gut and water follows by osmosis