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


Membrane-enclosed organelles

Absent Absent Present


Peptidogylcan

in cell wall Present Absent Absent


RNA polymerase

One kind Several kinds

Several kinds


Initiator amino acid for start of protein synthesis

Formyl-methionine

Methionine Methionine


Introns Absent Present in some genes

Present


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

Technology

Chapter 27