Transcript 2- prokaryotes

Prokaryotes
The world of prokaryotes
A- They’re everywhere
1- Collective prokaryote biomass outweighs all eukaryotes combined
by at least tenfold.
2. They exist almost everywhere, including places where eukaryotes
cannot.
3. Most prokaryotes are beneficial; we couldn’t live without
them. (e.g. Nitrogen-fixing bacteria)
4. Some cause illness à bubonic plague, diphtheria, salmonella
5. Approximately 5000 species have been identified. Estimates of
prokaryote diversity range from 400,000 to 4,000,000 species.
.
B. Bacteria and archaea are the two main branches of
prokaryote evolution
Archaea are thought to be more closely related to
eukaryotes than to bacteria.
II. Structure, function, and reproduction of prokaryotes
A. Most prokaryotes are unicellular.
1. Some species form aggregates of two or more individuals.
B. Three (3) common shapes: cocci (round); bacilli (rod);
helical (spiral)
C. Prokaryotes are typically 1-5 μm in diameter, but some
can be seen by the naked eye.
- Eukaryotic cells are typically 10-100 μm in diameter.
D. Almost all prokaryotes have cell walls external to the
plasma membrane.
1. Cell walls maintain cell shape.
2. Cell walls are composed of peptidoglycan.
3. There are two types of cell walls. Bacteria are grouped
according to cell wall type.
a. Gram-positive bacteria have simple, thick cell walls. Their
cell walls are composed of a relatively large amount of
peptidoglycan.
b. Gram-negative bacteria have less peptidoglycan and are
more complex. They have a peptidoglycan layer surrounded
by the plasma membrane and an outer membrane.
- Gram-negative bacteria are typically more resistant to host
immune defenses and antibiotics.
Note that the two types of bacteria can be stained to
determine which is gram-negative (pink) and gram-positive
(purple) using a Gram Stain.
Gram Positive
Peptidoglycan
Plasma membrane
Gram Negative
Lipopolysaccharide layer
Outer membrane
Peptidoglycan
Plasma membrane
4. Most prokaryotes secrete sticky substances that form a
protective layer and enable them to adhere to substrates.
a. The sticky protective layer secreted by prokaryotes is
called the capsule.
5. Some prokaryotes adhere to substrates using pili.
a. Some pili are specialized for DNA transfer. This process is
called conjugation; note for later in class.
E. Many prokaryotes are motile
- Some exceed speeds 100 times their body length per
second.
1. Modes of movement – Note the three types:
a. Flagellum - basal apparatus rotates the flagellum and
propels the cell
b. Corkscrew movement of spirochetes (helical)
c. Some prokaryotes glide over jets of slimy secretions.
2. Many prokaryotes move toward or away from a stimulus =
taxis. Chemotaxis is the movement toward or away from a
chemical.
F. Cellular and genomic organization of prokaryotes is
different from that of eukaryotes
1. Prokaryotes have no nucleus.
2. The nucleoid region in a prokaryotic cell consists of a
concentrated mass of DNA. This mass of DNA is usually one
thousand times less than what is found in a eukaryote.
3. A prokaryote may have a plasmid in addition to its major
chromosome. A plasmid is a small ring of DNA that carries
accessory genes.
Usually these genes are for antibiotic resistance!
Asexual
reproduction:
Fission
G. Prokaryotes grow and adapt rapidly - The doubling time
for E. coli is 20 minutes. Start with one E. coli cell. After 48
hours of doubling every 20 minutes, the mass of E. coli would
be 10,000 times the mass of the earth.
Bacteria do not have gene transfer by sexual reproduction,
but do transfer genes. Why? This is an aid in adapting
(evolving).
1. Three (3) ways for genes to be transferred between cells:
a. Transformation – cell takes up genes from the surrounding
environment.
b. Conjugation – direct transfer of genes from one
prokaryote to another. Use the sex pilus to conjugate.
c. Transduction – viruses transfer genes between
prokaryotes.
Prokaryotic
conjugation
Bacterial transduction
2. Endospores are resistant cells formed by some
bacteria as a way to withstand harsh conditions. The
cell replicates its chromosome and wraps it in a durable
wall that can protect the chromosome from adverse
conditions, e.g. boiling water, desiccation. When the
environment is good again, the cell will revive to a new
vegetative (growing) spore.
III. Nutritional and metabolic diversity
A. All prokaryotes (and eukaryotes too) are grouped
into four (4) categories according to how they obtain energy
and carbon .
1. Photoautotrophs
- Photosynthetic  use light as the energy source
- CO2 is the carbon source
Example: Cyanobacteria; plants (eukaryotic).
2. Chemoautotrophs
- Energy from oxidation of inorganic substances (e.g. NH4,
and S)
- CO2 is the carbon source
Example: Sulfolobus, Beggiatoa (shown on slide)
3. Photoheterotrophs
- Light as energy source
- Organic compounds are source of carbon
4. Chemoheterotrophs
- Organic compounds are energy source and source
of carbon (this includes humans)
Examples: Many prokaryotes; animals (eukaryotic);
fungi (eukaryotic)
B. Metabolic relationships to oxygen
1. Obligate aerobes
- Use O2 for respiration; cannot grow without it. (Humans
are obligate aerobes)
2. Facultative aerobes
- Use O2 when available; ferment when O2 isn’t available.
3. Obligate anaerobes
- Poisoned by O2; use fermentation or live by anaerobic
respiration. In anaerobic respiration, inorganic molecules
like SO4, NO3, and Fe3+ are used instead of oxygen.
C. Photosynthesis evolved early in prokaryotic life
1. Cyanobacteria started to produce O2 about 2.7 billion
years ago
Contrasting hypotheses for the taxonomic distribution of
photosynthesis among prokaryotes.
A. Great diversity of Archaea in extreme environments and
oceans
1. Two taxa of archae:
a. Euryarchaeota – most archae
b. Crenarcheota – most thermophilic species
2. Examples of extremophiles
a. Methanogens produce methane
- Energy is from hydrogen gas
- Strictly anaerobic
- Inhabit swamps and animal intestines
b. Extreme halophiles
- Live in salty environments (Great Salt Lake)
c. Extreme thermophiles
- 60- 80 °C optimum temperatures (hot springs)
- 105 °C for deep-sea hydrothermal vents
V. Ecological impacts of prokaryotes
A. Prokaryotes are links in the recycling of chemical
elements
B. Many prokaryotes are symbiotic (2 organisms living in
direct contact with each other).
There are three types of symbioses:
1. Mutualism – both symbiotic organisms benefit
- e.g. Nitrogen-fixing bacteria like Rhizobium:
plant obtain organic nitrogen, Rhizobium gets energy in the
form of sugars that the plant produces. Another example:
Are all prokaryotes
disease producing
germs?
Without prokaryotes
ecosystems would
collapse!
An overview of
ecosystem dynamics
2. Commensalism – one organism benefits and the other is
not harmed.
- e.g. Bacteria on our skin
3. Parasitism – parasite benefits and the host is harmed.
C. Pathogens cause human diseases
- Some pathogens are opportunistic. They may be normal
residents of the host, but if the host is weakened, then they
cause disease.
- How do we know if a particular organism is responsible for
a disease?
Robert Koch formed postulates as guidelines to establish
that a disease is caused by a particular pathogen:
a. Find same pathogen in each diseased individual.
b. Isolate the pathogen and grow it in pure culture.
c. Inoculate an individual with the isolated pathogen and the
disease is induced.
d. Isolate the same pathogen from the infected individual.
This procedure is called Koch’s Postulates and is used
widely to determine what infectious agent causes disease.
Most pathogens cause disease by producing poisons, these
are either:
- Exotoxins: proteins secreted by the pathogen that cause
illness.
-Endotoxins: poisons that are part of the pathogen that
causes illness. (e.g. bacterium’s outer membrane)
D. Humans use prokaryotes in research and technology
Examples:
Sewage treatment
Bioremediation
Chemical & Medical production
Research (genetic engineering, etc.)