Site hosted by Build your free website today!


Index  Notes  Labs  Web Quests  Assignments  Quizzes  Links  Student Work

I.  Overview

A.       4000 species have been identified but many think there are several thousands more that haven’t been identified.

B.       Structurally and metabolically diverse

C.      Differences


2. No membrane bound organelles

3. Cell walls but structure is different than plants.

4. Simple genomes

5. Different replication, protein synthesis and recombination

II.  Archae and bacteria are the two main branches of prokaryotic evolution

A.       Traditional 5-kingdom system emphasized prokaryotes vs. eukaryotes and looks at the structural differences.

B.       Recent research put the prokaryotes into two kingdoms.

1.       Archaebacteria

a. Evolved from earliest cells

b. Inhabit extreme environments that may resemble early habitats

2.       Eubacteria

a. More modern

b. More numerous

c. Differ structurally, biochemically, and physiologically

C.      Carol Woese proposed six-kingdom system.

1. Two prokaryotic kingdoms

2. Four eukaryotic kingdoms

D.      An eight kingdom system has also been proposed.

E.       A three domain system is another idea.

1.  Domain is higher than a kingdom

III.  The success of prokaryotic life is based on diverse adaptations of form and function.

A.       Morphological

1. Single celled

2. Some are multicellular aggregates

3. Diversity of shapes

        a. Spheres - cocci

        b. Rods - bacilli

        c. Spirals – spirilla and spirochetes

B.       Cell surface

1.       Prokaryotes have cell walls that

a. Maintain the cell shape

b. Protect the cell

c. Prevent the cell from bursting in a hypotonic environment

d. Often contain peptidoglycan

        1. Modified polymers cross-linked by short polypeptides

        2. Exact composition varies among species

        3. Some antibiotics work by preventing the cross-links.

2.       Gram stain is used to distinguish between two groups of eubacteria that have structural differences in the cell walls.

a.  Gram-positive bacteria

        1.  Have simple cell walls with large amounts of peptidoglycan.

        2. Stain blue

b.        Gram-negative bacteria

        1. Have more complex cell walls with smaller amounts of peptidoglycan

        2. An outer lipopolysaccharide-containing membrane over cell wall

        3. Stain pink

        4. More often disease causing

        5. Lipopolysaccharides are often toxic and protect from host

        6. Lipopolysaccharides impede entry of drugs into cells

3.       Some have a capsule

a. A gelatinous secretion which gives cells additional protection

b. Helps them adhere to hosts

c. Helps adhere to form an aggregate

4.       Some use a pili

a. Surface appendages used for adherence to a host

b. Transferring DNA during conjugation

C.      The motility of Prokaryotes

1.       Flagella

a. Do not contain the 9+2 arrangement

b. Rotate rather than whip back and forth

c. Not covered by and extension of the plasma membrane

d. Uses flagellin to attach to another protein hook at the basal apparatus

e. Basal apparatus has 36 different proteins arranged in rings.

2.       Filaments which are characteristic of spirochetes or helical shaped bacteria

a. Several filaments spiral around the cell inside the cell wall

b. Similar to flagella in structure but rotate cell like a corkscrew

3.       Gliding

a. Glide through a layer of slimy chemicals secreted by the organism

b. Movement may result from flagellar motors that lack filaments

D.      Prokaryotic movement is fairly random in homogenous environments but may become directional in a heterogeneous mixture

1.       Taxis is the movement to or away from a stimulus

a. Phototaxis – moving from light

b. Chemotaxis – moving from chemicals

c. Magnetotaxis – moving from magnetism

d. Movement toward a stimulus is positive taxis

e. Movement away from a stimulus is negative taxis

2.       During taxis, bacteria move by running and tumbling movements.

a. Enabled by rotation of flagella either counterclockwise or clockwise

b. Caused by flagella moving coordinately about each other or randomly

E.       Internal Membrane organization

1.       Prokaryotes lake diverse internal membranes.

2.       Some have specialized membranes formed by invaginations of the plasma membranes.

a. Infoldings of the plasma membrane function in cellular respiration

b. Cyanobacteria have thylakoid membranes containing chlorophyll

F.       Prokaryotic Genomes

1.       Has only 1/1000 of the DNA found in a eukaryotic cell.

2.       The bacterial chromosome is also known as the genophore

a. Double stranded and circular

b. DNA is concentrated in the nucleoid region and not membrane-bound

c. Has very little protein associated with it

3.       Many also have plasmids which are smaller rings of DNA having supplemental genes for functions such as resistance or metabolism of unusual nutrients.

a. Replicate independently

b. Can be transferred during conjugation

4.       Bacterial DNA replication and translation are similar to that of prokaryotes.

a. Bacterial ribosomes are smaller and have different protein and RNA

b. Difference allows antibiotics to block protein synthesis

G.      Growth, Reproduction, and Gene exchange.

1.       Neither mitosis or meiosis occurs in prokaryotes.

a. Reproduction is asexual by binary fission

b. DNA synthesis is almost continuous

2.       Growth in the number of cells is geometric in an environment with unlimited resources

a. Generation is usually 1-3 hours although some take 20 minutes

b. At high concentration of cells, growth slows due to waste and lack of food

c. Competition is reduced by release of antibiotic chemicals

d. Optimal growth requirements depend on species.

3.       Some bacteria survive adverse environmental conditions and toxins by producing endospores.

a. Resistant cell that contains one chromosome copy surround by a wall

b. Original cell replicates and surrounds one copy with a durable wall

c. Some can survive boiling water

d. May remain dormant for many years

4.       Prokaryotes do not go through meiosis or syngamy but genetic recombination can take place through three mechanisms.

a. Transformation

b. Transduction

c. Conjugation

5.       Short generation times allow prokaryotic populations to adapt to rapidly changing environmental conditions.

a.  New mutations are screened by natural selection quickly

b. Resulted in current diversity

IV.                  All major types of nutrition and metabolism evolved among prokaryotes.

A.       Major modes of nutrition

1.       Exhibit a great diversity in how they obtain the necessary resources to synthesize organic compounds.

2.       Some obtain energy from light while others use chemicals.

3.       Some utilize carbon dioxide while others require at least one organic nutrient as a carbon source.

a. Photoautotrophs

        1. Use light energy to make compounds from carbon dioxide.

b. Chemoautotrophs

        1. Require only CO2 as source and oxidize H2S, NH3, and Fe2+

        2. Unique to certain prokaryotes

c. Photoheterotrophs

        1. Use light to generate ATP from organic carbon source

        2. Unique to prokaryotes

d. Chemoheterotrophs

        1. Must obtain organic molecules for energy as a source of carbon

        2. Found in most eukaryotes as well.

B.      Nutritional Diversity among chemoheterotrophs

1. Most bacteria are chemoheterotrophs and can be divided into two groups

a.       Saprobes

1.  Decomposers that absorb nutrients from dead organics

b.       Parasites

1. Absorb nutrients from body fluids of living hosts

2. The chemoheterotrophs are very diverse groups and some have very strict requirements while others are versatile.

a.       Lactobacillus will grow well only when the medium contains all 20 amino acids, several vitamins, and other organic compounds.

b.       E. coli will grow on a medium which contains only a single organic ingredient.

3. Almost any organic molecules can serve as a carbon source for some species.

a.       Some bacteria are capable of degrading petroleum and are used to clean oil spills.

b.       Those compounds that cannot be used as a carbon source by bacteria are considered non-biodegradable.

C.      Nitrogen metabolism

1. While eukaryotes can only use some forms of nitrogen to produce proteins and nucleic acid, prokaryotes can metabolize most nitrogen compounds.

2. Examples

a. Some convert ammonia to nitrates

b.  Some convert nitrates to atmospheric nitrogen

c. Nitrogen fixation is unique to prokaryotes

d. Cyanobacteria are self sufficient with nitrogen fixation

D.      Metabolic relationships to oxygen

1. Prokaryotes differ in their growth response to the presence of oxygen.

2. Obligate aerobes

a. Prokaryotes needing oxygen for cellular respiration

3. Facultative anaerobes

a. Use oxygen when presence but can grow using fermentation

4. Obligate anaerobes

a. Prokaryotes that are poisoned by oxygen

b. Live exclusively by fermentation

c. Use inorganic molecules as electron acceptors

V.  The evolution of prokaryotic metabolism was both cause and effect of changing environments on Earth.

A.       Prokaryotes evolved all forms of nutrition and most metabolic pathways eons before eukaryotes arose.

B.       Evolution of these capabilities were a response to the changing environment of the early atmosphere.

C.      As these new capabilities evolved, they changed the environment for subsequent prokaryotic communities.

D.      The origin of Glycolysis.

1.       The first prokaryotes were probably chemoheterotrophs that absorbed free organic compounds generated by abiotic synthesis.

2.       The universal role of ATP implies that prokaryotes used that molecule for energy very early in their evolution.

3.       As ATP supplies were depleted, natural selection favored those prokaryotes that could regenerate ATP from ADP and thus leading to step by step evolution of Glycolysis.

4.       Glycolysis is the only metabolic pathway common to all modern organisms and does not require oxygen.

5.       Some Archaebacteria that live by fermentation have forms of nutrition believed to be similar to those of the original prokaryotes.

E.       The origin of Electron Transport Chains and Chemiosmosis.

1.       Chemiosmotic ATP synthesis probably evolved in early prokaryotes as it is a common mechanism in all three domains.

2.       Early prokaryotes may have used the transmembrane pumps to help regulate their internal pH by expelling hydrogen ions produced by fermentation.  Energy would have been necessary to drive these pumps.

3.       ATP may have been saved by the first electron transport chains by coupling oxidation of organic acids to the transport of H+ out of the cell.

4.       Some bacteria may have evolved electron transport chains so efficiently that more H= was extruded than necessary for pH regulation.  These cells could then utilize the influx of H+ to reverse the proton pump and generate ATP.

a.       Modern bacteria use this form of energy metabolism called aerobic respiration.

F.       The origin of Photosynthesis

1.       As the supply of free ATP and abiotically produced organic molecules was depleted, natural selection may have favored organisms that could make their own organic molecules from inorganic resources.

2.       Light absorbing pigments in the earliest prokaryotes may have provided protection to the cells by absorbing excels light energy that could be harmful.

3.       These energized pigments may have then been couples with electron transport systems to power ATP synthesis.

4.       Bacteriorhodopsin is a light energy capturing pigment in the membrane of extreme halophiles

5.       Components of the electron transport chains that functioned in anaerobic respiration in other prokaryotes may have been co-opted to also provide reducing power.

G.      Cyanobacteria, the oxygen revolution, and the origins of cellular respiration

1.       Eventually prokaryotes evolved that could use water as the electron source and thus evolved Cyanobacteria which released oxygen.

2.       They evolved about 2.5 –3.4 billion years ago.

3.       They lived with other bacteria in colonies that resulted in the formation of the stromatolites.

4.       Oxygen release by photosynthesis may have first reacted with dissolved iron ions to precipitate as iron oxide preventing accumulation of free oxygen.

5.       Precipitation of iron oxide would have eventually depleted the supply of dissolved iron and oxygen would have accumulated in the seas.

6.       As the oxygen accumulated, it was released into the atmosphere.

7.       As it accumulated, may species became extinct while others survived in anaerobic environments.

8.       Aerobic respiration may have originated as a modification of electron transport chains used in photosynthesis.

VI.  Molecular systematics is leading to a phylogenic classification of prokaryotes.

A.       The use of molecular systematics has shown that prokaryotes diverged into the Archaebacteria and Eubacteria lineages very early in prokaryotic evolution.

B.       Domain Archae

1.Cell walls lack petidoglycan.

2.Plasma membranes have a unique lipid composition.

3.       RNA polymerase and ribosomal protein are more like those of eukaryotes than of Eubacteria.

4.       Methogens

a. Named for their unique form of energy metabolism

b. Use hydrogen to reduce CO2 to methane and are strict anaerobes

c.  Decomposers in marshes, swamps, and sewage treatments     

d. Important digestive system symbionts

5.       Extreme halophiles

a. Inhabit high salinity environments

b. Some tolerate and others require this condition

c. Have Bacteriorhodopsin to absorb light and pump out H+

d. Pigment is responsible for purple-red color

6.       Extreme thermophiles

a. Inhabit hot environments

b. Habitats are 60 – 80oC

c. Inhabit deep sea hydrothermal vents

C.      Domain bacteria

1.Proteobacteria is the most diverse group of bacteria containing 3 main sub groups.

a. Purple bacteria

      1. Have bacterichlorophylls built into membrane invaginations

      2.  Extract electrons from water and release no oxygen

      3. Obligate anaerobes found in sediments

      4. Many are flagellated

b. Chemoautotrophic proteobacteria

      1. Includes free-living and symbiotic species

      2. Play roles in the nitrogen cycle

c.   Chemoheterotrophic proteobacteria

      1. Enteric bacteria

      2. Most are rod-shaped facultative anaerobes

D.      Gram-positive Eubacteria

                                1. Form endospores

2. Photosynthetic members

3. Mycoplasmas

      a. Smallest of all known cells

      b. Only eubacteria that lack cell walls

      c. Common in soil and some are pathogenic

           E. Actinomycetes

                          1. Soil bacteria that form branching colonies which resemble fungi

                          2. Many are important sources of antibiotics

3.       Cyanobacteria

a. Photoautotrophs with photosynthesis

b. Most inhabit fresh water, some marine, and some relate with fungi

c. Cell walls are often thick and gelatinous

d. Motile forms move by gliding

e. Many are unicellular while others are colonial

4.       Spirochetes

a. Helical cells that are long and thin

b. Internal flagellar filaments function in corkscrew-like movements

c. Chemoheterotrophs that are both free living and pathogens

5.       Chlamydias

a. Obligate intracellular parasites of animals

b. Obtain ATP from host cells

c. Have gram-negative walls but lack peptidoglycan

d. Chlamydia is most common cause of blindness and is an STD

VII.  Prokaryotes continue to have an enormous ecological impact.

A.       Prokaryotes and Chemical cycles.

1.       Prokaryotes are critical links in the recycling of chemical elements between the biological and physical components of ecosystems.

2.       Decomposers

a. Decompose dead organisms and waste of live organisms

b. Return elements to the environments

3.       Autotrophic bacteria

a. Fix carbon dioxide

b. Support food chains

B.       Symbiotic Bacteria

1.       Most prokaryotes form associations with other organisms usually with other bacterial species possessing complementary metabolisms.

2.       Symbiosis is the ecological relationships between organisms of different species that are in direct contact.  Usually the smaller organism is the symbiont and lives within the larger host.

3.       Three categories of symbiosis

a. Mutualism


b. Commensalism


c. Parasitism


C.      Bacteria and disease

1.       About half of human disease is caused by bacteria.

2.       To cause a disease, the bacteria must invade the host, evade or resist the host’s internal defenses long enough to grow, and harm the host.

3.       Some pathogens are opportunistic meaning they become pathogenic only when defenses are weakened by other factors such as poor nutrition or other infections.

4.       Louis Pasteur, Joseph Lister and others were linking disease to pathogenic microbes. 

5.       In the late 1800’s Robert Koch was the first to determine a direct connection between specific bacteria and certain diseases. 

6.       While doing this he determine four criteria used as guidelines in medical microbiology.

7.       Koch’s postulates

a. Find the same pathogen in each diseased individual

b. Isolate the pathogen and grow it in a pure culture

c. Use cultured pathogen to induce the disease

d. Isolate the same pathogen from experimental animal

8.       Some pathogens cause growth and invasion of tissues by production of a toxin.

9.       The two main types of toxins are

a.       Exotoxins

        1. Proteins secreted by bacteria cells

        2. Can cause disease without the organism itself being present

        3. Among the most potent poisons know.

        4. Elicits specific symptoms

b.       Endotoxins

        1. Toxic component of outer membranes in gram-negative bacteria

        2. Induce general symptoms of fever and aches

                3. May cause food poisoning