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General Biology 101
Chapter Outlines
Starr & Taggert
10th edition
Chapter 21:
Prokaryotes & Viruses
The Unseen Multitudes
Microorganisms – prokaryotic cells and single-celled protistans too small to be seen
without a microscope.
Bacteria are measured in a unit called micrometers – a thousandth of a millimeter.
Viruses are measured in a unit called nanometers – billions of a meter.
Bacteria are:
- Prokaryotes i.e. cells without nuclei.
- They are the smallest organism.
- Some can reproduce every 20 minutes – very quickly.
Pathogens – infectious, disease causing agents that invade target organisms and multiply
in or on them.
Good attributes of some bacteria include:
- Some are photosynthetic and make oxygen.
- Some are decomposers and consume waste and remains of dead organisms.
Section 21.1
Prokaryotes are the most abundant and far reaching of all life forms.
Photoautotrophic – build organic compounds (containing carbon atoms) by using
sunlight – called “self-feeding.”
Chemoautotrophs – “self-feeding” organisms that use CO2, which can be oxidized to
create organic compounds for energy. Some may use other compounds to obtain energy
as well such as sulfur derived materials.
Photoheterotrophs – capture sunlight but get carbon from sources other than CO2 e.g.
carbohydrates to address their ingredient needs to make new compounds.
Chemoheterotrophs – prokaryotic cells that are not self-feeding. May be parasitic, or
saprobic i.e. digesting organic wastes or remains.
Shapes of bacteria
 Cocci (pl) – spheres
 Bacilli – rod/staff
 Spirilla – twists/spirals
Characteristics of Prokaryotic cells.
1) No membrane bound nucleus, but has ribosomes that are important to make
proteins.
2) Single chromosome which is circular.
3) Cell walls – maintain shape and prevents rupture of the cell if swollen.
4) Reproduction by fission i.e. splitting.
5) Metabolically diverse i.e. they use many different ways to obtain energy/fuel.
Gram stain – a diagnostic tool using stains that helps identify different bacteria strains.
Positive – purple
Negative – pink
Glycocalyx – a mesh work arrangement on the surface of bacteria that provides them
with a slime capsule for adhesion purposes i.e. so that they stick to their substrate.
Bacterial flagella – whip like tails that rotate like propellers for the purpose of
movement.
Section 21.2
The nature of growth
Bacteria can live in extreme conditions:
- Freezing temperatures of Antarctica
- Scorching deserts
- Deep in the Earth’s crust
- They can live in or on rocks.
- Some can live in very acidic conditions
Prokaryotic fission – replication of cells that results in 2 genetically equivalent daughter
cells.
Conjugation – Involves a transfer of circular DNA material between 2 bacterial cells via
a structure/tube called a sex pili or conjugation tube. Purpose is for mixing or
recombination of variations in genetic characteristics.
Section 21.3
Prokaryotes are classified according to their physical characteristics as well as their
biochemical properties. The eubacteria or “true” bacteria are considered to be the most
primitive of the three domains of life based on these properties.
Numerical taxonomy – the method by which prokaryotes are classified. It is based on
the practice of comparing the number of traits of an unidentified cell to known
prokaryotic cells.
Section 21.4
Major Prokaryotic Groups
Defining species of bacteria is quite difficult because they don’t rely on sexual isolation
to define them as a species.
Table 21.2 in the text outlines some examples of important groups of prokaryotes
including both eukaryotes and archeabacteria.
Section 21.5
Archaebacteria
Archae – means “beginning.”
Unique group based on DNA sequences, structures and methods of metabolism. i.e. how
they burn their fuel.
I.
Extreme Thermophiles – “heat loving” forms. Some populations can exist
above boiling temperatures. They use anaerobic metabolism i.e. without the
presence of oxygen to utilize their fuel sources. Some live near hot sea vents
or hot springs.
II.
Methanogens – Strict anaerobes. They live in swamps, wetlands and the guts
of termites and mammals e.g. cows. Methane is produced as part of their
metabolic activity.
III.
Extreme halophiles – “salt loving” forms of archaebacteria that thrive in very
salty environments. E.g. Great Salt Lake, Dead Sea. Most are aerobic, can
change to photosynthesizing if necessary.
Section 21.6
Eubacteria – the “true” bacteria.
Most species of known prokaryotes are true bacteria.
What makes Eubacteria, different than Archaebacteria is that they have fatty acids in their
cell membranes. Nearly all have peptidoglycan in their cell walls that is different than in
the case of the archaeabacteria.
A sampling of the biodiversity of true bacteria.
1) Photoautotrophic bacteria – cyanobacteria, formerly called blue-green algae are
photosynthetic and aerobic. Some forms are able to fix nitrogen.
2) Chemoautotrophic bacteria – involved with nitrogen cycle e.g. nitrifying
bacteria. Ammonia  Nitrate conversion.
3) Chemoheterotrophic bacteria – Most known bacterial species are in this group.
Some are beneficial for decomposition services, involved with making food,
bioremediation, and some are sources of antibiotic drugs.
Some are pathogenic and cause disease or poisoning e.g. E. coli strains that
cause diarrhea or botulism respectively.
Endospore – A resting “dormant” structure within a prokaryote cell. They allow
resistance to heat, drying out, irradiation and even boiling water. Tetnus is such an
example.
Lyme disease and Rocky Mtn Spotted fever are chemoautotrophs that can be passed by
ticks that harbor the pathogen in their guts.
Some bacteria have become specially adapted to respond to stimuli in their environments.
-
Magnetotactic bacteria – can orient themselves in their environment because of
“inclusions” of magnetite in their cells.
Collective behavior – colonies of bacteria that can migrate together and move in
unison.
Spore bearing fruiting bodies – for maximum reproductive effect.
Section 21.7 – The Viruses
Defining Characteristics
Virus is derived from the Roman word that means poison or venomous secretion.
Characteristics:
- Non-cellular
- Infectious agent
- Structural composition is a protein coat and a nucleic acid core, either DNA or
RNA.
- Can’t reproduce by itself i.e.needs to invade a host’s cell.
Part of the shell of a virus is composed of viral proteins some of these are spiky projects
that can change or mutate with time making it particularly challenging for our immune
system to recognize and disable them.
Examples of Viruses:
Bacteriophages – a group of viruses that infect bacterial cells. The are often used in
genetic engineering to transfer genetic material.
Examples of some of the major viruses are listed in Table 21.3
Examples include: HIV, Herpes, Parvoviruses, Measles, Ebola
Some viruses specifically invade plants e.g. Tobacco Mosaic virus.
Section 21.8 Viral Multiplication Cycles
Basic steps of viral replication:
1) Attachment – via molecular connections on cell surface.
2) Penetration – virus (or its nucleic acid) goes into the cell its infecting.
3) Replication and Synthesis – virus hijacks cells protein making apparatus and
makes more of itself.
4) Assembly – new viral proteins and nucleic acids constructed.
5) Release – viruses leave host cell.
Method/Rate of replication:
Lysis – when the cell ruptures and viruses leave the cell. Death to the cell quickly
occurs. Common in the case of bacteriophages.
Lysogenic pathway – a latent period exists an the virus doesn’t kill its host outright. The
virus invades and becomes part of the host cell’s DNA/genetic material. Often the active
or reactivating phase occurs at a later time with some environmental cue triggering it.
E.g. Herpes.
Viroids – Small, strands or circles of RNA, that don’t have a protein coat. They are
essentially bits of “naked” RNA. Cause diseases in many plants.
Prions – abnormal, less soluble proteins found associated with nerve cells i.e. neurons.
Prions cause “Kuru” and “Creutzfeldt-Jakob” diseases which destroy brain function. In
sheep a prion disease is called “Scrapie.”
>>> Prions cause mad cow disease.
BSE – Bovine Spongiform encephalopathy (brain deterioration) is thought to have been
caused by animal feed given to cows that had sheep particles infected with the scrapie
causing prions.
Section 21.9
The nature of disease
Infection – when a pathogen invades the body and multiplies.
Disease – when he body’s defenses are unable to keep up with the pathogen’s activities
and the body is unable to function optimally.
Epidemic – when a disease quickly spreads through a population for a given time and
then subsides.
Pandemic – when an infectious disease spreads through several countries at the same
time.
-
Sporadic – occur irregularly and affect just a few individuals.
Endemic – occur continuously but don’t spread much.
A pathogen exists only
> for as long as it has access to outside energy.
> it has access to raw materials e.g. food, space, host is alive
Most pathogens are co-evolving with t heir host and so the host has built up defenses
against it.
The greater the population density of the host organism the more variety and the higher
the frequency of infectious diseases.
Case example of a disease with new human hosts:
- Ebola (a form of hemorrhagic fever).
- Monkey pox (a pox type disease that has passed from monkeys to humans. Has
the potential to become a pandemic).
 Bacteria have developed drug resistant forms that leave the host quite vulnerable. An
example is Streptococcus pneumoniae which causes pneumonia, meningitis and chronic
inner ear infections. It has become resistant to the drugs that are used to treat it; a major
factor is high population densities, and also overuse of the antibiotics.
Pathogens also exist in our food.
E. coli and Salmonella enteriditis are examples of bacteria that have slipped into our food
supply and caused many people to get sick. Common foods that tend to harbor such
pathogens include primarily meat products such as shellfish. Some forms make a toxin
that is deadly e.g. botulism.