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Worksheet 17.1
Chapter 17: Microbes and biotechnology – fifteen
summary facts
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After studying the newly mapped molecule of rRNA, Carl Woese realized that the five-kingdom
system of classification was inaccurate. By examining the sequence of nucleotides in ribosomal
RNA (rRNA), he reclassified all living things into three domains. The three domains are:
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Eubacteria – true bacteria with no organized nucleus and no membrane-bound organelles
Archaea bacteria or ‘ancient bacteria’ – bacteria living in extreme environments
Eukarya – single-celled and multicellular organisms with an organized nucleus and
membrane-bound organelles.
Distinguishing characteristics of the three domains can be found in the chart on page 514.
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The Archaea live in a wide range of habitats. They can be methanogens, thermophiles, and
halophiles.
The Eubacteria have wide diversity of shape. They can be rods, spheres or helices. They can also
be divided into two groups – Gram-positive or Gram-negative – based on their cell wall structure
(Figure 17.3, page 516).
The wide diversity of microscopic eukaryotes is illustrated by Saccharomyces, Amoeba,
Plasmodium, Paramecium, Euglena and Chlorella (table, page 519).
Viruses are not cellular and cannot reproduce without using the cell machinery of another
organism. They are not considered living things. Viruses do show great diversity in their
structure. Their protein coat (capsid) can be naked or enveloped.
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Diversity allows microbes to exploit many niches in the ecosystem. They may be producers (e.g.
bacteria) which use chlorophyll to trap sunlight, nitrogen fixers which take nitrogen gas from the
atmosphere and fix it into nitrogen usable for plants, or decomposers which release inorganic
nutrients back into the ecosystem. Bacteria play a huge role in the nitrogen cycle (Figure 17.6,
page 521).
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Release of raw sewage into the water supply can transmit pathogens to those who drink or bathe
in the water. Release of nitrates into the water supply can cause increased growth of algae (algal
blooms). As the algae die, they are decomposed by aerobic bacteria which use up the oxygen in
the water (high biochemical oxygen demand, BOD). Water becomes low in oxygen and many
other organisms die. To prevent disease and algal blooms, raw sewage is treated with
saprotrophic bacteria using a trickling filter system or reed bed system (page 523).
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Biomass can be used as raw material for the production of fuels such as methane and ethanol.
Manure from farm animals and cellulose from plants are put into an anaerobic digester with no
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free oxygen, a temperature of 35 °C, and regulated pH. Acidogenic bacteria produce organic
acids and alcohol. Next, acetogenic bacteria make acetate with carbon dioxide and hydrogen as
by-products. Finally, methanogenic bacteria produce methane from the carbon dioxide and
hydrogen or from the breakdown of acetate (Figure 17.7, page 524).
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Reverse transcriptase is an enzyme used by retroviruses (e.g. HIV) to catalyse the production of
DNA from RNA. Using reverse transcriptase, molecular biologists have made therapeutic
proteins such as insulin. First, a human DNA molecule with its introns is taken from a pancreas
cell. Next, mRNA copies the DNA without its introns. Reverse transcriptase, working backward,
produces a new single-stranded cDNA. From single-stranded cDNA, double-stranded DNA is
made using DNA polymerase. The double-stranded DNA is inserted into a plasmid and put into
a bacterial cell. The bacteria cell multiplies and makes insulin. Insulin is harvested and used by
diabetics (Figure 17.11, page 527).
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Gene therapy involves replacing a defective gene with an effective gene which will make a
correct protein. Genes are delivered to cells by vectors (e.g. a virus). If the vector delivers the
gene to a gamete cell, it is germ-line therapy; if the gene is delivered to a body cell, it is somatic
therapy.
Children affected with SCID do not produce a protein called ADA. This weakens their immune
system. Stem cells from 11 children with SCID were mixed with a virus containing the gene for
making ADA. The virus transferred the normal gene into the stem cells of each child. The stem
cells were infused back into the bone marrow of the children and their immune systems were
restored. However, two years later, two of the children contracted leukaemia as a result of this
procedure and one of them died. Risks of gene therapy can include: the virus vector inserting the
new gene in the wrong cell, a new gene making too much protein, the virus stimulating an
immune reaction in the patient or the risk of children being too underdeveloped to have this
procedure safely.
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Saccharomycetes cerevisae (yeast) is a common fungus used in the production of bread, wine
and beer.
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In beer-making, a grain is germinated to release sugars. This is called malt. Hops are added
and the mixture is boiled. After cooling, the yeast is added to break down the malt into
ethanol and carbon dioxide. The beer is then pasteurized to kill any remaining yeast cells.
In wine-making, yeast is added to the crushed grapes. The grape sugar is fermented by the
yeast to produce carbon dioxide and ethanol. The carbon dioxide escapes and 11% ethanol
is left in the wine.
In bread-making, yeast acts on sugars in the dough and produces carbon dioxide and
ethanol. The carbon dioxide makes the bread rise. The yeast is killed and the ethanol is
evaporated as the bread is baked in the oven.
Preserved foods like jams and pickled vegetables make seasonal fruits and vegetables available
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for longer periods of time. When preserving fruit with sugar by making jam, a hypertonic
environment is created to prevent the growth of mould and bacteria which would spoil the fruit.
An acid environment can also prevent the growth of mould and bacteria. Pickles are vegetables
preserved in vinegar. Spices like dill can be added to enhance the flavour. The hot vinegar and
spices poured over vegetables such as small onions or cucumbers create a very acidic
environment. The very low pH restricts the growth of microbes.
10 Microbes can be divided into four groups based on their type of metabolism.
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A photoautotroph traps light energy and transfers it to chemical bond energy. An example is
Anabaena, a cyanobacterium. Cyanobacteria contain chlorophyll. Be able to draw and label
Anabaena. (Figure 17.15, page 536)
A photoheterotroph can trap light energy when available or can break down glucose for
energy with or without oxygen. An example is a bacterium, Rhodobacter sphaeroides.
A chemoautotroph gets energy from the chemical bonds of an inorganic molecule. An
example is Nitrosomonas, a nitrogen-fixing bacterium which gets energy from the oxidation
of ammonia found in the soil.
A chemoheterotroph uses organic compounds as an energy source. Most bacteria have this
type of metabolism. Bacteria break down the organic compound they ingest either
anaerobically or aerobically.
For more comparisons, see the chart on page 536.
11 During bioremediation, bacteria and fungi are used to breakdown organic compounds such as
pesticides and oil in soil or contaminated water. The organisms are chemoheterotrophs, so they
benefit by deriving energy from the unwanted molecules while improving our environment.
After an oil spill by the tanker Exon Valdez, marine bacteria were used to break down the oil.
Several months’ later, visible reduction of oil was seen on the rocks and sand. In Southern
Mississippi, harmful chemicals were dumped in the soil over a period of years. Cleanup is now
progressing as bacteria are being use to remove the creosote and PCP.
12 Pathogens are disease-causing microbes that can be transmitted and gain entry to the body six
ways:
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food (E. coli can enter the digestive tract on food)
water (cholera-causing bacteria can be found in drinking water)
air (Streptococcus are found in droplets from a sneeze of an infected person)
animals (mosquitoes can carry the malaria parasite, Plasmodium)
puncture wounds (tetanus bacteria can be in dirt on an old nail)
sexual contact (Chlamydia and HIV can be transmitted by unprotected sexual contact).
Chlamydia (an intracellular pathogen) and Streptococcus (an extracellular pathogen) are
compared on the table on page 540. Bacteria damage their hosts by producing exotoxins or
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endotoxins.
13 Methods for controlling bacterial growth can be bactericidal (kill bacteria) or bacteriostatic
(inhibit bacterial growth). The following are bacteriostatic: irradiation (gamma radiation,
microwaves or UV radiation), disinfectants (strong chemicals like bleach), and antiseptics (weak
chemicals like 70% ethanol). Pasteurization can be bactericidal for pathogenic bacterial such as
TB, but only bacteriostatic for other non-pathogenic bacteria. Antibiotics are antimicrobial
agents made by microbes which each have a specific method of action (page 542).
14 Epidemiology is the study of the occurrence, distribution and control of disease in a population.
Pandemics are very widespread epidemics that affect a large geographical area, such as a
continent. The Spanish flu pandemic of 1918 was caused by a virus with a lytic cycle (Figure
17.16, page 544). The origin of this virus seems to have been in China. A rare genetic shift
caused recombination of viral proteins making a new virus not recognized by the human immune
system. Infected people carried this virus with them along trade routes and shipping lanes. The
movement was magnified by the mass movement of military personal involved in World War I.
It was finally controlled by a combination of policies which banned public gatherings, educated
the public about hygiene and hand-washing and the use of disinfectants and sterilization
methods. The spongiform encephalopathies are a new group of troubling diseases. The chart on
page 548 details the virino and prion hypotheses of the cause of these diseases.
15 According to the United Nations, malaria is a pandemic in Africa. One African child dies of
malaria every 30 seconds. This could be controlled by the use of mosquito nets because the
disease is carried by a mosquito infecting the human bloodstream with a protozoon,
Plasmodium, which causes this disease. When a mosquito bites an infected person, it takes up
the gametes of the protozoon. The gametes join together in the gut of the female mosquito and
eventually form new cells which move to the salivary glands of the mosquito. As she bites
another person, they are transferred to a new victim and the life cycle begins again.
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