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1 of 25 © Boardworks Ltd 2012
Bacteria and Viruses
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Comparing cell sizes
Most plant and animal cells are between 10µm and 100µm
in size – around the diameter of a human hair – and too
small to see without a microscope.
The largest cell in the human body
is the female egg cell, at around
100µm in diameter.
The smallest human cell is the
sperm cell – the head is around
5µm long.
Bacteria are single celled
organisms and are around 1µm in
diameter, even smaller than sperm.
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How big are micro-organisms?
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Microscopes
Due to advances in microscope technology, micro-organisms
can be magnified, allowing us to see them in greater detail.
size of image
actual size of the objectmagnification =
Now, with electron microscopes, scientists
can attain a magnification as high as 2×106. bacteria
When Antonie van Leeuwenhoek discovered
single-celled organisms in 1676, his microscope
could magnify an image up to 500 times.
Magnification is the measure of how many times bigger the
image is than the object being studied.
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Virtual microscope
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What are bacteria?
Bacteria can be classified according to their shape
under a microscope:
spherical spiral rod curved rod
Bacteria lack mitochondria and chloroplasts. As they also
have no distinct nucleus, their genetic material is contained
within a coiled cluster of chromosomal DNA and a single
circular strand of plasmid DNA.
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What do bacteria contain?
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How do bacteria feed?
A large number of different species of bacteria exist and
exploit a very wide range of habitats. Different species of
bacteria can survive on a range of energy sources:
Using photosynthesis to
produce their own food.
Consuming organic
compounds from the
environment, e.g. from dead
organisms, to obtain energy.
Using carbon dioxide and inorganic chemicals, such
as ammonia and nitrate, to produce their own food.
This method of obtaining food allows bacteria to survive
extreme conditions, such as boiling sulphur-rich water.
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Energy from decaying substances
Many species of bacteria can obtain energy from organic
compounds through the process of decomposition or decay.
A warm, moist, oxygen-rich environment is the most
favourable for decay to occur.
Which of the following might you
expect to decay at a faster rate:
cubes of bread that are
kept in a dry environment
cubes of bread that are
exposed to moisture?
The conditions in which bacteria reproduce and feed will
therefore affect the rate of food spoilage.
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Rates of decay experiment
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How bacteria reproduce
Bacteria can reproduce rapidly via asexual reproduction,
where a single bacterium makes a copy of itself with exactly
the same genetic code.
In particular, bacteria
use a special type of
asexual reproduction
called binary fission.
This involves
one bacteria
splitting in two.
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The growth rate of pathogens
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What do micro-organisms need to grow?
Bacteria can be grown in a liquid broth
or on solid agar. To reproduce at the
optimum growth rate, bacteria need to
be kept in ideal conditions, involving:
a continuous supply of food
the optimum pH
the removal of products
For example, when the pH or
temperature is above the optimum,
bacterial growth rates decline.
gro
wth
ra
te
pH or temperature
optimum
the optimum temperature.
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Culturing bacteria
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Virus fact file
Micro-organism: Virus (e.g. flu virus and HIV)
Size: They are smaller than bacteria
and fungi, between 10 nm and 100 nm.
Shape: Viruses have regular and
geometric shapes.
Structure: A virus is a simple
organism that does not display all
the characteristics of a living thing.
It is made up of a protein coating
and some genetic material.
Reproduction: Viruses can only grow
and reproduce within other living things.
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Structure of a virus
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A virus cannot read its own genes but it can make a host
cell copy them and make the proteins.
Using viruses
The virus on the right is a bacteriophage.
It infects bacteria by injecting its genetic
material down a special tube.
protein
coat
genetic
material
injection tube
Viruses can also attack some plant
cells and some animal cells.
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Human immunodeficiency virus (HIV)
HIV is a virus that causes acquired immunodeficiency
syndrome (AIDS) – a deterioration of the immune system.
In 2007, 33.2 million
people were estimated
to be living with HIV.
There is currently no
cure; however, antiviral
drugs have been
developed to help delay
the onset of AIDS.
The process of how the HIV infects a cell and reproduces
within the living cell is typical of a virus.
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How do viruses replicate?
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Glossary
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Multiple-choice quiz