NCERT Textbook: Microorganisms - Friend & Foe | Science Class 8 PDF Download

 Page 1


MICROORGANISMS : FRIEND AND FOE
MICROORGANISMS : FRIEND AND FOE
Y
ou have seen several kinds of
plants and animals. However,
there are other living organisms
around us which we normally cannot
see. These are called microorganisms
or microbes. For example, you might
have observed that during the rainy
season moist bread gets spoilt and its
surface gets covered with greyish white
patches. Observe these patches through
a magnifying glass. You will see tiny,
black rounded structures. Do you know
what these structures are and where do
these come from?
2.1 Microorganisms
Activity 2.1
Collect some moist soil from the
field in a beaker and add water to
it. After the soil particles have
settled down, observe a drop of
water from the beaker under a
microscope. What do you see ?
Activity 2.2
Take a few drops of water from a
pond. Spread on a glass slide and
observe through a microscope.
Do you find tiny organisms moving
around?
These observations show that water
and soil are full of tiny organisms,
though not all of them fall into the
category of microbes. These
microorganisms or microbes are so
small in size that they cannot be seen
with the unaided eye. Some of these,
such as the fungus that grows on bread,
can be seen with a magnifying glass.
Others cannot be seen without the help
of a microscope. That is why these are
called microorganisms or microbes.
Microorganisms are classified into
four major groups. These groups are
bacteria, fungi,  protozoa and some
algae. Some of these common
microorganisms are shown in
Figs. 2.1 - 2.4.
Viruses are also microscopic but are
different from other microorganisms.
They, however, reproduce only inside the
cells of the host organism, which may be
a bacterium, plant or animal. Some of the
viruses are shown in Fig. 2.5. Common
ailments like cold, influenza (flu) and
most coughs are caused by viruses.
Serious diseases like polio and chicken
pox are also caused by viruses.
Diseases like dysentery and malaria
are caused by protozoa(protozoans)
whereas  typhoid and tuberculosis (TB)
are bacterial diseases.
You have learnt about some of
these microorganisms in Classes VI
and VII.
2024-25
Page 2


MICROORGANISMS : FRIEND AND FOE
MICROORGANISMS : FRIEND AND FOE
Y
ou have seen several kinds of
plants and animals. However,
there are other living organisms
around us which we normally cannot
see. These are called microorganisms
or microbes. For example, you might
have observed that during the rainy
season moist bread gets spoilt and its
surface gets covered with greyish white
patches. Observe these patches through
a magnifying glass. You will see tiny,
black rounded structures. Do you know
what these structures are and where do
these come from?
2.1 Microorganisms
Activity 2.1
Collect some moist soil from the
field in a beaker and add water to
it. After the soil particles have
settled down, observe a drop of
water from the beaker under a
microscope. What do you see ?
Activity 2.2
Take a few drops of water from a
pond. Spread on a glass slide and
observe through a microscope.
Do you find tiny organisms moving
around?
These observations show that water
and soil are full of tiny organisms,
though not all of them fall into the
category of microbes. These
microorganisms or microbes are so
small in size that they cannot be seen
with the unaided eye. Some of these,
such as the fungus that grows on bread,
can be seen with a magnifying glass.
Others cannot be seen without the help
of a microscope. That is why these are
called microorganisms or microbes.
Microorganisms are classified into
four major groups. These groups are
bacteria, fungi,  protozoa and some
algae. Some of these common
microorganisms are shown in
Figs. 2.1 - 2.4.
Viruses are also microscopic but are
different from other microorganisms.
They, however, reproduce only inside the
cells of the host organism, which may be
a bacterium, plant or animal. Some of the
viruses are shown in Fig. 2.5. Common
ailments like cold, influenza (flu) and
most coughs are caused by viruses.
Serious diseases like polio and chicken
pox are also caused by viruses.
Diseases like dysentery and malaria
are caused by protozoa(protozoans)
whereas  typhoid and tuberculosis (TB)
are bacterial diseases.
You have learnt about some of
these microorganisms in Classes VI
and VII.
2024-25
SCIENCE 18
Fig. 2.4 : Fungi
Fig. 2.1: Bacteria
Chlamydomonas
Spirogyra
Fig. 2.2 : Algae
Amoeba
Paramecium
Fig. 2.3 : Protozoa
Bread mould
Penicillium
Aspergillus
2024-25
Page 3


MICROORGANISMS : FRIEND AND FOE
MICROORGANISMS : FRIEND AND FOE
Y
ou have seen several kinds of
plants and animals. However,
there are other living organisms
around us which we normally cannot
see. These are called microorganisms
or microbes. For example, you might
have observed that during the rainy
season moist bread gets spoilt and its
surface gets covered with greyish white
patches. Observe these patches through
a magnifying glass. You will see tiny,
black rounded structures. Do you know
what these structures are and where do
these come from?
2.1 Microorganisms
Activity 2.1
Collect some moist soil from the
field in a beaker and add water to
it. After the soil particles have
settled down, observe a drop of
water from the beaker under a
microscope. What do you see ?
Activity 2.2
Take a few drops of water from a
pond. Spread on a glass slide and
observe through a microscope.
Do you find tiny organisms moving
around?
These observations show that water
and soil are full of tiny organisms,
though not all of them fall into the
category of microbes. These
microorganisms or microbes are so
small in size that they cannot be seen
with the unaided eye. Some of these,
such as the fungus that grows on bread,
can be seen with a magnifying glass.
Others cannot be seen without the help
of a microscope. That is why these are
called microorganisms or microbes.
Microorganisms are classified into
four major groups. These groups are
bacteria, fungi,  protozoa and some
algae. Some of these common
microorganisms are shown in
Figs. 2.1 - 2.4.
Viruses are also microscopic but are
different from other microorganisms.
They, however, reproduce only inside the
cells of the host organism, which may be
a bacterium, plant or animal. Some of the
viruses are shown in Fig. 2.5. Common
ailments like cold, influenza (flu) and
most coughs are caused by viruses.
Serious diseases like polio and chicken
pox are also caused by viruses.
Diseases like dysentery and malaria
are caused by protozoa(protozoans)
whereas  typhoid and tuberculosis (TB)
are bacterial diseases.
You have learnt about some of
these microorganisms in Classes VI
and VII.
2024-25
SCIENCE 18
Fig. 2.4 : Fungi
Fig. 2.1: Bacteria
Chlamydomonas
Spirogyra
Fig. 2.2 : Algae
Amoeba
Paramecium
Fig. 2.3 : Protozoa
Bread mould
Penicillium
Aspergillus
2024-25
MICROORGANISMS : FRIEND AND FOE 19
2.2Where do Microorganisms
Live?
Microorganisms may be single-celled
like bacteria, some algae and protozoa,
or multicellular, such as many algae and
fungi. They live in all types of
environment, ranging from ice cold
climate to hot springs; and deserts to
marshy lands. They are also found
inside the bodies of animals including
humans. Some microorganisms
grow on other organisms while others
exist freely.
2.3 Microorganisms and Us
Microorganisms play an important role
in our lives. Some of them are beneficial
in many ways whereas some others are
harmful and cause diseases. Let us
study about them in detail.
Microorganisms have been used for
the production of alcohol since ages.
Friendly Microorganisms
Microorganisms are used for various
purposes. They are used in the
preparation of curd, bread and cake.
Curd contains several micro-
organisms. Of these, the bacterium,
Lactobacillus  promotes the formation
of curd. It multiplies in milk and
converts it into curd. Bacteria are also
involved in the making of cheese,
pickles and many other food items. An
important ingredient of rava (sooji)
idlis and bhaturas is curd. Can you
guess why? Bacteria and yeast are also
helpful for fermentation of rice idlis
and dosa batter.
Fig. 2.5 : Viruses
I saw that my mother
added a little curd to warm
milk to set curd for the
next day. I wonder why?
They are also used in cleaning up
of the environment. For example, the
organic wastes (vegetable peels, remains
of animals,  faeces, etc.) are broken
down into harmless and usable
substances by bacteria. Recall that
bacteria are also used in the
preparation of medicines. In agriculture
they are used to increase soil fertility
by fixing nitrogen.
Making of Curd and Bread
You have learnt in Class VII that milk is
turned into curd by bacteria.
2024-25
Page 4


MICROORGANISMS : FRIEND AND FOE
MICROORGANISMS : FRIEND AND FOE
Y
ou have seen several kinds of
plants and animals. However,
there are other living organisms
around us which we normally cannot
see. These are called microorganisms
or microbes. For example, you might
have observed that during the rainy
season moist bread gets spoilt and its
surface gets covered with greyish white
patches. Observe these patches through
a magnifying glass. You will see tiny,
black rounded structures. Do you know
what these structures are and where do
these come from?
2.1 Microorganisms
Activity 2.1
Collect some moist soil from the
field in a beaker and add water to
it. After the soil particles have
settled down, observe a drop of
water from the beaker under a
microscope. What do you see ?
Activity 2.2
Take a few drops of water from a
pond. Spread on a glass slide and
observe through a microscope.
Do you find tiny organisms moving
around?
These observations show that water
and soil are full of tiny organisms,
though not all of them fall into the
category of microbes. These
microorganisms or microbes are so
small in size that they cannot be seen
with the unaided eye. Some of these,
such as the fungus that grows on bread,
can be seen with a magnifying glass.
Others cannot be seen without the help
of a microscope. That is why these are
called microorganisms or microbes.
Microorganisms are classified into
four major groups. These groups are
bacteria, fungi,  protozoa and some
algae. Some of these common
microorganisms are shown in
Figs. 2.1 - 2.4.
Viruses are also microscopic but are
different from other microorganisms.
They, however, reproduce only inside the
cells of the host organism, which may be
a bacterium, plant or animal. Some of the
viruses are shown in Fig. 2.5. Common
ailments like cold, influenza (flu) and
most coughs are caused by viruses.
Serious diseases like polio and chicken
pox are also caused by viruses.
Diseases like dysentery and malaria
are caused by protozoa(protozoans)
whereas  typhoid and tuberculosis (TB)
are bacterial diseases.
You have learnt about some of
these microorganisms in Classes VI
and VII.
2024-25
SCIENCE 18
Fig. 2.4 : Fungi
Fig. 2.1: Bacteria
Chlamydomonas
Spirogyra
Fig. 2.2 : Algae
Amoeba
Paramecium
Fig. 2.3 : Protozoa
Bread mould
Penicillium
Aspergillus
2024-25
MICROORGANISMS : FRIEND AND FOE 19
2.2Where do Microorganisms
Live?
Microorganisms may be single-celled
like bacteria, some algae and protozoa,
or multicellular, such as many algae and
fungi. They live in all types of
environment, ranging from ice cold
climate to hot springs; and deserts to
marshy lands. They are also found
inside the bodies of animals including
humans. Some microorganisms
grow on other organisms while others
exist freely.
2.3 Microorganisms and Us
Microorganisms play an important role
in our lives. Some of them are beneficial
in many ways whereas some others are
harmful and cause diseases. Let us
study about them in detail.
Microorganisms have been used for
the production of alcohol since ages.
Friendly Microorganisms
Microorganisms are used for various
purposes. They are used in the
preparation of curd, bread and cake.
Curd contains several micro-
organisms. Of these, the bacterium,
Lactobacillus  promotes the formation
of curd. It multiplies in milk and
converts it into curd. Bacteria are also
involved in the making of cheese,
pickles and many other food items. An
important ingredient of rava (sooji)
idlis and bhaturas is curd. Can you
guess why? Bacteria and yeast are also
helpful for fermentation of rice idlis
and dosa batter.
Fig. 2.5 : Viruses
I saw that my mother
added a little curd to warm
milk to set curd for the
next day. I wonder why?
They are also used in cleaning up
of the environment. For example, the
organic wastes (vegetable peels, remains
of animals,  faeces, etc.) are broken
down into harmless and usable
substances by bacteria. Recall that
bacteria are also used in the
preparation of medicines. In agriculture
they are used to increase soil fertility
by fixing nitrogen.
Making of Curd and Bread
You have learnt in Class VII that milk is
turned into curd by bacteria.
2024-25
SCIENCE 20
spoon of yeast powder to the sugar
solution. Keep it covered in a warm
place for 4-5 hours. Now smell the
solution.  Can you get a smell?
Louis Pasteur
discovered
fermentation
in 1857.
In 1929, Alexander
Fleming was working
on a culture of disease-
causing bacteria.
Suddenly he found the
spores of a little green mould in one of
his culture plates. He observed that
the presence of mould prevented the
growth of bacteria. In fact, it also killed
many of these bacteria. From this the
mould penicillin was prepared.
This is the smell of alcohol as sugar
has been converted into alcohol by yeast.
This process of conversion of sugar into
alcohol is known as fermentation.
Activity 2.3
Take ½ kg flour (atta or maida),
add some sugar and mix with
warm water. Add a small amount
of yeast powder and knead to
make a soft dough. What do you
observe after two hours? Did you
find the dough rising?
Maida with yeast powder
Raised maida
Fig. 2.6
Yeast reproduces rapidly and
produces carbon dioxide during
respiration. Bubbles of the gas fill the
dough and increase its volume (Fig. 2.6).
This is the basis of the use of  yeast in
the baking industry for making breads,
pastries and cakes.
Commercial Use of Microorganisms
Microorganisms are used for the large scale
production of alcohol, wine and acetic acid
(vinegar). Yeast is used for commercial
production of alcohol and wine. For this
purpose yeast is grown on natural sugars
present in grains like barley, wheat, rice,
crushed fruit juices, etc.
Activity 2.4
Take a 500 mL beaker filled upto
¾ with water. Dissolve 2-3
teaspoons of sugar in it. Add half a
Medicinal Use of Microorganisms
Whenever you fall ill the doctor may
give you some antibiotic tablets,
capsules or injections such as of
penicillin. The source of these
medicines is microorganisms. These
medicines kill or stop the growth of the
disease-causing microorganisms. Such
medicines are called antibiotics. These
days a number of antibiotics are being
produced from bacteria and fungi.
Streptomycin, tetracycline and
erythromycin are some of the
2024-25
Page 5


MICROORGANISMS : FRIEND AND FOE
MICROORGANISMS : FRIEND AND FOE
Y
ou have seen several kinds of
plants and animals. However,
there are other living organisms
around us which we normally cannot
see. These are called microorganisms
or microbes. For example, you might
have observed that during the rainy
season moist bread gets spoilt and its
surface gets covered with greyish white
patches. Observe these patches through
a magnifying glass. You will see tiny,
black rounded structures. Do you know
what these structures are and where do
these come from?
2.1 Microorganisms
Activity 2.1
Collect some moist soil from the
field in a beaker and add water to
it. After the soil particles have
settled down, observe a drop of
water from the beaker under a
microscope. What do you see ?
Activity 2.2
Take a few drops of water from a
pond. Spread on a glass slide and
observe through a microscope.
Do you find tiny organisms moving
around?
These observations show that water
and soil are full of tiny organisms,
though not all of them fall into the
category of microbes. These
microorganisms or microbes are so
small in size that they cannot be seen
with the unaided eye. Some of these,
such as the fungus that grows on bread,
can be seen with a magnifying glass.
Others cannot be seen without the help
of a microscope. That is why these are
called microorganisms or microbes.
Microorganisms are classified into
four major groups. These groups are
bacteria, fungi,  protozoa and some
algae. Some of these common
microorganisms are shown in
Figs. 2.1 - 2.4.
Viruses are also microscopic but are
different from other microorganisms.
They, however, reproduce only inside the
cells of the host organism, which may be
a bacterium, plant or animal. Some of the
viruses are shown in Fig. 2.5. Common
ailments like cold, influenza (flu) and
most coughs are caused by viruses.
Serious diseases like polio and chicken
pox are also caused by viruses.
Diseases like dysentery and malaria
are caused by protozoa(protozoans)
whereas  typhoid and tuberculosis (TB)
are bacterial diseases.
You have learnt about some of
these microorganisms in Classes VI
and VII.
2024-25
SCIENCE 18
Fig. 2.4 : Fungi
Fig. 2.1: Bacteria
Chlamydomonas
Spirogyra
Fig. 2.2 : Algae
Amoeba
Paramecium
Fig. 2.3 : Protozoa
Bread mould
Penicillium
Aspergillus
2024-25
MICROORGANISMS : FRIEND AND FOE 19
2.2Where do Microorganisms
Live?
Microorganisms may be single-celled
like bacteria, some algae and protozoa,
or multicellular, such as many algae and
fungi. They live in all types of
environment, ranging from ice cold
climate to hot springs; and deserts to
marshy lands. They are also found
inside the bodies of animals including
humans. Some microorganisms
grow on other organisms while others
exist freely.
2.3 Microorganisms and Us
Microorganisms play an important role
in our lives. Some of them are beneficial
in many ways whereas some others are
harmful and cause diseases. Let us
study about them in detail.
Microorganisms have been used for
the production of alcohol since ages.
Friendly Microorganisms
Microorganisms are used for various
purposes. They are used in the
preparation of curd, bread and cake.
Curd contains several micro-
organisms. Of these, the bacterium,
Lactobacillus  promotes the formation
of curd. It multiplies in milk and
converts it into curd. Bacteria are also
involved in the making of cheese,
pickles and many other food items. An
important ingredient of rava (sooji)
idlis and bhaturas is curd. Can you
guess why? Bacteria and yeast are also
helpful for fermentation of rice idlis
and dosa batter.
Fig. 2.5 : Viruses
I saw that my mother
added a little curd to warm
milk to set curd for the
next day. I wonder why?
They are also used in cleaning up
of the environment. For example, the
organic wastes (vegetable peels, remains
of animals,  faeces, etc.) are broken
down into harmless and usable
substances by bacteria. Recall that
bacteria are also used in the
preparation of medicines. In agriculture
they are used to increase soil fertility
by fixing nitrogen.
Making of Curd and Bread
You have learnt in Class VII that milk is
turned into curd by bacteria.
2024-25
SCIENCE 20
spoon of yeast powder to the sugar
solution. Keep it covered in a warm
place for 4-5 hours. Now smell the
solution.  Can you get a smell?
Louis Pasteur
discovered
fermentation
in 1857.
In 1929, Alexander
Fleming was working
on a culture of disease-
causing bacteria.
Suddenly he found the
spores of a little green mould in one of
his culture plates. He observed that
the presence of mould prevented the
growth of bacteria. In fact, it also killed
many of these bacteria. From this the
mould penicillin was prepared.
This is the smell of alcohol as sugar
has been converted into alcohol by yeast.
This process of conversion of sugar into
alcohol is known as fermentation.
Activity 2.3
Take ½ kg flour (atta or maida),
add some sugar and mix with
warm water. Add a small amount
of yeast powder and knead to
make a soft dough. What do you
observe after two hours? Did you
find the dough rising?
Maida with yeast powder
Raised maida
Fig. 2.6
Yeast reproduces rapidly and
produces carbon dioxide during
respiration. Bubbles of the gas fill the
dough and increase its volume (Fig. 2.6).
This is the basis of the use of  yeast in
the baking industry for making breads,
pastries and cakes.
Commercial Use of Microorganisms
Microorganisms are used for the large scale
production of alcohol, wine and acetic acid
(vinegar). Yeast is used for commercial
production of alcohol and wine. For this
purpose yeast is grown on natural sugars
present in grains like barley, wheat, rice,
crushed fruit juices, etc.
Activity 2.4
Take a 500 mL beaker filled upto
¾ with water. Dissolve 2-3
teaspoons of sugar in it. Add half a
Medicinal Use of Microorganisms
Whenever you fall ill the doctor may
give you some antibiotic tablets,
capsules or injections such as of
penicillin. The source of these
medicines is microorganisms. These
medicines kill or stop the growth of the
disease-causing microorganisms. Such
medicines are called antibiotics. These
days a number of antibiotics are being
produced from bacteria and fungi.
Streptomycin, tetracycline and
erythromycin are some of the
2024-25
MICROORGANISMS : FRIEND AND FOE 21
commonly known antibiotics which are
made from fungi and bacteria. The
antibiotics are manufactured by
growing specific microorganisms and
are used to cure a variety of diseases.
Antibiotics are even mixed with the
feed of livestock and poultry to check
microbial infection in animals. They are
also used to control many plant
diseases.
It is important to remember that
antibiotics should be taken only on
the advice of a qualified doctor. Also
you must complete the course
prescribed by the doctor. If you take
antibiotics when not needed or in
wrong doses, it may make the drug
less effective when you might need
it in future. Also antibiotics taken
unnecessarily may kill the beneficial
bacteria in the body. Antibiotics,
however, are not effective against
cold and flu as these are caused by
viruses.
Vaccine
from the disease-causing microbes for
ever. This is how a vaccine works.
Several diseases, including cholera,
tuberculosis, smallpox and hepatitis can
be prevented by vaccination.
Edward Jenner
discovered the
vaccine for small-
pox in 1798.
Why are children/infants
given vaccination?
When a disease-carrying microbe enters
our body, the body produces antibodies
to fight the invader. The body also
remembers how to fight the microbe if
it enters again. If dead or weakened
microbes are introduced into a healthy
body, the body fights and kills
the invading bacteria by producing
suitable antibodies. The antibodies
remain in the body and we are protected
In your childhood, you must have
been given injections to protect
yourself against several diseases.
Can you prepare a list of these
diseases? You may take help from
your parents.
It is essential to protect all children
against these diseases. Necessary
vaccines are available in the nearby
hospitals. You might have seen the
advertisement on TV and newspapers
regarding protection of children against
polio under the Pulse Polio Programme.
Polio drops given to children are
actually a vaccine.
A worldwide campaign against
smallpox has finally led to its eradication
from most parts of the world.
These days vaccines are made on a
large scale from microorganisms  to
protect humans and other animals from
several diseases.
Increasing Soil Fertility
Some bacteria  (Fig. 2.7) are able to fix
nitrogen from the atmosphere to enrich
soil with nitrogen and increase its
fertility. These microbes are commonly
called biological nitrogen fixers.
2024-25