NCERT Solutions for Class 8 Science
Chapter 2, Microorganisms: Friend and Foe
June 20, 2024, by Md. Farrukh Asif
·
Those
living organisms around us that we cannot be seen with the naked eyes are called
microorganisms or microbes.
·
Some
of these can be seen with a magnifying glass while some cannot be seen without
the help of a microscope. That is why these are called microorganisms or
microbes.
Microorganisms:
Microorganisms are classified into four major groups. These
groups are
Bacteria, Fungi, Protozoa and some
Algae.
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|
REPRODUCTION IN ANIMALS NIMALS
Chap 6 |
Microorganisms play an important role in our lives. Some of
them are benecial in many ways whereas some others are harmful and cause
diseases. In this chapter, you will study about them in detail. Useful
Micro-organisms (Friends) Micro-organisms are used for various purposes, such
as
a.
Preparation of curd, bread, cake, idli, dosa, and cheese.
b.
In industry – to produce alcohol, wine, and vinegar (acetic acid).
c.
In medicines – to produce antibiotics and vaccines.
d.
In agriculture – to increase soil fertility.
e.
Cleaning the environment.
f.
As food.
Classification of Microorganism
·
Single-celled
organisms
·
Found
in a wide range of habitats ranging from glaciers to deserts and hot springs
·
For
example curd bacteria (Lactobacillus)
(ii) Fungi
·
Multicellular,
heterotrophic organisms
·
Lack
chlorophyll and are generally found in colonies
·
For
example: Penicillium, Aspergillus
(iii) Protozoa
·
Unicellular
or multicellular microorganisms
·
Usually
found in water
·
For
example Amoeba and Paramecium
(iv) Algae
·
Unicellular
or multicellular autotrophic organisms
·
Contain
chlorophyll pigment and carry out photosynthesis
·
For
example Chlamydomonas and Spirogyra
(v) Viruses
·
Viruses
are also microscopic and reproduce only inside the cells of the host
organism, which may be a bacterium, plant or animal.
·
Ultramicroscopic
organisms
·
Require
host cells to reproduce and complete their life cycle.
·
For
example Influenza virus, and polio virus.
Food Preservation:
· The process of preventing the spoilage of food items by the action of microbes is called
food preservation.
·
Microorganisms
act on food items and spoil them.
Methods of food preservation
(i) Chemical methods
·
The
chemicals that control the growth of microorganisms in food are called
preservatives.
For example: sodium benzoate, sodium
metabisulphite, salts, and edible oil.
·
Common
salt is used as a preservative in pickles. It is also used to preserve meat and
fish.
·
Sugar
is used as a preservative in jams and jellies.
·
Oil
and vinegar are used as preservatives in pickles and vegetables.
(ii) Heat and cold treatments
·
Boiling
the milk helps in killing microorganisms present in it.
·
Pasteurization
is a technique of preserving milk in which it is boiled to about 70°C for 15 to
30 seconds and then suddenly chilled and stored.
(iii) Storage and packing
·
Dry
fruits and vegetables are stored in sealed air-tight packets to prevent
microbial attack.
Importance of microorganisms:
(i) In the food industry
·
Lactobacillus
bacteria promote the conversion of milk into curd.
·
Yeast
is used in the preparation of breads, pastries, and cakes.
(ii) In the beverage industry
·
Yeast
is used for the commercial production of alcohol, wine, and vinegar (acetic acid).
·
Yeast
acts on sugar and converts it into alcohol by the process of fermentation.
Louis Pasteur discovered fermentation.
(iii) In medicine production
·
Medicines
produced by certain microorganisms to kill or stop the growth of other
disease-causing microorganisms are called antibiotics.
·
Antibiotics
are obtained from bacteria and fungi.
·
Commonly
used antibiotics are streptomycin, tetracycline, and erythromycin.
· The first antibiotic penicillin was prepared by Alexander Fleming
(iv) In vaccine production
·
Protection
of the body from the attack of various disease-causing microorganisms through
vaccines is known as vaccination.
· Vaccines include dead or weakened microbes that trigger the production of antibodies in
the body.
·
These
antibodies help in preventing the attack from disease-causing microorganisms.
·
Vaccination
helps in controlling diseases such as cholera, polio, smallpox, hepatitis, etc.
·
Vaccine
for smallpox was discovered by Edward Jenner.
(v) In increasing soil fertility
· Blue-green algae and Rhizobium bacteria are called biological nitrogen fixers.
·
They
fix free atmospheric nitrogen to enhance soil fertility.
(vi) In cleaning the environment
·
Microorganisms
(decomposers) help in converting the dead waste of plants and animals into simpler
substances by the process of decomposition.
Nitrogen cycle:
Our atmosphere has 78% nitrogen gas. Nitrogen is one of the
essential constituents of all living organisms as part of proteins, chlorophyll,
nucleic acids and vitamins. The atmospheric nitrogen cannot be taken directly
by plants and animals. Certain bacteria and blue green algae present in the
soil fix nitrogen from the atmosphere and convert it into compounds of
nitrogen.
Once nitrogen is
converted into these usable compounds, it can be utilized by plants from the
soil through their root system. Nitrogen is then used for the synthesis of
plant proteins and other compounds. Animals feeding on plants get these
proteins and other nitrogen compounds
·
It
involves the circulation of nitrogen through living and non-living components of
nature.
·
Nitrogen
gas comprises 78% of the atmosphere.
· The first process of the nitrogen cycle is the fixation of nitrogen gas into nitrogenous
compounds caused by the bacterium Rhizobium and lightning.
·
Nitrogen
compounds in soil are taken up by the plants through roots and used up in the synthesis of plant proteins. Animals obtain nitrogen by feeding on plants.
·
Waste
of plants and animals are converted to nitrogenous compounds by the action of
bacteria and fungi in the soil.
·
Some
bacteria convert nitrogenous compounds back to nitrogen to maintain atmospheric
levels of nitrogen.
Harmful microorganisms:
·
Disease-causing
microorganisms are called pathogens.
·
Diseases
in humans caused by microorganisms
·
Diseases
caused by microorganisms that spread from an infected person to a healthy
person through air, water, or food are called communicable diseases.
·
The
example includes cholera, chicken pox, and tuberculosis.
·
The
organisms that transmit diseases from one place to the other are called
carriers.
Medium of carriers:
·
Housefly
spreads diseases such as cholera, dysentery, and typhoid.
·
Female
Anopheles mosquito spreads malarial parasites.
·
Female
Aedes mosquito spreads dengue virus.
Human diseases caused by:
Tuberculosis - bacteria:
Cholera - bacteria:
Typhoid - bacteria:
Measles - virus
Chicken pox - virus
Polio - virus
Hepatitis-B - virus
Malaria - protozoa
Sleeping Sickness - protozoa
Diseases in Animals caused by Microorganisms:
Anthrax - bacteria.
Foot and mouth disease in cattle - virus
Diseases in plants caused by Microorganisms:
Citrus canker disease i- bacteria
Rust of wheat - fungi
Yellow vein mosaic of Bhindi (Okra) – virus
Question 1. Fill in the blanks.
(a) Microorganisms can be seen with the help of
a________________.
(b) Blue-green algae fix ______________ directly from air to
enhance fertility.
(c) Alcohol is produced with the help of __________.
(d) Cholera is caused by ____________.
Solution :
(a) Microorganisms can be seen with the help of a microscope.
(b) Blue-green algae fix nitrogen
directly from air to enhance fertility.
(c) Alcohol is produced with the help of yeast.
(d) Cholera is caused by bacteria.
Question 2. Tick the correct answer.
(a) Yeast is used in the production of
(i) Sugar (ii) alcohol (iii) hydrochloric acid (iv) oxygen
(b) The following is an antibiotic
(i) Sodium carbonate (ii) Streptomycin (iii) Alcohol (iv)
Yeast
(c) Carrier of malaria-causing protozoan is
(i) Female Anopheles mosquito (ii) cockroach
(iii) housefly (iv) butterfly
(d) The most common carrier of communicable diseases is
(i) ant (ii) housefly (iii) dragonfly (iv) spider
(e) The bread or idli dough rises because of
(i) heat (ii) grinding (iii) growth of yeast cells (iv)
kneading
(f) The process of conversion of sugar into alcohol is called
(i) nitrogen fixation (ii) moulding (iii) fermentation (iv)
infection.
Solution :
(a) —> (ii), (b)
—> (ii), (c) —> (i)
(d) —> (ii), (e)
—> (iii), (f) —> (iii)
Question 3. Match the organisms in Column A with their action
in Column B.
|
Column I |
Column II |
|
(a) Bacteria (b) Rhizobium (c) Lactobacillus (d) Yeast (e) A protozoan (f) A virus |
(i) Fixing nitrogen (ii) Setting of curd (iii) Baking of bread (iv) Causing malaria (v) Causing cholera (vi) Causing AIDS |
Solution :
|
Column I |
Column II |
|
(a) Bacteria (b) Rhizobium (c) Lactobacillus (d) Yeast (e) A protozoan (f) A virus |
(v) Causing cholera (i) Fixing nitrogen (ii) Setting of curd (iii) Baking of bread (iv) Causing malaria (vi) Causing AIDS |
Question 4. Can microorganisms be seen with the naked eyes? If not, how can they be seen?
Solution :
Micro-organisms are
too small to be seen through the naked eye. They can be seen with the help of a
magnifying glass or microscope.
For example, the fungus that grows on bread is so small that it
can be seen only with the help of a magnifying glass or microscope.
No, microorganisms are impossible to be seen with the naked eye.
They can only be seen with the help of a microscope.
Question 5. What are the major groups of microorganisms?
Solution :
There are five major groups of micro-organisms:
·
Bacteria
- They are single-celled disease-causing microorganisms. They can be spiral or
rod-shaped.
·
Fungi
- They are mostly multicellular disease-causing microbes. Bread molds are
common examples of fungi.
·
Protozoa
- They mainly include organisms such as Amoeba, Plasmodium, etc. They can be
unicellular or multicellular.
·
Virus
-Viruses are disease-causing microbes that reproduce only inside the host
organism.
·
Algae
- They include multicellular, photosynthetic organisms such as Spirogyra,
Chlamydomonas, etc.
Question 6. Name the microorganism which can fix atmospheric
nitrogen in the soil.
Solution :
Certain bacteria such as Rhizobium and blue-green algae
present in the soil fix atmospheric nitrogen and convert it into compounds of
nitrogen.
Question 7. Write 10 lines on the usefulness of
microorganisms in our lives.
Solution :
Microorganisms are too small to be seen through naked eye.
However, they are vital to plants and the environment.
1. Rhizobium present in the roots of
pulse plants fix nitrogen from air and supply nitrogen compounds to the pulse
plants.
2. Micro-organisms are also used in
winemaking, baking, pickling, and other food-making processes.
3. Alcoholic fermentation by yeast is
widely used in the preparation of wine and bread.
4. Microbes are used to reduce
pollution. For example, decomposers such as bacteria and fungi break down dead
bodies and excreta to form inorganic compounds, which can be absorbed by
plants.
5. Microbes also play an important role
in the preparation of medicines. Antibiotics are chemicals produced by
microorganisms to kill bacteria. Penicilin is an antibotic made from
Penicilium.
6. Bacteria present in our intestine
help in proper digestion and release Vitamin B which is absorbed by the intestine.
7. Many vaccines are prepared from
microorganisms. These vaccines are given to children to protect them from
disease.
8. Certain microbes are also used in the
biological treatment of sewage and industrial effluents.
9. Yeast is used in making idlis,
bhaturas, bread, pastries, and cakes.
10. Lactobacillus is used to form curd
from milk.
Question 8. Write a short paragraph on the harms caused by
microorganisms.
Solution :
The harm caused by microorganisms is as
follows-
a. They cause many communicable diseases like cholera, common cold, tuberculosis etc.
b. Malaria is caused by a
microorganism called plasmodium.
c. Several plants diseases are caused by bacteria and fungi.
d. Food poisoning is caused by microorganism.
e. Microorganism are
responsible for spoiling of food.
Question 9. What are antibiotics? What precautions must be
taken while taking antibiotics?
Solution :
The medicines that
kill or stop the growth of the disease-causing microorganism are called antibiotic. Streptomycin, tetracycline,
erythromycin etc. are common antibiotics.
They are manufactured by growing specific microorganisms and
are used to cure a variety of diseases.
Precautions to be taken while using antibiotics
:
* Antibiotics should be taken under the supervision of a well-qualified doctor.
* Course (intake) of antibiotics should be completed as per
the prescription given by the doctor.
* Antibiotics should be taken in the right amount and at the
right time. A wrong dose of antibiotics makes the drug ineffective. Also,
excessive consumption of drugs may kill the useful bacteria present in our
body.
|
ALGAE |
BACTERIA |
FUNGI |
NITROGEN CYCLE |
|
ANTIBIOTICS |
CARRIER COMMUNICABLE DISEASES |
MICROORGANISM |
NITROGEN FIXATION |
|
ANTIBODIES |
FERMENTATION |
LACTOBACILLUS |
PASTEURISATION |
|
PATHOGEN |
PRESERVATION |
PROTOZOA |
RHIZOBIUM |
|
VACCINE |
VIRUS |
YEAST |
|
Frequently asked MCQ’s for the Examination:
Microorganisms: Friend and Foe (Chapter-02)
1. Diseases like polio
and chicken pox are caused by _______
(a) Bacteria (b) Fungi
(c) Virus (d) Worms
Answer: (c) Virus
2. Examples of
Multicellular Microorganism are
(a) Algae, Bacteria
(b) Bacteria and Fungi (c) Bacteria and Viruses (d) Algae and Fungi
Answer: (d) Algae and Fungi
3. Some medicines
obtained from microorganisms are applied to kill or stop the growth of
disease-causing microorganisms. Such medicines are called ______.
(a) Antibodies (b)
Antibiotics (c) Antiseptics (d) All of the above
Answer: (b)
Antibiotics
4. A common preservative used in jam and pickles is
(a) Sodium benzoate (b) Nitric acid (c) Sodium Chloride (d)
Copper Sulphate
Answer: (a) Sodium
benzoate
5. Rhizobium found in root nodules of leguminous roots is an
(a) Atmospheric Carbon fixer (b) Atmospheric Oxygen fixer (c)
Atmospheric Nitrogen fixer (d) All of the above
Answer: (c)
Atmospheric Nitrogen fixer
6. Lactobacillus is commonly found in
(a) Cake (b) Curd (c) Bread (d) All of the above
Answer: (d) All of the above<strong>
7. The process of conversion of sugar into alcohol by yeast
is called
(a) Fermentation (b) Pasteurisation (c) Alcoholism (d) All of
the above
Answer: (a)
Fermentation
8. The pores in the bread is due to gas bubbles of
(a) Oxygen (b) Nitrogen di-oxide (c) Nitrogen (d) Carbon di-oxide
Answer: (d) Carbon dioxide
9. Deliberately injecting weak microbes into a healthy body
and producing antibodies to fight against strong microbes is called
_______ (a) Medication (b) Antibiotics
(c) Vaccination (d) All of the above
Answer: (c)
Vaccination
10. The microbe for Malaria is carried by
(a) Male Anopheles mosquito (b) Female Anopheles Mosquito (c)
Male Aedes mosquito (d) Female Aedes mosquito
Answer: (b) Female
Anopheles Mosquito
11. Which of the
following is a disease caused by protozoans?
(a) Tuberculosis (b) Polio (c) Malaria (d) Typhoid
Answer: (c) Malaria
12. Which of the following diseases is caused by bacteria?
(a) Chickenpox (b) Tuberculosis (c) Dengue (d) Polio
Answer: (b) Tuberculosis
13. Some plants have
nitrogen-fixing bacteria in their root
nodules. What are these bacteria called?
(a) Blue green algae (b) Nitrosomonas (c) Azotobacter (d)
Rhizobium
Answer: (d) Rhizobium
14. In order to take
precautionary steps to control dengue, we must take measures to stop the
breeding of:
(a) Aedes mosquito (b) Fleas (c) Fire ants (d) Anopheles
mosquito
Answer: (a) Aedes mosquito
15. Partial
sterilization of a product such as milk at a high temperature is known as:
(a) Pasteurization (b) Filtration (c) Pickling (d)
Refrigeration
Answer: (a) Pasteurization
16. Living organisms
which are invisible to the naked eye are called _________.
(a) Particles (b) Molecules (c) Microorganisms (d)
Macroorganisms
Answer: (c) Microorganisms
17. Which of the following is a communicable disease?
(a) Diabetes (b)
Chicken pox (c) Alzheimer’s (d) Cancer
Answer: (b)
Chickenpox
18. Vaccines are made up of:
(a) Chemicals (b) Weak microorganisms (c) Viruses (d) Drugs
Answer: (b) Weak microorganisms
19. The bacterium
present in curd is:
(a) Salmonella typhi (b) Lactobacillus (c) Penicillin (d)
Vibrio cholera
Answer: (b) Lactobacillus
20. While baking
cakes, yeast reproduces rapidly and produces ___ gas.
(a) Hydrogen (b) Oxygen (c) carbon dioxide (d) nitrogen
Answer: (c) carbon dioxide
21. ______ is used for
the production of alcohol and wine.
(a) Yeast (b) Mosquito (c) Ant (d) Algae
Answer: (a) Yeast
22. Which of the
following diseases can be cured using antibiotics?
(a) AIDS (b) Dengue (c) Typhoid (d) Malaria
Answer: (c) Typhoid
23. Which organisms
are microscopic and dependent on host organisms for reproduction?
(a) Algae (b) Protozoa (c) Viruses (d) Bacteria
Answer: (c) Viruses
24. _______ are the
agents that act as carriers of a pathogen and spread diseases.
(a) Vectors (b) Microbes (c) Parasites (d) DNA
Answer: (a) Vectors
25. An infectious
disease is ___________.
(a) caused only by bacteria (b) spread from person to person
(c) spread from animal to animal (d) caused only by viruses
Answer: (b) spread from person to person
26. Which one of the
following is not a method of food
preservation?
(a) Salting (b) Drying
(c) Boiling (d) Pickling
Answer: (c)
Boiling
27. Which among the
following statements is not associated with food preservation?
(a) Prevention of decay or spoilage (b) Decreased shelf life
(c) Air-tight sealing (d) Storage for
future use
Answer: (b) Decreased shelf life
28. Which one of the
following is not produced by the process of fermentation?
(a) Cheese (b) Milk (c) Yoghurt (d) Wine
Answer: (b) Milk
29. A plant disease
called citrus canker is caused by a _________.
(a) Bacteria (b) Fungi (c) Protozoa (d) Virus
Answer: (a) Bacteria
30. Which among the
following pathogens causes cholera?
(a) Bacteria (b) Protozoan (c) Fungus (d) Virus
Answer: (a) Bacteria
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