NCERT Solutions for Class 8 Science Chapter 2 Microorganisms Friend and Foe

NCERT
SOLUTIONS
CLASS - 8TH

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Book : Science Ncert Solutions | Chapter-2 Science

Class : 8th
Subject : Science
Chapter : 2
Chapter Name : MICROORGANISMS : FRIEND AND FOE

Q1 Fill in the blanks.
(a) Microorganisms can be seen with the help of a ____________.
(b) Blue green algae �x __________ directly from air and enhance fertility of soil.
(c) Alcohol is produced with the help of __________.
(d) Cholera is caused by __________.

Answers. (a) Microorganisms can be seen with the help of a microscope
(b) Blue green algae �x nitrogen _ directly from air to enhance fertility of soil.
(c) Alcohol is produced with the help of yeast
(d) Cholera is caused by bacteria

Q2 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 bicarbonate
(ii) Streptomycin
(iii) Alcohol
(iv) Yeast

(c) Carrier of malaria-causing protozoan is
(i) female Anopheles mosquito
(ii) cockroach
(iii) house�y
(iv) butter�y

(d) The most common carrier of communicable diseases is
(i) ant
(ii) house�y
(iii) dragon�y
(iv) spider

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Book : Science Ncert Solutions | Chapter-2 Science

(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 �xation
(ii) moulding
(iii) fermentation
(iv) infection

Answer. (a) — (ii)
(b) — (ii)
(c) — (i)
(d) — (ii)
(e) — (iii)
(f) — (iii)

Q3 Match the organisms in Column A with their action in Column B.

Answer.

Q4 Can microorganisms be seen with the naked eye? If not, how can they be seen?

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Book : Science Ncert Solutions | Chapter-2 Science

Answer. Micro-organisms are too small to be seen through naked eyes. They can be seen with
the help of a magnifying glass or microscope.
For example, fungus that grows on bread is so small that it can be seen only with the help of a
magnifying glass or microscope.

Q5 What are the major groups of microorganisms?

Answer. There are �ve major groups of micro-organisms:
(i) Bacteria — They are single celled disease-causing micro-organisms. They can be spiral or rod-
shaped.
(ii) Fungi — They are mostly multicellular disease-causing microbes. Bread moulds are common
examples of fungi.
(iii) Protozoa - They mainly include organisms such as Amoeba, Plasmodium,etc.
They can be unicellular or multicellular.
(iv) Virus -Viruses are disease-causing microbes that reproduce only inside the host organism.
(v) Algae — They include multicellular, photosynthetic organisms such as Spirogyra,
Chlamydomonas, etc.

Page : 30 , Block Name : Exercise

Q6 Name the microorganisms which can �x atmospheric nitrogen in the soil

Answer. Bacteria such as Rhizobium and certain blue-green algae present in the soil can �x
atmospheric nitrogen and convert it into usable nitrogenous compounds. These nitrogenous
compounds can be easily utilized by plants for the synthesis of plant proteins and other
compounds.

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Q7 Write 10 lines on the usefulness of microorganisms in our lives

Answer. Micro-organisms are too small to be seen through naked eyes. However, theyare vital to
plants and the environment. Importance of micro-organisms: They are used in winemaking,
baking, pickling, and other food making processes. Alcoholic fermentation by yeast is widely
used in the preparation of wine and bread. A bacterium Lactobacillus, promotes the formation of
curd. 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.They are used to increase the soil fertility by �xing the atmospheric nitrogen with the
help of bacterium Rhizobium and some other blue-green algae. Microbes also play an important
role in the preparation of medicines. Antibiotics are chemicals produced by micro-organisms to
kill bacteria. Streptomycin, for example, is an antibiotic. Certain microbes are also used in the
biological treatment of sewage and industrial ef�uents.

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Book : Science Ncert Solutions | Chapter-2 Science

Page : 30 , Block Name : Exercise

Q8 Write a short paragraph on the harmful effects of microorganisms.

Answer. Harmful effects of micro-organisms:
Micro-organisms cause diseases in animals. For example, in humans, bacteria cause diseases
such as tuberculosis, cholera, typhoid, etc. In cattle, the foot and mouth disease is caused by a
virus. Also, several microbes cause diseases in plants. For example, the productivity of wheat,
orange, apple, etc. is reduced due to microbial diseases in plants. Certain microbes, on entering
into our body, produce toxic substances. This leads to food poisoning. Some micro-organisms
such as fungus spoil our food. For example, bread when left unused under moist conditions gets
spoilt by fungus, producing a white cotton-like growth on the bread.

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Q9 What are antibiotics? What precautions must be taken while taking antibiotics?

Answer. Antibiotics are medicines produced by certain microorganisms to kill other disease
causing microorganisms. These medicines are commonly obtained from bacteria and fungi.
Streptomycin, tetracycline, penicillin, etc. are common antibiotics.
Precautions to be taken while using antibiotics:
(i) Antibiotics should be taken under the supervision of a well quali�ed doctor.
(ii) Course (intake) of antibiotics should be completed as per the prescription given by the
doctor.
(iii) 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.

Page : 30 , Block Name : Exercise

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