NCERT Solutions for Class 8th: Ch 2 Microorganisms : Friend and Foe Science

NCERT Solutions for Class 8th: Ch 2 Microorganisms : Friend and Foe Science

Page No: 29

Excercise

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 of soil.
(c) Alcohol is produced with the help of _________.
(d) Cholera is caused by _________.

Answer

(a) Microorganisms can be seen with the help of a microscope.

(b) Blue green algae fix nitrogen directly from air to enhance fertility of soil.

(c) Alcohol is produced with the help of yeast.

(d) Cholera is caused by bacteria.

2. Tick the correct answer:

(a) Yeast is used in the production of
(i) sugar
(ii) alcohol
(iii) hydrochloric acid
(iv) oxygen
► (ii) alcohol

(b) The following is an antibiotic
(i) Sodium bicarbonate
(ii) Streptomycin
(iii) Alcohol
(iv) Yeast
► (ii) Streptomycin

(c) Carrier of malaria-causing protozoan is
(i) female Anopheles mosquito
(ii) cockroach
(iii) housefly
(iv) butterfly
► (i) female Anopheles mosquito

(d) The most common carrier of communicable diseases is
(i) ant
(ii) housefly
(iii) dragonfly
 (iv) spider
► (ii) housefly

(e) The bread or idli dough rises because of
(i) heat
(ii) grinding
(iii) growth of yeast cells
(iv) kneading
► (iii) growth of yeast cells

(f) The process of conversion of sugar into alcohol is called
(i) nitrogen fixation
(ii) moulding
(iii) fermentation
(iv) infection
► (iii) fermentation

3. Match the organisms in Column I with their action in Column II.
Column I
Column II
(i)
Bacteria
(a)
Fixing nitrogen
(ii)
Rhizobium
(b)
Setting of curd
(iii)
Lactobacillus
(c)
Baking of bread
(iv)
Yeast
(d)
Causing malaria
(v)
A protozoan
(e)
Causing cholera
(vi)
A virus
(f)
Causing AIDS
(g)
Producing antibodies

Answer

Column I
Column II
(i)
Bacteria
(e)
Causing cholera
(ii)
Rhizobium
(a)
Fixing nitrogen
(iii)
Lactobacillus
(b)
Setting of curd
(iv)
Yeast
(c)
Baking of bread
(v)
A protozoan
(d)
Causing malaria
(vi)
A virus
(f)
Causing AIDS

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

Answer

Microorganisms are too small so they cannot be seen with naked eye. They can be seen with the help of a magnifying glass or microscope.

Page No: 30

5. What are the major groups of microorganisms?

Answer

There are five major groups of micro-organisms:
→ Bacteria - They are single celled disease-causing micro-organisms. They can be spiral or rod- shaped.
→ Fungi - They are mostly multicellular disease-causing microbes. Bread moulds 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.

6. Name the microorganisms which can fix atmospheric nitrogen in the soil.

Answer

Bacteria such as Rhizobium and certain blue-green algae can fix atmospheric nitrogen in the soil.

7. Write 10 lines on the usefulness of microorganisms in our lives.

Answer

Micro-organisms are too small to be seen through naked eyes. However, they are vital to plants and the environment.→ Lactobacillus is used to form curd from milk.
→ Rhizobium present in the roots of pulse plants fix nitrogen from air and supply nitrogen compounds to the pulse plants.
→ Micro-organisms are also used in winemaking, baking, pickling, and other food making processes.
→ Alcoholic fermentation by yeast is widely used in the preparation of wine and bread.
→ 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.
→ Microbes also play an important role in the preparation of medicines. Antibiotics are chemicals produced by micro-organisms to kill bacteria. Penicilin is an antibotic made from Penicilium.
→ Bacteria present in our intestine helps in proper digestion and release Vitmain B which is absorbed by intestine.
→ Many vaccines are prepared from micro-organisms. These vaccines are given to children to protect them from disease.
→ Certain microbes are also used in the biological treatment of sewage and industrial effluents.
→ Yeast is used in making idlis, bhaturas, bread, pastries and cakes.

8. Write a short paragraph on the harms caused by microorganisms.

Answer

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.

9. What are antibiotics? What precautions must be taken while taking antibiotics?

Answer

Antibiotics are medicines produced by certain micro-organisms to kill other disease-causing micro-organisms.
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.


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