Science NCERT Grade 8, Chapter 2, Microorganisms: Friends and Foe encompasses all the essential concepts of microorganisms. The chapter starts with a basic definition of microorganisms/microbes. Further understanding of microorganisms is developed with the help of two activities which shows the presence of microorganisms in water and soil. Classification of microorganisms into bacteria, fungi, protozoa and algae along with viruses is included. Apart from this, the diseases they cause are also mentioned in the chapter, Microorganisms: Friends and Foe. Different types of bacteria, algae, protozoa and fungi are shown with the help of figures. Thereafter, the sources of microorganisms and the places where they live are discussed. Explanation of the positive and negative effects of microorganisms on humans is covered in detail. The microorganisms are divided into two categories in chapter, Microorganisms: Friends and Foe for the sake of understanding -

  • Friendly microorganisms

  • Harmful microorganisms 

The following uses of friendly microorganisms are discussed:

  • Making of curd and bread

  • Increasing soil fertility

  • Large scale production of alcohol, wine and acetic acid

  • Fermentation

  • As antibiotics

  • As vaccines

  • Cleaning the environment

Harmful microorganisms called pathogens are discussed along with the communicable diseases they cause. The information on some common diseases caused by microorganisms in humans, animals and plants are presented in a tabular format. Apart from this, food preservation and some methods associated with it are covered using examples. The chapter, Microorganisms: Friends and Foe also discuss nitrogen cycle in detail along with its diagrammatic representation.

The topics covered in the chapter, Microorganisms: Friends and Foe are -
  1. Microorganisms
  2. Where do Microorganisms Live?
  3. Microorganisms and Us,
  4. Harmful Microorganisms
  5. Food Preservation
  6. Nitrogen Fixation and Nitrogen cycle

Page No 29:

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 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 _.

Page No 29:

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 bicarbonate (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

Answer:

(a) Yeast is used in the production of

(i) sugar (ii) alcohol&mnTick;

(iii) hydrochloric acid (iv) oxygen

(b) The following is an antibiotic

(i) sodium bicarbonate (ii) streptomycin&mnTick;

(iii) alcohol (iv) yeast

(c) Carrier of malaria-causing protozoan is

(i) female Anopheles mosquito&mnTick; (ii) cockroach

(iii) housefly (iv) butterfly

(d)The most common carrier of communicable diseases is

(i) ant (ii) housefly&mnTick;

(iii) dragonfly (iv) spider

(e)The bread or idli dough rises because of

(i) heat (ii) grinding

(iii) growth of yeast cells&mnTick; (iv) kneading

(f) The process of conversion of sugar into alcohol is called

(i) nitrogen fixation (ii) moulding

(iii) fermentation&mnTick; (iv) infection

Page No 29:

Question 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

Page No 29:

Question 4:

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

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.



Page No 30:

Question 5:

What are the major groups of microorganisms?

Answer:

There are five 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) Algae − They include multicellular, photosynthetic organisms such as Spirogyra, Chlamydomonas, etc.

Page No 30:

Question 6:

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

Answer:

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

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 fixing 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 effluents.

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Question 8:

Write a short paragraph on the harms caused by 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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Question 9:

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

Answer:

Antibioticsare medicines produced by certain micro-organisms to kill other disease-causing micro-organisms. 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 qualified 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.



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