Microorganisms: Friend And Foe | NCERT Solution | Class VIII

Q.1. Fill in the blanks:

a. Microorganisms can be seen with the help of a ____________.

b. Blue green algae fix __________ directly from air and enhance                fertility of soil.

c. Alcohol is produced with the help of __________.

d. Cholera is caused by __________.

Ans :

(a) Microscope

(b) Nitrogen 

(c) Yeast

(d) Bacteria

Q.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

Ans : (a) — (ii) (b) — (ii) (c) — (i) (d) — (ii) (e) — (iii) (f) — (iii)

Q.3: Match the organisms in Column I with their action in

Column II.

Ans :

(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

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

Ans : Microorganisms 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.

Q.5: What are the major groups of microorganisms?

Ans : There are five major groups of microorganisms:

(i) Bacteria-

  • They are very small and the simplest living organisms.
  • They are single-celled micro-organisms.
  • They can be spiral, spherical, comma or rod-shaped.

(ii) Virus- 

  • The word 'virus' means poisonous liquid.
  • They are the smallest microbes.
  • They can only be studied under an electron microsope.
  • They only contain a small nuclear material wrapped in a protein coat.
  • Virus cannot grow on their own. This indicates that they are non-living.
  • For e.g. Vaccinia, Influenza, Polio and HIV-I.

(iii) Protozoa-

  • Protozoan means the 'first animal'.
  • Protozoa are either saprophytic or parasitic. Euglena is an exception. It is an autotroph.
  • They are unicellular.
  • They mainly include organisms such as Amoeba, etc.

(iv) Fungi-

  • Fungi is a group of diverse organisms that lack chlorophyll. 
  • They grow in warm, moist and dark conditions.
  • Mostly fungi are saprophytes while some are parasites.
  • For e.g. Yeast, Bread Mould and Mushroom.

(v) Algae-

  • They vary in size, shape and habitat.
  • Some are unicellular and microscopic while others are multicellular and can be a few metres long.
  • They do not have a stem, root or leaves like other plants.
  • They include plants such as Spirogyra, Chlamydomonas, etc.

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

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

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

Ans : 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.
  • Tanning is done with the help of bacteria.
  • Bacteria produce biogas.
  • Some microbes live in digestive system of ruminants and help them digest cellulose of plants.
  • Certain microbes are also used in the biological treatment of sewage and industrial effluents.

Q.8: Write a short paragraph on the harmful effects of microorganisms.

Ans : 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.

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

Ans : Antibiotics are medicines produced by certain living organisms to kill other disease causing microorganisms (pathogens). 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) We must complete the entire cource of antibiotics prescribed by the doctor.

(iii) We should not take antibiotics when not needed or in wrong doses. 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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