Class 8 - Biology - Microorganisms: Friend and Foe

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 __________.



(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 microorganisms.

(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 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



(a) Alcohol

(b) Streptomycin

(c) Female Anopheles mosquito

(d) Housefly

(e) Growth of yeast cells

(f) Fermentation


 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



(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



Question 4:

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



No, microorganisms cannot be seen by naked eye as they are very small and are called microbes. They can be seen with the help of microscope.



Question 5:

What are the major groups of microorganisms?



Microorganisms are classified into four major groups are bacteria, fungi, protozoa and some algae. They vary in size and mode of nutrition.


  • Further divided into archaebacteria and eubacteria.
  • Archaebacteria-They are composed of a single cell and are called prokaryotes. Archaebacteria are only found in hot boiling water or other types of extreme environment.
  • Eubacteria- a bacterium of a large group typically having simple cells with rigid cell walls and often flagella for movement.


  • Heterotrophic multicellular organisms.
  • It includes microorganisms such as yeasts and molds, as well as the more familiar mushrooms.
  • The mode of formation and shape of these spores is traditionally used to classify molds.
  • Many of these spores are colored, making the fungus much more obvious to the human eye at this stage in its life-cycle


  • Unicellular eukaryotic organisms.
  • They are often grouped in the kingdom Protista together with the plant-like algae and fungus-like water molds and slime molds.
  • Protozoa are dangerous parasites.


  • Large, diverse group of photosynthetic organisms.
  • Any of numerous groups of chlorophyll-containing, mainly aquatic eukaryotic organisms ranging from microscopic single-celled forms to multicellular forms.
  • "Algae" covers many different organisms capable of producing oxygen through photosynthesis (the process of harvesting light energy from the sun to generate carbohydrates).


Question 6:

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



 Bacteria such as rhizobium and certain blue-green algae present in the soil can fix atmospheric nitrogen and convert into usable nitrogenous compounds,

which are used by plants for the synthesis of plant proteins and other compounds.


 Question 7:

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



Microorganisms are too small to be seen through naked eyes. However, they are vital to plants and the environment. Importance of microorganisms:

  1. They are used in winemaking, baking, pickling and other food making processes.
  2. Alcoholic fermentation by yeast is widely used in the preparation of wine and bread.
  3. A bacterium lactobacillus promotes the formation of curd.
  4. Microbes are used to reduce pollution.
  5. They are used to increase the soil fertility by fixing the atmospheric nitrogen.
  6. Microbes are also useful in preparing many medicines and antibiotics.
  7. Certain microbes are also used in the biological treatment of sewage and industrial effluents.


 Question 8:

Write a short paragraph on the harms caused by microorganisms.



Microorganisms are harmful in many ways.

  • Some of the microorganism cause diseases in human beings, plants and animals. Such disease-causing microorganisms are called pathogens.
  • Some microorganisms spoil food, clothing and leather.
  • Some of the common diseases affecting humans are cholera, common cold, chicken pox and tuberculosis.
  • Several microorganisms not only cause diseases in humans but also in animals. For example, anthrax is a dangerous human and cattle disease caused by a bacterium.
  • Disease causing microorganisms in plants like wheat, rice, potato, sugarcane, orange, apple and others reduce the yield of crops.



Question 9:

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



The medicines that kill or stop the growth of the bacteria are called antibiotics.

They are used only against bacterial infections. For example: Streptomycin, tetracycline, etc.

Following precautions must be taken while taking antibiotics:

  • Antibiotics should be taken only on the advice of a qualified doctor.
  • One must finish the course prescribed by the doctor.
  • Antibiotics must be avoided when not needed or in wrong doses.

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