top of page




Question 1: Bacteria cannot be seen with the naked eyes, but these can be seen with the help of a microscope.  If you have to carry a sample from your home to your biology laboratory to demonstrate the presence of microbes under a microscope, which sample would you carry and why?

Ans: The most common household product that we would like to carry is curd which contains lactic acid bacteria ( sps.).


Question 2: Give examples to prove that microbes release gases during metabolism.

Ans: Puffed-up appearance of dough which is used for making 'dosa', 'idli' and bread is due to gas (CO2) production. Methanogens (bacteria) in the biogas plant produce methane and carbon dioxide. Also, large holes in the 'Swiss cheese' are due to production of a large amount of CO2 during its production.


Question 3: In which food would you find lactic acid bacteria? Mention some of their useful applications.

Ans: The lactic acid bacteria (LAB) are found in curd.

  • LAB convert the lactose sugar of milk into lactic acid. Lactic acid coagulates the milk protein called casein.

  • It also increases the nutritional quality of curd as the curd contains vitamin B12 along with other vitamins.

  • They also check the disease-causing microbes in stomach.


Question 4: Name some traditional Indian foods made of wheat, rice and Bengal gram (or their products) which involve use of microbes.

Ans: 'Dosa' and 'idli' (from rice), bread (from wheat) and 'dhokla' (from Bengal gram) are the traditional Indian foods which involve use of microbes.


Question 5: In which way have microbes played a major role in controlling diseases caused by harmful bacteria?

Ans: The major role of microbes in controlling the diseases is the 'antibiotic production'. Antibiotics have been used against pathogenic bacteria, e.g., penicillin from , Streptomycin from , etc.


Question 6: Name any two species of fungus, which are used in the production of the antibiotics.

Ans: Two species of fungus, used in the production of the antibiotics:

  1. Penicillium notatum (for penicillin production).

  2. Aspergillus fumigatus (for fumagillin production).


Question 7: What is sewage? In which way can sewage be harmful to us?

Ans: Sewage is the municipal waste water containing large quantities of human excreta and other organic wastes. Sewage could be harmful to us as it contains many pathogenic microbes and produces foul smell. It is the cause of many water-borne diseases. It is also the cause of eutrophication of water bodies thereby killing many aquatic organisms.


Question 8: What is the key difference between primary and secondary sewage treatment?

Ans: The key difference between primary and secondary treatment of sewage is that primary treatment is the physical process of removing grit and large pieces of organic matter while secondary treatment is a biological process that involves digestion of organic matter by microbes.


Question 9: Do you think microbes can also be used as source of energy? If yes, how?

Ans: Yes, microbes can be used to produce energy indirectly. Methanogens (bacteria) like are involved in the production of biogas which is commercially used as source of energy.


Question 10: Microbes can be used to decrease the use of chemical fertilizers and pesticides. Explain how this can be accomplished.

Ans: Microbes can be used both as fertilizers and pesticides called biofertilizers and biopesticides, respectively.

Microbes are used as biofertilizers to enrich the soil nutrients, e.g., , , etc., which can fix atmospheric nitrogen in the soil.

Bacillus thuringiensis bacteria act as biopesticide to control the growth of insect pests.

Trichoderma, fungal species, is an effective biocontrol agent of several plant pathogens.

Baculoviruses used as biological control agents in genus Nucleopolyhedrovirus are excellent for species-specific, narrow spectrum insecticidal applications.

Question 11: Three water samples namely river water, untreated sewage water and secondary effluent discharged from a sewage treatment plant were subjected to BOD test. The samples were labelled A, B and C; but the laboratory attendant did not note which was which. The BOD values of the three samples A, B and C were recorded as 20 mg/L, 8 mg/L and 400 mg/L, respectively. Which sample of the water is most polluted? Can you assign the correct label to each assuming the river water is relatively clean?

Ans: Sample C is most polluted (Highest BOD).

Sample A — River water

Sample B — Secondary effluent (Least BOD)

Sample C — Untreated sewage (Highest BOD)


Question 12: Find out the name of the microbes from which cyclosporin A (an immuno-suppressive drug) and statins (blood cholesterol lowering agents) are obtained.


  1. Cyclosporin A is obtained from Trichoderma polysporum.

  2. Statins are obtained from the yeast Monascus Purpureus.


Question 13: Find out the role of microbes in the following and discuss it with your teacher:

  1. Single cell protein (SCP)

  2. Soil.


  1. Single cell protein (SCP): It is an alternative protein source for animal and human nutrition (protein-rich microbial biomass which can be used as food) from certain beneficial microbe like Spirulina. SCP contains essential amino acids and low fat. Bacteria, filamentous fungi, algae, yeast, etc., are used as "single cell proteins" (SCPs). Spirulina is taken as a tablet having 60 per cent proteins, all minerals, vitamins, etc.

  2. Soil: Soil is the habitat of numerous microbes. Microbes in the soil increase the fertility of soil by decomposing organic matter. Some microbes convert nitrates into free nitrogen that escapes into atmosphere for replenishment.


Question 14: Arrange the following in the decreasing order (most important first) of their importance, for the welfare of human society. Give reasons for your answer.

Biogas, Citric acid, Penicillin and Curd.


  1. Penicillin: It is an antibiotic used in curing numerous bacterial diseases.

  2. Biogas: It is a source of energy in rural areas, produced by anaerobic degradation of organic matter.

  3. Curd: It is vitamin-rich milk preparation which is easily digested.

  4. Citric acid: It is an organic acid used as preservative in juices, jams and jellies, etc.



Question 15: How do biofertilizer enrich the fertility of the soil?

Ans: Biofertilizers are microorganisms which bring about nutrient enrichment of soil by enhancing the availability of nutrients to crops. They are of following types:

  1. Nitrogen fixing bacteria and cyanobacteria: They form symbiotic association with plants. They get food and shelter form plants and on the other hand, plants get nitrogen faxed by these bacteria, For example, Rhizobium.

  2. Mycorrhiza: It is an association between a fungus and roots of higher plants. It takes part in the solubilization and absorption of nutrients from organic matter. Many members of the genus Glomus form Mycorrhiza.

  3. Manures: They are semi-decayed organic remains  of various types – manures, green manure, compost and vermicompost,


MHW ex
  1. Which bacteria is commonly found in the anaerobic sludge during sewage treatment? (1 mark)

  2. Explain the biogas plant with a neat labelled diagram. (5 marks)

  3. Write the scientific name of the source organism for citric acid. (1 marks)

  4. a) Explain the role of three organisms as biocontrol agents. (3)
    b) What is the significance of BOD? (1)
    c) Give an example for a fungus found in mycorrhiza. (1)

  5. Mention the role of Methanobacterium in rumen of cattle. (1)

  6. Describe the role of Microbes in sewage treatment plant (5)

  7. What are microbial flocs? (1)

  8. Mention any five microbes and their commercial or industrial products. (5)

  9. Name the bio active molecule used in organ transplantation. (1)

  10. Give an account of microbes used as biofertilizers. (5)

  11. Write the role of microbes in household food products. (5)

  12. Define BOD. (1)

bottom of page