NCERT Solutions for Class 12 Biology Chapter 10: Microbes in Human Welfare

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    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 with the help of a microscope, which sample would you carry and why?

    Ans. Soil sample/water sample as they are the natural habitat of micro-organisms. Tap water contains many kinds of bacteria by (≈80,000 bacteria/ml) which can be observed under microscope by staining process.

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

    Ans.There are lots of examples which prove, that microbes release gases during their metabolism. Some examples are as follows :
    (i) Dough, which is used for making dosa and idli is fermented by bacteria. The puffy appearance of dough is due to production of CO2 by fermentation process.
    (ii) Swiss cheese is characterized by having the characteristic flavour and large holes. The large holes are formed due to the amount of CO2 released by a bacterium Propionibacterium sharmanii.

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

    Ans. Milk, Curd and Cheese are coagulated product where lactic acid bacteria found.
    Useful Applications:
    (i) Lactic acid bacteria (LAB) convert lactose sugar into lactic acid. They also improve the nutritional quality of curd by increasing vitamin B12.
    (ii) Lactic acid bacteria are also found in the stomach where they check disease causing microbes.

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

    Ans.Some important traditional dishes made up of wheat, rice and Bengal gram or black gram (vernacular urad) are as follows :
    (i) Bread: It is prepared by fermentation of wheat using yeast Saccharomyces cerevisiae.
    (ii) Dosa, upma and idli: They are fermented preparation of rice and black gram (urad). The two are allowed to ferment for 3-12 hrs with Leuconostoc and Streptococcus species of bacteria.

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

    Ans. Microbes have played a major role in controlling diseases caused by harmful bacteria by producing a chemical substance called antibiotic that can be used to kill or retard the growth of disease causing bacteria Ex-Streptomycin from streptomyces bacteria.

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

    Ans. Penicillium chrysogenum, P. notatum, produce penicillin and Cephalosporium produces cephalosporins.

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

    Ans.Sewage is waste water release from household and industrial applications. It is harmful as:
    (i) It contains large amount of organic matter and many pathogenic microbes.
    (ii) When sew age enters a water stream it destroys or pollutes water body.
    (iii) dissemination of pathogenic bacteria.

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

    Ans. Primary treatment is a physical process removes large impurities while secondary is a biological process that remove organic matter of sewage through the action of microbes.

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

    Ans.Yes, microbes can be used us a source of energy, methanogenic bacteria are used for the production of biogas.
    These bacteria under anaerobic condition convert carbon dioxide and hydrogen into methane gas (65%). The hydrogen serves as an electron above for the reduction of CO2.

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

    Ans. Microbes can be used to decrease the use of chemical fertilizers and pesticides. This can be accomplished by the use of biofertilizers. Biofertilizer are organisms that enrich the nutrient quality of soil. The main source of biofertilizers are bacteria, fungi and cyanobacteria.
    (i) Use of leguminous plant with Rhizobium.
    (ii) Use of sulphur fixing bacteria.
    (iii) Use of hydrogen fixing bacterium.

    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/Land 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.BOD values indicates the amount of organic wastes present in the matter. The greater the BOD of waste water, more is its polluting potential. On the basis of this we can conclude that:
    (i) Sample C is of untreated sew age water has highest quantities of organic matter and so has highest BOD value (i.e., 400mg/L).
    (ii) Sample B is of water of secondary effluent chamber of STP because it comes after primary treatment and with less organic matter than sewage water so it has highest BOD value (i.e., 20mg/L).
    (iii) Sample A is of river water because it has minimum BOD value (i.e., 8mg/L). Thus, from given 3 samples, sample C will be more polluted because having high BOD value and organic load.

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

    Ans. Cyclosporin A (an immuno suppressive drug) is obtained from fungus Trichoderma polysporum while statins (blood cholesterol lowering agent) is obtained from yeast Monascus purpureus.

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

    Ans. NSCP: single cell protein is microbial yield/cell crop of bacterial, yeast, algae rich in protein. The protein content of microbial cell is very high. Dried cell of Pseudomonas grown on petroleum product has 69% protein and these proteins have all essential amino acids.
    Soil: Microbes take part in formation and maintenance of soil. They add organic matter to freshly formed soil. The process is called humification. Some microbes act as biofertilizers and biopesticides. 

    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.

    Ans. Curd: Less important for society – as it depends on individual use and has only nutritions value.
    Citric acid: Industrial use, not for dissipation in community.
    Penicillin: Medicinal use of microbes, good for health of society, commercially more usable.
    Biogas: Most important for community welfare as:
    (i) It reduces excreta, waste from community.
    (ii) It produces inflammable gases, can be used as energy source.
    (iii) It is a renewable source.
    (iv) It has multidimensional utility.
    (v) It is easily maintained and dissipated for community purpose.
    So, Penicillin > Biogas > Curd > Citric acid.

    15. How do biofertilisers enrich the fertility of the soil?

    Ans. Biofertilisers enrich the fertility of soil in following ways:
    (i) Manures: They are semi-decayed organic remains of various types– formyard manure, green manure, compost and vermicomposite. As the manure decay, the release of nutrient takes place.
    (ii) Nitrogen fixing organisms: Certain bacteria and cyanobacteria are capable to converting atmospheric nitrogen to its salts.
    (iii) Mycorrhiza: It is able to pick up inorganic nutrients from organic matter

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