SolveItClass 12 · NCERT

NCERT Solutions · Class 12 Biology Microbes in Human Welfare

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Exercises 8.11–8.15 (part 2 of 2)

  1. Exercise 8.11

    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 20mg/L, 8mg/L and 400mg/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?

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    Sample C is the most polluted, with a BOD of $\displaystyle 400$ mg/L.
    Labels
    A ($\displaystyle 20$ mg/L) — secondary effluent discharged from the sewage treatment plant.
    B ($\displaystyle 8$ mg/L) — river water (the cleanest of the three, as assumed).
    C ($\displaystyle 400$ mg/L) — untreated sewage water.
    Reasoning
    BOD is the amount of oxygen that would be consumed if all the organic matter in one litre of water were oxidised by bacteria; it is therefore an indirect measure of the organic matter present.
    The greater the BOD, the greater the polluting potential, so the samples rank in order of pollution as C > A > B.
    Untreated sewage carries the heaviest organic load, hence the highest BOD; secondary treatment is carried on until the BOD is significantly reduced, so the treated effluent sits well below raw sewage but still above relatively clean river water.

  2. Exercise 8.12

    Find out the name of the microbes from which Cyclosporin A (an immunosuppressive drug) and Statins (blood cholesterol lowering agents) are obtained.

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    Cyclosporin A — obtained from the fungus *Trichoderma polysporum*; it is used as an immunosuppressive agent in organ-transplant patients.
    Statins — produced by the yeast *Monascus purpureus*; commercialised as blood-cholesterol lowering agents, acting by competitively inhibiting the enzyme responsible for the synthesis of cholesterol.

  3. Exercise 8.13

    Find out the role of microbes in the following and discuss it with your teacher.
    (a)
    Single cell protein (SCP)
    (b)
    Soil

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    (a)
    Single cell protein (SCP)
    This chapter does not mention single cell protein at all — the exercise sends the student to find it out and discuss it with the teacher.
    Beyond the chapter, the accepted idea: microbial biomass (bacteria, yeasts, algae, filamentous fungi) is grown in bulk and used as a protein-rich food or animal feed — an alternative protein source that multiplies fast on cheap substrates and needs little land. Confirm the named examples with your teacher before writing them.
    (b)
    Soil
    Microbes enrich the nutrient quality of the soil — this is exactly what the chapter calls biofertilisers, whose main sources are bacteria, fungi and cyanobacteria.
    *Rhizobium* in the root nodules of legumes, and free-living *Azospirillum* and *Azotobacter*, fix atmospheric nitrogen into forms plants can use.
    Mycorrhizal fungi (many species of *Glomus*) absorb phosphorus from the soil and pass it to the plant, and confer resistance to root-borne pathogens and tolerance to salinity and drought.
    Cyanobacteria (Anabaena, Nostoc, Oscillatoria) fix nitrogen in paddy fields, and blue green algae add organic matter to the soil and increase its fertility.
    *Trichoderma*, free-living in the root ecosystem, controls several soil-borne plant pathogens.
    The spent slurry from a biogas plant is drawn off and used as fertiliser, returning organic matter to the soil.
    Together these reduce the use of chemical fertilisers and pesticides, which otherwise pollute the soil and ground water.

  4. Exercise 8.14

    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

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    A defensible order: Penicillin > Biogas > Curd > Citric acid.
    Reasons
    Penicillin first: antibiotics are called one of the most significant discoveries of the twentieth century; penicillin and the antibiotics that followed transformed our capacity to treat plague, whooping cough, diphtheria, leprosy and pneumonia, diseases that used to kill millions all over the globe. It saves life, so it outranks the rest.
    Biogas second: a fuel produced from cattle dung and bio-wastes by methanogens, used for cooking and lighting in rural areas, while the spent slurry serves as fertiliser — it meets an energy need and disposes of waste at the same time.
    Curd third: a daily food made by LAB, which improve its nutritional quality by increasing vitamin B₁₂, and which also check disease-causing microbes in our stomach — valuable, but nutrition rather than survival.
    Citric acid last: an industrial chemical produced by Aspergillus niger; useful in food and industry, but the least critical of the four to human welfare.
    Any other order is acceptable if each place is justified from what the chapter says the item does.

  5. Exercise 8.15

    How do biofertilisers enrich the fertility of the soil?

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    Biofertilisers are organisms that enrich the nutrient quality of the soil; the main sources are bacteria, fungi and cyanobacteria.
    How they enrich it
    *Rhizobium*: forms a symbiotic association with the roots of leguminous plants, making nodules, and fixes atmospheric nitrogen into organic forms that the plant uses as nutrient.
    *Azospirillum* and *Azotobacter*: fix atmospheric nitrogen while free-living in the soil, thus enriching its nitrogen content.
    Mycorrhiza: fungi such as many members of the genus *Glomus* form symbiotic associations with plants; the fungal symbiont absorbs phosphorus from the soil and passes it to the plant. Such plants also show resistance to root-borne pathogens, tolerance to salinity and drought, and an overall increase in growth and development.
    Cyanobacteria: autotrophic microbes such as *Anabaena, Nostoc, Oscillatoria*, many of which fix atmospheric nitrogen; in paddy fields they are an important biofertiliser, and blue green algae also add organic matter to the soil and increase its fertility.
    The result: soil nutrients are replenished naturally, so farmers can reduce their dependence on chemical fertilisers and the pollution these cause.