SolveItNCERT · CBSE Boards

NCERT Solutions · Class 12 Biology Microbes in Human Welfare

15 exercises · 15 still being checked

Exercises 8.1–8.10 (part 1 of 2)

  1. Exercise 8.1

    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?

    The book prints no answer for this

    NCERT publishes no answers for this textbook, so there is nothing in the book to check this working against. It has also not yet been read through by hand.

    Carry a small sample of curd from home.
    Curd is made by lactic acid bacteria (LAB) such as Lactobacillus, which grow in milk and convert it to curd.
    Even a spoonful of curd used as an inoculum or starter contains millions of LAB, so a drop mounted on a slide is almost certain to show bacteria.
    Under the microscope they appear as tiny rod-shaped bacteria, which the naked eye cannot resolve.
    A bit of the fermented dough kept for idli or dosa would serve equally well, since that dough too is fermented by bacteria.

  2. Exercise 8.2

    Give examples to prove that microbes release gases during metabolism.

    The book prints no answer for this

    NCERT publishes no answers for this textbook, so there is nothing in the book to check this working against. It has also not yet been read through by hand.

    Yes — several everyday examples show gas being released as microbes metabolise; the gas is mostly CO₂, and in anaerobic conditions methane.
    Dough for dosa and idli: its puffed-up appearance is due to CO₂ produced by the fermenting bacteria.
    Bread dough: fermented by baker's yeast (Saccharomyces cerevisiae), which again puffs the dough up with CO₂.
    'Swiss cheese': the large holes in it are due to a large amount of CO₂ produced by the bacterium Propionibacterium sharmanii.
    Fermented beverages: in dough fermentation, cheese making and beverage production alike, the main gas produced is CO₂.
    Methanogens such as Methanobacterium, growing anaerobically on cellulosic material, produce a large amount of methane along with CO₂ and H₂.
    Anaerobic sludge digesters in a sewage plant: the bacteria there produce a mixture of methane, hydrogen sulphide and carbon dioxide — this mixture is biogas.

  3. Exercise 8.3

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

    The book prints no answer for this

    NCERT publishes no answers for this textbook, so there is nothing in the book to check this working against. It has also not yet been read through by hand.

    Lactic acid bacteria (LAB) are found in curd, where they grow in milk and convert it to curd; a little curd added to fresh milk as a starter carries millions of them.
    They also occur naturally in our stomach.
    Useful applications
    They produce acids that coagulate and partially digest the milk proteins, converting milk into curd.
    They improve the nutritional quality of the product by increasing vitamin B₁₂.
    In our stomach they play a beneficial role in checking disease-causing microbes.
    Lactobacillus is used for the commercial production of lactic acid.

  4. Exercise 8.4

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

    The book prints no answer for this

    NCERT publishes no answers for this textbook, so there is nothing in the book to check this working against. It has also not yet been read through by hand.

    Bread — dough of wheat flour fermented by baker's yeast, Saccharomyces cerevisiae.
    Idli and dosa — made from a dough (rice and gram) that is fermented by bacteria; the CO₂ they release gives the dough its puffed-up appearance.
    These are the traditional foods of this kind that the chapter names. The other fermented items it mentions — 'toddy' from palm sap, and fermented fish, soyabean and bamboo-shoots — are not made from wheat, rice or Bengal gram.
    Note: the chapter names the foods but does not spell out which grain goes into each dough.

  5. Exercise 8.5

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

    The book prints no answer for this

    NCERT publishes no answers for this textbook, so there is nothing in the book to check this working against. It has also not yet been read through by hand.

    By giving us antibiotics, regarded as one of the most significant discoveries of the twentieth century.
    Antibiotics are chemical substances produced by some microbes that kill or retard the growth of other, disease-causing microbes.
    Penicillin, the first antibiotic, was a chance discovery: Alexander Fleming saw a mould on an unwashed culture plate around which Staphylococci would not grow, and named the chemical after the mould Penicillium notatum.
    Ernest Chain and Howard Florey established its full potential; it was used extensively on American soldiers wounded in World War II, and the three shared the Nobel Prize in $\displaystyle 1945$.
    Antibiotics greatly improved our capacity to treat deadly bacterial diseases such as plague, whooping cough, diphtheria, leprosy and pneumonia, which used to kill millions across the globe.

  6. Exercise 8.6

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

    The book prints no answer for this

    NCERT publishes no answers for this textbook, so there is nothing in the book to check this working against. It has also not yet been read through by hand.

    *Penicillium notatum* and *Penicillium chrysogenum*.
    Penicillium notatum is the mould of the chapter's own account: Alexander Fleming saw it growing on an unwashed Staphylococci culture plate and saw the bacteria around it fail to grow. He named the chemical responsible Penicillin, after the mould.
    Its full potential as an antibiotic was established later by Ernest Chain and Howard Florey, and penicillin was used to treat wounded soldiers in the Second World War. Fleming, Chain and Florey shared the Nobel Prize in 1945.
    Penicillium chrysogenum is the species used for industrial penicillin production.

  7. Exercise 8.7

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

    The book prints no answer for this

    NCERT publishes no answers for this textbook, so there is nothing in the book to check this working against. It has also not yet been read through by hand.

    Sewage is the municipal waste water generated every day in cities and towns; its major component is human excreta.
    It contains large amounts of organic matter and microbes, many of which are pathogenic.
    How it is harmful
    Being loaded with pathogens, it is a direct source of water-borne diseases.
    It cannot be discharged into rivers and streams as it is; untreated sewage discharged directly leads to river pollution and an increase in water-borne diseases.
    Its heavy organic load gives it a high BOD, and the greater the BOD of waste water, the more is its polluting potential — the organic matter is oxidised by bacteria, using up the oxygen of the water body.
    This is why sewage is treated in sewage treatment plants (STPs) before disposal, and why the Ganga Action Plan and Yamuna Action Plan propose building many more of them.

  8. Exercise 8.8

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

    The book prints no answer for this

    NCERT publishes no answers for this textbook, so there is nothing in the book to check this working against. It has also not yet been read through by hand.

    The key difference: primary treatment is a physical process, secondary treatment is a biological (microbial) one.
    (a)
    Primary treatment
    Involves the physical removal of particles, large and small, from the sewage by filtration and sedimentation.
    Floating debris is removed by sequential filtration, then grit (soil and small pebbles) by sedimentation.
    All the solids that settle form the primary sludge; the supernatant is the effluent, which goes on to secondary treatment.
    (b)
    Secondary (biological) treatment
    The primary effluent is passed into large aeration tanks, agitated mechanically while air is pumped in.
    This lets useful aerobic microbes grow vigorously as flocs — masses of bacteria associated with fungal filaments in a mesh-like structure.
    While growing, these microbes consume the major part of the organic matter, which significantly reduces the BOD of the effluent.
    The flocs are then sedimented in a settling tank as activated sludge; a small part is pumped back to the aeration tank as inoculum, the rest goes to anaerobic sludge digesters, where biogas is produced.

  9. Exercise 8.9

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

    The book prints no answer for this

    NCERT publishes no answers for this textbook, so there is nothing in the book to check this working against. It has also not yet been read through by hand.

    Yes — microbes are a source of energy in the form of biogas, a mixture of gases containing predominantly methane, which is inflammable and is used as fuel.
    How it is produced
    Certain bacteria that grow anaerobically on cellulosic material produce a large amount of methane along with CO₂ and H₂; they are collectively called methanogens, a common one being *Methanobacterium*.
    Methanogens occur in the anaerobic sludge of sewage treatment, and in the rumen of cattle, where they break down cellulose and help in the nutrition of cattle.
    Cattle excreta (gobar) is therefore rich in these bacteria and is used to generate biogas, commonly called gobar gas.
    The biogas plant
    A concrete tank $\displaystyle 10$–$\displaystyle 15$ feet deep is fed with bio-wastes and a slurry of dung.
    A floating cover over the slurry keeps rising as gas is produced by microbial activity.
    An outlet connected to a pipe supplies the biogas to nearby houses for cooking and lighting; the spent slurry is removed through another outlet and used as fertiliser.
    Such plants are built mostly in rural areas, where cattle dung is plentiful. The technology was developed in India largely by IARI and KVIC.

  10. Exercise 8.10

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

    The book prints no answer for this

    NCERT publishes no answers for this textbook, so there is nothing in the book to check this working against. It has also not yet been read through by hand.

    By replacing them with biofertilisers (for nutrients) and biocontrol agents (for pests and diseases).
    (a)
    Replacing chemical fertilisers — biofertilisers
    Biofertilisers are organisms that enrich the nutrient quality of the soil; the main sources are bacteria, fungi and cyanobacteria.
    *Rhizobium*, in the root nodules of leguminous plants, fixes atmospheric nitrogen into organic forms the plant can use.
    *Azospirillum* and *Azotobacter* fix atmospheric nitrogen while free-living in the soil, enriching its nitrogen content.
    Mycorrhiza — many members of the genus *Glomus* — absorb phosphorus from the soil and pass it to the plant, and also give resistance to root-borne pathogens, tolerance to salinity and drought, and an overall increase in growth.
    Cyanobacteria such as Anabaena, Nostoc and Oscillatoria fix atmospheric nitrogen and are important biofertilisers in paddy fields; blue green algae also add organic matter to the soil.
    (b)
    Replacing pesticides — biocontrol
    Biocontrol is the use of biological methods, relying on natural predation instead of introduced chemicals, to control plant diseases and pests.
    *Bacillus thuringiensis* (Bt) is sold as sachets of dried spores; mixed with water and sprayed on brassicas and fruit trees, it is eaten by insect larvae, and the toxin released in their gut kills them while leaving other insects unharmed.
    Bt toxin genes have been introduced into plants by genetic engineering, giving pest-resistant crops such as Bt-cotton.
    *Trichoderma*, free-living fungi common in root ecosystems, are effective biocontrol agents of several plant pathogens.
    Baculoviruses (genus *Nucleopolyhedrovirus*) are species-specific, narrow-spectrum insecticides with no negative impact on plants, mammals, birds, fish or non-target insects — valuable in integrated pest management (IPM).
    Ladybird beetles and dragonflies get rid of aphids and mosquitoes respectively.
    The gain: chemical insecticides, pesticides and weedicides are toxic to human beings and animals and pollute soil, ground water, fruits, vegetables and crop plants; biocontrol greatly reduces our dependence on them.