Microbes in Human Welfare Class 12 Biology Exam Questions

Please refer to Biology For Microbes in Human Welfare Class 12 Biology Exam Questions provided below. These questions and answers for Class 12 Biology have been designed based on the past trend of questions and important topics in your class 12 Biology books. You should go through all Class 12 Biology Important Questions provided by our teachers which will help you to get more marks in upcoming exams.

Class 12 Biology Exam Questions Microbes in Human Welfare

Case-based MCQs

Read the following text and answer the following questions on the basis of the same :
Attempt any four sub-parts from each question. Each sub-part carries 1 mark.

I. Read the following and answer any four questions below :
Villagers in a place near Chambur started planning to make power supply for agricultural purpose from cow dung. They have started a biogas plant for the purpose. Study the flow chart for biogas production given below and answer the following questions.  (Image 70)

Question. What is represented by ‘B’ in the flow chart?
(A) Carbohydrates
(B) Protein polymers
(C) Organic acids
(D) Fat globules
Answer : (C)

Question. In the given flow chart, ‘A’ denotes :
(A) aerobic bacteria
(B) methanogenic bacteria
(C) cellulose degrading bacteria
(D) yeast and protozoa
Answer : (B)

Question. If ‘A’ is not added in the procedure
(A) methane will not be formed
(B) CO₂ will not be formed
(C) organic compounds will not be converted to H2S
(D) O₂ will not be formed
Answer : (A)

Question. Which is the first organic acid produced by microbial fermentation ?
(A) Acetic acid
(B) Lactic acid
(C) Tartaric acid
(D) Citric acid.
Answer : (C)

Question. Biogas is composed of majorly :
(A) methane, CO₂ and O₂
(B) CO2, H2S, and H
(C) methane, CO2
(D) H2S, H and O2
Answer : (C)

Read the following text and answer any four questions on the basis of the same :

Ecological Indicators :
The presence of dragonflies can reveal changes in the water ecosystems more quickly than studying other animals or plants. In fact, from the nymph to the adult stage, the dragonfly has a significant,positive ecological impact. Dragonfly eggs are laid and hatched in or near water, so their lives show impact both on water and land ecosystems. Once hatched, dragonfly nymphs can breathe in the air or underwater which enables them to eat mosquito larvae, other aquatic insects and worms, and even small aquatic vertebrates like tadpoles and small fish. Adult dragonflies capture and eat adult mosquitoes.
Community wide mosquito control programs that spray insecticides to kill adult mosquitoes also kill dragonflies.

Question. The most effective stages in the life cycle of dragonfly that eradicates mosquitoes are.
(A) Larvae and adult
(B) Caterpillar and adult
(C) Nymph and adult
(D) Pupa and adult
Answer : (C)

Question. Dragonflies indicate positive ecological impact as :
(A) The presence of dragonflies indicates polluted water.
(B) Dragonfly nymphs selectively eat mosquito larvae.
(C) They help to decrease the probability of diseases spread by vectors.
(D) Dragonfly do not cause any harm to beneficial species.
Answer : (C)

Question. The approach to biological control includes :
(A) Import and release of an insect pest to a new area to provide hosts for natural enemies.
(B) Import and release of natural enemies from the native home of an alien insect pest that has invaded a new area.
(C) Preservation of natural enemies (predators & parasites) that are already established in an area.
(D) Use of insecticides to reduce alien insect pests to establish new equilibrium position.
Answer : (C)

Question. Assertion (A) : Releasing dragonflies in areas where there is an outbreak of malarial diseases can be an environment friendly method of control.
Reason (R) : Dragonflies are dominant species and will not allow mosquitoes to reproduce.
(A) Both assertion and reason are true, and the reason is the correct explanation of the assertion.
(B ) Both assertion and reason are true, but the reason is not the correct explanation of the assertion.
(C) Assertion is true but reason is false.
(D) Assertion is false but reason is true.
Answer : (B)

Question. Two diseases less likely to occur in a region with plenty of dragonflies are_____
(A) Yellow fever and amoebic dysentery
(B) Malaria and Yellow fever
(C) Anthrax and typhoid
(D) Cholera and typhoid
Answer : (B)

Read the following text and answer any four questions on the basis of the same :

Large quantities of sewage is generated every day in cities and towns, which is treated in Sewage Treatment Plants (STPs) to make it less polluted.
Given below is the flow diagram of one of the stages of STP.
Observe the given flow diagram and answer the questions accordingly. (Image 71)

Question. Identify the correct set of gases produced during digestion of sludge :
(A) methane, hydrogen sulphide and carbon dioxide
(B) methane, sulphur and carbon dioxide
(C) carbon monooxide, methane, hydrogen sulphide
(D) methane, hydrogen sulphide and nitrogen
Answer : (A)

Question. Technical term used for sediment formed in sewage treatment is :
(A) Flocs
(B) Effluents
(C) Activated sludge
(D) Anaerobic sludge
Answer : (C)

Question. Primary effluent is passed into large aeration tanks :
(A) To allow bacterial flocs to settle down
(B) To allow fast growth of useful aerobic microbes into flocs
(C) To allow anaerobic sludge digestion
(D) None of these.
Answer : (B)

Question. Which of the following in sewage treatment removes suspended solids?
(A) Tertiary treatment
(B) Secondary treatment
(C) Primary treatment
(D) Sludge treatment
Answer : (C)

Question. The significance of the above sediment formed is that :
(A) It acts as inoculum
(B) It serves in formation of flocs
(C) It helps in formation of natural water bodies
(D) It helps in anaerobic sludge digestion
Answer : (A)

Very Short Answer Type Questions

Question. Why do we add an inoculum of curd to milk for curdling it ?
Answer : The inoculum contains Lactobacilli which curdles milk into curd with lactic acid. Lactobacillus converts lactose sugar of milk into lactic acid which causes coagulation and partial digestion of milk protein casein. As a result, milk gets changed into curd.

Question. What are ‘flocs’, formed during secondary treatment of sewage ? 
Answer : Masses of bacteria associated with fungal filament (to form mesh like structure).

Question. How is lactic acid bacteria beneficial to us other than helping in curdling the milk ?
Answer : Lactic acid bacteria improves the nutritional quality by increasing Vitamin B12.

Question. Name any two free nitrogen fixing bacteria.
Answer : Azotobacter and Azospirillum.

Question. Write any two places where methanogens can be found.
Answer : Anaerobic sludge (digester), rumen of cattle / ruminants / stomach of cattle / gut of cattle, marshy area, flooded rice field, biogas plant.

Question. Large holes in ‘Swiss-Cheese’ are due to which micro-organism.
Answer : Large holes in ‘Swiss-Cheese’ are due to Propionibacterium shermanii.

Question. Name the organism which forms symbiotic association with plants and helps them in their nutrition.
Answer : Glomus, is a genus of arbuscular mycorrhizal fungi which form a symbiotic relationship with plant roots.

Short Answer Type Questions

Question. Make a list of three household products along with the names of the microorganisms producing them.
Answer : The three household products along with the names of the microorganisms producing them are as given below Items Microorganisms
(i) Curd – Lactobacillus
(ii) Bread – Saccharomyces cerevisiae
(iii) Swiss cheese – Propionibacterium shermanii

Question. Name two organisms belonging to two different kingdoms that are commonly used as biofertilisers and how ? 
Answer : The two microorganisms belonging to two different kingdoms that are commonly used as biofertiliser are 1. Fungi
Many members of genus–Glomus form mycorrhiza. Plants with mycorrhizal association show benefits such as
(i) Resistance to root-borne pathogens.
(ii) Tolerance to salinity and drought.
(iii) Increase in plant growth and development.
2. Bacteria
Nitrogen-fixing bacteria fix atmospheric nitrogen into organic form, which is used by the plant as nutrient, e.g.
Rhizobium is a symbiotic bacterium that lives in the root nodules of legumes and fixes atmospheric nitrogen into organic compounds. Azotobacter and Azospirillum are free-living bacteria, which absorb free nitrogen from the soil, air and convert it into salts of nitrogen compounds.

Question. Choose any three microbes, from the following, which are suited for organic farming which is in great demand these days for various reasons.
Mention one application of each one chosen.
Mycorrhiza, Monascus, Anabaena, Rhizobium Methanobacterium, Trichoderma 
Answer : The three microbes that can be chosen for organic farming are
(i) Rhizobium The nodules on the roots of leguminous plants are formed by the symbiotic association of Rhizobium bacteria. These bacteria fix atmospheric nitrogen into organic forms, which is used by the plants as nutrient.
(ii) Mycorrhiza Fungi form symbiotic association with plants called mycorrhiza. The fungal symbiont absorbs phosphorus from soil and passes it to the plant. Such plant shows resistance to root-borne pathogens, tolerance to salinity and drought and overall increase in plant growth and development.
(iii) Anabaena It is a cyanobacteria that is used as a biofertiliser. It fixes atmospheric nitrogen.

Question. Write a note on fermentation by microbes and its applications.
Answer : Fermentation is the process of conversion of carbohydrates to alcohol and CO2 by some microorganisms in the absence ofO2. Microbes via fermentation are utilised for the synthesis of a number of products valuable for human beings.
Some of the applications of fermentation by microbes are
(i) Production of bread using baker’s yeast.
(ii) Microbes are used to ferment fish, soybean, bamboo shoots, etc.
(iii) Different varieties of cheese are produced by fermentation via microbes.
(iv) Wine, beer and other alcoholic drinks are also produced by fermentation.
(v) Vinegar is also produced by fermentation.

Question. In which way, microbes have played a major role in controlling diseases caused by harmful bacteria?
Answer : Microbes play major role in controlling diseases by harmful bacteria as they are used in preparing medicines.
The most important medicines for which microbes are used, are antibiotics. These are chemical compounds produced by some microbes, which are used to kill or to subside the growth of other diseases causing microbes.
Streptomycin, tetracycline and penicillin are some of the commonly used antibiotics. Antibiotics have significantly contributed towards the welfare of the human society.

Question. Read the statements given below and identify them as true or false. Also, correct the statements which are false.
(i) Lactobacillus acidophilus is used in fermentation of milk.
(ii) LAB is grown on molasses and sold as a food flavouring agent.
(iii) Ernst Chain and Howard Florey discovered antibiotic penicillin.
Answer : (i) True
(ii) False, yeast is grown on molasses and sold as a flavouring agent because it produces a characteristic smell and taste in food.
(iii) False, Alexander Fleming discovered penicillin.
Ernst Chain and H Florey discovered the full potential of this antibiotic.

Question. (i) What would happen if a large volume of untreated sewage is discharged into a river?
(ii) In what way, anaerobic sludge diagestion is important in sewage treatments?
Answer : (i) If untreated sewage is discharged directly into rivers it will lead to serious pollution of the waters with organic matter and pathogenic bacteria, Protozoa and many other diseases.
This water, if used, will cause outbreaks of water borne diseases.
(ii) In anaerobic sludge digestion, anaerobic bacteria, digest the aerobic bacteria and the fungi in the sludge and the remaining organic matter.
During this digestion, bacteria produce a mixture of gases such as methane, hydrogen sulphide and carbon dioxide. These gases (biogas) can be used as source of energy as it is inflammable.

Question. Do you think microbes can be used as a source of energy? If yes how? 
Answer : Yes, microbes can be used as a source of energy, e.g. methanogens like Methanobacterium are used to produce biogas, which is a source of energy.

Question. Name the enzyme produced by Streptococcus bacterium. Explain its importance in medical sciences.
Answer : Streptokinase enzyme is produced by the bacterium, Streptococcus. It is modified by genetic engineering and is used as a clot buster for removing clots from the blood vessels of patients who have suffered from myocardial infarction.

Question. In which food would you find lactic acid bacteria? Mention some of their useful applications.
Answer : Generally, Lactic Acid Bacteria (LAB) are found in milk and its products.
Some useful applications of LAB are n LAB play very beneficial role in checking disease causing microbes in our stomach. n They also improve nutritional quality by increasing vitamin-B 12 content of the curd.

Question. Why are some molecules called bioactive molecules? Give two examples of such molecules.
Answer : Bioactive molecules are produced from microbes that are useful to other living organisms in modifying their metabolism, e.g. streptokinase, cyclosporin-A, statins, etc.

Question. Find out the name of microbes from which cyclosporin-A and statins are obtained.
Answer : (i) Cyclosporin-A is obtained from the fungus, Trichoderma polysporum.
(ii) Statins is obtained from yeast, Monascus purpureus.

Question. What is the pathogenic property of baculovirus, used as a biological agents ?
Name the genus of these organisms.
Answer : Attack insects and other arthropods.
Nucleopolyhedrovirus.

Question. Why does the insecticidal protein produced by Bacillus thuringiensis not kill the bacterium, but kills the cotton bollworm? Explain.
Answer : The Bt toxin protein exists as inactive protoxins, it becomes active due to the alkaline pH of the gut of cotton bollworm.

Question. “Large scale cultivation of Spirulina is highly advantageous for human population.”Explain giving two reasons.
Answer : Source of good protein, fats, carbohydrates, minerals and vitamins for undernourished humans and animal population, reduce environmental pollution / can be grown in wastewater / molasses / straw / animal manure, easy to cultivate.

Question. Your advice is sought to improve the nitrogen content of the soil to be used for cultivation of a non-leguminous terrestrial crop.
(a) Recommend two microbes that can enrich the soil with nitrogen.
(b) Why do leguminous crops not require such enrichment of the soil?
Answer : (a) Azospirillum/ Azotobacter/ Anabaena/ Nostoc / Oscillatoria /Frankia (Any two correct names of microbes).
(If cyanobacteria mentioned=½, but if along with cyanobacteria-Anabaena / Nostoc / Oscillatoria mentioned then no mark on cyanobacteria).
(b) They can fix atmospheric nitrogen, due to presence of Rhizobium/N2 fixing bacteria in their root nodules.

Question. Name a bioactive molecule, its source organism and the purpose for which it is given to organ transplant patients.
Answer : Cyclosporin A.
Source – Trichoderma polysporum.
Purpose – Immunosuppressive agent.

Question. Mention a product of human welfare obtained with the help of each one of the following microbes :
(i) LAB
(ii) Saccharomyces cerevisiae
(iii) Propionibacterium shermanii
(iv) Aspergillus niger
Answer : (i) Milk to curd
(ii) Bread / ethanol / alcoholic drinks / whisky / brandy/ beer/ rum
(iii) Swiss cheese
(iv) Citric acid

Question. Name a free-living symbiotic bacterium that serve as bio-fertilizer. Why are they so called?
Answer : Azospirillum/Azotobacter, Rhizobium. They enrich soil nutrient/nitrogen fixation.

Question. Name the microbes that help the production of the following products commercially :
(i) Statins
(ii) Citric acid
(iii) Penicillin
(iv) Butyric acid
Answer : (i) Monascus purpureus
(ii) Aspergillus niger
(iii) Penicillium notatum
(iv) Clostridium butylicum

Question. List the events that lead to biogas production from wastewater whose BOD has been reduced significantly.
Answer : Sedimentation of flocs to form activated sludge, sludge pumped to anaerobic sludge digester, growth of anaerobic bacteria, digestion of sludge by bacteria to release biogas.

Question. How does the application of the fungal genus, Glomus, to the agricultural farm increase the farm output ?
Answer : Glomus forms mycorrhizal association, absorbs phosphorus, provide resistance to root borne pathogens, enhanced to tolerate salinity / drought.

Question. Explain the function of ‘‘anaerobic sludge digester’’ in a sewage treatment plant.
Answer : In an aerobic sludge digester, the anaerobic bacteria the methanogen, digest the flocs of bacteria and fungi in activated sludge and produce methane along with H2S and CO2 i.e., biogas which is a source of energy as it is inflammable.

Question. How do mycorrhiza help the plants to grow better?
Answer : Mycorrhiza refers to symbiotic association between the fungus and the root of higher plants. These fungi in these associations absorb water, phosphorus,nitrogen, potassium, calcium from soil and pass it to the plant. The fungus brings about the solubilization of organic matter of soil humus, release of inorganic nutrients absorption and their transfer to roots. The genus Glomus forms a mycorrhizal association with plants. 

Question. Describe the contributions of Alexander Fleming, Ernest Chain and Howard Florey in the field of microbiology.
Answer : Alexander Fleming while working on Staphylococci bacteria, once observed a mould growing in one of his unwashed culture plates around which Staphylococci could not grow. He found out that it was due to a chemical produced by the mould and he named it penicillin after the mould Pencillium notatum. Later, Ernest Chain and Howard Florey made its full potential effective antibiotic. 

Question. Name a genus of baculovirus. Why are they considered good biocontrol agents ?
Name the genus to which baculoviruses belong. Describe their role in the integrated pest management programmes.
Answer : Nucleopolyhedrovirus Species specific, narrow spectrum insecticidal application, no negative impact on non-target organisms.

Question. List the events that reduce the biological oxygen demand (BOD) of a primary effluent during sewage treatment.
Answer : Effluent from the primary settling tank passed into aeration tank, agitated mechanically and air is pumped into it, vigorous growth of aerobic microbes into flocs, microbes consume major part of the organic matter in effluent.

Question. Spirulina is a rich source of proteins. Mention the two ways by which large scale culturing of these microbes is possible.
Answer : Spirulina is a source of food rich in protein, minerals,fats, carbohydrates and vitamins. It can grow on wastewater from potato processing plants, straw, molasses, animal manure and even sewage, so it also reduces water pollution. 

Question. Explain the different steps involved during primary treatment phase of sewage. 
Answer : Physical removal of particles (large and small), by filtration and sedimentation, forming primary sludge / sedimented solids, forming effluent (supernatant) for secondary treatment.

Long Answer Type Questions

Question. Describe the process of secondary treatment given to municipal waste water (sewage) before it can be released into fresh water bodies. Mention another benefit provided by this process.
Answer : Process of secondary treatment :
Passing of primary effluent into large aeration tank which is constantly agitated mechanically & air is pumped into it that allows vigorous growth of useful aerobic microbes into flocs.
                                                                    ↓
Microbes consume major part of organic matter in effluent which significantly reduces BOD.
                                                                   ↓
Now effluent is passed into settling tank where flocs are allowed to settle/ sediment called activated sludge.
                                                                   ↓
Digestion of activated sludge by anaerobic microbes and effluents from secondary treatment can be released into river/ stream.
                                                                   ↓
Resulted in production of Bio gas (CH₄, H₂S and CO₂) which can be used as source of energy.

Question. Baculoviruses are good example of biocontrol agents. Justify giving three reasons.
Answer : i. Species specific / narrow spectrum insecticidal application. 
ii. They have no negative impact on plants / mammals / birds / fish / non-target insects. 
iii. They are beneficial for IPM (Integrated Pest Management ) / Pest Management Programme.

Question. Name the group of bacteria involved in setting milk into curd. Explain the process they carry in doing so. Write another beneficial role of such bacteria.
Answer : (group of) LAB / (group of) Lactic acid Bacteria /Lactobacillus species LAB produce acids that coagulate and partially digest the milk proteins. 
Increases Vitamin B12 / Checks disease causing microbes in the stomach.

Question. The three microbes are listed below. Name the product produced by each one of them and mention their use.
(a) Aspergillus niger
(b) Trichoderma polysporum
(c) Monascus purpureus
Answer : (a) Aspergillus niger – Citric Acid, natural preservative / flavouring agent. 
(b) Trichoderma polysporum – Cyclosporin A , immunosuppressive agent.
(c) Monascus purpureus – Statin, blood cholesterol lowering agent.

Question. (a) How does Bacillus thuringiensis act as a biocontrol agent for protecting Brassica and fruit trees? Explain.
(b) List the components of biogas.
(c) What makes methanogens a suitable source for biogas production?
Answer : (a) Bacterium Bacillus thuringiensis (Bt) are available in sachets as dried spores, mixed with water and sprayed onto vulnerable plants, these are eaten up by the insect larvae,
the toxins are released in the gut and larvae gets killed.
(b) Methane, H₂S, CO₂ , H₂.
(any two = , any three = 1)
(c) Methanogens grow anaerobically, on cellulosic material, produce large amount of methane, along with CO₂ and H₂.

Question. (a) Match the microbes listed under Column-A with the products mentioned under Column-B.

Column-A	Column-B
(H) Penicillium notatum	(i) Statin
(I) Trichoderma polysporum	(ii) ethanol
(J) Monascus purpureus	(iii) antibiotic
(K) Saccharomyces cerevisiae	(iv) Cyclosporin-A

(b) Why does ‘Swiss Cheese’ develop large holes ?
Answer : ( a) The correct matches are as follows :
(H) Penicillium notatum             (iii) Antibiotic
(I) Trichoderma polysporum       (iv) Cyclosporin–A
(J) Monascus purpureus             (i) Statin
(K) Saccharomyces cerevisiae    (ii) Ethanol 
(b) Due to production of large amount of CO₂ (by Propionibacterium shermanii)

Question. How does the activity of each one of the following help in organic farming ?
(a) Mycorrhiza
(b) Cyanobacteria
(c) Rhizobium
Answer : (a) Mycorrhiza : The fungal symbionts in these association absorb phosphorous from soil and pass it to plant. Plants also show resistance to root borne pathogens , tolerance to salinity /drought , an overall increase in plant growth and development. (Any two)
(b) Cyanobacteria : Serve as an important biofertilizers by fixing atmospheric nitrogen, also add organic matter to the soil, and increase its fertility. (Any two)
(c) Rhizobium : Fix atmospheric nitrogen into organic forms , which is used by plant as nutrient / increase soil fertility / symbiotic association in root nodules of leguminous plants.

Question. Name the two different categories of aerobic microbes naturally occurring in sewage water. Explain their role in cleaning sewage water into usable water. U [O.E.B.]
Answer : Microbes are tiny structures that are not visible to the naked eye, they are microscopic in nature.
Some microbes are beneficial but some are fatal for life. Human health and the environment are mainly affected by the direct disposal of industrial and human effluents into natural resources without any treatment. Sewage contains a huge amount of toxic organic matters. The two categories of aerobic microbes present in sewage are bacteria and fungi.
Aerobic microbes grow excessively in the aeration tank and it consumes a major part of the organic matter. It helps in reducing the BOD of the effluent.
Microorganisms that are natural to the wastewater environment play a vital role in the wastewater treatment process. Bacteria and fungi feed on organic material in wastewater & break it down.
Bacteria clump together, or floc, forming masses that settle and separate from the waste water. This settled mass is called sludge.

Question. (a) Write the scientific name of methanogen bacteria. Where are these bacteria generally found? Explain their role in biogas production.
(b) Name the components of biogas.
Answer : (a) The scientific name of methanogens : Methanobacterium.
Methanobacterium is found in the anaerobic sludge and rumen of cattle (for cellulose digestion). A lot of cellulosic material present in the food of cattle is also present in the rumen. In the rumen, these bacteria help in the breakdown of cellulose and play an important role in the nutrition of cattle. Thus, the excreta (dung) of cattle, commonly called Gobar, is rich in these bacteria. Dung can be used for the generation of biogas commonly called gobar gas.
(b) Biogas contains methane (CH₄), CO₂ and H₂.

Question. Some microbes act as biofertilizers. Explain with the help of three suitable examples.
Answer : (i) Rhizobium : Symbiotically fix atmospheric nitrogen into organic forms which can be used by the plant as nutrients.
(ii) Azospirillum / Azotobacter : Free living bacteria fix atmospheric nitrogen.
(iii) Glomus / Fungi : Symbiotic with plants absorbs phosphorus from soil and passes it to plant.
(iv) Cyanobacteria / Anabaena / Nostoc / Oscillatoria : Fix atmospheric nitrogen in aquatic and terrestrial environment.

Question. Effluent from the primary treatment of sewage is passed for secondary treatment.
Explain the process till the water is ready to be released into natural water bodies.
Answer : During treatment (after adding a small amount of inoculum) primary effluent is constantly agitated mechanically in (large) aeration tanks and the air is pumped into it, this allows the vigorous growth of useful microbes into flocs, the microbes consume the major part of the organic matter in the effluent, it reduces the BOD of the effluent, the effluent is then passed into a settling tank where the bacterial flocs are allowed to sediment, major part of the activated sludge is pumped into aerobic sludge digester and the remaining water is released into natural water bodies.

Question. “Determination of Biological Oxygen Demand (BOD) can help in suggesting the quality of a water body.” Explain.
Answer : High BOD in a water body indicates more number of micro-organisms in water, resulting in bad quality of water / death of aquatic creatures, more polluting potential.
Lower BOD of water body indicates less number of micro-organisms in water, good quality of water / aquatic life flourishes / less polluting potential.

Question. Explain Microbe as Biocontrol agents.
Answer : Biocontrol means the use of biological methods for controlling plant diseases and pests. To meet the increasing demand for food for the growing population the use of insecticides and pesticides has increased tremendously. But their ill effects and the chemicals which get released from these pesticides and insecticides are very harmful too. They are polluting the soil, water, fruit and vegetables. 
So many biological methods were developed to overcome their ill-effect. 
(i) Biological control of pests and diseases : In this method, natural predation methods are used rather than the use of chemicals. Example :
(a) Ladybird, a beetle with red and black markings, is used in controlling aphids. 
(b) Dragonflies, are used to get rid of mosquitoes.
(c) Gambusia fishes are used to kill the larvae of mosquitoes. 
(d) Bacillus thuringiensis (Bt) is a microbial agent, which is used to control butterfly caterpillars. 
(e) Transgenic plants are used in agriculture which are resistant to attack by insect pests.
e.g., BT cotton.
(f) Trichoderma species are free living fungi, they are effective against several plant pathogens.
(g) Baculoviruses are pathogens that attack insects and other arthropods. 
(h) Biopesticides are used to destroy the weeds as well as the insect pests. They are of two types bioherbicides and bioinsecticides.

Question. Describe how do ‘flocs‘ and ‘activated sludge‘ help in Sewage Treatment.
Answer : Flocs : Aerobic microbes consume the major part of the organic matter in the effluent, which significantly reduces BOD.
Activated sludge : Small part of activated sludge is used as inoculum and pumped back to aeration tank / pumped into anaerobic sludge digesters where microbes or bacteria grow anaerobically to produce CH4 or H₂S or CO₂ or biogas.

Question. Choose any three microbes, from the following which are suited for organic farming which is in great demand these days for various reasons. Mention one application of each one choosen. Mycorrhiza; Monascus; Anabaena; Rhizobium; Methanobacterium; Trichoderma.
Answer : Mycorrhiza : Fungal symbiont of the association/absorb phosphorus from soil.
Anabaena : Fix atmospheric nitrogen / Adds organic matter to the soil.
Rhizobium : It is a symbiotic root nodule bacterium which fixes atmospheric nitrogen (in leguminous plants) in organic form which is used by plant as nutrient.
Methanobacterium : They digest cellulosic material and the product / spent slurry can be used as fertilizer.