CBSE Class 12 Biology Ch 8 Practice Paper with Solutions

Apna CareerApna CareerApna Career

Class 12 Biology · Chapter 8 · Practice Paper + Solutions

CBSE · NCERT

BIOLOGY – CHAPTER 8 : MICROBES IN HUMAN WELFARE

Chapter-wise Practice Question Paper (Board + Competitive / NEET Level)

www.careerinformationportal.in

Class : XIISubject : BiologyRoll No. : ..................

Time : 3:00 HoursSession : 2026–27Max. Marks : 100

General Instructions

1. All questions are compulsory. This paper is based on NCERT Class XII Biology, Chapter 8, and includes NEET-style MCQs and Assertion–Reason questions.
2. Q.1 to Q.5 are Section A (40 marks): Q.1 has 20 MCQs; Q.2, Q.3, Q.4 and Q.5 have 5 questions each; every question carries 1 mark.
3. Q.6 to Q.13 carry 2 marks each; Q.14 to Q.20 carry 3 marks each; Q.21 and Q.22 are case-based questions of 4 marks each; Q.23 to Q.25 carry 5 marks each.
4. Internal choice (OR) is given in Q.14 to Q.20 and Q.23 to Q.25. Attempt only one of the alternatives.
5. Draw neat, labelled diagrams wherever required. Solutions to all questions are given after the paper.

SECTION – A (Objective Questions · 40 Marks)

Q.1 Choose the correct option.20 × 1 = 20

(i) Milk is converted into curd by the action of –

(a) Saccharomyces(b) Lactobacillus (lactic acid bacteria)(c) Rhizobium(d) Penicillium

(ii) During the formation of curd, the nutritional quality of milk is improved by an increase in –

(a) Vitamin A(b) Vitamin C(c) Vitamin D(d) Vitamin B₁₂

(iii) The large holes in ‘Swiss cheese’ are due to –

(a) CO₂ produced by Propionibacterium sharmanii(b) Methane produced by methanogens(c) Alcohol produced by yeast(d) O₂ released by algae

(iv) ‘Roquefort cheese’ is ripened by growing –

(a) Lactic acid bacteria(b) Yeast(c) Specific fungi on the cheese(d) Viruses

(v) The puffed-up appearance of dough (and idli/dosa batter) is due to –

(a) Methane(b) CO₂ produced during fermentation(c) Oxygen(d) Ethanol vapour

(vi) Which of the following alcoholic drinks is produced with distillation?

(a) Wine(b) Beer(c) Toddy(d) Whisky

(vii) Penicillin, the first antibiotic, was discovered by –

(a) Alexander Fleming(b) Louis Pasteur(c) Robert Koch(d) Edward Jenner

(viii) Citric acid is commercially produced by –

(a) Acetobacter aceti(b) Clostridium butylicum(c) Aspergillus niger(d) Lactobacillus

(ix) Streptokinase, used as a ‘clot buster’ in myocardial infarction patients, is produced by –

(a) Trichoderma(b) Streptococcus(c) Monascus(d) Aspergillus

(x) Cyclosporin A, an immunosuppressive agent in organ transplantation, is produced by –

(a) Monascus purpureus(b) Streptococcus(c) Saccharomyces(d) Trichoderma polysporum

(xi) Statins, which lower blood cholesterol, are produced by –

(a) Monascus purpureus (yeast)(b) Penicillium(c) Aspergillus(d) Trichoderma

(xii) A high BOD (biochemical oxygen demand) of water indicates that the water is –

(a) Cleaner(b) Rich in dissolved oxygen(c) More polluted with organic matter(d) Free of microbes

(xiii) ‘Flocs’ in the secondary treatment of sewage are –

(a) Masses of algae(b) Masses of bacteria associated with fungal filaments(c) Protozoan cysts(d) Clumps of viruses

(xiv) The gases produced in the anaerobic sludge digester are –

(a) O₂ and N₂(b) CO₂ and N₂(c) H₂ and O₂(d) Methane, H₂S and CO₂

(xv) Biogas is produced by –

(a) Methanogens such as Methanobacterium(b) Rhizobium(c) Azotobacter(d) Bacillus

(xvi) The main constituent of biogas is –

(a) Carbon dioxide(b) Hydrogen sulphide(c) Methane(d) Nitrogen

(xvii) Bacillus thuringiensis (Bt) is used to control –

(a) Fungal diseases(b) Butterfly caterpillars (lepidopteran larvae)(c) Mosquito adults(d) Weeds

(xviii) Baculoviruses are used as biocontrol agents because they –

(a) Kill all insects(b) Are harmful to plants(c) Are broad-spectrum insecticides(d) Are species-specific with a narrow spectrum and no harm to non-target organisms

(xix) Which of the following is a symbiotic nitrogen-fixing bacterium?

(a) Rhizobium(b) Azotobacter(c) Azospirillum(d) Nostoc

(xx) Mycorrhizal fungi (Glomus) help the host plant mainly by absorbing –

(a) Nitrogen(b) Water only(c) Phosphorus from the soil(d) Potassium

Q.2 Fill in the blanks.5 × 1 = 5

(i) Baker’s yeast is the fungus ______.

(ii) The ‘clot buster’ enzyme obtained from Streptococcus is ______.

(iii) The amount of oxygen consumed if all the organic matter in one litre of water were oxidised by bacteria is called ______.

(iv) Biogas is also called ______ gas.

(v) The symbiotic association of a fungus with plant roots is called ______.

Q.3 Write True or False.5 × 1 = 5

(i) Lactic acid bacteria increase the vitamin B₁₂ content of curd.

(ii) Wine and beer are produced by the distillation of the fermented broth.

(iii) A high BOD indicates that the water is less polluted.

(iv) Bacillus thuringiensis is used as a biological insecticide.

(v) Azotobacter is a symbiotic nitrogen-fixer living in root nodules.

Q.4 Match Column ‘A’ with Column ‘B’.5 × 1 = 5

Column ‘A’	Column ‘B’
(i) Penicillium notatum	(a) Cyclosporin A
(ii) Monascus purpureus	(b) Citric acid
(iii) Trichoderma polysporum	(c) Streptokinase
(iv) Aspergillus niger	(d) Penicillin
(v) Streptococcus	(e) Statins

Q.5 Assertion (A) – Reason (R) type questions. Choose: (a) Both A and R true, R explains A; (b) Both true, R does not explain A; (c) A true, R false; (d) A false, R true.5 × 1 = 5

(i) Assertion (A): Curd is more nutritious than milk.
Reason (R): Lactic acid bacteria increase the vitamin B₁₂ content during curd formation.

(ii) Assertion (A): ‘Swiss cheese’ has large holes.
Reason (R): ‘Roquefort cheese’ is ripened by growing specific fungi on it.

(iii) Assertion (A): Activated sludge is partly used as an inoculum in the aeration tank.
Reason (R): The entire activated sludge is used as inoculum and nothing goes to the digester.

(iv) Assertion (A): Baculoviruses are broad-spectrum insecticides.
Reason (R): Baculoviruses are species-specific with a narrow spectrum of action.

(v) Assertion (A): Mycorrhiza improves the growth of the host plant.
Reason (R): The fungal partner absorbs phosphorus from the soil and passes it to the plant.

SECTION – B (Very Short Answer · 2 Marks each)

6. How do lactic acid bacteria help in making curd and in our health?(2)

7. What makes the dough and idli/dosa batter rise? Name the organisms involved.(2)

8. Differentiate between the production of wine/beer and whisky/brandy/rum.(2)

9. Who discovered penicillin and how? Name two scientists who established its use.(2)

10. State the uses of streptokinase and cyclosporin A.(2)

11. Define BOD. What does a high BOD signify?(2)

12. What are biofertilisers? Give two examples.(2)

13. Why are baculoviruses considered good biocontrol agents in integrated pest management?(2)

SECTION – C (Short Answer · 3 Marks each)

14. Describe the role of microbes in the production of curd, dough, toddy and cheese.(3)

OR

Describe the role of microbes in the production of fermented beverages.

15. What are antibiotics? Describe the discovery of penicillin.(3)

OR

Write the names of microbes and the products obtained from them: organic acids, ethanol, enzymes (any six).

16. Describe the primary treatment of sewage.(3)

OR

Describe the secondary (biological) treatment of sewage.

17. Describe the working of a biogas plant.(3)

OR

Which microbes produce biogas? How is biogas produced and what are its uses?

18. What is biological control? Describe the use of Bacillus thuringiensis and ladybird as biocontrol agents.(3)

OR

Describe Trichoderma and baculoviruses as biocontrol agents. What is IPM?

19. Name the bacteria and cyanobacteria that act as biofertilisers and mention their role.(3)

OR

What is mycorrhiza? State its role in plant growth.

20. Write short notes on the bioactive molecules cyclosporin A, statins and streptokinase.(3)

OR

What is the Ganga Action Plan? Why is sewage treatment important before discharge into water bodies?

SECTION – D (Case-Based Questions · 4 Marks each)

21. Read the passage and answer the questions that follow.(4)

At a sewage treatment plant (STP), the primary effluent is passed into large aeration tanks, constantly agitated and supplied with air. Masses of microbes develop and consume the organic matter. After treatment, the BOD of the effluent is found to be much lower than that of the incoming sewage.

(i) What are the ‘masses of microbes’ formed in the aeration tank called? Name their components.(1)

(ii) What is BOD? What does a lower BOD of the treated effluent indicate?(1)

(iii) What happens to the activated sludge? Name the gases produced.(2)

22. Read the passage and answer the questions that follow.(4)

A farmer feeds cattle dung slurry into a gobar-gas plant and uses the spent slurry as manure. In his paddy field he adds Anabaena, he inoculates his pulse crop with Rhizobium and sprays dried spores of Bacillus thuringiensis on his cabbage crop to control caterpillars.

(i) Name the main gas produced in the gobar-gas plant and the group of microbes producing it.(1)

(ii) State the role of Anabaena and Rhizobium in the farmer’s fields.(1)

(iii) How does Bt control caterpillars? Why is this method called eco-friendly?(2)

SECTION – E (Long Answer · 5 Marks each)

23. Describe the role of microbes in household products and in the production of fermented beverages, with examples.(5)

OR

Describe the microbes used for the production of antibiotics, organic acids, enzymes and bioactive molecules, with examples.

24. Describe the treatment of sewage in a sewage treatment plant (STP). What is BOD?(5)

OR

Describe the production of biogas: the microbes, structure and working of a biogas plant and its advantages.

25. Describe the role of microbes in organic farming as biofertilisers and biocontrol agents.(5)

OR

Compare chemical and biological methods of pest control. Explain the biocontrol agents with examples and the concept of IPM.

********

CBSE · NCERT

BIOLOGY – CHAPTER 8 : COMPLETE SOLUTIONS

Microbes in Human Welfare · Answer Key with Step-by-Step Explanations

www.careerinformationportal.in

Class : XIISubject : BiologyMax. Marks : 100

Q.1 Quick Answer Key (MCQ)20 × 1 = 20

i	(b)	ii	(d)	iii	(a)	iv	(c)	v	(b)
vi	(d)	vii	(a)	viii	(c)	ix	(b)	x	(d)
xi	(a)	xii	(c)	xiii	(b)	xiv	(d)	xv	(a)
xvi	(c)	xvii	(b)	xviii	(d)	xix	(a)	xx	(c)

Q.1 Solutions with Explanation

(i) Ans. (b) Lactobacillus (lactic acid bacteria)

LAB (Lactobacillus and others) in a small amount of curd (inoculum/starter) grow in milk and convert it to curd; lactic acid coagulates and partially digests the milk proteins.

(ii) Ans. (d) Vitamin B₁₂

LAB increase vitamin B₁₂ in curd; in our stomach they also check disease-causing microbes.

(iii) Ans. (a) CO₂ produced by Propionibacterium sharmanii

Propionibacterium sharmanii produces a large amount of CO₂, which forms the large holes.

(iv) Ans. (c) Specific fungi on the cheese

Roquefort cheese is ripened with specific fungi (Penicillium roqueforti) which give it a particular flavour.

(v) Ans. (b) CO₂ produced during fermentation

Baker’s yeast (Saccharomyces cerevisiae) in dough, and bacteria in idli/dosa batter, produce CO₂ by fermentation.

(vi) Ans. (d) Whisky

Wine and beer are produced without distillation; whisky, brandy and rum are produced by distillation of the fermented broth.

(vii) Ans. (a) Alexander Fleming

Fleming noticed a mould (Penicillium notatum) killing Staphylococci in a culture plate; Chain and Florey established its use as an antibiotic; all three won the Nobel Prize in 1945.

(viii) Ans. (c) Aspergillus niger

Aspergillus niger – citric acid; Acetobacter aceti – acetic acid; Clostridium butylicum – butyric acid; Lactobacillus – lactic acid.

(ix) Ans. (b) Streptococcus

Streptokinase from the bacterium Streptococcus is used to remove clots from the blood vessels of patients.

(x) Ans. (d) Trichoderma polysporum

Cyclosporin A is produced by the fungus Trichoderma polysporum.

(xi) Ans. (a) Monascus purpureus (yeast)

Statins are competitive inhibitors of the enzyme responsible for the synthesis of cholesterol.

(xii) Ans. (c) More polluted with organic matter

BOD is the oxygen consumed by bacteria if all organic matter in 1 litre of water is oxidised; the higher the BOD, the more polluted the water.

(xiii) Ans. (b) Masses of bacteria associated with fungal filaments

Aerobic microbes grow as flocs (mesh-like masses of bacteria with fungal filaments) and consume the organic matter, reducing the BOD.

(xiv) Ans. (d) Methane, H₂S and CO₂

Anaerobic bacteria digest the activated sludge and release a mixture of gases – methane, hydrogen sulphide and CO₂ (biogas).

(xv) Ans. (a) Methanogens such as Methanobacterium

Methanogens grow anaerobically on cellulosic material (also in the rumen of cattle) and produce methane with CO₂ and H₂.

(xvi) Ans. (c) Methane

Biogas (gobar gas) is mainly methane (CH₄) with CO₂; it is used for cooking and lighting.

(xvii) Ans. (b) Butterfly caterpillars (lepidopteran larvae)

Dried Bt spores mixed with water are sprayed on plants (e.g. Brassicas, fruit trees); the larvae eat them and are killed by the toxin.

(xviii) Ans. (d) Are species-specific with a narrow spectrum and no harm to non-target organisms

Nucleopolyhedrovirus has no negative impact on plants, mammals, birds, fish or non-target insects – ideal for integrated pest management.

(xix) Ans. (a) Rhizobium

Rhizobium lives in the root nodules of leguminous plants; Azotobacter and Azospirillum are free-living; Nostoc is a free-living cyanobacterium.

(xx) Ans. (c) Phosphorus from the soil

The fungal symbiont absorbs phosphorus from the soil and passes it to the plant; it also gives resistance to root pathogens, salinity and drought.

Q.2 Fill in the blanks5 × 1 = 5

(i) Saccharomyces cerevisiae

(ii) Streptokinase

(iii) BOD (biochemical oxygen demand)

(iv) Gobar

(v) Mycorrhiza

Q.3 True / False5 × 1 = 5

(i) True.

LAB improve the nutritional quality of milk by increasing vitamin B₁₂.

(ii) False.

Wine and beer are produced without distillation; whisky, brandy and rum require distillation.

(iii) False.

High BOD means more organic matter and therefore more pollution.

(iv) True.

It is a bacterial biocontrol agent against butterfly caterpillars.

(v) False.

Azotobacter is a free-living nitrogen-fixing bacterium; Rhizobium is the symbiotic one.

Q.4 Matching5 × 1 = 5

(i) – (d)  ·  (ii) – (e)  ·  (iii) – (a)  ·  (iv) – (b)  ·  (v) – (c)

Penicillium notatum – penicillin; Monascus purpureus – statins; Trichoderma polysporum – cyclosporin A; Aspergillus niger – citric acid; Streptococcus – streptokinase.

Q.5 Assertion – Reason5 × 1 = 5

(i) Ans. (a)

Both true; the rise in vitamin B₁₂ makes curd nutritionally better.

(ii) Ans. (b)

Both true but about different cheeses; R does not explain A (holes are due to CO₂ from Propionibacterium sharmanii).

(iii) Ans. (c)

A is true (a small part is the inoculum). R is false: the major part is sent to the anaerobic sludge digesters.

(iv) Ans. (d)

A is false; R is true – they are species-specific and so safe for non-target organisms.

(v) Ans. (a)

Both true; improved phosphorus uptake (and resistance to stress) promotes the growth.

SECTION – B (2 Marks each)

6. Ans.

A small amount of curd (starter with LAB) is added to warm milk; the LAB multiply, produce lactic acid, which coagulates and partially digests milk proteins to form curd. [1] LAB also increase vitamin B₁₂ and in the stomach check disease-causing microbes. [1]

7. Ans.

Fermentation produces CO₂, which puffs up the dough/batter. [1] Dough: baker’s yeast (Saccharomyces cerevisiae); idli/dosa batter: bacteria (lactic acid bacteria). [1]

8. Ans.

Wine and beer: yeast (Saccharomyces cerevisiae) ferments malted cereals/fruit juice and the product is used without distillation. [1] Whisky, brandy and rum: the fermented broth is distilled to give a higher alcohol content. [1]

9. Ans.

Alexander Fleming observed a mould (Penicillium notatum) killing Staphylococci in a contaminated culture plate. [1] Ernest Chain and Howard Florey established its full potential as an effective antibiotic (Nobel Prize, 1945). [1]

10. Ans.

Streptokinase (from Streptococcus) acts as a ‘clot buster’ – removes clots from the blood vessels of patients with myocardial infarction. [1] Cyclosporin A (from Trichoderma polysporum) is an immunosuppressive agent used in organ-transplant patients. [1]

11. Ans.

BOD (biochemical oxygen demand) is the amount of oxygen that would be consumed if all the organic matter in one litre of water were oxidised by bacteria. [1] A higher BOD means a greater amount of organic matter, i.e. more polluted water. [1]

12. Ans.

Biofertilisers are organisms (bacteria, fungi, cyanobacteria) that enrich the nutrient quality of the soil. [1] Examples: Rhizobium, Azospirillum, Azotobacter, Anabaena, Nostoc, mycorrhiza (Glomus). (Any two) [1]

13. Ans.

Baculoviruses (e.g. nucleopolyhedrovirus) are species-specific, narrow-spectrum pathogens of insects and other arthropods. [1] They have no negative impact on plants, mammals, birds, fish or non-target insects, so they suit IPM and organic farming. [1]

SECTION – C (3 Marks each)

14. Ans.

Curd: LAB convert milk to curd, increase vitamin B₁₂. [¾] Dough: baker’s yeast produces CO₂ by fermentation, which puffs up the dough; idli/dosa batter is fermented by bacteria. [¾] Toddy: a traditional drink made by fermenting sap from palms. [¾] Cheese: microbes act on milk curd; Swiss cheese gets large holes from CO₂ of Propionibacterium sharmanii; Roquefort cheese is ripened with specific fungi for flavour. [¾]

OR

Saccharomyces cerevisiae (brewer’s yeast) has been used from ancient times for fermenting malted cereals and fruit juices to produce ethanol. [1] Wine and beer are produced without distillation. [1] Whisky, brandy and rum are produced by distillation of the fermented broth. [1]

15. Ans.

Antibiotics are chemical substances produced by some microbes that can kill or retard the growth of other (disease-causing) microbes. [1] Penicillin: Alexander Fleming, while working on Staphylococci, observed a mould (Penicillium notatum) around which the bacteria did not grow because of a chemical it produced, which he named penicillin. [1] Its full potential as an antibiotic was established by Ernest Chain and Howard Florey; it was widely used to treat the wounded soldiers of World War II. Fleming, Chain and Florey received the Nobel Prize in 1945. [1]

OR

Aspergillus niger – citric acid; Acetobacter aceti – acetic acid; Clostridium butylicum – butyric acid; Lactobacillus – lactic acid; Saccharomyces cerevisiae – ethanol; lipases (detergents), pectinases and proteases (clarify bottled juices), streptokinase (Streptococcus). [½ × 6]

16. Ans.

Primary treatment is the physical removal of particles from sewage. [½] Sequential filtration removes floating debris, and sedimentation removes grit (soil and small pebbles). [1] The settled solids form the primary sludge; the supernatant is the effluent, which is taken for secondary treatment. [1½]

OR

The primary effluent is passed into large aeration tanks where it is agitated mechanically and air is pumped in. [1] This allows vigorous growth of useful aerobic microbes into flocs (masses of bacteria with fungal filaments), which consume the organic matter and reduce the BOD. [1] The effluent goes to a settling tank where the flocs sediment as activated sludge; a small part is used as inoculum and the major part goes to anaerobic sludge digesters. The effluent is released into rivers/streams. [1]

17. Ans.

A biogas plant is a concrete tank (10–15 feet deep) in which bio-wastes are collected and a slurry of dung is fed. [1] A floating cover is placed over the slurry; it rises as the gas is produced. The outlet is connected to a pipe to supply biogas to nearby houses. [1] The spent slurry is removed through another outlet and used as fertiliser. The dung of cattle is plentiful, hence the name gobar gas. [1]

OR

Methanogens (e.g. Methanobacterium), which grow anaerobically on cellulosic material, produce methane along with CO₂ and H₂; they are present in anaerobic sludge and in the rumen of cattle (cellulose digestion). [1½] In a biogas plant the microbes act on cattle dung/bio-waste slurry to produce methane-rich biogas. [½] Uses: fuel for cooking and lighting; the spent slurry is a manure. IARI and KVIC popularised biogas plants in India. [1]

18. Ans.

Biological control is the use of living organisms to control pests and diseases, instead of chemicals; it is eco-friendly. [1] Bacillus thuringiensis: dried spores mixed with water are sprayed on vulnerable plants such as brassicas and fruit trees; caterpillars eat the leaves and are killed. [1] Ladybird controls aphids and dragonflies control mosquitoes. [1]

OR

Trichoderma: a free-living fungus, common in root ecosystems, effective biocontrol agent of several plant pathogens. [1] Baculoviruses (Nucleopolyhedrovirus): species-specific, narrow-spectrum insect pathogens with no harm to non-target organisms. [1] IPM (integrated pest management): combining biological control, natural predators and limited chemicals for sustainable pest control. [1]

19. Ans.

Rhizobium: symbiotic nitrogen-fixer in the root nodules of leguminous plants. [¾] Azospirillum and Azotobacter: free-living bacteria that fix atmospheric nitrogen and enrich the soil. [¾] Cyanobacteria (Anabaena, Nostoc, Oscillatoria): autotrophic nitrogen-fixers, important in paddy fields; they also add organic matter to the soil. [¾] They reduce the need for chemical fertilisers and are eco-friendly. [¾]

OR

Mycorrhiza is the symbiotic association of a fungus (e.g. Glomus) with the roots of higher plants. [1] The fungus absorbs phosphorus from the soil and passes it to the plant. [1] Plants show resistance to root-borne pathogens, tolerance to salinity and drought, and increased growth and development. [1]

20. Ans.

Cyclosporin A: from the fungus Trichoderma polysporum; an immunosuppressive agent used in organ-transplant patients to prevent rejection. [1] Statins: from the yeast Monascus purpureus; blood-cholesterol lowering agents (competitive inhibitors of the cholesterol-synthesising enzyme). [1] Streptokinase: from Streptococcus; a ‘clot buster’ to remove clots from blood vessels of myocardial infarction patients. [1]

OR

The Ganga Action Plan and Yamuna Action Plan were started by the Ministry of Environment and Forests (MoEF) to save rivers from pollution by treating sewage before it is discharged. [1½] Treatment is important because untreated sewage with high BOD pollutes water bodies, reduces dissolved oxygen, harms aquatic life and spreads waterborne diseases. [1½]

SECTION – D (Case-Based · 4 Marks each)

21. Ans.

(i) Flocs – masses of bacteria associated with fungal filaments forming mesh-like structures. [1]

(ii) BOD = the amount of oxygen consumed if all the organic matter in 1 litre of water were oxidised by bacteria. A lower BOD means the effluent has less organic matter and is less polluted. [1]

(iii) A small part of the activated sludge is used as inoculum; the major part is passed to anaerobic sludge digesters. [1] There, anaerobic bacteria digest it and produce biogas – methane, hydrogen sulphide and CO₂. [1] [2]

22. Ans.

(i) Methane; produced by methanogens (e.g. Methanobacterium). [1]

(ii) Both are biofertilisers: Anabaena (cyanobacterium) fixes atmospheric nitrogen in paddy fields; Rhizobium fixes nitrogen symbiotically in the root nodules of pulses. [1]

(iii) Caterpillars eat the sprayed leaves; the Bt toxin kills the larvae. [1] It is a biological, species-specific method which avoids chemical pesticides and their harm to the environment and non-target organisms. [1] [2]

SECTION – E (Long Answer · 5 Marks each)

23. Ans.

Household products: curd – LAB (Lactobacillus) coagulate and partially digest milk protein and increase vitamin B₁₂. [1] Dough – baker’s yeast (Saccharomyces cerevisiae) releases CO₂; idli/dosa batter – bacterial fermentation puffs up the batter. [1] Toddy – fermented palm sap. [½] Cheese – Swiss cheese (Propionibacterium sharmanii, CO₂ holes); Roquefort cheese (fungi for flavour). [1]
Beverages: S. cerevisiae ferments malted cereals and fruit juices to give ethanol; wine and beer – without distillation; whisky, brandy and rum – with distillation. [1½]

OR

Antibiotics: Penicillium notatum – penicillin (Fleming; Chain and Florey; Nobel 1945), used in World War II. [1]
Organic acids and alcohol: Aspergillus niger – citric acid; Acetobacter aceti – acetic acid; Clostridium butylicum – butyric acid; Lactobacillus – lactic acid; Saccharomyces – ethanol. [1½]
Enzymes: lipases (detergents – remove oily stains), pectinases and proteases (clarify bottled juices), streptokinase (clot buster). [1]
Bioactive molecules: cyclosporin A (Trichoderma polysporum) – immunosuppressant; statins (Monascus purpureus) – lower blood cholesterol. [1½]

24. Ans.

Sewage contains large amounts of organic matter and microbes (many pathogenic); it is treated in STPs before release. [½]
Primary treatment: physical – filtration and sedimentation remove floating debris and grit; settled solids = primary sludge; supernatant = effluent. [1½]
Secondary (biological) treatment: the effluent is passed to aeration tanks; vigorous growth of aerobic microbes as flocs consumes organic matter, reducing BOD; the effluent is passed to a settling tank where flocs settle as activated sludge. [1½]
Sludge: a small part is used as inoculum; the major part goes to anaerobic digesters, where anaerobic bacteria produce biogas (methane, H₂S, CO₂). The effluent is released into rivers/streams. [1]
BOD: oxygen consumed if all organic matter in 1 litre of water were oxidised by bacteria; higher BOD = more polluted. [½]

OR

Microbes: methanogens (Methanobacterium) – anaerobic, grow on cellulosic wastes; found in sludge and the rumen of cattle; produce methane, CO₂ and H₂. [1]
Structure and working: a concrete tank 10–15 feet deep; slurry of dung is fed; a floating cover rises as gas accumulates; an outlet pipe supplies the gas; spent slurry is removed from another outlet. [2]
Uses/advantages: a clean fuel (mainly methane) for cooking and lighting; the spent slurry is a manure; it uses cattle dung and wastes, reduces pollution. IARI and KVIC popularised it in India. [2]

25. Ans.

Biofertilisers are organisms that enrich soil nutrients – bacteria, fungi and cyanobacteria. [½] Rhizobium – symbiotic nitrogen fixation in legume nodules; Azospirillum and Azotobacter – free-living nitrogen-fixers; Anabaena, Nostoc, Oscillatoria – cyanobacteria in paddy fields (also Azolla–Anabaena); mycorrhiza (Glomus) – absorbs phosphorus, gives resistance to pathogens and tolerance to salinity and drought. [2½]
Biocontrol agents: Bacillus thuringiensis (kills caterpillars), Trichoderma (controls plant pathogens), baculoviruses (species-specific insecticide), ladybird (aphids), dragonflies (mosquitoes). [2]

OR

Chemical control uses pesticides and insecticides that are toxic, non-specific, pollute the environment and lead to resistant pests; biological control uses living organisms, is eco-friendly and target-specific. [2]
Examples: ladybird – aphids; dragonfly – mosquitoes; Bacillus thuringiensis – caterpillars; Trichoderma – fungal pathogens; baculovirus – insects. [2]
IPM: integrated pest management combines biocontrol, natural predators and minimal chemicals for sustainable crop protection and organic farming. [1]

QUICK REVISION · HIGH-YIELD POINTS (NEET / CBSE)

1. Household: curd – Lactobacillus (LAB; ↑ vitamin B12); dough – Saccharomyces cerevisiae (CO2); idli/dosa batter – bacteria; Swiss cheese – Propionibacterium sharmanii (CO2 holes); Roquefort – fungi (Penicillium roqueforti); toddy – fermented palm sap.
2. Beverages: wine and beer – no distillation; whisky, brandy, rum – distillation (Saccharomyces cerevisiae).
3. Antibiotics: penicillin – Fleming (Penicillium notatum), Chain and Florey; Nobel 1945; used in World War II.
4. Acids: Aspergillus niger – citric; Acetobacter aceti – acetic; Clostridium butylicum – butyric; Lactobacillus – lactic. Ethanol – Saccharomyces.
5. Enzymes: lipase (detergents), pectinase and protease (juice clarification), streptokinase (clot buster, Streptococcus). Cyclosporin A – Trichoderma polysporum (immunosuppressant); statins – Monascus purpureus (cholesterol).
6. Sewage: primary (physical – sedimentation, filtration) → secondary (aeration tank, flocs, BOD ↓) → activated sludge → anaerobic digester (biogas) → effluent to water body. High BOD = high pollution. Ganga Action Plan (MoEF).
7. Biogas: methanogens (Methanobacterium; also rumen); mainly methane + CO2 + H2; gobar gas plant; spent slurry = manure; IARI and KVIC.
8. Biocontrol: Bt (caterpillars), Trichoderma (fungal pathogens), baculovirus (species-specific, IPM), ladybird (aphids), dragonfly (mosquito).
9. Biofertilisers: Rhizobium (symbiotic), Azospirillum and Azotobacter (free-living), Anabaena, Nostoc, Oscillatoria (cyanobacteria), mycorrhiza – Glomus (phosphorus).
10. Organic farming relies on biofertilisers + biocontrol + IPM; reduces chemical fertilisers and pesticides.

********