Microbes in Human Welfare: Class 12 Biology Practice Questions

A rural community in Rajasthan set up a biogas plant using cattle dung slurry. The plant produced biogas that was used for cooking and lighting. The spent slurry left behind after biogas production was used directly on agricultural fields. Nearby, a small dairy cooperative started producing curd on a large scale and noticed that the curd made in the evening using a small amount of previously prepared curd (starter/inoculum) was ready by morning. The cooperative manager wondered whether the same microbes responsible for curd formation could also be used to treat the wastewater generated from washing dairy equipment.

Read the following passage and answer the questions that follow:

A rural community in Rajasthan set up a biogas plant using cattle dung slurry. The plant produced biogas that was used for cooking and lighting. The spent slurry left behind after biogas production was used directly on agricultural fields. Nearby, a small dairy cooperative started producing curd on a large scale and noticed that the curd made in the evening using a small amount of previously prepared curd (starter/inoculum) was ready by morning. The cooperative manager wondered whether the same microbes responsible for curd formation could also be used to treat the wastewater generated from washing dairy equipment.

(a) Name the main component of biogas and the group of microorganisms responsible for its production in the biogas plant. (1 mark)

(b) The spent slurry from the biogas plant is considered more useful than untreated cattle dung when applied to agricultural fields. Give one reason to justify this. (1 mark)

(c) Name the microorganism used as inoculum/starter for curd formation and state ONE role it plays beyond converting milk to curd that makes it beneficial to human health. (1 mark)

(d) The cooperative manager suggested using curd-forming bacteria (LAB) to treat dairy wastewater. Evaluate this suggestion — is it appropriate? Give a scientific reason to support your answer. (1 mark)

A municipal corporation in a mid-sized Indian city has been struggling with two interconnected problems. First, the city's sewage treatment plant (STP) is receiving wastewater with a BOD of 280 mg/L from a new food-processing industrial zone. After secondary treatment, the effluent BOD drops to 18 mg/L before being discharged into a nearby river. Second, the city's agricultural hinterland has seen a sharp decline in soil fertility over the past decade due to excessive use of chemical fertilisers, which has also led to eutrophication of local water bodies. A team of environmental biotechnologists has proposed a two-pronged solution: (A) retrofitting the STP with an enhanced biological treatment stage, and (B) launching a biofertiliser programme for local farmers using a combination of Rhizobium, Azospirillum, and Anabaena.

Read the following passage carefully and answer the questions that follow:

A municipal corporation in a mid-sized Indian city has been struggling with two interconnected problems. First, the city's sewage treatment plant (STP) is receiving wastewater with a BOD of 280 mg/L from a new food-processing industrial zone. After secondary treatment, the effluent BOD drops to 18 mg/L before being discharged into a nearby river. Second, the city's agricultural hinterland has seen a sharp decline in soil fertility over the past decade due to excessive use of chemical fertilisers, which has also led to eutrophication of local water bodies. A team of environmental biotechnologists has proposed a two-pronged solution: (A) retrofitting the STP with an enhanced biological treatment stage, and (B) launching a biofertiliser programme for local farmers using a combination of Rhizobium, Azospirillum, and Anabaena.

(i) The food-processing effluent has a BOD of 280 mg/L. After secondary treatment the BOD is 18 mg/L. Is the treated effluent safe to discharge into the river? Justify your answer using the concept of BOD as a water quality indicator. (1 mark)

(ii) During secondary (biological) treatment at the STP, large masses of 'flocs' are formed. (a) What are flocs? (b) How do they help reduce BOD? (1 mark)

(iii) The biotechnologists recommend Rhizobium, Azospirillum, and Anabaena as biofertilisers. For EACH organism, state: (a) its mode of nitrogen fixation (symbiotic / free-living / associative), and (b) one specific host or habitat it is associated with. (1 mark)

(iv) Explain how replacing chemical nitrogenous fertilisers with the proposed biofertiliser combination would help reduce eutrophication of local water bodies. (1 mark)

A farmer in Punjab noticed that his wheat crop was being severely damaged by a fungal pathogen. Instead of using chemical fungicides, an agricultural scientist advised him to use a soil bacterium as a biocontrol agent. The scientist also suggested that the farmer inoculate his legume crop (chickpea) with a specific microorganism to improve soil nitrogen levels naturally, and add another free-living microorganism to the soil to further enhance nitrogen availability. After one crop season, the farmer observed: (i) significant reduction in fungal disease, (ii) improved soil fertility, and (iii) reduced expenditure on chemical fertilisers and pesticides.

Read the following passage carefully and answer the questions that follow:

A farmer in Punjab noticed that his wheat crop was being severely damaged by a fungal pathogen. Instead of using chemical fungicides, an agricultural scientist advised him to use a soil bacterium as a biocontrol agent. The scientist also suggested that the farmer inoculate his legume crop (chickpea) with a specific microorganism to improve soil nitrogen levels naturally, and add another free-living microorganism to the soil to further enhance nitrogen availability. After one crop season, the farmer observed: (i) significant reduction in fungal disease, (ii) improved soil fertility, and (iii) reduced expenditure on chemical fertilisers and pesticides.

(a) Identify the soil bacterium recommended as a biocontrol agent against the fungal pathogen. Briefly explain the mechanism by which it controls fungal diseases. (2 marks)

(b) Name the microorganism used to inoculate the chickpea crop and the free-living microorganism added to the soil. State one role of each in improving soil nitrogen levels. (2 marks)

A dairy cooperative in Rajasthan was facing two problems simultaneously. First, their curd-making unit was not producing curd of consistent quality — sometimes the curd was too sour, sometimes it did not set properly. A microbiologist visited the unit and advised them to always add a small amount of previously prepared good-quality curd (called a 'starter') to fresh, warm milk before incubation. She explained that the microorganism responsible for curd formation also produces a vitamin that is beneficial for human health.

Second, the cooperative had a large amount of liquid whey (a byproduct of cheese-making) that was being discharged into a nearby pond, causing the water to turn greenish and foul-smelling within a few weeks.

Read the following passage carefully and answer the questions that follow:

A dairy cooperative in Rajasthan was facing two problems simultaneously. First, their curd-making unit was not producing curd of consistent quality — sometimes the curd was too sour, sometimes it did not set properly. A microbiologist visited the unit and advised them to always add a small amount of previously prepared good-quality curd (called a 'starter') to fresh, warm milk before incubation. She explained that the microorganism responsible for curd formation also produces a vitamin that is beneficial for human health.

Second, the cooperative had a large amount of liquid whey (a byproduct of cheese-making) that was being discharged into a nearby pond, causing the water to turn greenish and foul-smelling within a few weeks.

(i) Name the microorganism responsible for curd formation. What vitamin does it produce?
(ii) Why is it necessary to add a 'starter' (inoculum) to fresh milk for curd formation?
(iii) Identify the phenomenon occurring in the pond due to whey discharge. Give ONE reason why the water becomes foul-smelling.
(iv) Suggest ONE microbiological method by which the cooperative can treat the whey before discharging it, so that the pond is not harmed.

A rural biogas plant operator noticed that his biogas plant was producing significantly less methane than expected, even though he was regularly feeding it with cattle dung slurry. On investigation, a microbiologist found that the plant had been accidentally contaminated with dissolved oxygen due to a faulty inlet seal. The microbiologist also noted that the liquid effluent (slurry) remaining after biogas production was being discarded into a nearby pond, causing excessive algal growth and a sharp decline in fish population. Meanwhile, a nearby organic farm was successfully using microbes to enrich its soil and reduce fertiliser costs — the farmer had inoculated legume seeds with a specific bacterium and was also applying a preparation of blue-green algae to his waterlogged paddy fields.

Read the following passage carefully and answer the questions that follow:

A rural biogas plant operator noticed that his biogas plant was producing significantly less methane than expected, even though he was regularly feeding it with cattle dung slurry. On investigation, a microbiologist found that the plant had been accidentally contaminated with dissolved oxygen due to a faulty inlet seal. The microbiologist also noted that the liquid effluent (slurry) remaining after biogas production was being discarded into a nearby pond, causing excessive algal growth and a sharp decline in fish population. Meanwhile, a nearby organic farm was successfully using microbes to enrich its soil and reduce fertiliser costs — the farmer had inoculated legume seeds with a specific bacterium and was also applying a preparation of blue-green algae to his waterlogged paddy fields.

A wastewater treatment plant in a city receives large volumes of domestic sewage daily. The plant manager observes that the incoming sewage has a BOD of 300 mg/L. After passing through the treatment stages, the effluent released into a nearby river has a BOD of 2 mg/L. The plant uses large aeration tanks where sewage is constantly agitated and air is pumped in. Over time, the sewage develops visible mesh-like masses of bacteria and fungal filaments. The treated sludge from the secondary settling tanks is then transferred to large anaerobic tanks where it is further processed by a different group of microbes, producing a combustible gas mixture that the plant uses to generate electricity for its own operations.

Read the following passage carefully and answer the questions that follow:

A wastewater treatment plant in a city receives large volumes of domestic sewage daily. The plant manager observes that the incoming sewage has a BOD of 300 mg/L. After passing through the treatment stages, the effluent released into a nearby river has a BOD of 2 mg/L. The plant uses large aeration tanks where sewage is constantly agitated and air is pumped in. Over time, the sewage develops visible mesh-like masses of bacteria and fungal filaments. The treated sludge from the secondary settling tanks is then transferred to large anaerobic tanks where it is further processed by a different group of microbes, producing a combustible gas mixture that the plant uses to generate electricity for its own operations.

(a) What does the change in BOD values (300 mg/L to 2 mg/L) indicate about the quality of water? (1 mark)
(b) Name the mesh-like masses formed in the aeration tanks. What is their role in the treatment process? (1 mark)
(c) Name the group of microbes responsible for processing the sludge in the anaerobic tanks and identify the combustible gas mixture produced. (1 mark)
(d) The treated effluent (BOD = 2 mg/L) is released into the river. However, the plant manager is still advised to avoid releasing it near a lake with high nutrient levels. Explain the biological reason for this caution. (1 mark)

A municipal corporation in a rapidly growing city was struggling with the management of large volumes of sewage generated daily. The treatment plant used a two-stage process. In the first stage, sewage was passed through settling tanks where heavy solids settled as primary sludge. In the second stage, the effluent from the first stage was pumped into large aeration tanks where it was vigorously agitated and air was continuously pumped in. After several hours, the BOD of the water was found to have dropped from 250 mg/L to below 10 mg/L. The resulting sludge from the second stage was then transferred to large anaerobic digesters. A gas mixture collected from the digesters was used to generate electricity for the treatment plant itself, making the plant partially energy self-sufficient.

Read the following passage carefully and answer the questions that follow:

A municipal corporation in a rapidly growing city was struggling with the management of large volumes of sewage generated daily. The treatment plant used a two-stage process. In the first stage, sewage was passed through settling tanks where heavy solids settled as primary sludge. In the second stage, the effluent from the first stage was pumped into large aeration tanks where it was vigorously agitated and air was continuously pumped in. After several hours, the BOD of the water was found to have dropped from 250 mg/L to below 10 mg/L. The resulting sludge from the second stage was then transferred to large anaerobic digesters. A gas mixture collected from the digesters was used to generate electricity for the treatment plant itself, making the plant partially energy self-sufficient.

(a) Identify the type of treatment described in the second stage and name the biological agents responsible for the reduction in BOD. (1 mark)

(b) What does the drop in BOD from 250 mg/L to below 10 mg/L indicate about the quality of the water? Explain why BOD is used as a measure of water pollution. (1 mark)

(c) Name the gas mixture collected from the anaerobic digesters and identify the specific group of microorganisms responsible for its production. (1 mark)

(d) The treated effluent is released into a nearby river. Despite the BOD being within safe limits, an ecologist warns that the release could still cause ecological damage. Suggest ONE reason for this warning and name the ecological phenomenon it could trigger. (1 mark)

A municipal corporation in a rapidly growing city commissioned a new Sewage Treatment Plant (STP) to manage the increasing load of domestic and industrial wastewater. The plant manager reported that after primary treatment, the effluent was passed into large aeration tanks. After several hours of aeration, the liquid developed dense, brownish, mesh-like aggregates that settled at the bottom of the settling tank. The settled material was divided into two portions: a small fraction was pumped back into the aeration tank, while the bulk was transferred to large, sealed, oxygen-free digesters. After digestion, the remaining material was used on agricultural fields. Simultaneously, the gas produced in the digesters was captured and used to generate electricity for the plant itself.

During a routine water-quality audit, the environmental officer measured the BOD of the final effluent released into the nearby river. She found it had dropped from an initial value of 270 mg/L to 3 mg/L and declared the treatment satisfactory.

Read the following passage carefully and answer the questions that follow:

A municipal corporation in a rapidly growing city commissioned a new Sewage Treatment Plant (STP) to manage the increasing load of domestic and industrial wastewater. The plant manager reported that after primary treatment, the effluent was passed into large aeration tanks. After several hours of aeration, the liquid developed dense, brownish, mesh-like aggregates that settled at the bottom of the settling tank. The settled material was divided into two portions: a small fraction was pumped back into the aeration tank, while the bulk was transferred to large, sealed, oxygen-free digesters. After digestion, the remaining material was used on agricultural fields. Simultaneously, the gas produced in the digesters was captured and used to generate electricity for the plant itself.

During a routine water-quality audit, the environmental officer measured the BOD of the final effluent released into the nearby river. She found it had dropped from an initial value of 270 mg/L to 3 mg/L and declared the treatment satisfactory.

A municipal corporation collected sewage sludge from a secondary treatment plant and used it as manure in a public park. After six months, soil samples showed a significant increase in nitrogen-fixing bacterial colonies and improved plant growth, but also tested positive for coliform bacteria above the safe limit. Which of the following BEST explains why the treated sludge simultaneously promoted plant growth AND posed a public health risk?

A dairy technologist notices that freshly prepared curd from a batch of pasteurised milk has a sharp, tangy flavour and a firm texture after incubation at 40°C for 6 hours. She then uses a small amount of this curd to inoculate the next batch of milk. Which of the following best explains the biochemical basis of curd formation and why the inoculum (starter culture) is essential?

Which one of the following microorganisms is correctly matched with its industrial product?

Assertion (A): Biogas produced in biogas plants contains methane as its major component and is used as a source of energy in rural areas.
Reason (R): Methanogenic bacteria such as Methanobacterium are strict anaerobes that produce methane by the decomposition of cellulosic material in the presence of oxygen.

A food technologist tests four different beverages and records their microbial origin. Which of the following correctly identifies a product and the microorganism responsible for its production?

| Beverage | Microorganism |
|---|---|
| (A) Beer | Aspergillus niger |
| (B) Toddy | Saccharomyces cerevisiae |
| (C) Curd | Clostridium butyricum |
| (D) Vinegar | Lactobacillus acidophilus |

A food technologist is testing two milk samples. Sample P was kept at 37°C after adding a small amount of previously prepared curd, while Sample Q was kept at 37°C with no addition. After 6 hours, Sample P had set into curd but Sample Q remained liquid. The technologist concluded that the setting of Sample P was due to a drop in pH caused by microbial activity. Which of the following correctly identifies the microorganism responsible AND the metabolic product that caused the pH drop in Sample P?

A dairy farmer notices that the milk she left at room temperature overnight has turned sour and formed a semi-solid curd. Her young daughter asks why this happened. Which of the following best explains the biological basis of this transformation?

Assertion (A): Biogas produced in a biogas plant contains methane as its major component and can be used directly as a fuel.
Reason (R): Methanogenic bacteria such as Methanobacterium are strict anaerobes that convert acetic acid and H₂ + CO₂ into methane during biogas production.

Assertion (A): Cyclosporin A, used as an immunosuppressant drug in organ transplant patients, is produced by the fungus Trichoderma polysporum.

Reason (R): Immunosuppressants suppress the immune response of the recipient so that the transplanted organ is not rejected.

Which of the following correctly pairs a microorganism with its role in human welfare?

A dairy farmer noticed that her freshly prepared milk turned into curd overnight when left at room temperature. She observed that the curd had a slightly sour taste. Which of the following correctly explains this observation?

A dairy farmer noticed that the curd prepared at home using a small amount of previous day's curd as 'starter' sets faster in summer than in winter. Which of the following BEST explains this observation?

Draw a labelled diagram of a biogas plant. Name the group of microorganisms responsible for biogas production.

Name the biofertiliser that forms a symbiotic association with the roots of leguminous plants and state ONE way in which it benefits the plant.

A village in Rajasthan installed a community biogas plant two years ago. Initially it produced enough biogas to meet cooking needs of 10 families. Recently, the villagers started adding only water (no cattle dung) to the mixing tank for several weeks due to a shortage of cattle. The gas output from the plant dropped sharply.

Study the labelled diagram of the biogas plant given below and answer the questions that follow:

[DIAGRAM IN QUESTION — Biogas Plant]

Mixing tank
(Slurry input)
│
▼
┌─────────────────────┐ ← Gas holder (floating dome)
│ │──────────────► Gas outlet pipe
│ Digester tank │ (to stove/lamp)
│ (underground, │
│ sealed) │
└─────────────────────┘
│
▼
Outlet for
spent slurry

(i) Using the diagram, identify which part of the plant is directly affected by the shortage of cattle dung, and name the group of microorganisms whose activity is reduced as a result.
(ii) The spent slurry collected from the outlet is not discarded by the villagers. State ONE reason why.

A farmer stops using chemical fungicides on his wheat crop and instead drenches the soil around the roots with a suspension of Trichoderma harzianum. A month later, the crop shows resistance to Fusarium wilt, even though Trichoderma was never directly applied to the aerial parts of the plant. (i) Name the mechanism by which Trichoderma protects the plant roots from Fusarium. (ii) Apart from biocontrol, state ONE additional benefit Trichoderma provides to the plant in the rhizosphere.

Distinguish between primary and secondary sewage treatment on the basis of the process involved and the type of impurities removed.

A dairy farmer notices that the milk he adds to a warm container every evening turns into curd overnight. He observes that if he uses a freshly cleaned, unused container (with no leftover curd from the previous batch), the milk does NOT set into curd even after the same time. (i) Name the microorganism responsible for curd formation and state ONE role it plays beyond converting milk to curd. (ii) Explain why the milk fails to set in the freshly cleaned container.

During the secondary (biological) treatment of sewage, flocs are formed in the aeration tank. (i) What are flocs? (ii) What happens to the BOD of sewage water after secondary treatment, and why?

A student observed the following diagram of a biogas plant used in a rural village. Study the diagram and answer the questions:

(i) Name the group of microorganisms responsible for biogas production in such a plant.
(ii) Name the main combustible gas produced in this plant that makes biogas useful as a fuel.

A dairy farmer noticed that the curd prepared at home using a small amount of previously made curd as a starter (inoculum) was ready within 3–4 hours in summer, but took nearly 8–10 hours in winter to set. The farmer also observed that the curd prepared was slightly sour and had a pleasant aroma.

(i) Name the microorganism responsible for curd formation and state its role in the process.
(ii) Explain why curd sets faster in summer than in winter.
(iii) State ONE nutritional benefit of consuming curd over fresh milk.

Riya's family runs a small dairy business. During summer, they noticed that the curd prepared using a small amount of previous day's curd as 'starter' set faster and tasted more sour than usual. Her younger brother asked, 'If microbes make curd sour, why do we deliberately add them? Won't that spoil the food?'

(a) Name the microorganism used as a starter culture for curd formation and state the product it releases that causes the sour taste. (1 mark)
(b) Explain why adding a small amount of old curd to warm milk is essential for curd formation. (1 mark)
(c) Riya's brother thinks all microbes spoil food. Do you agree? Justify your answer using ONE example other than curd formation where microbes are beneficial to humans in the food/health sector. (1 mark)