CUET UG Biology Booster Test 3- Biotechnology and Biofertilisers -

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CUET UG Biology Booster Test 3- Biotechnology and Biofertilisers

⏱️ 30 Minutes

❓ 20 Questions

📌 Answers are locked once submitted — results and explanations appear at the end.

CUET UG Biology Booster Test 3- Biotechnology and Biofertilisers

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QUESTION 1 OF 20

Arrange the analytical steps representing the transition from conventional biocontrol to modern biotechnological applications:
1. Introduction of toxin genes into target crop plants.
2. Identification of Bacillus thuringiensis as an insecticidal bacterium.
3. Cultivation of inherently pest-resistant crops like Bt-cotton.
4. Application of genetic engineering methods to isolate the toxin gene.

Answer Review

1 Arrange the analytical steps representing the transition from conventional biocontrol to modern biotechnological applications:
1. Introduction of toxin genes into target crop plants.
2. Identification of Bacillus thuringiensis as an insecticidal bacterium.
3. Cultivation of inherently pest-resistant crops like Bt-cotton.
4. Application of genetic engineering methods to isolate the toxin gene.

A 2, 1, 4, 3

B 2, 4, 1, 3

C 4, 2, 1, 3

D 1, 4, 2, 3

💡 Short Explanation

Identification of the bacterium (2) is the foundational step. Isolation of the gene (4) follows through biotechnology. Introduction of the gene into the crop (1) is the transformation step. Cultivation (3) is the final agricultural application.

✅ Correct Answer Explanation

The logical progression of biotechnology involves first discovering a biological agent (B. thuringiensis), then using recombinant DNA technology to isolate the specific insecticidal gene. This gene is then integrated into the genome of a crop plant, which is subsequently field-cultivated. Sequence A follows this chronological and scientific progression.

❓ Why Other Options Are Incorrect

Option A → Places introduction (1) before isolation (4), which is technically impossible.
Option C → Places isolation (4) before identification (2), which is illogical.
Option D → Starts with the final product (introduction/cultivation), which is the reverse of the development process.

🎯 Strategy

Used: Substitution

Application: Start with the foundational discovery (2). This leaves A and B. Between A and B, gene isolation (4) must precede introduction (1).

Final Logic: B is the only sequence that aligns with the steps of genetic engineering.

🧠 Memory Trick

"Discover -> Isolate -> Integrate -> Cultivate."

📚 Class XII – Biology – Chapter: Microbes in Human Welfare – Topic: Microbes as Biocontrol Agents

2 From an integrated pest management perspective, cultivating Bt-cotton is superior to routinely spraying B. thuringiensis spore sachets primarily because:

A Spores sprayed on leaves also kill beneficial ladybirds, whereas Bt-cotton does not.

B The plant inherently produces the toxin, providing continuous resistance without relying on repeated manual applications.

C Spores require root nodules to function, which cotton plants lack.

D Bt-cotton also fixes its own atmospheric nitrogen.

💡 Short Explanation

Bt-cotton provides internal, continuous protection. Spraying is external, discontinuous, and labor-intensive. The toxin is expressed in the plant tissues, eliminating the need for periodic re-application.

✅ Correct Answer Explanation

The primary advantage of genetically modified Bt-crops is the persistent expression of the toxin gene within the plant. Unlike manual spraying, which is subject to environmental degradation (rain/sunlight) and requires repetitive labor, the plant's endogenous toxin production provides reliable, season-long protection against specific pests.

❓ Why Other Options Are Incorrect

Option A → B. thuringiensis spores are generally species-specific and don't typically kill ladybirds; the advantage of Bt-cotton is operational efficiency, not necessarily safety over spores.
Option C → Bt is not linked to root nodules; this is a misconception mixing up Rhizobium functions.
Option D → Bt-cotton does not fix nitrogen; it only provides insect resistance.

🎯 Strategy

Used: Elimination

Application: Eliminate C and D as biological fallacies. Between A and B, B describes the economic/operational superiority of a transgenic crop over a manual spray.

Final Logic: B describes the fundamental "internalized" benefit of transgenic biotechnology.

🧠 Memory Trick

"Internalized Toxin = Continuous Protection."

📚 Class XII – Biology – Chapter: Microbes in Human Welfare – Topic: Microbes as Biocontrol Agents

3 Match the specific ecological niche/domain with the appropriate microbial agent:

Column 1	Column 2
1. Inside the root nodules of legumes	I. Cyanobacteria
2. Free-living in the root ecosystems	II. Rhizobium
3. Inside the gut of insect larvae	III. Trichoderma species
4. Widely distributed in aquatic and terrestrial environments	IV. B. thuringiensis toxin

A 1-II, 2-IV, 3-III, 4-I

B 1-III, 2-II, 3-I, 4-IV

C 1-II, 2-III, 3-IV, 4-I

D 1-IV, 2-III, 3-II, 4-I

💡 Short Explanation

Rhizobium (1) resides in nodules (II). Trichoderma (2) lives in roots (III). B. thuringiensis toxin (3) activates in the gut (IV). Cyanobacteria (4) are widespread (I).

✅ Correct Answer Explanation

Rhizobium is the classic legume symbiont (1-II). Trichoderma are fungi commonly associated with root ecosystems (2-III). B. thuringiensis toxins are activated specifically in the gut of insect larvae (3-IV). Cyanobacteria are autotrophic microbes that inhabit both terrestrial and aquatic ecosystems (4-I).

❓ Why Other Options Are Incorrect

Option A, B, D → All misplace one or more agents from their correct biological niche.

🎯 Strategy

Used: Substitution

Application: Identify Rhizobium (1) with nodules (II). This eliminates A, B, and D immediately.

Final Logic: C is the only choice that matches all biological niches accurately.

🧠 Memory Trick

"Rhizo-Nodule, Tricho-Root, Bt-Gut, Cyanobacteria-Everywhere."

📚 Class XII – Biology – Chapter: Microbes in Human Welfare – Topic: Biocontrol/Biofertilisers

4 An agronomist attempts to cure a severe outbreak of leaf-eating caterpillars by applying a high concentration of Trichoderma directly to the crop canopy. Why is this Strategy NOT biologically sound?

A Trichoderma is an autotrophic microbe that only works in paddy fields.

B Trichoderma is a root ecosystem fungus and an effective biocontrol agent for plant pathogens, not an insecticidal agent.

C Trichoderma will kill all non-target insects.

D Trichoderma causes root diseases and will kill the crop.

💡 Short Explanation

Trichoderma is a fungal antagonist. It targets soil-borne pathogens, not insects. Applying it to leaves to kill caterpillars is a mismatch of biological function.

✅ Correct Answer Explanation

Trichoderma species are specialized fungi used as biocontrol agents against root pathogens. Their mechanism involves parasitic or antagonistic interactions with soil-borne fungi. They are not insecticides; therefore, applying them to target caterpillars is useless.

❓ Why Other Options Are Incorrect

Option A → Trichoderma is heterotrophic; autotrophic aquatic microbes are cyanobacteria.
Option C → Trichoderma does not act as an insecticide for either target or non-target insects.
Option D → Trichoderma is beneficial for crop health, not a root pathogen.

🎯 Strategy

Used: Substitution

Application: The question asks why a specific Strategy (using a root fungus for leaf insects) is wrong. B correctly identifies the functional limitation of the microbe.

Final Logic: Biological tools must be matched to their specific target (Pathogen vs. Insect).

🧠 Memory Trick

"Trichoderma = Pathogen Killer, not Insecticide."

📚 Class XII – Biology – Chapter: Microbes in Human Welfare – Topic: Microbes as Biocontrol Agents

5 In an ecologically sensitive area, a manager needs to eliminate a specific arthropod pest while strictly conserving populations of ladybirds and dragonflies. Why is a virus from the genus Nucleopolyhedrovirus the optimal choice?

A It is a broad-spectrum pathogen that resets the entire ecosystem.

B It has a narrow spectrum insecticidal application and lacks negative impacts on non-target insects.

C It fixes nitrogen, giving beneficial insects an alternative food source.

D It genetically engineers the pest to become a beneficial predator.

💡 Short Explanation

Baculoviruses (Nucleopolyhedrovirus) are species-specific. They do not affect beneficial insects like ladybirds. This makes them ideal for sensitive habitats.

✅ Correct Answer Explanation

The genus Nucleopolyhedrovirus is prized in Integrated Pest Management for its high species specificity. It targets only the intended pests, sparing the beneficial predators (ladybirds/dragonflies) that are crucial for a balanced ecosystem. This narrow-spectrum action is essential for ecologically sensitive areas.

❓ Why Other Options Are Incorrect

Option A → Baculoviruses are narrow-spectrum, not broad-spectrum.
Option C → Baculoviruses do not fix nitrogen; they are viral insecticides.
Option D → Viruses do not genetically engineer pests into predators; they kill the host.

🎯 Strategy

Used: Contextual/Tonal Matching

Application: "Sensitive area" + "Conserving ladybirds" necessitates specificity. Only (B) highlights "narrow spectrum" and safety for non-target insects.

Final Logic: B describes the key benefit of species-specific baculoviruses.

🧠 Memory Trick

"Nucleopolyhedrovirus = Narrow Spectrum = Safe."

📚 Class XII – Biology – Chapter: Microbes in Human Welfare – Topic: Microbes as Biocontrol Agents

6 Which scenario does NOT reflect the intended agricultural or ecological use of baculoviruses?

A Aiding in an overall integrated pest management (IPM) programme.

B Conserving beneficial insects like ladybirds.

C Eradicating all insect life in a field to ensure zero crop damage.

D Treating ecologically sensitive areas without harming fish or mammals.

💡 Short Explanation

Baculoviruses are specific, not universal eradicators. IPM goal is balance, not "zero insect life." Eradicating everything is contrary to organic/ecological principles.

✅ Correct Answer Explanation

Baculoviruses are meant for species-specific management, not mass eradication of all insect life. Total eradication is an ecological disaster and is neither the intended use of IPM nor the goal of using biological controls.

❓ Why Other Options Are Incorrect

Option A, B, D → These are all recognized benefits and intended uses of baculoviruses.

🎯 Strategy

Used: Extreme Word Filter

Application: The phrase "Eradicating all insect life" is an extreme and ecologically unsound goal. It contradicts the IPM philosophy of maintaining ecological balance.

Final Logic: C is the false statement regarding IPM/Biocontrol intent.

🧠 Memory Trick

"Baculoviruses = Targeted, not Universal."

📚 Class XII – Biology – Chapter: Microbes in Human Welfare – Topic: Microbes as Biocontrol Agents

7 Evaluate the following statements regarding the ecological safety of Nucleopolyhedrovirus:
I. They are safe for mammals, birds, and fish.
II. They negatively impact the root ecosystem by competing with Trichoderma.
III. They are highly desired when beneficial insects must be conserved.

A I and II only

B I, II and III

C II and III only

D I and III only

💡 Short Explanation

Baculoviruses are safe for non-insects (I). They do not compete with Trichoderma (II is false). They are highly specific, aiding conservation (III).

✅ Correct Answer Explanation

Baculoviruses are indeed safe for mammals, birds, and fish (I). They are used in IPM because they don't harm beneficial insects (III). Statement (II) is false; there is no competition between viral insecticides and root fungi (Trichoderma).

❓ Why Other Options Are Incorrect

Option A, B, C → All contain the false claim (II).

🎯 Strategy

Used: Elimination

Application: (II) is false because baculoviruses (insect viruses) and Trichoderma (soil fungi) inhabit completely different ecological domains and mechanisms. Eliminating options with (II) leaves only D.

Final Logic: D includes the two correct safety/utility statements.

🧠 Memory Trick

"Virus (Insecticide) != Fungi (Bio-guard)."

📚 Class XII – Biology – Chapter: Microbes in Human Welfare – Topic: Microbes as Biocontrol Agents

8 The use of baculoviruses in an ecologically sensitive area aligns perfectly with the core belief of the organic farmer because:

A It introduces toxic chemical compounds to ensure high yields.

B It entirely eradicates pests, which is the ultimate goal of organic farming.

C It keeps pests at manageable levels by relying on targeted biological checks rather than indiscriminate eradication.

D It relies entirely on genetic engineering to create sterile environments.

💡 Short Explanation

Organic farming emphasizes natural/biological control. Baculoviruses provide targeted management. This aligns with the "live and let live" ecological balance.

✅ Correct Answer Explanation

Organic farming seeks to maintain ecological health by using biological agents to keep pests at manageable levels. Baculoviruses fit this philosophy perfectly because they are specific, sustainable, and don't introduce toxic chemicals into the environment, unlike conventional pesticides.

❓ Why Other Options Are Incorrect

Option A → Organic farming rejects toxic chemicals.
Option B → Indiscriminate eradication is the opposite of organic philosophy.
Option D → Organic farming is not centered on genetic engineering.

🎯 Strategy

Used: Contextual/Tonal Matching

Application: "Organic farmer" values are the key. Match "Organic" with "Biological checks" and "Manageable levels."

Final Logic: C correctly identifies the philosophy of integrated/organic biological control.

🧠 Memory Trick

"Organic = Balance, not Eradication."

📚 Class XII – Biology – Chapter: Microbes in Human Welfare – Topic: Microbes as Biocontrol Agents

9 Which of the following practices does NOT align with the goal of using biofertilisers to enrich nutrient quality?

A Applying blue-green algae to a paddy field.

B Encouraging the proliferation of Azotobacter in the soil.

C Utilizing the symbiotic associations of Glomus.

D Increasing the reliance on chemical fertilisers to maximize crop yield.

💡 Short Explanation

Biofertilisers are sustainable alternatives to chemical fertilisers. Chemical fertilisers are associated with pollution, not biofertiliser goals. Option D contradicts the core purpose of organic/biofertiliser adoption.

✅ Correct Answer Explanation

The entire goal of using biofertilisers is to provide a sustainable, eco-friendly way to enrich soil nutrients. Increasing reliance on chemical fertilisers is the antithesis of the organic and biofertiliser-led movement, which aims to mitigate the pollution caused by such chemicals.

❓ Why Other Options Are Incorrect

Option A, B, C → These are all examples of biofertilisers (Cyanobacteria, Azotobacter, Glomus).

🎯 Strategy

Used: Extreme Word Filter

Application: The question asks for what does NOT align. Chemical fertilisers are the "odd one out" compared to the biological methods listed in A, B, and C.

Final Logic: D is fundamentally opposite to the biofertiliser philosophy.

🧠 Memory Trick

"Bio = Organic; Chemical = Pollution."

📚 Class XII – Biology – Chapter: Microbes in Human Welfare – Topic: Microbes as Biofertilisers

10 Arrange the steps by which a farmer might transition a depleted field into a self-sustaining organic system:
1. Reduction of dependence on chemical fertilisers.
2. Introduction of commercially available bacteria, fungi, and cyanobacteria.
3. Natural replenishment of soil nutrients and organic matter.
4. Identification of severe environmental pollution due to chemical overuse.

A 1, 2, 3, 4

B 2, 4, 1, 3

C 4, 1, 2, 3

D 4, 2, 3, 1

💡 Short Explanation

Identify problem (4). Introduce biofertilisers (2). Soil recovers (3). Reduce dependency on chemicals (1).

✅ Correct Answer Explanation

The process begins with identifying the environmental harm of existing practices (4). Once understood, the farmer introduces biofertilisers (2). These agents naturally replenish the soil's organic matter (3), which then logically allows the farmer to stop relying on chemical fertilisers (1).

❓ Why Other Options Are Incorrect

Option A, B, C → These orders don't follow the clear progression from problem identification to natural replenishment of the soil system.

🎯 Strategy

Used: Substitution

Application: Identify the start (4) and the end (1). D matches this frame.

Final Logic: D represents the logical shift from chemical dependence to organic sustainability.

🧠 Memory Trick

"Pollution -> Introduce Bio -> Replenish -> Stop Chemicals."

📚 Class XII – Biology – Chapter: Microbes in Human Welfare – Topic: Microbes as Biofertilisers

11 Biochemically, the symbiotic relationship involving Rhizobium is crucial for leguminous plants because it bypasses the plant's inability to:

A Absorb water from the soil.

B Directly utilize atmospheric nitrogen.

C Produce its own carbohydrates through photosynthesis.

D Resist broad-spectrum arthropod pests.

💡 Short Explanation

Plants cannot break the triple bond of N2. Rhizobium uses the nitrogenase enzyme to do this. This provides nitrogen to the plant.

✅ Correct Answer Explanation

Atmospheric nitrogen gas (N2) is held by an extremely strong triple bond. Plants do not possess the enzymes required to break this bond and utilize nitrogen in gaseous form. Rhizobium provides the nitrogenase enzyme to perform this fixation, which plants cannot do themselves.

❓ Why Other Options Are Incorrect

Option A → Plants absorb water through roots using osmosis, not symbiosis.
Option C → Plants produce their own carbohydrates via photosynthesis.
Option D → Nitrogen fixation has nothing to do with pest resistance.

🎯 Strategy

Used: Substitution

Application: The question defines the biochemical problem: "plant's inability to..." Nitrogen fixation (B) is the only biochemical process in the list that plants are globally incapable of.

Final Logic: B describes the exact biological limitation Rhizobium solves.

🧠 Memory Trick

"Rhizobium = N2 Fixer."

📚 Class XII – Biology – Chapter: Microbes in Human Welfare – Topic: Microbes as Biofertilisers

12 Match the nitrogen-fixing organisms with their ecological Strategy

Column 1	Column 2
1. Rhizobium	I. Free-living bacterium in the soil
2. Azospirillum	II. Symbiotic association in leguminous roots
3. Anabaena	III. Autotrophic microbe fixing nitrogen in aquatic environments
4. Azotobacter	IV. Another free-living bacterium in the soil

A 1-II, 2-III, 3-IV, 4-I

B 1-I, 2-II, 3-IV, 4-III

C 1-II, 2-I, 3-III, 4-IV

D 1-III, 2-IV, 3-I, 4-II

💡 Short Explanation

Rhizobium (II) = Symbiotic. Azospirillum (I) = Free-living soil. Anabaena (III) = Aquatic autotroph. Azotobacter (IV) = Free-living soil.

✅ Correct Answer Explanation

Rhizobium forms a symbiosis (II). Azospirillum is a free-living bacterium (I). Anabaena is an aquatic autotrophic cyanobacterium (III). Azotobacter is another free-living nitrogen-fixing bacterium (IV).

❓ Why Other Options Are Incorrect

Option A, B, D → All misplace one or more of these well-defined ecological strategies.

🎯 Strategy

Used: Substitution

Application: Start with Rhizobium (1) = Symbiotic (II). Only A and C fit. Now check Anabaena (3) = Aquatic (III). Only C fits.

Final Logic: C provides a perfect match for all organisms.

🧠 Memory Trick

"R-Symbiont, Anabaena-Aquatic, Azos/Azot-Free living."

📚 Class XII – Biology – Chapter: Microbes in Human Welfare – Topic: Microbes as Biofertilisers

📄 Passage

"You have studied about the nodules on the roots of leguminous plants formed by the symbiotic association of Rhizobium. These bacteria fix atmospheric nitrogen into organic forms, which is used by the plant as nutrient. Other bacteria can fix atmospheric nitrogen while free-living in the soil (examples Azospirillum and Azotobacter), thus enriching the nitrogen content of the soil."

Based on the passage, the defining difference between Azospirillum and Rhizobium lies in their:

A Ability to fix atmospheric nitrogen.

B Requirement for a host plant to form nodules.

C Ability to enrich the nitrogen content of the soil.

D Classification as bacteria.

💡 Short Explanation

Both fix nitrogen. Rhizobium requires nodules (symbiotic). Azospirillum is free-living.

✅ Correct Answer Explanation

The passage establishes Rhizobium as forming nodules in symbiotic associations, whereas Azospirillum is explicitly mentioned as a "free-living" fixer. This difference in their ecological dependency (host vs. free) is their defining distinction in this context.

❓ Why Other Options Are Incorrect

Option A, C, D → Both microbes share these traits (fixing nitrogen, enriching soil, being bacteria).

🎯 Strategy

Used: Odd One Out

Application: Compare the traits. Nitrogen fixation, soil enrichment, and bacterial classification are similarities. Only (B) represents a difference.

Final Logic: B is the only differential characteristic.

🧠 Memory Trick

"Rhizo-NeedsHost, Azos-LivesFree."

📚 Class XII – Biology – Chapter: Microbes in Human Welfare – Topic: Microbes as Biofertilisers

📄 Passage

According to the passage, the ultimate result of Azotobacter activity in a non-leguminous crop field would be:

A The formation of parasitic nodules.

B The depletion of soil organic matter.

C The enrichment of the nitrogen content of the soil.

D Enhanced resistance to root-borne pathogens.

💡 Short Explanation

Azotobacter is a free-living fixer. Its main purpose is adding nitrogen. The passage explicitly mentions "enriching the nitrogen content."

✅ Correct Answer Explanation

The passage states that free-living bacteria like Azotobacter "enrich the nitrogen content of the soil." This is the core agricultural utility of these microbes in non-leguminous fields.

❓ Why Other Options Are Incorrect

Option A → Parasitic nodules are not formed by Azotobacter.
Option B → They don't deplete organic matter; they help fertility.
Option D → While Glomus (mycorrhiza) provides resistance, Azotobacter is specifically for nitrogen fixation.

🎯 Strategy

Used: Contextual/Tonal Matching

Application: The passage provides the direct answer: "Azospirillum and Azotobacter ... thus enriching the nitrogen content of the soil."

Final Logic: C is directly pulled from the passage.

🧠 Memory Trick

"Azotobacter = Nitrogen Enricher."

📚 Class XII – Biology – Chapter: Microbes in Human Welfare – Topic: Microbes as Biofertilisers

15 If a fungicide completely eliminated the genus Glomus from a terrestrial ecosystem, which of the following would NOT be a resulting consequence?

A Decreased phosphorus absorption by plants.

B Decreased plant tolerance to salinity.

C Increased susceptibility of plants to root-borne pathogens.

D A total failure of legumes to fix atmospheric nitrogen.

💡 Short Explanation

Glomus (mycorrhiza) manages Phosphorus. Legume nitrogen fixation is handled by Rhizobium. Eliminating Glomus won't stop Rhizobium.

✅ Correct Answer Explanation

Glomus is a mycorrhizal fungus. Eliminating it would remove benefits like phosphorus absorption, pathogen resistance, and salinity tolerance. However, nitrogen fixation in legumes is handled by Rhizobium (a bacterium), which is independent of the mycorrhizal fungus. Thus, nitrogen fixation would NOT fail.

❓ Why Other Options Are Incorrect

Option A, B, C → These are all direct results of losing mycorrhiza.

🎯 Strategy

Used: Elimination

Application: Identify the role of Glomus (Phosphorus/Stress/Pathogen resistance). Nitrogen fixation (D) is unrelated to the Glomus genus.

Final Logic: D is the function of a different microbe (Rhizobium).

🧠 Memory Trick

"Glomus (P) != Rhizobium (N)."

📚 Class XII – Biology – Chapter: Microbes in Human Welfare – Topic: Microbes as Biofertilisers

16 Consider the dynamics of mycorrhizal associations:
I. The fungal symbiont absorbs phosphorus from the soil.
II. The plant provides the fungus with resistance to root-borne pathogens.
III. The association results in an overall increase in plant growth and development.

A I and III only

B I and II only

C II and III only

D I, II, and III

💡 Short Explanation

Fungi absorb phosphorus (I). Fungi provide pathogen resistance (Plant does not give resistance) (II is false). Mycorrhiza improves growth (III).

✅ Correct Answer Explanation

The fungus absorbs phosphorus for the plant (I). The mycorrhizal association overall increases plant growth (III). Statement (II) is inverted: the fungus gives the plant resistance to pathogens, not the other way around.

❓ Why Other Options Are Incorrect

Option B, C, D → All contain the false claim (II).

🎯 Strategy

Used: Elimination

Application: Analyze the direction of "resistance." The fungal symbiont is the protective agent. Thus, (II) is factually inverted. Eliminating (II) leaves A.

Final Logic: A reflects the correct functional direction of the symbiosis.

🧠 Memory Trick

"Mycorrhiza = Protector, Phosphorus provider."

📚 Class XII – Biology – Chapter: Microbes in Human Welfare – Topic: Microbes as Biofertilisers

17 From an evolutionary perspective, mycorrhizal associations are highly advantageous for plants colonizing arid (dry) environments primarily because the fungi confer:

A Narrow-spectrum insecticidal properties.

B Tolerance to drought and salinity.

C The ability to fix atmospheric nitrogen.

D The capacity to perform autotrophic photosynthesis.

💡 Short Explanation

Arid environments are dry and often salty. Mycorrhiza (Glomus) explicitly provide drought/salinity tolerance. These are essential traits for arid survival.

✅ Correct Answer Explanation

Mycorrhiza significantly improve a plant's ability to survive abiotic stressors. In arid environments, the increased tolerance to drought and salinity provided by Glomus allows plants to colonize areas that would otherwise be inhospitable.

❓ Why Other Options Are Incorrect

Option A → This is for baculoviruses.
Option C → This is for Rhizobium.
Option D → The plant does this, not the fungus.

🎯 Strategy

Used: Substitution

Application: Link "Arid (dry)" environment to the matching biological advantage in the options. Drought tolerance (B) is the direct match.

Final Logic: B addresses the environmental challenges of an arid site.

🧠 Memory Trick

"Arid = Drought = Mycorrhiza."

📚 Class XII – Biology – Chapter: Microbes in Human Welfare – Topic: Microbes as Biofertilisers

18 If a plant biologist observes that a specific crop strongly resists infections from soil-dwelling fungal pathogens despite the absence of chemical treatments, this resistance is most likely mediated by:

A Bacillus thuringiensis toxin genes.

B Symbiotic mycorrhizal associations.

C Genus Nucleopolyhedrovirus.

D Blue-green algae forming root nodules.

💡 Short Explanation

Resistance to root-borne fungal pathogens is a key benefit of Glomus (mycorrhiza). Bt is for insects. Viruses are for insects. BGA is for nitrogen.

✅ Correct Answer Explanation

The text explicitly notes that mycorrhizal associations provide "resistance to root-borne pathogens." This is an intrinsic biological defense mechanism where the fungal symbiont protects the roots.

❓ Why Other Options Are Incorrect

Option A → Bt acts against insects.
Option C → Nucleopolyhedrovirus acts against insects.
Option D → Blue-green algae fix nitrogen; they don't form root nodules.

🎯 Strategy

Used: Contextual/Tonal Matching

Application: Match "soil-dwelling fungal pathogens" with the bio-agent identified as the "root-borne pathogen" defender (mycorrhiza).

Final Logic: B is the only logical agent for root-pathogen defense.

🧠 Memory Trick

"Pathogen Resistance = Mycorrhiza."

📚 Class XII – Biology – Chapter: Microbes in Human Welfare – Topic: Microbes as Biofertilisers

19 A rice farmer introduces Anabaena and Nostoc into a flooded paddy field. What dual agricultural benefit is the farmer analytically securing?

A Broad-spectrum pest eradication and phosphorus absorption.

B Atmospheric nitrogen fixation and the addition of organic matter to the soil.

C Pathogen resistance and drought tolerance.

D Extermination of aquatic weeds and oxygen depletion.

💡 Short Explanation

Anabaena and Nostoc are cyanobacteria. They perform N2 fixation. They add organic matter (soil fertility).

✅ Correct Answer Explanation

Cyanobacteria like Anabaena and Nostoc are autotrophic nitrogen fixers. When applied to paddy fields, they not only increase soil nitrogen but also add organic matter, which enhances the overall fertility of the soil.

❓ Why Other Options Are Incorrect

Option A → Pests/Phosphorus are handled by Bt/Mycorrhiza, not cyanobacteria.
Option C → Pathogens/Drought are handled by Mycorrhiza.
Option D → They do not kill weeds or deplete oxygen; they are beneficial biofertilisers.

🎯 Strategy

Used: Substitution

Application: Match "Anabaena and Nostoc" (Cyanobacteria) to their known biofertiliser functions (N2 fixation + organic matter).

Final Logic: B is the textbook role of cyanobacteria in agriculture.

🧠 Memory Trick

"Cyanobacteria = Nitrogen + Organic Matter."

📚 Class XII – Biology – Chapter: Microbes in Human Welfare – Topic: Microbes as Biofertilisers

20 In the context of reducing environmental pollution, why is the commercial availability of blue-green algae significant for a nation's agricultural sector?

A It provides a sustainable method to replenish soil nutrients, reducing the heavy reliance on chemical fertilisers.

B thuringiensis toxin.

C It replaces the need for genetically engineered crops like Bt-cotton entirely.

D It acts as a biological substitute for the

💡 Short Explanation

Chemical fertilisers cause pollution. Blue-green algae act as renewable, sustainable biofertilisers. This reduces the environmental burden.

✅ Correct Answer Explanation

The agricultural sector relies heavily on synthetic chemicals, which cause significant water and soil pollution. Blue-green algae provide a natural, sustainable, and renewable way to replenish nitrogen, thereby directly reducing the need for pollutant-heavy chemical fertilisers.

❓ Why Other Options Are Incorrect

Option B → Biofertilisers are for crops, not sewage disposal.
Option C → They don't replace Bt (which is for insects); they replace nitrogen fertilisers.
Option D → They fix nitrogen, they don't produce Bt toxin.

🎯 Strategy

Used: Contextual/Tonal Matching

Application: "Reducing environmental pollution" is the goal. Replacing chemical fertilisers (A) is the direct way to achieve this reduction in an agricultural context.

Final Logic: A directly addresses the pollution reduction goal mentioned in the question.

🧠 Memory Trick

"Sustainable Bio = Reduced Chemical Dependency."

📚 Class XII – Biology – Chapter: Microbes in Human Welfare – Topic: Microbes as Biofertilisers

CUET UG Biology Booster Test 3- Biotechnology and Biofertilisers

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