CUET UG Booster Biology Unit 8Test (M5) -

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CUET UG Booster Biology Unit 8Test (M5)

⏱️ 30 Minutes

❓ 20 Questions

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

CUET UG Booster Biology Unit 8Test (M5)

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QUESTION 1 OF 20

Why does the Bacillus thuringiensis toxin kill the insect caterpillar but leave the vulnerable plant unharmed?

Answer Review

1 Why does the Bacillus thuringiensis toxin kill the insect caterpillar but leave the vulnerable plant unharmed?

A The toxin is only released when mixed with water on the leaf surface.

B The toxin is specifically released inside the gut of the insect larvae after consumption.

C The plant naturally produces antibodies against the Bt toxin.

D The bacteria only reproduce inside the plant's root system.

💡 Short Explanation

B. thuringiensis produces protoxin crystals. These crystals require an alkaline pH, found only in the insect larval gut, to be activated. The plant lacks this specific gut environment, keeping it safe.

✅ Correct Answer Explanation

The correct answer is (B). The Bt toxin exists as an inactive protoxin in the bacteria. Once ingested by the susceptible insect larvae, the highly alkaline environment of the larval gut solubilizes the crystals and activates the toxin, which then perforates the gut wall, leading to insect death. The plant, lacking this alkaline gut-like environment, remains unaffected.

❓ Why Other Options Are Incorrect

Option A → The toxin is not triggered by water; it is a protein crystal activated by gut pH.
Option C → Plants do not produce antibodies; the safety mechanism is due to the lack of toxin activation conditions.
Option D → Bt is used to engineer whole-plant resistance, not just root systems, and reproduction is not the mechanism of action.

🎯 Strategy

Application: The question asks for the mechanism of specificity. Only (B) explains the chemical/biological activation requirement (gut pH) that ensures specificity.

Final Logic: The specificity of Bt toxin is strictly dependent on the alkaline gut environment of the insect.

🧠 Memory Trick

"Bt = Alkaline Gut Activation."

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

2 The cultivation of Bt-cotton eliminates the need for farmers to rely heavily on which of the following practices for managing insect caterpillars?

A Crop rotation

B Spraying conventional chemical insecticides

C Irrigating fields

D Using blue-green algae

💡 Short Explanation

Bt-cotton is genetically engineered to produce its own insecticidal protein. This intrinsic resistance reduces the need for external chemical applications. It specifically targets insect caterpillars like bollworms.

✅ Correct Answer Explanation

The correct answer is (B). The primary agricultural advantage of Bt-cotton is its internal production of Bt toxin, which acts as a bio-pesticide. This significantly decreases the farmer's dependency on spraying external, broad-spectrum chemical insecticides to control cotton bollworms.

❓ Why Other Options Are Incorrect

Option A → Crop rotation is a general agronomic practice still recommended even with Bt-crops.
Option C → Irrigation is unrelated to pest management strategies.
Option D → Blue-green algae are used for nitrogen fixation, not insect control.

🎯 Strategy

Application: The question asks about managing "insect caterpillars." Chemical insecticides (B) are the direct alternative to Bt toxins for this specific purpose.

Final Logic: Bt-cotton acts as a substitute for chemical insecticide applications.

🧠 Memory Trick

"Bt = Less Chemicals."

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

3 A farmer is looking for a biofertiliser to fix atmospheric nitrogen. Why should the farmer NOT choose Trichoderma?

A Because it is a free-living fungus effective as a biocontrol agent for pathogens, not a nitrogen fixer.

B Because it only acts as an autotrophic microbe in aquatic environments.

C Because it forms symbiotic root nodules rather than living freely.

D Because it is a virus that only targets insects.

💡 Short Explanation

Trichoderma is a fungus, not a nitrogen fixer. Its primary agricultural role is biocontrol of plant diseases. Nitrogen fixation is performed by different microbes (e.g., Rhizobium).

✅ Correct Answer Explanation

The correct answer is (A). Trichoderma species are beneficial fungi common in root ecosystems. They function as biocontrol agents by inhibiting or attacking soil-borne plant pathogens. They do not possess the nitrogenase enzyme required for nitrogen fixation.

❓ Why Other Options Are Incorrect

Option B → Autotrophic aquatic microbes are cyanobacteria, not Trichoderma.
Option C → Trichoderma is free-living, not a nodule-former.
Option D → Trichoderma is a fungus, not a virus.

🎯 Strategy

Application: Eliminate (B), (C), and (D) as they are factually incorrect definitions of Trichoderma. (A) accurately reflects its function as a biocontrol agent.

Final Logic: Trichoderma is exclusively a biocontrol fungus, not a nitrogen-fixing bacterium.

🧠 Memory Trick

"Trichoderma = Pathogen Defender, not Nitrogen Provider."

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

4 If a crop is suffering from a fungal root pathogen, introducing Trichoderma species would be considered an application of:

A Genetic engineering

B Integrated Pest Management for arthropods

C Biological control for plant diseases

D Chemical soil sterilization

💡 Short Explanation

Trichoderma is a fungus that attacks other plant pathogens. This is the definition of biological control. It is not genetic engineering or chemical control.

✅ Correct Answer Explanation

The correct answer is (C). Using one living organism (Trichoderma) to control another harmful organism (a fungal root pathogen) is the textbook definition of biological control. This approach is highly effective in managing soil-borne diseases.

❓ Why Other Options Are Incorrect

Option A → Genetic engineering involves modifying DNA, not simply introducing a beneficial microbe.
Option B → Trichoderma targets fungi, while IPM usually focuses on insect/arthropod pests.
Option D → Chemical sterilization involves synthetic toxins, which Trichoderma avoids.

🎯 Strategy

Application: Match the biological role (Trichoderma controlling pathogens) to the technique (Biological control).

Final Logic: The interaction between Trichoderma and pathogens is biological, not chemical or genetic.

🧠 Memory Trick

"Trichoderma = Fungal Bio-guard."

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

5 Arrange the rational steps a farmer would take when implementing an Integrated Pest Management (IPM) program using Nucleopolyhedrovirus:
1. Beneficial non-target insects are conserved.
2. The specific target arthropod pest is identified.
3. Nucleopolyhedrovirus is applied as a narrow-spectrum insecticide.
4. The pest population is successfully controlled without ecological damage.

A 1, 2, 3, 4

B 3, 2, 4, 1

C 2, 3, 1, 4

D 2, 1, 3, 4

💡 Short Explanation

Identify pest (2). Apply narrow-spectrum biocontrol (3). Non-target insects are preserved (1). Control achieved (4).

✅ Correct Answer Explanation

The correct answer is (C). The logical flow of an IPM program is: First, identify the specific pest (2). Then, apply the narrow-spectrum biocontrol agent (3). Because of its specificity, beneficial non-target insects are conserved (1), leading to effective control without damaging the ecosystem (4).

❓ Why Other Options Are Incorrect

Option A, B, D → These sequences skip the necessary identification step (2) or incorrectly order the conservation and application steps.

🎯 Strategy

Application: Identification (2) must come first in any professional management program. Only (C) and (D) start with 2. C is logically superior because application (3) precedes the resulting conservation of species (1).

Final Logic: C represents the standard scientific methodology for IPM.

🧠 Memory Trick

"ID Pest → Apply Specific Agent → Protect Others → Profit/Control."

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

6 Match the biocontrol application scenario with the appropriate microbial agent:

Column 1	Column 2
1. Treating a plant disease in the root ecosystem.	I. Azotobacter
2. Targeting a specific insect pest without harming local fish.	II. Genus Nucleopolyhedrovirus
3. Protecting a plant from caterpillars via introduced genes.	III. Trichoderma
4. Enriching soil nitrogen levels without a host plant.	IV. Bacillus thuringiensis toxin gene

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

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

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

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

💡 Short Explanation

Root pathogen control = Trichoderma (1-III). Specific insect targeting = Baculovirus (2-II). Genetic Caterpillar protection = Bt (3-IV). Free-living N-fixer = Azotobacter (4-I).

✅ Correct Answer Explanation

The correct answer is (B). Trichoderma is used for root pathogens (1-III). The genus Nucleopolyhedrovirus is used for species-specific insect control (2-II). The Bacillus thuringiensis toxin gene is used in genetically modified crops to resist caterpillars (3-IV). Azotobacter is a free-living nitrogen-fixing bacterium (4-I).

❓ Why Other Options Are Incorrect

Option A, C, D → These options misplace the functional roles of the microbes/genes.

🎯 Strategy

Application: Trichoderma (1) is famously for root ecosystems (III). Only B and D start with 1-III. Check Bacillus thuringiensis (3) as the gene source (IV). Only B fits.

Final Logic: B provides the only consistent set of matches.

🧠 Memory Trick

"Tricho=Root, Baculo=Specific Pest, Bt=Caterpillar Gene, Azotobacter=Free Nitrogen."

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

7 When applying baculoviruses in a field, an organic farmer expects safety for all organisms except the target pest. Which of the following is NOT an advantage of using baculoviruses over chemical pesticides?

A They have no negative impact on plants.

B They act as broad-spectrum killers of all field fauna.

C They conserve beneficial insects.

D They have no negative impact on birds and mammals.

💡 Short Explanation

Baculoviruses are narrow-spectrum, not broad-spectrum. They are highly selective for specific insect pests. Option B describes the negative effect of chemical pesticides, not the advantage of biocontrol.

✅ Correct Answer Explanation

The correct answer is (B). Baculoviruses are narrow-spectrum insecticidal agents. Being "broad-spectrum" and killing "all field fauna" is a disadvantage of chemical pesticides, not an advantage of baculoviruses.

❓ Why Other Options Are Incorrect

Option A, C, D → These are all recognized ecological advantages of using baculoviruses.

🎯 Strategy

Application: The question asks for what is NOT an advantage. "Broad-spectrum killers of all field fauna" is an ecological negative, not an advantage.

Final Logic: Option B is the polar opposite of the specificity provided by baculoviruses.

🧠 Memory Trick

"Baculovirus = Specific, Not Broad."

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

8 Why is the genus Nucleopolyhedrovirus specifically chosen when treating ecologically sensitive areas?

A It permanently eradicates all microbial life in the soil.

B It has species-specific, narrow spectrum insecticidal applications that preserve biodiversity.

C It genetically modifies local plants to withstand chemical herbicides.

D It functions as a biofertiliser while simultaneously killing pathogens.

💡 Short Explanation

Baculoviruses are highly specific to target pests. They do not affect non-target organisms. This makes them perfect for sensitive environments.

✅ Correct Answer Explanation

The correct answer is (B). Nucleopolyhedrovirus is highly species-specific. In sensitive ecosystems, you want to kill the pest while leaving beneficial pollinators and predators alive; this virus achieves that balance.

❓ Why Other Options Are Incorrect

Option A → Biocontrol does not aim to eradicate microbial life.
Option C → Baculoviruses do not modify plant genes.
Option D → They are insecticides, not fertilisers.

🎯 Strategy

Application: "Ecologically sensitive" implies the need for high selectivity. Only (B) mentions "narrow spectrum" and "biodiversity," which fits this context.

Final Logic: Option B explains the ecological utility of species-specific baculoviruses.

🧠 Memory Trick

"Sensitive area = Narrow spectrum = Safe."

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

9 Match the biofertiliser source with its specific mechanism of nutrient enrichment:

Column 1	Column 2
1. Rhizobium	I. Absorbs phosphorus from soil and passes it to the plant
2. Genus Glomus	II. Free-living atmospheric nitrogen fixation in soil
3. Cyanobacteria	III. Symbiotic atmospheric nitrogen fixation into organic forms
4. Azospirillum	IV. Autotrophic addition of organic matter to soil

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

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

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

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

💡 Short Explanation

Rhizobium (III): Symbiotic Nitrogen fixation. Glomus (I): Phosphorus absorption. Cyanobacteria (IV): Organic matter/Autotrophic enrichment. Azospirillum (II): Free-living Nitrogen fixation.

✅ Correct Answer Explanation

The correct answer is (C). Rhizobium forms a symbiosis for nitrogen fixation (1-III). Glomus is a mycorrhiza that absorbs phosphorus (2-I). Cyanobacteria are autotrophic and add organic matter (3-IV). Azospirillum is a free-living nitrogen fixer (4-II).

❓ Why Other Options Are Incorrect

Option A, B, D → These incorrectly map the mechanisms of the named biofertilisers.

🎯 Strategy

Application: Rhizobium (1) = Symbiotic Nitrogen fixation (III). Only A starts with 1-III. Check Glomus (2) = Phosphorus (I). A matches perfectly.

Final Logic: A is the only choice that matches all functions correctly.

🧠 Memory Trick

"R-S (Rhizobium-Symbiotic), G-P (Glomus-Phosphorus), C-O (Cyanobacteria-Organic), A-F (Azospirillum-Free)."

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

10 Arrange the logical progression of shifting agricultural practices:
1. Use of chemical fertilisers to meet agricultural demands.
2. Commercial use of bacteria, fungi, and cyanobacteria.
3. Environmental pollution becomes a major cause of concern.
4. Large pressure to switch to organic farming and biofertilisers.

A 1, 3, 4, 2

B 3, 1, 2, 4

C 1, 4, 3, 2

D 4, 3, 1, 2

💡 Short Explanation

Chemical use (1) leads to pollution (3). Pollution creates demand for organic solutions (4). Biofertilisers become standard (2).

✅ Correct Answer Explanation

The correct answer is (A). The shift followed a clear timeline: First, chemicals were used for high yields (1). Then, the environmental consequences (pollution) were realized (3). This triggered public and scientific pressure for organic farming (4), leading to the current commercial integration of biofertilisers (2).

❓ Why Other Options Are Incorrect

Option B, C, D → These sequences fail to show the logical cause-effect chain from chemical use to environmental backlash and subsequent adoption of biofertilisers.

🎯 Strategy

Application: The use of chemicals (1) clearly predates the shift to biofertilisers (2). Only (A) and (C) respect this order. Between them, pollution (3) must cause the pressure for change (4). A captures this chain perfectly.

Final Logic: A follows the historical and environmental logic of the agricultural shift.

🧠 Memory Trick

"Chemicals → Pollution → Pressure → Biofertilisers."

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

11 Which of the following statements correctly apply to the nodules found on leguminous plants?
I. They are formed by the symbiotic association of Rhizobium.
II. They serve as sites for fixing atmospheric nitrogen into organic forms.
III. They are mainly caused by free-living fungi like Trichoderma.

A I only

B II and III only

C I and II only

D I, II, and III

💡 Short Explanation

Nodules are symbiotic (Rhizobium). Nodules fix N2 to amino acids (organic). Fungi (Trichoderma) do not cause nodules.

✅ Correct Answer Explanation

The correct answer is (C). Root nodules on legumes are formed specifically by the symbiosis between the plant and Rhizobium (I). These nodules are the specialized environment where nitrogen fixation occurs (II). Statement (III) is false; Trichoderma is not involved in nodule formation.

❓ Why Other Options Are Incorrect

Option A → Incomplete; misses the function (II).
Option B → Incorrect; (III) is false.
Option D → Incorrect; (III) is false.

🎯 Strategy

Application: (III) is false because nodule formation is a bacterial symbiosis, not a fungal infection. Eliminate all options containing (III).

Final Logic: (C) is the only choice that remains.

🧠 Memory Trick

"Rhizobium = Symbiosis, Nodule = Nitrogen Fixation."

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

12 The association between leguminous plants and Rhizobium is termed "symbiotic" because:

A The bacteria kill pests while the plant provides phosphorus.

B The bacteria provide organic nitrogen forms used as nutrients by the plant.

C The bacteria cause nodules that harm the plant's roots.

D The plant fixes nitrogen for the bacteria to consume.

💡 Short Explanation

Symbiosis = Mutual benefit. Bacteria provide fixed nitrogen to the plant. Plant provides shelter/nutrients to the bacteria.

✅ Correct Answer Explanation

The correct answer is (B). A "symbiotic" or "mutualistic" association means both organisms benefit. The Rhizobium bacteria convert atmospheric nitrogen into organic forms that the plant can directly absorb, while the plant provides the bacteria with a home (nodules) and energy (carbohydrates).

❓ Why Other Options Are Incorrect

Option A → Rhizobium doesn't kill pests; that is the role of biocontrol agents.
Option C → The association is mutually beneficial, not harmful.
Option D → The bacteria do the nitrogen fixing, not the plant.

🎯 Strategy

Application: Symbiosis means both benefit. Only (B) correctly describes the primary benefit the plant receives (nitrogen) from the bacteria.

Final Logic: B matches the functional definition of mutualism.

🧠 Memory Trick

"Symbiosis = Shared Benefit (Nitrogen for Housing)."

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

13 A soil sample is tested and found to be rich in Azospirillum. Which biological feature is NOT associated with this microbe?

A It lives freely in the soil.

B It fixes atmospheric nitrogen.

C It depends entirely on root nodules for survival.

D It enriches the nitrogen content of the soil.

💡 Short Explanation

Azospirillum is a free-living bacterium. It does not need root nodules to live. This is the opposite of a symbiotic fixer.

✅ Correct Answer Explanation

The correct answer is (C). Azospirillum is a classic "free-living" nitrogen fixer. It does not inhabit or depend on root nodules. Relying on nodules is the characteristic of symbiotic bacteria like Rhizobium.

❓ Why Other Options Are Incorrect

Option A, B, D → These are all true features of free-living nitrogen fixers.

🎯 Strategy

Application: The question asks for what is NOT associated. (C) describes a total dependency on nodules, which is the definition of a symbiotic bacterium, directly contradicting the nature of a "free-living" bacterium.

Final Logic: C is the false statement.

🧠 Memory Trick

"Azospirillum = Free, No Nodules."

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

14 How does the agricultural application of Azotobacter fundamentally differ from that of Rhizobium?

A Azotobacter enriches soil nitrogen while free-living, whereas Rhizobium requires a symbiotic association.

B Azotobacter requires a leguminous host, whereas Rhizobium does not.

C Azotobacter is a fungus, whereas Rhizobium is a bacterium.

D Azotobacter absorbs phosphorus, whereas Rhizobium absorbs nitrogen.

💡 Short Explanation

Azotobacter is free-living. Rhizobium is symbiotic (requires nodules). Both are nitrogen fixers.

✅ Correct Answer Explanation

The correct answer is (A). The fundamental difference is their ecological Strategy Azotobacter fixes nitrogen as a free-living microbe in the soil, whereas Rhizobium must form a symbiotic relationship within a leguminous root nodule to function as an effective fixer.

❓ Why Other Options Are Incorrect

Option B → Incorrect; Rhizobium is the one requiring the host.
Option C → Both are bacteria.
Option D → Both fix nitrogen; they don't primarily "absorb phosphorus" (that's Glomus).

🎯 Strategy

Application: Differentiate the microbes by their ecological niche. Azotobacter = Free-living; Rhizobium = Symbiotic.

Final Logic: A accurately contrasts the ecological strategies defined in the text.

🧠 Memory Trick

"Azotobacter = Free; Rhizobium = House-bound (Nodules)."

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

15 Which of the following is NOT an advantage derived by a plant forming a mycorrhizal association with the genus Glomus?

A Tolerance to salinity and drought

B Resistance to root-borne pathogens

C An overall increase in plant growth and development

D Increased atmospheric nitrogen fixation directly by the fungus

💡 Short Explanation

Glomus (mycorrhiza) helps with phosphorus absorption. Glomus does NOT fix nitrogen. Nitrogen fixation is the role of bacteria, not mycorrhizal fungi.

✅ Correct Answer Explanation

The correct answer is (D). The genus Glomus is a mycorrhizal fungus, not a nitrogen-fixing organism. Its primary benefit is phosphorus absorption. Nitrogen fixation is performed by bacteria or cyanobacteria.

❓ Why Other Options Are Incorrect

Option A, B, C → These are all documented benefits of mycorrhizal associations provided in the text.

🎯 Strategy

Application: The question asks for what is NOT an advantage. Phosphorus absorption is the Glomus advantage; nitrogen fixation is a Rhizobium advantage. (D) is a categorical error.

Final Logic: D incorrectly attributes the role of bacteria to a fungus.

🧠 Memory Trick

"Glomus = Phosphorus, not Nitrogen."

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

16 In a phosphorus-deficient soil, which microbial intervention would be most directly beneficial for plant growth?

A Application of baculoviruses

B Introduction of Glomus species

C Spraying Bacillus thuringiensis spores

D Inoculating the leaves with Trichoderma

💡 Short Explanation

Glomus = Mycorrhiza. Mycorrhiza = Phosphorus absorption. This is the direct solution to phosphorus deficiency.

✅ Correct Answer Explanation

The correct answer is (B). The genus Glomus forms mycorrhizal associations with plants. The primary functional benefit of these associations, as highlighted in NCERT, is the increased absorption of phosphorus from the soil.

❓ Why Other Options Are Incorrect

Option A → Baculoviruses are for insects.
Option C → Bt is for insects.
Option D → Trichoderma is for pathogens.

🎯 Strategy

Application: Phosphorus deficiency requires a phosphorus-solubilizing or absorbing microbe. Glomus is the only choice that matches this functional profile.

Final Logic: B is the only choice that addresses the specific nutritional deficiency.

🧠 Memory Trick

"P-Deficiency → Glomus (P-Absorber)."

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

📄 Passage

"Fungi are also known to form symbiotic associations with plants (mycorrhiza). Many members of the genus Glomus form mycorrhiza. The fungal symbiont in these associations absorbs phosphorus from soil and passes it to the plant. Plants having such associations show other benefits also, such as resistance to root-borne pathogens, tolerance to salinity and drought, and an overall increase in plant growth and development."

Based on the passage, a plant subjected to a severe lack of water would significantly benefit from mycorrhiza due to its enhanced:

A Pathogen resistance

B Drought tolerance

C Phosphorus secretion

D Nitrogen fixation

💡 Short Explanation

The passage states: "tolerance to... drought." This is a direct benefit of the Glomus mycorrhizal association. It helps the plant survive water-scarce conditions.

✅ Correct Answer Explanation

The correct answer is (B). The text specifically lists "tolerance to salinity and drought" as benefits provided by the mycorrhizal association. Therefore, in conditions of water scarcity, drought tolerance is the specific advantage provided.

❓ Why Other Options Are Incorrect

Option A, C, D → While these are related to other benefits or functions, (B) is the one directly addressing the "lack of water."

🎯 Strategy

Application: The question asks about "severe lack of water." (B) is the direct synonym for "drought tolerance."

Final Logic: B is derived directly from the provided passage.

🧠 Memory Trick

"Lack of Water = Drought Tolerance."

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

📄 Passage

According to the passage, the defensive benefit provided by the fungal symbiosis is specifically localized to protect against:

A Airborne viruses

B Leaf-eating insects

C Root-borne pathogens

D Stem-boring caterpillars

💡 Short Explanation

The passage states: "resistance to root-borne pathogens." This matches the symbiotic location (root mycorrhiza). This is the only defensive benefit listed in the passage.

✅ Correct Answer Explanation

The correct answer is (C). The passage explicitly mentions "resistance to root-borne pathogens" as a benefit of mycorrhizal associations. This makes biological sense because the fungus colonizes the root system, creating a protective zone.

❓ Why Other Options Are Incorrect

Option A, B, D → These threats are not mentioned as being affected by mycorrhiza in the provided text.

🎯 Strategy

Application: The passage contains the exact phrase "resistance to root-borne pathogens."

Final Logic: C is the exact localized threat mentioned in the provided text.

🧠 Memory Trick

"Mycorrhiza = Root Zone Defender."

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

19 Why are cyanobacteria uniquely suited to serve as biofertilisers in paddy fields?

A Because they require extremely dry terrestrial conditions to thrive.

B Because they are autotrophic microbes widely distributed in aquatic environments capable of fixing atmospheric nitrogen.

C Because they actively prey on aquatic arthropods.

D Because they act as fungi that absorb phosphorus from waterlogged soils.

💡 Short Explanation

Paddy fields are aquatic/wet. Cyanobacteria are autotrophic and aquatic. They fix nitrogen, enriching the soil.

✅ Correct Answer Explanation

The correct answer is (B). Cyanobacteria (like Anabaena and Nostoc) are naturally found in aquatic habitats. Since paddy fields provide a similar aquatic environment, they are the perfect setting for these nitrogen-fixing autotrophs to flourish and fertilize the crops.

❓ Why Other Options Are Incorrect

Option A → They are aquatic, not terrestrial/dry-adapted.
Option C → They are autotrophic, not predators.
Option D → They are bacteria, not fungi, and their role is nitrogen fixation, not phosphorus absorption.

🎯 Strategy

Application: The question asks why they are "uniquely suited." Aquatic environments + nitrogen fixation is the unique combination for cyanobacteria.

Final Logic: B correctly summarizes the biological traits that make cyanobacteria effective in paddy ecosystems.

🧠 Memory Trick

"Cyanobacteria = Water-loving N-fixers."

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

20 Which of the following statements correctly highlight the role of blue-green algae in agriculture?
I. They act as autotrophic microbes.
II. They add organic matter to the soil.
III. They decrease the fertility of the soil by consuming nutrients.

A I and II only

B II and III only

C I and III only

D I, II, and III

💡 Short Explanation

BGA (Cyanobacteria) are autotrophic (I). BGA add organic matter (II). They INCREASE (not decrease) fertility (III is false).

✅ Correct Answer Explanation

The correct answer is (A). Blue-green algae are autotrophic (photosynthetic) and they contribute significantly to soil fertility by adding organic matter and fixing atmospheric nitrogen. Statement (III) is completely incorrect as they improve, not decrease, fertility.

❓ Why Other Options Are Incorrect

Option B, C, D → All these options include the false statement (III).

🎯 Strategy

Application: Biofertilisers, by definition, improve fertility. Any statement saying they "decrease the fertility" (III) is logically false.

Final Logic: A is the only choice that captures the positive agricultural impact.

🧠 Memory Trick

"Blue-green = Good for soil (Autotrophic + Organic matter addition)."

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

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