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Microbes in Human Welfare Set-5 MCQ Online Practice Test | Class 12 | Quiz Tweek

Microbes in Human Welfare Set-5

Practice Important MCQs for Microbes in Human Welfare — Biology, Class 12.

Microbes in Human Welfare is a high-scoring chapter because it links microbiology with real-life applications such as fermentation (food and alcohol), sewage treatment, biogas production, and antibiotic use. Board and competitive exams repeatedly test both core concepts (growth conditions, microbial metabolism, biogeochemical roles) and application-based reasoning (why treatments work or fail under different conditions like oxygen limitation, nitrate presence, or heavy-metal contamination).

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Q1. Anaerobic fermentation of glucose by yeast follows the stoichiometry $C_6H_{12}O_6 \rightarrow 2\,C_2H_5OH + 2\,CO_2$ . If $1$ mole of glucose (molar mass $=180\ \mathrm{g\,mol^{-1}}$ ) is fermented completely, what is the maximum theoretical mass percent yield of ethanol produced? (molar mass of ethanol $=46\ \mathrm{g\,mol^{-1}}$ )

(A)

$50.0\%$

(B)

$66.7\%$

(C)

$51.1\%$

(D)

$25.6\%$

Q2. Influent sewage has biochemical oxygen demand (BOD) $=300\ \mathrm{mg\,L^{-1}}$ . Primary sedimentation removes $40\%$ of BOD; secondary treatment (activated sludge) removes $90\%$ of the remaining BOD. What is the BOD of effluent after secondary treatment (in $\mathrm{mg\,L^{-1}}$ )?

(A)

$18\ \mathrm{mg\,L^{-1}}$

(B)

$30\ \mathrm{mg\,L^{-1}}$

(C)

$60\ \mathrm{mg\,L^{-1}}$

(D)

$90\ \mathrm{mg\,L^{-1}}$

Q3. A process for producing single-cell protein (SCP) uses carbohydrate substrate. Microbe A yields $0.6\ \mathrm{kg}$ biomass per kg substrate with protein content $45\%$ . Microbe B yields $0.4\ \mathrm{kg}$ biomass per kg substrate with protein content $70\%$ . Which microbe gives the higher protein yield per kg substrate and by how many grams?

(A)

Microbe A by $10\ \mathrm{g}$

(B)

Microbe B by $100\ \mathrm{g}$

(C)

Both give equal protein ( $270\ \mathrm{g}$ per kg)

(D)

Microbe B by $10\ \mathrm{g}$

Q4. An antibiotic-producing Streptomyces fermentation gives high titre in a $2\ \mathrm{L}$ lab fermenter but when scaled up to $2000\ \mathrm{L}$ the antibiotic concentration falls by ~40% despite identical strain and medium. Which of the following is the most plausible combined explanation?

(A)

Reduced volumetric oxygen transfer coefficient ( $k_La$ ) in the large vessel causing overall oxygen limitation and reduced activity of oxidative steps in antibiotic biosynthesis

(B)

Increased shear in the large reactor fragments mycelial pellets, improving mass transfer and thereby increasing antibiotic production

(C)

Both reduced $k_La$ (leading to oxygen limitation) and formation of substrate-rich microzones in the large tank that cause local catabolite repression of secondary metabolism

(D)

Increased hydrostatic pressure in the large tank causes loss of biosynthetic plasmids encoding the antibiotic pathway

Q5. Groundwater contaminated with dissolved heavy metals ( $Me^{2+}$ ) also contains high sulfate ( $SO_4^{2-}$ ) and high nitrate ( $NO_3^{-}$ ). The remediation plan is to precipitate metals as metal sulfides ( $MeS$ ) by stimulating sulfate-reducing bacteria (SRB). Which intervention is most appropriate to enable effective metal sulfide precipitation?

(A)

Add excess sulfate to the aquifer so SRB will produce enough sulfide to precipitate metals despite the nitrate

(B)

First promote denitrification (e.g., add labile organic carbon to stimulate denitrifiers) to remove $NO_3^{-}$ , and then allow anaerobic SRB activity to generate $H_2S$ for $MeS$ precipitation

(C)

Aerate the groundwater to oxidize nitrate and thereby stimulate SRB activity

(D)

Introduce SRB strains engineered to use nitrate as electron acceptor so they produce sulfide in the presence of high $NO_3^{-}$

Q6. In an anaerobic fermentation carried out by yeast, glucose is converted as: $C_6H_{12}O_6 \rightarrow 2\,C_2H_5OH + 2\,CO_2$ . If $180\ \mathrm{g}$ of pure glucose is completely fermented, the theoretical mass of ethanol produced is:

(A)

$46\ \mathrm{g}$

(B)

$184\ \mathrm{g}$

(C)

$92\ \mathrm{g}$

(D)

$138\ \mathrm{g}$

Q7. Per kilogram of substrate, cattle dung contains $40\%$ carbon and $1.6\%$ nitrogen (C/N = 25) while vegetable waste contains $48\%$ carbon and $0.6\%$ nitrogen (C/N = 80). To obtain an overall C/N ratio of $30$ suitable for anaerobic digestion, the mass ratio (cattle dung : vegetable waste) that should be mixed is nearest to:

(A)

$15:4$

(B)

$1:3$

(C)

$1:1$

(D)

$3:1$

Q8. Assertion (A): Routine use of sub‑therapeutic antibiotics (e.g., tetracycline) in livestock feed accelerates the emergence of multidrug‑resistant bacteria in the farm microbiome.
Reason (R): Antibiotic resistance determinants are frequently carried on mobile genetic elements (plasmids, transposons, integrons) that encode conjugation or recombination functions, enabling horizontal transfer of multiple resistance genes among commensal and pathogenic bacteria.

(A)

Both A and R are true but R is not a correct explanation of A.

(B)

A is true but R is false.

(C)

A is false but R is true.

(D)

Both A and R are true and R correctly explains A.

Q9. A sewage influent has biochemical oxygen demand (BOD) = $300\ \mathrm{mg/L}$ . Primary settling removes $30\%$ of BOD. An activated sludge secondary treatment then removes $85\%$ of the remaining BOD. The expected final BOD after both treatments is approximately:

(A)

$45\ \mathrm{mg/L}$

(B)

$31.5\ \mathrm{mg/L}$

(C)

$63\ \mathrm{mg/L}$

(D)

$100\ \mathrm{mg/L}$

Q10. In industrial submerged fermentation for citric acid using Aspergillus niger, an accidental contamination raised the $Mn^{2+}$ concentration to $0.1\ \mathrm{mg/L}$ . Citric acid yield dropped sharply despite unchanged sugar concentration, pH and aeration. Which explanation plus immediate corrective action (without changing strain) best fits the observation?

(A)

Trace $Mn^{2+}$ acts as a cofactor for enzymes (e.g., aconitase/isocitrate dehydrogenase) that divert citrate into the TCA cycle and favour biomass instead of extracellular citric acid; the fastest effective corrective action is to remove or chelate $Mn^{2+}$ (e.g., ion‑exchange/adsorbent) to restore metal‑depleted conditions.

(B)

Excess $Mn^{2+}$ activates citrate‑consuming enzymes (e.g., aconitase), lowering extracellular citrate accumulation; the fastest correction is to increase sugar concentration to outcompete citrate consumption.

(C)

Trace $Mn^{2+}$ lowers dissolved oxygen solubility and thereby reduces citric acid production; immediate remedy is to increase aeration rate several‑fold.

(D)

Trace $Mn^{2+}$ stimulates sporulation diverting carbon into spores; immediate remedy is to raise fermentation pH to neutral to inhibit sporulation.

Q11. A sewage sample initially contains $1.0\times10^{3}$ viable bacteria. Under optimal conditions the bacteria divide by binary fission with generation time $g=30\ \text{min}$ . After $t=5\ \text{h}$ , estimate the number of bacteria. Use $N=N_0 2^n$ and $n=\dfrac{t}{g}$ .

(A)

$1.00\times10^{6}$

(B)

$1.024\times10^{9}$

(C)

$1.024\times10^{6}$

(D)

$2.048\times10^{5}$

Q12. A full-scale anaerobic digester treating protein-rich animal waste develops elevated free ammonia. Microbial analyses over weeks show decline of acetoclastic methanogens and enrichment of syntrophic acetate-oxidising bacteria (SAOB) together with hydrogenotrophic methanogens; overall methane yield remains largely sustained. Which explanation best accounts for this shift?

(A)

High free ammonia selectively inhibits acetoclastic methanogens; SAOB oxidize acetate to $H_2$ and $CO_2$ , and hydrogenotrophic methanogens convert $H_2+CO_2$ to $CH_4$ , maintaining methane production.

(B)

Free ammonia acts as a nitrogen nutrient that stimulates acetoclastic methanogens, increasing methane production.

(C)

Ammonia chemically converts acetate to methane without microbial mediation, so microbial composition changes are irrelevant.

(D)

Ammonia precipitates essential trace metals, irreversibly halting all methanogenesis and drastically reducing methane yield.

Q13. In a 1 L batch fermentation, a mixed culture of lactic acid bacteria ferments $0.100\ \text{mol}$ of glucose. Homofermentative LAB convert glucose by $C_6H_{12}O_6 \rightarrow 2\,\text{lactate}$ (no $CO_2$ ), while heterofermentative LAB convert glucose by $C_6H_{12}O_6 \rightarrow \text{lactate} + \text{ethanol} + CO_2$ (one mole $CO_2$ per glucose). If the total $CO_2$ evolved is $0.020\ \text{mol}$ , what percentage of the glucose was fermented heterofermentatively?

(A)

$2\%$

(B)

$10\%$

(C)

$50\%$

(D)

$20\%$

Q14. Soil contaminated with highly chlorinated polychlorinated biphenyls (PCBs) was treated by two remediation sequences: (I) anaerobic reductive treatment followed by aerobic biodegradation; (II) aerobic biodegradation followed by anaerobic treatment. Sequence (I) produced near-complete mineralization while (II) left persistent chlorinated intermediates. Which statement best explains why sequence (I) succeeds?

(A)

Aerobic oxygenases initially cleave the aromatic rings of highly chlorinated PCBs, producing simpler compounds that are then dechlorinated anaerobically.

(B)

Under anaerobic conditions microbes catalyse reductive dechlorination (removing $Cl$ substituents) converting highly chlorinated congeners to less-chlorinated biphenyls; these less-chlorinated congeners are then susceptible to oxygenase-mediated ring hydroxylation and cleavage under aerobic conditions, enabling mineralization.

(C)

Aerobic microbes alone can mineralize highly chlorinated PCBs efficiently without any prior dechlorination.

(D)

Anaerobic treatment alone is sufficient to mineralize highly chlorinated PCBs because reductive dechlorination directly yields $CO_2$ and harmless products.

Q15. Assertion (A): Addition of nitrate-rich fertiliser (high $NO_3^-$ ) to soil often suppresses nodulation and biological nitrogen fixation in legumes. Reason (R): Presence of available nitrate reduces a legume's release of flavonoid signals and carbon allocation to roots, downregulating rhizobial nod gene induction and nodule development. Choose the correct option.

(A)

Both A and R are true, and R correctly explains A.

(B)

Both A and R are true, but R does not correctly explain A.

(C)

A is true but R is false.

(D)

A is false but R is true.

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