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Microbes in Human Welfare Set-4 MCQ Online Practice Test | Class 12 | Quiz Tweek

Microbes in Human Welfare Set-4

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

Microbes in Human Welfare explains how beneficial microorganisms are used to produce food, medicines, and industrial products, and also how microbial processes are applied for wastewater and waste management. For CBSE and competitive exams, this chapter is crucial because it links microbial metabolism (aerobic/anaerobic), biotechnological applications (fermentation, antibiotics, enzymes), and public health/environmental safety (biogas, sewage treatment, composting) through both concepts and calculations.

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Q1. Yeast (Saccharomyces cerevisiae) ferments glucose according to the reaction $C_6H_{12}O_6 \rightarrow 2\,C_2H_5OH + 2\,CO_2$ . If $180\ \text{g}$ of glucose is completely fermented, the theoretical mass of ethanol produced is:

(A)

$46\ \text{g}$

(B)

$92\ \text{g}$

(C)

$138\ \text{g}$

(D)

$180\ \text{g}$

Q2. A polluted lake has initial pollutant concentration $C_0 = 120\ \text{mg L}^{-1}$ . Biodegradation follows first-order kinetics with rate constant $k = 0.08\ \text{day}^{-1}$ (i.e., $C_t = C_0 e^{-kt}$ ). After how many days will the pollutant concentration fall below $30\ \text{mg L}^{-1}$ ? (Round to one decimal place.)

(A)

$10.0\ \text{days}$

(B)

$23.1\ \text{days}$

(C)

$30.0\ \text{days}$

(D)

$17.3\ \text{days}$

Q3. An anaerobic digester treats $1000\ \text{kg}$ of cattle dung containing $30\%$ volatile solids (VS) by mass. If biogas yield is $0.5\ \text{m}^3$ per kg VS removed, methane fraction of biogas is $60\%$ , and lower heating value (LHV) of methane is $35.8\ \text{MJ m}^{-3}$ , estimate the total energy content (in MJ) of methane produced. (Assume all VS are removed.)

(A)

$3.22\times10^{3}\ \text{MJ}$

(B)

$1.61\times10^{3}\ \text{MJ}$

(C)

$4.50\times10^{3}\ \text{MJ}$

(D)

$1.50\times10^{2}\ \text{MJ}$

Q4. Assertion (A): Thermophilic composting of sewage sludge (temperatures $>55^\circ\text{C}$ ) substantially reduces viable pathogenic vegetative bacteria but may not eliminate endospores of Bacillus; surviving spores can germinate later when the sludge is applied as manure.
Reason (R): Temperatures above $55^\circ\text{C}$ during thermophilic composting reliably destroy all bacterial endospores, so any pathogen detected later must be due to environmental recontamination.

(A)

Both A and R are true and R is a correct explanation of A

(B)

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

(C)

A is true but R is false

(D)

A is false but R is true

Q5. At a poultry farm sub-therapeutic (low) doses of antibiotics are routinely added to feed as growth promoters. Which of the following best explains how this practice increases the risk of multi-drug resistant bacteria entering the human food chain?

(A)

Sub-therapeutic antibiotics directly induce high rates of point mutations in bacterial chromosomes, rapidly creating resistance alleles.

(B)

Low antibiotic concentrations select for resistant strains and promote horizontal gene transfer (conjugation, transposons, integrons) within dense gut microbial communities and biofilms, amplifying multi-drug resistance that can transfer to humans.

(C)

Antibiotics in feed sterilize the gut leaving only beneficial bacteria, which prevents resistance spread.

(D)

Antibiotic residues are completely metabolized in animals and therefore do not exert selection pressure on gut or environmental microbes.

Q6. A bacterial culture in exponential phase has a doubling time of $30\ \text{min}$ . If the culture starts with $1\times10^{6}$ cells, how many cells will be present after $2$ hours?

(A)

$6.4\times10^{6}$

(B)

$3.2\times10^{6}$

(C)

$1.6\times10^{7}$

(D)

$3.2\times10^{7}$

Q7. Raw sewage has biochemical oxygen demand (BOD) of $200\ \text{mg L}^{-1}$ . Primary sedimentation removes $30\%$ of BOD; subsequent activated sludge (secondary biological treatment) removes $85\%$ of the remaining BOD. What is the BOD (in $\text{mg L}^{-1}$ ) after these two stages?

(A)

$21\ \text{mg L}^{-1}$

(B)

$70\ \text{mg L}^{-1}$

(C)

$30\ \text{mg L}^{-1}$

(D)

$14\ \text{mg L}^{-1}$

Q8. In a chemostat with a single limiting substrate the steady-state substrate concentration for a species is given by $S^{*}=\dfrac{K_s\,D}{\mu_{\max}-D}$ , where $D$ is dilution rate. Two species compete: A with $\mu_{\max}=0.8\ \text{h}^{-1},\ K_s=50\ \text{mg L}^{-1}$ and B with $\mu_{\max}=0.4\ \text{h}^{-1},\ K_s=10\ \text{mg L}^{-1}$ . Calculate $S^{*}$ for both species at $D=0.1\ \text{h}^{-1}$ and at $D=0.35\ \text{h}^{-1}$ and predict which species will dominate at low and at high $D$ (assume $D<\min(\mu_{\max})$ ).

(A)

Species A will dominate at both $D=0.1$ and $D=0.35\ \text{h}^{-1}$ .

(B)

Species B will dominate at both dilution rates.

(C)

The two species will coexist stably at both dilution rates.

(D)

Species B wins at low $D$ (e.g. $D=0.1\ \text{h}^{-1}$ ) while species A wins at higher $D$ (e.g. $D=0.35\ \text{h}^{-1}$ ).

Q9. Penicillin is produced as a secondary metabolite by Penicillium chrysogenum. Which of the following fermentation conditions is most likely to maximise penicillin yield in an industrial batch process?

(A)

Excess nitrogen, limited aeration and high sugar concentration to maximise rapid biomass growth.

(B)

Ample carbon source with deliberately limited nitrogen and sulphur, good aeration (high dissolved O $_2$ ) and controlled pH near 6–6.5 to push cells into secondary metabolism.

(C)

Very low carbon with high nitrogen to keep cells in prolonged exponential growth before harvest.

(D)

Very high agitation and elevated temperature to maximise growth rate and continuous nutrient feed.

Q10. A recombinant microbial culture produces an extracellular enzyme only after induction. Before induction the specific growth rate is $\mu=0.6\ \text{h}^{-1}$ ; after induction it drops to $\mu'=0.3\ \text{h}^{-1}$ . Specific enzyme production rate after induction is $q_p=0.02\ \text{g enzyme g}^{-1}\text{cell h}^{-1}$ . Initial biomass $X_0=0.01\ \text{g L}^{-1}$ ; harvest time $t_h=8\ \text{h}$ . Assuming exponential growth before and after induction, total enzyme produced by harvest when induced at time $t_{\text{ind}}$ is

E(t_{\text{ind}})=q_p X_0 e^{\mu t_{\text{ind}}}\,\frac{e^{\mu'(t_h-t_{\text{ind}})}-1}{\mu'}.

Which induction time among the choices will maximise total enzyme at harvest $t_h=8\ \text{h}$ ?

(A)

Induce at $t\approx 5.7\ \text{h}$

(B)

Induce immediately at $t=0\ \text{h}$

(C)

Induce at $t=2\ \text{h}$

(D)

Induce late at $t=7\ \text{h}$

Q11. In alcoholic fermentation by Saccharomyces cerevisiae, glucose is converted according to $C_6H_{12}O_6 \rightarrow 2\,C_2H_5OH + 2\,CO_2$ . If $180\ \mathrm{g}$ of glucose is completely fermented, the maximum mass of ethanol produced is approximately:

(A)

$46\ \mathrm{g}$

(B)

$138\ \mathrm{g}$

(C)

$92\ \mathrm{g}$

(D)

$180\ \mathrm{g}$

Q12. A household biogas plant produces $1.2\ \text{m}^3$ of biogas per day with methane fraction $f_{CH_4}=65\%$ by volume. Given the calorific value of pure methane is $35.8\ \text{MJ/m}^3$ , the approximate daily energy content of the produced biogas is (use $E=V\times f_{CH_4}\times 35.8\ \text{MJ/m}^3$ ):

(A)

$\approx 28\ \text{MJ}$

(B)

$\approx 21\ \text{MJ}$

(C)

$\approx 34\ \text{MJ}$

(D)

$\approx 8\ \text{MJ}$

Q13. During scale-up fermentation of Penicillium chrysogenum the medium with excess nitrogen produced high biomass but very low penicillin titer. Which change in process conditions is most likely to increase penicillin (a secondary metabolite) yield?

(A)

Maintain continuous excess glucose and ammonium to sustain exponential growth and fast turnover.

(B)

Switch to anaerobic conditions with minimal aeration to force stress-induced production.

(C)

Apply large intermittent glucose pulses and allow wide pH fluctuations to stress the culture.

(D)

Limit nitrogen to allow cells to enter stationary phase while supplying adequate carbon and oxygen and maintain controlled $pH$ — promoting secondary metabolite (penicillin) synthesis.

Q14. Statement I: Bacillus thuringiensis (Bt) formulations are used as selective biopesticides effective against specific insect pests and are considered safe for humans and many non‑target organisms. Statement II: Cry ( $\delta$ ‑endotoxin) proteins are synthesized by Bt as crystalline parasporal inclusions during sporulation.

(A)

Both I and II are true, and II explains why Bt formulations are selective and safe.

(B)

Both I and II are true, but II does not explain why Bt formulations are selective and safe.

(C)

I is true but II is false.

(D)

I is false but II is true.

Q15. A farmer wants an overall C:N ratio of $30:1$ for compost by mixing high‑carbon straw (C:N $80:1$ ) with fresh green material (C:N $15:1$ ). What fraction by mass of straw ( $x$ ) should be mixed with green material to achieve this target? (Solve $(x\times 80)+(1-x)\times 15 = 30$ .)

(A)

$\approx 23\%$

(B)

$30\%$

(C)

$46\%$

(D)

$15\%$

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