Microbes in Human Welfare is a high-yield Class 12 Biology chapter because it links core microbiology with real-life applications like fermentation, sewage and wastewater treatment, nitrogen fixation, and disease prevention strategies. In board and competitive exams, questions commonly test stoichiometry/yield calculations, treatment efficiency logic, and conceptual understanding of how microbes support agriculture and human health.
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Marking
Q1. Complete anaerobic fermentation of glucose by Saccharomyces follows . If of glucose is fermented completely, what volume of (at STP, ) will be produced?
Q2. An industrial fermenter of volume initially contains of glucose. After fermentation the ethanol concentration is v/v. Density of ethanol . Theoretical yield of ethanol from glucose is . Approximately how much unfermented glucose (in kg) remains at the end?
Q3. A household requires for cooking. A biogas plant produces biogas per day with methane content . Calorific value of methane is . How much additional biogas (in /day) is required to meet the demand? (Assume only methane contributes to usable energy.)
No additional biogas is needed
Q4. Assertion (A): Application of Azotobacter as a biofertilizer in legume fields increases soil nitrogen more effectively than inoculation with Rhizobium.
Reason (R): Azotobacter fixes atmospheric nitrogen as free-living aerobes, whereas Rhizobium requires root nodule symbiosis; therefore Azotobacter supplies larger net N per hectare.
Both A and R are true and R is the correct explanation for A.
Both A and R are true but R is not the correct explanation for A.
A is true but R is false.
Both A and R are false.
Q5. A wastewater contains BOD = and –N = . For aerobic treatment, nitrification consumes per g –N oxidized, and heterotrophic BOD removal needs per g BOD removed. If aeration supplies dissolved O and complete nitrification is required, what percentage of the initial BOD can be oxidized?
Q6. Complete fermentation of glucose by yeast is represented by . If of glucose is fermented and the process has a mass-yield efficiency of , how many grams of ethanol are produced? (Atomic masses: )
Q7. A sewage influent has biochemical oxygen demand (BOD) . Primary sedimentation reduces BOD by . The secondary (activated sludge) unit removes of the BOD entering it. What is the final BOD after secondary treatment, and is tertiary treatment needed if the regulatory discharge limit is ?
; tertiary not required
; tertiary required
; tertiary required
; yes, tertiary required
Q8. For a disease with basic reproduction number and a vaccine efficacy of (i.e., vaccine reduces susceptibility by factor ), the critical vaccination coverage required to interrupt transmission can be estimated by . What minimum fraction of the population must be vaccinated?
Q9. A homofermentative Lactobacillus strain (producing mainly lactic acid from hexoses) is switched from glucose feed to an equimolar sucrose feed (sucrose = glucose + fructose) without changing strain or process parameters. Observed outcomes: total lactic acid yield falls, ethanol and by-products increase, and optical purity of lactic acid decreases. Which explanation best accounts for these observations?
The strain cannot hydrolyse sucrose (no invertase), so sugar uptake drops, lowering lactic acid yield.
Fructose is metabolised exclusively via the homofermentative pathway giving more optically pure lactic acid, so sucrose should increase yield and purity.
Fructose metabolism perturbs NADH/NAD^+ balance, diverting part of carbon into heterofermentative side reactions (ethanol + ), reducing lactic acid yield and optical purity.
Increased osmotic pressure of sucrose causes cell lysis and random by-product accumulation, explaining the observed changes.
Q10. In a recombinant E. coli culture antibiotic selection is removed. Plasmid segregational loss rate is per cell division (each dividing plasmid-bearing cell produces on average plasmid-free daughters per division) and plasmid-free cells have a growth advantage (their per-generation multiplication factor is while plasmid-bearing cells multiply by ). If the culture starts with all cells plasmid-bearing, approximately what fraction of the population will retain the plasmid after generations? (Assume discrete synchronous generations and no back-mutation.)
Q11. A household biogas plant produces of biogas per day composed of CH and CO. Given the calorific value of methane is and CO contributes negligible calorific value, estimate the daily usable energy output (MJ/day). Use .
Q12. A bacterial population of cells contains resistant mutants at a frequency of in . An antibiotic kills of susceptible cells each hour (only of susceptibles survive per hour); resistant cells are unaffected and double every hour. Approximately what fraction of the population will be resistant after hours?
Q13. A sewage influent has biochemical oxygen demand (BOD) and ammonia . Primary treatment removes of BOD and of ammonia. Secondary (activated sludge) treatment subsequently removes of the remaining BOD and of the remaining ammonia. What are the approximate final BOD and ammonia concentrations (in ) after both stages?
BOD ; NH
BOD ; NH
BOD ; NH
BOD ; NH
Q14. Assertion (A): In submerged fermentation by Aspergillus niger for industrial citric acid production, maintaining a very low manganese concentration (e.g. ) together with high sugar concentration maximizes citric acid yield.
Reason (R): Low manganese inhibits enzymes such as aconitase in the TCA cycle, reducing conversion of citrate to isocitrate, while excess sugar increases glycolytic flux supplying abundant acetyl-CoA and oxaloacetate so that citrate accumulates and is secreted.
Both A and R are true, and R correctly explains A.
Both A and R are true, but R does not correctly explain A.
A is true but R is false.
Both A and R are false.
Q15. Consider a bacterial population of size . The spontaneous mutation rates per generation to resistance against antibiotic A and antibiotic B are and respectively. If resistance to both antibiotics requires two independent point mutations occurring in the same lineage, estimate the expected number of bacteria resistant to both antibiotics present before treatment.