201. Assertion: Continuous aeration during secondary sewage treatment helps lower the BOD of the effluent.
Reason: Aeration supports vigorous growth of aerobic microbial flocs that consume a substantial amount of biodegradable organic matter.
ⓐ. Assertion is false, but Reason is true
ⓑ. Both Assertion and Reason are true, but Reason does not correctly explain Assertion
ⓒ. Assertion is true, but Reason is false
ⓓ. Both Assertion and Reason are true, and Reason correctly explains Assertion
Correct Answer: Both Assertion and Reason are true, and Reason correctly explains Assertion
Explanation: Both statements are correct, and the Reason provides the biological explanation for the Assertion. In the aeration tank, continuous air supply and mechanical agitation create favourable conditions for aerobic heterotrophic microorganisms. These organisms form flocs and actively consume the biodegradable organic matter present in primary effluent. Because BOD reflects the oxygen demand associated with bacterial oxidation of biodegradable organic material, removal of that material causes BOD to fall. Aeration therefore lowers BOD indirectly by supporting the microbial population responsible for organic-matter removal; air pumping alone is not the purification mechanism.
202. A sewage-treatment plant records the following BOD values for the same wastewater before and after effective secondary treatment.
| Stage | BOD in relative units |
|---|
| Before secondary treatment | 240 |
| After secondary treatment | 60 |
Which inference is best supported by the change?
ⓐ. The lower BOD shows that the biodegradable organic load increased during treatment
ⓑ. BOD changed only because grit and pebbles settled from the wastewater
ⓒ. The values show that secondary treatment had little effect on the organic load
ⓓ. The large BOD decline indicates substantial microbial removal of biodegradable organic matter
Correct Answer: The large BOD decline indicates substantial microbial removal of biodegradable organic matter
Explanation: BOD reflects the oxygen microorganisms would consume while oxidising biodegradable organic matter in water. Effective secondary treatment promotes aerobic microbial flocs that consume much of this organic load. A fall from a high value before treatment to a much lower value afterward therefore indicates substantial biological removal of organic matter and a corresponding reduction in polluting potential. The important conclusion comes from the direction and biological meaning of the BOD change rather than from arithmetic alone.
203. Use the activated-sludge arrangement described below.
After microbial flocs settle, the activated sludge is divided into two branches. Branch P carries a small fraction back to the aeration tank, while Branch Q carries the major fraction to another treatment chamber.
Which combination correctly identifies the purposes of P and Q?
ⓐ. P provides microbial inoculum; Q undergoes anaerobic digestion
ⓑ. P produces primary sludge; Q undergoes filtration
ⓒ. P removes floating debris; Q produces curd
ⓓ. P undergoes distillation; Q is used for aerobic bread fermentation
Correct Answer: P provides microbial inoculum; Q undergoes anaerobic digestion
Explanation: Activated sludge consists of the microbial flocs that settle after secondary treatment. Only a small fraction is returned to the aeration tank, where it functions as an inoculum by supplying an active population of microorganisms for continued biological treatment of incoming primary effluent. The major fraction follows a different route and is transferred to anaerobic sludge digesters. There, anaerobic bacteria digest much of the bacterial and fungal biomass and produce gases including methane, \(H_2S\) and \(CO_2\). The two branches therefore serve distinct biological functions: microbial reseeding of the aerobic stage and anaerobic digestion of excess microbial biomass.
204. A functioning gobar-gas plant contains fresh dung slurry and has an unobstructed gas outlet, but microbial activity inside the digester becomes severely suppressed. Which change would most directly be expected?
ⓐ. The floating gas holder would rise more rapidly because less microbial activity produces more gas
ⓑ. Gas accumulation would decrease, so the floating gas holder would rise less or may fail to rise normally
ⓒ. The digester would automatically become an aeration tank producing activated sludge
ⓓ. Gas accumulation would remain unchanged because the gas holder responds only to slurry volume rather than microbial activity
Correct Answer: Gas accumulation would decrease, so the floating gas holder would rise less or may fail to rise normally
Explanation: The floating gas holder rises because gases generated by anaerobic microbial digestion accumulate beneath it. Methanogenic activity is particularly important for production of methane-rich biogas from dung and other suitable organic material. If microbial activity inside the digester is strongly suppressed, gas generation decreases even when the physical structure of the plant and gas outlet remain intact. Less gas accumulates beneath the holder, so its upward movement is reduced. This changed-condition prediction connects the visible movement of the plant component directly with the underlying microbial process rather than treating the gas holder as an independent mechanical device.
205. Three biological-control agents are tested against one target insect pest and several non-target organisms.
| Agent | Target-pest mortality | Beneficial-insect mortality | Effect on birds and fish |
|---|
| P | Very high | Very high | Adverse effects observed |
| Q | Very high | Minimal | No adverse effect observed |
| R | Low | Minimal | No adverse effect observed |
If the aim is effective pest control in an ecologically sensitive area while conserving non-target organisms, which agent shows the pattern most characteristic of a suitably specific baculovirus?
ⓐ. Agent P
ⓑ. Agent Q
ⓒ. Agent R
ⓓ. All three are equally suitable
Correct Answer: Agent Q
Explanation: A desirable baculovirus used in biological control combines strong action against the target insect with a narrow host range and minimal effects on non-target organisms. Agent Q fits this pattern because target-pest mortality is very high while beneficial insects, birds and fish remain largely unaffected. Agent P behaves more like a broad-spectrum agent and would create unnecessary ecological damage, whereas Agent R is comparatively safe but provides poor control of the intended pest. The narrow specificity of baculoviruses is therefore an ecological advantage, especially in integrated pest management programmes and sensitive ecosystems where conservation of beneficial organisms is important.
206. Bacillus thuringiensis is ingested normally by susceptible caterpillars, but toxin action in the larval gut is experimentally blocked. What outcome is most likely?
ⓐ. Caterpillar mortality will increase because Bt acts mainly by fixing nitrogen in the leaf
ⓑ. The larvae will die through methane released from the bacterial preparation
ⓒ. Caterpillar mortality will remain unchanged because gut toxin action is unrelated to Bt control
ⓓ. Caterpillar mortality will decrease because ingestion must be followed by toxin action in the larval gut
Correct Answer: Caterpillar mortality will decrease because ingestion must be followed by toxin action in the larval gut
Explanation: Conventional Bt control depends on more than simple contact with the sprayed preparation. Susceptible larvae ingest the bacterial material, after which the insecticidal toxin acts in the larval gut and causes death. If ingestion occurs but the gut-level toxin action is prevented, the biological sequence is interrupted after entry into the larva, so effective caterpillar mortality should fall. This distinguishes the ingestion step from the subsequent toxic action that actually kills the susceptible insect.
207. A farmer claims, "Mycorrhizal fungi improve crop growth mainly because they fix atmospheric nitrogen in exactly the same way as Rhizobium." Which evaluation is most accurate?
ⓐ. The claim is correct because all biofertilisers improve plants through the same mechanism
ⓑ. The claim is incorrect because the major mycorrhizal benefit emphasised here is improved phosphorus acquisition, together with additional disease- and stress-related benefits
ⓒ. The claim is correct because the major mycorrhizal benefit emphasised here is improved atmospheric nitrogen fixation, together with additional disease- and stress-related benefits
ⓓ. The claim is incorrect only because mycorrhiza is a bacterial association rather than a fungal one
Correct Answer: The claim is incorrect because the major mycorrhizal benefit emphasised here is improved phosphorus acquisition, together with additional disease- and stress-related benefits
Explanation: Mycorrhiza is a symbiotic association between fungal partners such as Glomus and plant roots. Its major nutritional role is enhanced phosphorus acquisition: fungal hyphae absorb phosphorus from soil and transfer it to the plant. Mycorrhizal plants can also show increased resistance to root-borne pathogens and improved tolerance to stresses such as salinity and drought. Rhizobium, in contrast, is a bacterial symbiont of legume root nodules and is associated with atmospheric nitrogen fixation. Both are biofertiliser relationships, but their organisms, nutrient functions and biological mechanisms are different.
208. The material flow through a simplified biogas plant is represented as follows:
Dung and water -> P -> microbial gas accumulation -> Q
Residual digested material -> R
Which identification of P, Q and R is correct?
ⓐ. P = aeration tank; Q = activated-sludge return pipe; R = primary sludge
ⓑ. P = anaerobic digester; Q = gas outlet supplying usable biogas; R = spent slurry that cannot be used as fertiliser
ⓒ. P = primary settling tank; Q = \(CO_2\) outlet for bread making; R = sewage grit
ⓓ. P = anaerobic digester; Q = gas outlet supplying usable biogas; R = spent slurry that can be used as fertiliser
Correct Answer: P = anaerobic digester; Q = gas outlet supplying usable biogas; R = spent slurry that can be used as fertiliser
Explanation: Dung mixed with water enters the anaerobic digester of the biogas plant. Inside this chamber, microorganisms, including methanogens associated with dung, act on the organic material and generate methane-rich biogas. The gas accumulates beneath the gas holder and is removed through an outlet pipe for practical uses such as cooking and lighting. The digested residual slurry leaves through a separate outlet and can be applied to agricultural fields as fertiliser. The plant therefore produces two useful outputs from organic waste: a combustible microbial fuel and a nutrient-containing residual material suitable for agricultural reuse.
209. A microbiologist isolates organism P from an extremely acidic habitat and organism Q from ordinary soil. Both can be grown on suitable nutritive media, where repeated multiplication produces colonies visible to the naked eye. Which conclusion is best supported?
ⓐ. P cannot be a microorganism because it lives under extreme conditions
ⓑ. Microorganisms may occupy very different habitats, and microscopic cells cannot collectively form visible colonies during culture
ⓒ. Microorganisms may occupy very different habitats, and microscopic cells can collectively form visible colonies during culture
ⓓ. Visible colonies prove that individual microbial cells are macroscopic
Correct Answer: Microorganisms may occupy very different habitats, and microscopic cells can collectively form visible colonies during culture
Explanation: The observations combine two independent properties of microbial life. First, microorganisms are not restricted to ordinary environments such as soil and water; some occur in extreme habitats, including highly acidic environments, deep soil, thick snow and thermal regions approaching \(100^\circ\text{C}\). Second, individual microbial cells may be microscopic while a large population produced through repeated multiplication forms a colony visible to the naked eye. The visibility of a colony therefore reflects the accumulation of many cells rather than enlargement of each individual microbe. Habitat extremity and colony formation are separate biological features and neither implies pathogenicity.
210. Equal volumes of milk are placed in four containers.
Container P receives viable LAB and is kept at a suitable temperature.
Container Q receives viable LAB but is kept under conditions that severely restrict their multiplication.
Container R receives no starter culture.
Container S receives viable LAB at a suitable temperature, but acid accumulation is experimentally prevented.
In which container should normal curd formation be most strongly expected?
ⓐ. Container Q
ⓑ. Container R
ⓒ. Container P
ⓓ. Container S
Correct Answer: Container P
Explanation: Normal curd formation requires both an active inoculum of lactic acid bacteria and conditions that allow those bacteria to multiply and produce acids. Container P provides viable LAB as well as a suitable temperature, so bacterial growth and acid production can proceed efficiently. The acids produced by LAB cause coagulation and partial digestion of milk proteins, producing the characteristic curd structure. Container Q restricts bacterial multiplication, R lacks the microbial starter, and S blocks the acid-mediated step even though viable LAB are present. This reasoning separates microbial presence, growth conditions and acid production as distinct requirements in the curd-forming process.
211. Study the microbial food relationships below.
| Food process | Major microbial relation | Characteristic outcome |
|---|
| P. Curd formation | LAB | Acid-mediated change in milk proteins |
| Q. Dosa/idli fermentation | Bacteria | \(CO_2\)-associated puffing |
| R. Bread making | Saccharomyces cerevisiae | Yeast fermentation |
| S. Swiss cheese | Propionibacterium sharmanii | \(CO_2\)-associated holes |
Which evaluation is correct?
ⓐ. P, Q, R and S are correct
ⓑ. Only P, Q and R are correct
ⓒ. Only Q, R and S are correct
ⓓ. Only P and S are correct
Correct Answer: P, Q, R and S are correct
Explanation: Each row correctly links a food product with the relevant microorganism or microbial group and the biological outcome of its activity. LAB produce acids in milk, causing coagulation and partial digestion of milk proteins during curd formation. Bacterial fermentation of dosa and idli batter produces \(CO_2\), contributing to the puffed appearance. Saccharomyces cerevisiae functions as baker's yeast in bread fermentation, while Propionibacterium sharmanii produces large quantities of \(CO_2\) that create the characteristic holes in Swiss cheese. The important reasoning is that similar outcomes such as gas formation can arise in different foods through different microorganisms.
212. During preparation of two cheeses, microbial growth proceeds normally in both. In cheese P, \(CO_2\) formation by the characteristic bacterium is experimentally suppressed. In cheese Q, the specific fungal growth required during ripening is prevented. Which prediction is most appropriate?
ⓐ. P should lose only its vitamin \(B_{12}\) enrichment, while Q should lose methane production
ⓑ. P should retain its characteristic holes because cheese structure is independent of bacterial \(CO_2\), while Q should lose its fungal-ripened flavour
ⓒ. P should show reduced formation of characteristic holes, while Q should fail to develop its characteristic fungal-ripened flavour normally
ⓓ. Both cheeses should remain completely unchanged because microbes affect only preservation
Correct Answer: P should show reduced formation of characteristic holes, while Q should fail to develop its characteristic fungal-ripened flavour normally
Explanation: Swiss cheese and Roquefort-type cheese illustrate two different microbial contributions to cheese characteristics. In Swiss cheese, Propionibacterium sharmanii produces \(CO_2\), and gas accumulation creates the characteristic holes. Suppressing this gas production would therefore reduce that structural feature. In Roquefort cheese, the characteristic flavour develops through the growth of specific fungi during ripening. Preventing that fungal growth would interfere with flavour development rather than primarily affecting gas-hole formation. The distinction concerns mechanism-to-product reasoning rather than simple recognition of cheese names.
213. Saccharomyces cerevisiae is used in bread making, alcoholic beverage production and commercial ethanol manufacture. Which statement best explains why these are considered different applications of the same microorganism?
ⓐ. The organism changes from a fungus into a bacterium depending on the product
ⓑ. Different substrates and processing give the same yeast different applications
ⓒ. Bread fermentation requires a different species that happens to have the same name
ⓓ. Commercial ethanol is produced without fermentation
Correct Answer: Different substrates and processing give the same yeast different applications
Explanation: Saccharomyces cerevisiae demonstrates that one microorganism can be used in several distinct human-welfare contexts. In bread making it functions as baker's yeast, in alcoholic beverage production it ferments substrates such as malted cereals and fruit juices, and it can also be cultivated for commercial ethanol production. The microbial species does not need to change for the application to change. Instead, the substrate, scale of operation, subsequent processing and intended final product determine how the same fermentative ability is harnessed.
214. Four fermented products undergo the following post-fermentation treatments:
Product P is bottled after fermentation without distillation.
Product Q is distilled after fermentation.
Product R is bottled after fermentation without distillation.
Product S is distilled after fermentation.
Which grouping is most consistent with the processing distinction described for alcoholic beverages?
ⓐ. P and Q resemble wine/beer; R and S resemble whisky/brandy/rum
ⓑ. Only P resembles a distilled beverage
ⓒ. All four must belong to the same beverage category because all underwent fermentation
ⓓ. P and R fit the non-distilled class, whereas Q and S fit the distilled class
Correct Answer: P and R fit the non-distilled class, whereas Q and S fit the distilled class
Explanation: Fermentation and distillation are separate processing stages. Wine and beer are produced through fermentation without the additional distillation step emphasised for whisky, brandy and rum. Therefore, products P and R, which are not distilled after fermentation, correspond to the non-distilled processing class. Q and S undergo an additional distillation step and therefore correspond to the distilled class. This distinction shows why alcoholic beverages cannot be classified only by the presence of ethanol; the raw material and subsequent processing also influence beverage identity.
215. Which claim about antibiotics is biologically valid?
ⓐ. A microbial antibiotic may kill or retard a pathogen's growth
ⓑ. An antibiotic must kill every microorganism that it contacts
ⓒ. Any microbial product used in medicine is automatically an antibiotic
ⓓ. A substance cannot be an antibiotic unless it is produced by a bacterium
Correct Answer: A microbial antibiotic may kill or retard a pathogen's growth
Explanation: Antibiotics are microbial chemicals that act against disease-causing microorganisms, and their action need not always involve immediate killing. A substance may kill the target microorganism or inhibit and retard its growth. This distinction is important because it prevents an overly narrow definition of antibiotic action. Microbial products such as streptokinase, cyclosporin A and statins have important medical uses but are not classified as antibiotics simply because they originate from microorganisms. The biological function of the product is therefore essential to its classification.
216. On a Staphylococci culture plate contaminated by a mould, bacterial growth is normal far from the mould but absent in a clear zone surrounding it. Which observation most strongly supports the inference that the mould released an inhibitory substance into the surrounding medium?
ⓐ. The mould and bacteria occupy different taxonomic groups
ⓑ. Inhibition is confined to the mould colony with no surrounding clear zone
ⓒ. Inhibition extends beyond the mould colony into surrounding medium
ⓓ. The mould colony is visible to the naked eye
Correct Answer: Inhibition extends beyond the mould colony into surrounding medium
Explanation: The decisive feature is the zone of bacterial inhibition extending beyond the mould colony. If bacterial growth were absent only exactly where the mould physically occupied the plate, simple competition for space could explain the pattern. Instead, inhibition in the surrounding region supports the idea that a substance released by the mould diffused through the medium and prevented Staphylococcal growth nearby. This observation formed the key biological inference behind Fleming's discovery of penicillin. This reasoning distinguishes mere coexistence on a plate from evidence of a diffusible antimicrobial effect.
217. Study the following microbial products.
| Product | Microbial source | Major biological action |
|---|
| P. Streptokinase | Streptococcus | Clot-busting action |
| Q. Cyclosporin A | Trichoderma polysporum | Immunosuppression |
| R. Statins | Monascus purpureus | Reduction of cholesterol synthesis |
| S. Penicillin | Penicillium notatum | Antimicrobial action |
Which option correctly identifies the row whose function depends specifically on competitive inhibition of an enzyme involved in a human biosynthetic pathway?
ⓐ. R
ⓑ. Q
ⓒ. S
ⓓ. P
Correct Answer: R
Explanation: Statins produced by Monascus purpureus lower blood cholesterol by competitively inhibiting an enzyme involved in cholesterol synthesis. This mechanism distinguishes them from the other medically useful microbial products in the table. Streptokinase functions as a clot buster, cyclosporin A suppresses immune activity and penicillin acts against susceptible microorganisms. This reasoning requires linking source, product, physiological effect and mechanism rather than recognising only a product name.
218. A modified microbial product still binds to its target enzyme but no longer competes effectively with the enzyme's normal substrate. Which useful effect would be most directly weakened if this product were a statin?
ⓐ. Suppression of immune responses following organ transplantation
ⓑ. Reduced cholesterol synthesis by competitive inhibition
ⓒ. Dissolution of a blood clot
ⓓ. Killing of susceptible Staphylococci
Correct Answer: Reduced cholesterol synthesis by competitive inhibition
Explanation: The cholesterol-lowering effect of statins depends on competitive inhibition of an enzyme involved in cholesterol synthesis. Competitive inhibition requires the inhibitor to compete effectively with the normal substrate for interaction with the enzyme. If this competitive ability is greatly reduced, the enzyme would be less strongly inhibited and cholesterol synthesis would be less effectively suppressed. This would weaken the physiological cholesterol-lowering effect. The molecular mechanism directly explains the medical outcome without requiring additional pharmacological detail.
219. Which sequence correctly places major materials or events from early to later stages of sewage treatment?
ⓐ. Raw sewage -> primary effluent -> aerobic microbial flocs -> activated sludge
ⓑ. Raw sewage -> activated sludge -> primary sludge -> untreated river water
ⓒ. Primary sludge -> raw sewage -> primary effluent -> curd starter
ⓓ. Primary effluent -> raw sewage -> grit -> alcoholic broth
Correct Answer: Raw sewage -> primary effluent -> aerobic microbial flocs -> activated sludge
Explanation: Raw sewage first undergoes physical primary treatment. After filtration and sedimentation, the liquid supernatant is primary effluent and is sent to secondary treatment. Aeration then supports vigorous growth of aerobic microbial flocs that consume biodegradable organic matter. After BOD has fallen substantially, these flocs settle and the sediment is called activated sludge. The sequence therefore follows the actual order of the named materials and microbial stage without treating the final liquid effluent as though it were formed from activated sludge.
220. A technician records the following proposed sewage-treatment flow:
Filtration -> aeration -> sedimentation of grit -> floc formation -> primary effluent -> activated sludge
Which corrected sequence restores the proper order?
ⓐ. Activated sludge -> filtration -> primary effluent -> aeration -> sedimentation of grit
ⓑ. Aeration -> floc formation -> filtration -> sedimentation of grit -> primary effluent
ⓒ. Filtration -> sedimentation of grit -> primary effluent -> aeration and floc formation -> activated sludge
ⓓ. Filtration -> primary effluent -> anaerobic digestion -> aeration and floc formation -> activated sludge
Correct Answer: Filtration -> sedimentation of grit -> primary effluent -> aeration and floc formation -> activated sludge
Explanation: Sewage treatment begins with physical primary treatment before biological secondary treatment. Filtration removes floating debris, and sedimentation removes grit such as soil and small pebbles. After the settled solids are separated, the remaining liquid forms the primary effluent. This effluent enters aeration tanks, where vigorous growth of aerobic microorganisms produces flocs that consume organic matter and reduce BOD. After sufficient treatment, the flocs settle and form activated sludge. Therefore, the physical-removal steps must precede aeration, floc formation and activated-sludge formation.
