Grade 13 — Term 3
14 study notes · 140 practice questions · Aligned with NIE syllabus
Counted in the Idasara Academy app, which you open with a free account. The free notes, revision, practice and library pages for each lesson are linked below.
This term in the Idasara Academy app has 14 lessons (NIE Units 9, 10). For each lesson, this page gives you the points you must know and one exam-style question with its answer. None of it needs an account.
Lessons in This Term
Diversity of microorganisms
What you must know
- Gram stain protocol sequence: Crystal violet (primary stain) ⟶ Iodine (mordant) ⟶ Alcohol/acetone (decolorizer) ⟶ Safranin (counterstain).
- Viral replication cycles: Lytic cycle results in host cell lysis and release of new virions; Lysogenic cycle involves integration of viral DNA into the host bacterial genome as a latent prophage.
- Prion resilience: Prions are extraordinarily resistant to heat, radiation, and standard chemical disinfectants because they lack nucleic acids and are already stably misfolded.
Try one question
Which structural feature definitively distinguishes Domain Archaea from Domain Bacteria?
- A. Presence of 70S ribosomes in cytoplasm
- B. Absence of a membrane-bound nucleus
- C. Membrane lipids containing branched hydrocarbons with ether linkages
- D. Presence of circular double-stranded DNA
Show the answer
Answer: C. Membrane lipids containing branched hydrocarbons with ether linkages. Archaea have unique plasma membranes composed of branched isoprenoid chains linked to glycerol by ether bonds (not ester bonds), allowing survival in extreme environments. Bacteria have unbranched fatty acids with ester linkages and peptidoglycan cell walls.
A model question written by Idasara on the exam pattern. It is not from a past paper.
Basic microbiology techniques
What you must know
- Autoclave parameters: 121°C at 15 pounds per square inch (psi) gauge pressure for 15-20 minutes; moist heat coagulates and denatures essential microbial enzymes and destroys resistant endospores.
- Flaming inoculating loop: Heat the wire loop until red hot along its entire length; allow it to cool inside the sterile zone before touching bacterial cultures to avoid killing the inoculum.
- Petri dish incubation orientation: Always incubate agar plates inverted (upside-down, lid at bottom) to prevent condensation droplets from falling onto the agar surface and smearing colonies.
Try one question
In standard autoclave sterilization of nutrient agar media, what temperature, pressure, and time duration are officially recommended?
- A. 121 °C, 15 psi (103 kPa), 15-20 minutes
- B. 160 °C, atmospheric pressure, 2 hours
- C. 100 °C, atmospheric pressure, 30 minutes
- D. 134 °C, 30 psi, 5 minutes
Show the answer
Answer: A. 121 °C, 15 psi (103 kPa), 15-20 minutes. Autoclaving uses moist heat under pressure at 121 °C and 15 pounds per square inch (psi) gauge pressure for 15-20 minutes to reliably destroy bacterial endospores.
A model question written by Idasara on the exam pattern. It is not from a past paper.
Microorganisms as pathogens
What you must know
- Exotoxins vs Endotoxins clinical comparison: Exotoxins are lethal in microgram doses, target specific cellular receptors (e.g., neurotoxins, enterotoxins), and can be neutralized into protective toxoid vaccines; Endotoxins produce generalized inflammatory fevers, require high doses, and cannot be converted into toxoids.
- Exceptions to Koch's Postulates: Obligate intracellular microbes (e.g., Treponema pallidum, Mycobacterium leprae, all viruses) cannot be cultured on cell-free synthetic media; asymptomatic carrier states exist (e.g., typhoid Mary).
- Incubation Period definition: The time interval between initial exposure/entry of pathogen and the first appearance of clinical signs and symptoms.
Try one question
Which structural and biological property correctly distinguishes bacterial endotoxins from bacterial exotoxins?
- A. Endotoxins are heat-stable lipopolysaccharide complexes integrated into the outer membrane of Gram-negative bacteria
- B. Endotoxins are readily neutralized by formalin to produce effective toxoid vaccines
- C. Endotoxins exhibit extreme tissue-specific pharmacological toxicity at sub-microgram dosages
- D. Endotoxins are enzymatic proteins actively secreted into extracellular fluid by viable Gram-positive bacteria
Show the answer
Answer: A. Endotoxins are heat-stable lipopolysaccharide complexes integrated into the outer membrane of Gram-negative bacteria. Endotoxins are lipopolysaccharides (LPS, specifically Lipid A) located within the outer membrane of Gram-negative bacteria, released only upon cell lysis or death. They are heat-stable, weakly immunogenic (cannot form toxoids), and cause systemic fever/septic shock via interleukin release. Exotoxins are heat-labile soluble proteins secreted by living bacteria.
A model question written by Idasara on the exam pattern. It is not from a past paper.
Microbial control methods
What you must know
- Autoclave operating standard: 121°C (250°F) at 15 psi (103 kPa) for 15-20 minutes is the universally validated minimum standard required to denature proteins and incinerate resilient Bacillus stearothermophilus / Clostridium botulinum endospores.
- 70% Alcohol efficacy paradox: 70% aqueous ethanol/isopropanol penetrates bacterial cell walls more effectively than 100% absolute alcohol, because water is essential to hydrolyze cellular proteins and facilitate intracellular denaturation.
- Antimicrobial Resistance (AMR) crisis: Overuse and misuse of antibiotics selects for resistant bacterial mutants harboring beta-lactamases, efflux pumps, or altered target enzymes (e.g., MRSA, multidrug-resistant TB), threatening modern surgical and therapeutic procedures.
Try one question
Which operating parameters are standardly prescribed for routine autoclaving to ensure total destruction of resistant bacterial endospores (such as Geobacillus stearothermophilus)?
- A. 72 °C for 15 seconds followed by rapid chilling
- B. 160 °C in a dry-air oven for 30 minutes
- C. 100 °C at atmospheric pressure for 10 minutes
- D. 121 °C at 15 psi (103 kPa) pure steam pressure for 15–20 minutes
Show the answer
Answer: D. 121 °C at 15 psi (103 kPa) pure steam pressure for 15–20 minutes. Moist heat sterilization in an autoclave operates with saturated steam at 121 °C under 15 pounds per square inch (psi / 103 kPa) above atmospheric pressure for at least 15 to 20 minutes. This achieves complete denaturation of proteins and thermal coagulation of all vegetative cells, viruses, and endospores.
A model question written by Idasara on the exam pattern. It is not from a past paper.
Use of microorganisms in industry and agriculture
What you must know
- Primary vs Secondary Metabolite kinetics: Primary metabolites (ethanol, amino acids) are synthesized during the log / exponential growth phase (trophophase) as essential metabolic byproducts; Secondary metabolites (antibiotics, toxins) are non-essential for vegetative growth and accumulate during the stationary phase (idiophase).
- Bioreactor batch vs continuous culture: Batch fermentation operates as a closed system with a fixed nutrient charge; Continuous culture (chemostat) pumps fresh sterile nutrients continuously while withdrawing effluent to keep microbes permanently in exponential growth phase.
- Bacillus thuringiensis (Bt) Cry toxin specificity: Inactive protoxins solubilize only in alkaline insect midgut (pH > 9.0), cleaved by proteases into active Cry delta-endotoxin pores that lyse gut epithelial cells, proving harmless to humans and non-target vertebrates with acidic stomachs.
Try one question
Which filamentous fungus is industrially cultivated under submerged aerobic conditions on carbohydrate substrates for the commercial production of citric acid?
- A. Aspergillus niger
- B. Saccharomyces cerevisiae
- C. Penicillium chrysogenum
- D. Rhizopus stolonifer
Show the answer
Answer: A. Aspergillus niger. Aspergillus niger is the primary industrial organism for citric acid production. When grown on high-sugar media (such as beet or cane molasses) with limited iron and manganese at low pH (<2.5), its Krebs cycle is overflowed, causing massive excretion of citric acid.
A model question written by Idasara on the exam pattern. It is not from a past paper.
Soil microorganisms and soil health
What you must know
- Vesicular-Arbuscular Mycorrhizae (VAM / AMF): Glomeromycota fungi penetrate root cortical cells forming branched arbuscules (nutrient exchange sites) and storage vesicles; vastly increase effective root absorption surface area for immobile phosphate ions.
- Actinobacteria role: Filamentous Gram-positive bacteria (e.g., Streptomyces) that degrade complex organic matter, produce soil odor geosmin, and synthesize critical clinical antibiotics.
- Soil microbial biomass indicator: Active soil microbial biomass carbon (SMBC) serves as the most sensitive early biological indicator of soil degradation, pesticide toxicity, or regenerative restoration.
Try one question
Which bacterial genus forms symbiotic root nodules in leguminous crops and reduces atmospheric dinitrogen into ammonia using an oxygen-sensitive nitrogenase enzyme complex protected by leghemoglobin?
- A. Nitrosomonas
- B. Rhizobium
- C. Pseudomonas denitrificans
- D. Azotobacter
Show the answer
Answer: B. Rhizobium. Rhizobium enters legume root hairs and induces the formation of root nodules containing bacteroids. Within the nodules, the nitrogenase enzyme complex fixes N₂ into NH₃, while plant-synthesized leghemoglobin buffers oxygen concentration to prevent irreversible inactivation of nitrogenase.
A model question written by Idasara on the exam pattern. It is not from a past paper.
Microbiology in water and waste management
What you must know
- Biochemical Oxygen Demand (BOD) metric: BOD measures the amount of dissolved O2 consumed by aerobic microorganisms to decompose organic matter in a water sample over 5 days at 20°C (BOD5); high BOD indicates severe organic pollution.
- E. coli as ideal indicator criteria: Consistently present in massive numbers in mammalian feces, does not proliferate naturally in clean freshwater, survives longer than bacterial pathogens, and is readily detected via simple standard selective cultures.
- Activated Sludge biological dynamic: High aerobic microbial flock (Zoogloea ramigera, protozoans, rotifers) kept suspended by constant air bubbling, converting dissolved organics into microbial biomass that rapidly settles as secondary sludge.
Try one question
Why is Escherichia coli widely adopted as the universal indicator organism for assessing the biological safety and sanitary quality of potable water supplies?
- A. It reproduces and multiplies rapidly in pure nutrient-free deep well water
- B. It is the direct causative agent of all waterborne outbreaks of cholera and typhoid
- C. Its presence indicates recent fecal contamination and the potential presence of enteric waterborne pathogens
- D. It survives chlorine disinfection at doses that destroy all other viruses and bacteria
Show the answer
Answer: C. Its presence indicates recent fecal contamination and the potential presence of enteric waterborne pathogens. E. coli resides exclusively as a normal inhabitant of the intestinal tract of warm-blooded animals and humans. Finding E. coli in treated water provides definitive evidence of recent fecal contamination, signaling that dangerous pathogens sharing the same fecal origin (e.g., Salmonella, Shigella, hepatitis A) may be present.
A model question written by Idasara on the exam pattern. It is not from a past paper.
Recycling of solid wastes
What you must know
- Thermophilic stage of composting: Internal core temperatures rise to 55-65°C due to intense microbial exothermic metabolic activity, effectively pasteurizing the compost by killing human enteric pathogens, weed seeds, and insect larvae.
- Carbon-to-Nitrogen (C:N) optimum ratio: Optimum aerobic composting requires a balanced C:N ratio around 25:1 to 30:1; excess carbon slows decomposition, while excess nitrogen leads to foul-smelling ammonia (NH3) gas emissions.
- Leachate generation & groundwater hazard: Rainwater percolating through unlined open garbage dumps dissolves toxic organic and heavy metal pollutants into dark toxic liquid called 'Leachate', severely contaminating subterranean aquifers.
Try one question
During the anaerobic biomethanation of organic municipal solid waste in a commercial biodigester, which microbial group performs the terminal biochemical step converting acetate, hydrogen, and carbon dioxide into methane gas?
- A. Acetogenic sulfate-reducing bacteria
- B. Acidogenic fermentative bacteria
- C. Methanogenic archaea (Methanogens)
- D. Hydrolytic cellulolytic fungi
Show the answer
Answer: C. Methanogenic archaea (Methanogens). Anaerobic digestion proceeds through four stages: hydrolysis, acidogenesis, acetogenesis, and methanogenesis. In the final stage, obligate anaerobic Methanogenic archaea (e.g., Methanobacterium, Methanosarcina) reduce acetate (acetoclastic methanogenesis) or CO₂ + H₂ (hydrogenotrophic methanogenesis) to produce combustible CH₄ and CO₂ (biogas).
A model question written by Idasara on the exam pattern. It is not from a past paper.
Microbes and food spoilage
What you must know
- Clostridium botulinum neurotoxin hazard: Obligate anaerobic spore-former producing botulinum toxin (lethal dose < 1 μg), blocking acetylcholine release at neuromuscular junctions causing flaccid paralysis; prevented by commercial pressure canning at 121°C ('botulinum cook') to sterilize endospores.
- Danger Zone for food storage: The temperature range between 5°C and 60°C (41°F - 140°F) allows exponential bacterial replication; hot foods must be kept >60°C and cold foods <5°C to arrest microbial division.
- Staphylococcal enterotoxin heat-stability: Staphylococcus aureus enterotoxin A is remarkably heat-stable, withstanding 100°C boiling for 30 minutes; boiling spoilt custard or cream cakes kills vegetative bacteria but leaves the enterotoxin fully intact to cause explosive vomiting and diarrhea.
Try one question
Foodborne botulism is a severe, flaccid paralytic neurointoxication caused by consuming preformed exotoxins synthesized by which anaerobic, endospore-forming bacterium commonly associated with improperly canned low-acid foods?
- A. Staphylococcus aureus
- B. Listeria monocytogenes
- C. Clostridium botulinum
- D. Salmonella enterica
Show the answer
Answer: C. Clostridium botulinum. Clostridium botulinum is a Gram-positive, obligately anaerobic, endospore-forming bacillus. In anaerobic, low-acid canned food environments, its germinated vegetative cells synthesize botulinum neurotoxin—one of the most lethal biological toxins known, which blocks acetylcholine release at neuromuscular junctions.
A model question written by Idasara on the exam pattern. It is not from a past paper.
Ornamental fish culture
What you must know
- Nitrogen cycle in aquarium biofiltration: Fish excrete toxic ammonia (NH3) -> Nitrosomonas bacteria oxidize ammonia to toxic nitrite (NO2-) -> Nitrobacter bacteria oxidize nitrite to relatively harmless nitrate (NO3-); regular partial water exchanges remove accumulated nitrate.
- Induced breeding via hypophysation: Injecting pituitary gland extract or synthetic GnRH analogues (e.g., Ovaprim) into mature broodstock stimulates gonadal maturation and spontaneous spawning in recalcitrant egg-scattering ornamental fish.
- Biosecurity quarantine protocol: Newly acquired fish stock must be quarantined in isolated prophylactic tanks for 14-21 days with observation and anti-parasitic treatment (e.g., against Ichthyophthirius multifiliis - White spot disease) before introduction to main facilities.
Try one question
Which commercially significant ornamental aquarium fish species reproduces by ovoviviparity (internal fertilization giving birth directly to free-swimming fry)?
- A. Angel fish (Pterophyllum scalare)
- B. Guppy (Poecilia reticulata)
- C. Goldfish (Carassius auratus)
- D. Zebra danio (Danio rerio)
Show the answer
Answer: B. Guppy (Poecilia reticulata). The guppy (Poecilia reticulata) is an ovoviviparous livebearing fish. Males possess a modified anal fin called a gonopodium to transfer spermatophores into the female's genital tract. Eggs are fertilized internally and develop inside the female without placental connection, emerging as free-swimming fry.
A model question written by Idasara on the exam pattern. It is not from a past paper.
Horticultural practices
What you must know
- Cell Totipotency concept: The inherent genetic capacity of any living nucleated plant cell to differentiate and regenerate into a complete, fertile adult organism when supplied with appropriate nutrient and hormonal stimuli (discovered by Gottlieb Haberlandt).
- Auxin-to-Cytokinin ratio in tissue culture: High auxin : low cytokinin ratio induces root formation (rhizogenesis); Low auxin : high cytokinin ratio induces shoot bud regeneration (caulogenesis); Balanced intermediate ratio produces undifferentiated callus tissue.
- Cambium alignment requirement in grafting: Success of grafting strictly depends on intimate vascular cambium contact between rootstock and scion to permit cell division and continuous xylem/phloem bridge regeneration.
Try one question
In plant tissue micropropagation, what phytohormone concentration ratio in the sterile nutrient culture medium is applied to stimulate the induction and development of adventitious roots from an undifferentiated callus?
- A. High gibberellin to low abscisic acid ratio
- B. High cytokinin to low auxin ratio
- C. High auxin to low cytokinin ratio
- D. High ethylene to low brassinosteroid ratio
Show the answer
Answer: C. High auxin to low cytokinin ratio. Skoog and Miller demonstrated that organogenesis in plant tissue culture is governed by the auxin-to-cytokinin ratio. A high auxin to cytokinin ratio (e.g., elevated NAA or IBA relative to BAP/kinetin) induces root differentiation, while a high cytokinin to auxin ratio induces shoot proliferation.
A model question written by Idasara on the exam pattern. It is not from a past paper.
Vector-borne diseases
What you must know
- Mosquito breeding site segregation: Aedes breeds in small artificial clean water containers (tires, roof gutters, discarded plastic cups); Anopheles breeds in slow-moving unpolluted natural waters (stream margins, paddy fields); Culex breeds in heavily polluted organic wastewater and blocked drains.
- Sri Lanka's Malaria-free status: Sri Lanka was certified malaria-free by the World Health Organization (WHO) in 2016 through sustained rigorous active and passive surveillance, though Anopheles vector competence persists.
- Dengue Shock Syndrome (DSS) pathogenesis: Antibody-dependent enhancement (ADE) occurs when secondary infection with a heterologous dengue serotype triggers non-neutralizing cross-reactive antibodies that facilitate massive viral entry into monocytes, causing cytokine storm and plasma vascular collapse.
Try one question
Which mosquito genus serves as the definitive biological host and primary transmission vector for human malaria caused by protozoan parasites of the genus Plasmodium?
- A. Female Aedes mosquito
- B. Mansonia mosquito
- C. Female Anopheles mosquito
- D. Female Culex mosquito
Show the answer
Answer: C. Female Anopheles mosquito. Malaria is transmitted solely by female mosquitoes of the genus Anopheles (most notably Anopheles culicifacies in Sri Lanka). The sexual cycle (syngamy and sporogony) occurs within the mosquito gut, making the female Anopheles the definitive host.
A model question written by Idasara on the exam pattern. It is not from a past paper.
Food preservation and postharvest losses
What you must know
- Water Activity (aw) concept: Water activity represents the ratio of water vapor pressure of food to pure water; bacteria generally cannot grow below aw = 0.91, yeasts below aw = 0.88, and common molds below aw = 0.80.
- Controlled Atmosphere Storage (CAS): Regulating warehouse atmospheric composition (lowering O2 to 1-3%, elevating CO2 to 2-5% at low temperature) drastically suppresses cellular respiration and ethylene synthesis, extending fruit/vegetable shelf life for months.
- Proper harvest maturity & packaging: Harvesting at physiological maturity rather than overripe stage, and using rigid plastic crates instead of compressing gunny sacks, slashes transport transit mechanical losses by over 70%.
Try one question
Which of the following fruits is classified as a climacteric fruit that exhibits a marked upsurge in cellular respiration accompanied by peak ethylene production during postharvest ripening?
- A. Watermelon (Citrullus lanatus)
- B. Banana (Musa acuminata)
- C. Orange (Citrus sinensis)
- D. Pineapple (Ananas comosus)
Show the answer
Answer: B. Banana (Musa acuminata). Climacteric fruits (e.g., banana, mango, papaya, tomato, avocado) continue ripening after harvest, characterized by an autocatalytic burst of ethylene synthesis and a sharp respiratory climacteric peak. Non-climacteric fruits (e.g., citrus, pineapple, grapes) do not exhibit this burst and do not ripen after harvest.
A model question written by Idasara on the exam pattern. It is not from a past paper.
Emerging biotechnologies
What you must know
- CRISPR-Cas9 Nobel significance: Awarded the 2020 Nobel Prize in Chemistry (Emmanuelle Charpentier and Jennifer Doudna) for developing a tool that makes genome editing as simple and programmable as a word processor cut-and-paste function.
- Yamanaka Factors (iPSCs): Shinya Yamanaka demonstrated that introducing four transcription factors (Oct4, Sox2, Klf4, c-Myc) into adult differentiated mouse/human somatic fibroblasts reprograms them back into embryonic-like induced Pluripotent Stem Cells (iPSCs), bypassing ethical controversies associated with blastocyst destruction.
- Bioethics & Gene Drives: Emerging technologies raise profound ethical considerations: germline genome editing creates inheritable genetic alterations in unborn generations; ecological gene drives engineered via CRISPR can theoretically force altered genes through entire wild populations, risking irreversible ecosystem consequences.
Try one question
In the CRISPR-Cas9 targeted genome editing technology, what component specifically guides the Cas9 endonuclease to introduce a double-strand break at the intended genomic DNA locus?
- A. Zinc-finger transcription factor recognizing double-stranded palindromes
- B. Taq DNA polymerase recognizing the Pribnow box
- C. Single guide RNA (sgRNA) containing a matching sequence complementary to the target DNA
- D. Reverse transcriptase synthesized by retroviral vectors
Show the answer
Answer: C. Single guide RNA (sgRNA) containing a matching sequence complementary to the target DNA. The CRISPR-Cas9 genome editing platform utilizes an engineered single guide RNA (sgRNA) that pairs via standard Watson-Crick base-pairing with a 20-nucleotide target sequence in the genomic DNA. The guide RNA directs the Cas9 endonuclease, which creates a site-specific double-strand break (DSB) next to the protospacer adjacent motif (PAM).
A model question written by Idasara on the exam pattern. It is not from a past paper.
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