Class 9 Science Chapter 2 Microorganisms: Friend and Foe — 30 MCQ Questions with Answers & Explanations

The 2024–25 CBSE restructuring has shifted assessment heavily toward objective questions. For Class 9 Science, MCQs now account for 30–40% of the total paper—a significant jump from previous years. Why? MCQs test both conceptual depth and speed, aligning with the aim to reduce rote learning and encourage critical thinking. Chapter 2 (Microorganisms: Friend and Foe) is a high-frequency MCQ zone because: 1. **Multiple correct interpretations**: A question on "beneficial microorganisms" can probe bacteria (Lactobacillus in yogurt), fungi (Penicillium for antibiotics), or algae (Spirulina) simultaneously. 2. **Trap-friendly topics**: Students often confuse anaerobic and aerobic respiration in fermentation, or mix up nitrogen fixation with denitrification. 3. **Real-world connections**: Questions link to food safety, disease prevention, and agriculture—topics examiners love because they reward understanding over memorisation. 4. **Assertion-Reason format**: CBSE loves these in Class 9. You may see: "Assertion: Yeast is used in bread-making. Reason: Yeast respires anaerobically, producing CO₂." Both statements must be correct AND logically linked. To score consistently on Chapter 2 MCQs, you need: - **Concept clarity** (types, roles, processes) - **Vocabulary precision** (saprophyte vs. parasite; fermentation vs. putrefaction) - **Process understanding** (how nitrogen cycle works, why pasteurization works) - **Speed without sacrifice** (skip guessing; eliminate two wrong options methodically)

**Question 1:** Which of the following is a unicellular microorganism? (A) Virus (B) Bacteria (C) Fungus (D) Algae **Correct Answer:** (B) Bacteria **Reason:** Bacteria are single-celled prokaryotes; viruses are not cells, fungi and algae are typically multicellular. --- **Question 2:** Lactobacillus is used in the production of— (A) Cheese and yogurt (B) Bread and cakes (C) Wine and beer (D) Antibiotics **Correct Answer:** (A) Cheese and yogurt **Reason:** Lactobacillus ferments lactose in milk, producing lactic acid, which curdles and preserves dairy products. --- **Question 3:** Which process kills most microorganisms in milk? (A) Boiling (B) Freezing (C) Drying (D) Smoking **Correct Answer:** (A) Boiling **Reason:** Heat denatures microbial proteins; boiling at 100°C is most effective for rapid sterilization. --- **Question 4:** Nitrogen fixation is carried out by— (A) Nitrosomonas (B) Azotobacter (C) Nitrobacter (D) E. coli **Correct Answer:** (B) Azotobacter **Reason:** Azotobacter is a free-living bacterium that converts atmospheric N₂ into ammonia (NH₃) in soil. --- **Question 5:** Which of the following is a harmful use of microorganisms? (A) Brewing beer (B) Making insulin (C) Causing typhoid (D) Producing cheese **Correct Answer:** (C) Causing typhoid **Reason:** Typhoid is caused by Salmonella bacteria and is a pathogenic (disease-causing) microorganism. --- **Question 6:** Penicillin is produced by— (A) Bacteria (B) Virus (C) Fungus (D) Protozoa **Correct Answer:** (C) Fungus **Reason:** Penicillium notatum, a fungus, produces penicillin, a widely-used antibiotic. --- **Question 7:** Which preservation method works by removing water? (A) Refrigeration (B) Drying (C) Pickling (D) Canning **Correct Answer:** (B) Drying **Reason:** Microorganisms require water for growth; removing moisture (as in sun-drying fruits) halts microbial activity. --- **Question 8:** The nitrogen cycle includes— (A) Fixation, assimilation, and denitrification (B) Respiration, photosynthesis, and fermentation (C) Boiling, freezing, and canning (D) Aerobic, anaerobic, and facultative respiration **Correct Answer:** (A) Fixation, assimilation, and denitrification **Reason:** These are the three main steps in converting atmospheric N₂ to usable compounds and back. --- **Question 9:** Which type of microorganism causes malaria? (A) Bacterium (B) Virus (C) Protozoan (D) Fungus **Correct Answer:** (C) Protozoan **Reason:** Plasmodium, a protozoan parasite transmitted by mosquitoes, causes malaria. --- **Question 10:** Vinegar is produced from— (A) Milk using Lactobacillus (B) Fruit juices using acetic acid bacteria (C) Grains using yeast (D) Vegetables using lactic acid bacteria **Correct Answer:** (B) Fruit juices using acetic acid bacteria **Reason:** Acetic acid bacteria (Acetobacter) oxidize ethanol to acetic acid, preserving and flavoring vinegar.

**Question 11:** Pasteurization of milk involves heating to 72°C for 15 seconds. Why does it NOT sterilize milk completely? (A) The temperature is too low to kill all spores (B) Bacteria are heat-resistant (C) Only aerobic bacteria are killed (D) Milk contains protective proteins **Correct Answer:** (A) The temperature is too low to kill all spores **Reason:** Pasteurization kills vegetative cells but not heat-resistant spores of some bacteria; sterilization requires higher temperatures or pressure. --- **Question 12:** A bacterium that can survive in both oxygen-rich and oxygen-poor environments is called— (A) Obligate aerobe (B) Obligate anaerobe (C) Facultative anaerobe (D) Microaerophile **Correct Answer:** (C) Facultative anaerobe **Reason:** Facultative anaerobes like E. coli use oxygen when available but can switch to anaerobic respiration when needed. --- **Question 13:** In the nitrogen cycle, denitrification— (A) Converts atmospheric N₂ into nitrates (B) Converts nitrates back to atmospheric N₂ (C) Breaks down organic nitrogen in dead organisms (D) Transfers nitrogen to plants through roots **Correct Answer:** (B) Converts nitrates back to atmospheric N₂ **Reason:** Denitrifying bacteria (e.g., Pseudomonas) reduce nitrates to N₂, completing the cycle and releasing nitrogen to the atmosphere. --- **Question 14:** Why is salt used as a food preservative in pickles? (A) It kills all microorganisms instantly (B) It creates a hypertonic environment, causing microbial dehydration (C) It lowers the pH permanently (D) It provides nutrients that inhibit spoilage **Correct Answer:** (B) It creates a hypertonic environment, causing microbial dehydration **Reason:** High salt concentration draws water out of microbial cells (osmosis), preventing growth without killing them. --- **Question 15:** Rhizobium bacteria live in root nodules of legumes because— (A) They obtain carbohydrates from the plant; the plant obtains fixed nitrogen (B) Legumes provide a toxic environment unsuitable for free-living bacteria (C) Nitrogen fixation requires high temperatures in root tissues (D) Legumes produce antibiotics that preserve the bacteria **Correct Answer:** (A) They obtain carbohydrates from the plant; the plant obtains fixed nitrogen **Reason:** This is a mutualistic symbiosis: Rhizobium fixes N₂ into usable compounds; the host plant provides sugars for bacterial energy. --- **Question 16:** A food spoils due to fungal growth. Which preservation method would be MOST effective? (A) Refrigeration alone (B) Drying + low temperature (C) Exposure to ultraviolet light (D) Adding salt only **Correct Answer:** (B) Drying + low temperature **Reason:** Fungi require moisture and warmth; combined drying and cooling eliminate both conditions, preventing germination and growth. --- **Question 17:** When yeast ferments glucose anaerobically, the main products are— (A) Ethanol and CO₂ (B) Lactic acid and CO₂ (C) Acetic acid and H₂O (D) Ethanol and lactic acid **Correct Answer:** (A) Ethanol and CO₂ **Reason:** Yeast undergoes alcoholic fermentation: C₆H₁₂O₆ → 2C₂H₅OH + 2CO₂ (glucose → ethanol + carbon dioxide). --- **Question 18:** In food canning, why is the container sealed immediately after heating? (A) To trap heat inside (B) To create a vacuum that prevents microbial entry (C) To reduce oxygen levels (D) To allow slow cooling **Correct Answer:** (B) To create a vacuum that prevents microbial entry **Reason:** Cooling under a sealed lid creates negative pressure (vacuum); microorganisms cannot enter without air, preserving sterility. --- **Question 19:** Which organism is used to produce beta-carotene (vitamin A precursor) in biotechnology? (A) Lactobacillus (B) Algae (Spirulina) (C) E. coli (D) Penicillium **Correct Answer:** (B) Algae (Spirulina) **Reason:** Spirulina, a cyanobacterium/algae, is a natural source of beta-carotene and is cultivated for nutritional supplements. --- **Question 20:** Saprophytic bacteria differ from parasitic bacteria because— (A) Saprophytes are always unicellular; parasites are multicellular (B) Saprophytes feed on dead organic matter; parasites live inside living hosts (C) Saprophytes are always harmful; parasites are always beneficial (D) Saprophytes reproduce asexually; parasites reproduce sexually **Correct Answer:** (B) Saprophytes feed on dead organic matter; parasites live inside living hosts **Reason:** Saprophytes are decomposers (essential for nutrient cycling); parasites harm living organisms for nutrition.

**Question 21:** **Assertion:** Fermentation by Lactobacillus in milk produces lactic acid, which lowers pH and prevents pathogenic bacterial growth. **Reason:** Lactic acid is toxic to most pathogenic bacteria and acts as a natural preservative. (A) Both A and R are true, and R is the correct explanation of A (B) Both A and R are true, but R is NOT the correct explanation of A (C) A is true, but R is false (D) A is false, and R is false **Correct Answer:** (A) Both A and R are true, and R is the correct explanation of A **Reason:** Lactic acid lowers pH (acidic environment inhibits pathogens) AND is antimicrobial; both statements correctly connect cause and effect. --- **Question 22:** **Assertion:** The nitrogen cycle cannot proceed without nitrogen-fixing bacteria. **Reason:** Nitrogen-fixing bacteria are the only organisms that can break the N≡N triple bond in atmospheric nitrogen. (A) Both A and R are true, and R is the correct explanation of A (B) Both A and R are true, but R is NOT the correct explanation of A (C) A is true, but R is false (D) A is false, and R is false **Correct Answer:** (A) Both A and R are true, and R is the correct explanation of A **Reason:** Only nitrogen-fixing bacteria possess nitrogenase enzyme to cleave the N≡N bond; without them, atmospheric N₂ remains unavailable to living organisms. --- **Question 23:** **Assertion:** Viruses are classified as microorganisms in biology textbooks. **Reason:** Viruses are living cells that replicate independently outside host organisms. (A) Both A and R are true, and R is the correct explanation of A (B) Both A and R are true, but R is NOT the correct explanation of A (C) A is true, but R is false (D) A is false, and R is false **Correct Answer:** (C) A is true, but R is false **Reason:** Viruses ARE studied with microorganisms (convenient classification), but they are NOT living cells and CANNOT replicate independently—only within host cells. --- **Question 24:** **Assertion:** Smoking preserves meat by reducing water content and creating an antimicrobial environment. **Reason:** Smoke contains compounds like creosol and formaldehyde that inhibit microbial growth and are harmful to human consumers. (A) Both A and R are true, and R is the correct explanation of A (B) Both A and R are true, but R is NOT the correct explanation of A (C) A is true, but R is false (D) A is false, and R is false **Correct Answer:** (B) Both A and R are true, but R is NOT the correct explanation of A **Reason:** Smoke DOES preserve meat and contains antimicrobial compounds (true), but modern smoking uses safe concentrations; R is not the reason for preservation efficacy. --- **Question 25:** **Assertion:** In anaerobic respiration by bacteria, glucose yields less ATP than in aerobic respiration. **Reason:** Anaerobic respiration uses ethanol and lactate as electron acceptors instead of oxygen, resulting in incomplete oxidation. (A) Both A and R are true, and R is the correct explanation of A (B) Both A and R are true, but R is NOT the correct explanation of A (C) A is true, but R is false (D) A is false, and R is false **Correct Answer:** (A) Both A and R are true, and R is the correct explanation of A **Reason:** Aerobic respiration yields ~30 ATP per glucose; anaerobic (fermentation) yields only 2 ATP because the Krebs cycle and electron transport chain are bypassed. --- **Question 26:** **Assertion:** Mycorrhizal fungi form associations with plant roots to improve nutrient absorption. **Reason:** Fungi break down organic matter in soil and transfer phosphorus and nitrogen to plant roots in exchange for carbohydrates. (A) Both A and R are true, and R is the correct explanation of A (B) Both A and R are true, but R is NOT the correct explanation of A (C) A is true, but R is false (D) A is false, and R is false **Correct Answer:** (A) Both A and R are true, and R is the correct explanation of A **Reason:** Mycorrhizae extend the root's absorptive surface and mobilize minerals; this mutualistic exchange is well-documented in soil microbiology. --- **Question 27:** **Assertion:** Sterilization by autoclaving works at 121°C under 15 psi pressure and is more effective than pasteurization. **Reason:** High pressure increases the boiling point of water, allowing temperatures high enough to denature even bacterial spore proteins and kill all microorganisms. (A) Both A and R are true, and R is the correct explanation of A (B) Both A and R are true, but R is NOT the correct explanation of A (C) A is true, but R is false (D) A is false, and R is false **Correct Answer:** (A) Both A and R are true, and R is the correct explanation of A **Reason:** Autoclaves use pressure to achieve higher temperatures and penetration; this denatures spore proteins and achieves true sterilization (no viable microbes). --- **Question 28:** **Assertion:** Biofortification of rice using recombinant DNA technology is an example of beneficial use of microorganisms. **Reason:** Bacteria like Agrobacterium act as vectors to insert genes for beta-carotene (Golden Rice) into plant cells. (A) Both A and R are true, and R is the correct explanation of A (B) Both A and R are true, but R is NOT the correct explanation of A (C) A is true, but R is false (D) A is false, and R is false **Correct Answer:** (A) Both A and R are true, and R is the correct explanation of A **Reason:** Agrobacterium naturally transfers DNA into plant genomes; scientists exploit this for genetic engineering to enhance crop nutrition. --- **Question 29:** **Assertion:** Denitrification by bacteria reduces nitrogen fertility in waterlogged soil. **Reason:** In anaerobic conditions, denitrifying bacteria use nitrate as an electron acceptor, converting it to N₂ gas that escapes into the atmosphere. (A) Both A and R are true, and R is the correct explanation of A (B) Both A and R are true, but R is NOT the correct explanation of A (C) A is true, but R is false (D) A is false, and R is false **Correct Answer:** (A) Both A and R are true, and R is the correct explanation of A **Reason:** Waterlogged (anaerobic) soils favor denitrifiers; this loss of bioavailable nitrogen is a real agricultural challenge in wetlands. --- **Question 30:** **Assertion:** A person exposed to cholera vibrios develops antibodies within days, but can still contract cholera again if exposed to a different strain. **Reason:** Cholera is caused by a virus that mutates rapidly; bacterial strains do not show genetic variation. (A) Both A and R are true, and R is the correct explanation of A (B) Both A and R are true, but R is NOT the correct explanation of A (C) A is true, but R is false (D) A is false, and R is false **Correct Answer:** (C) A is true, but R is false **Reason:** Cholera is bacterial (Vibrio cholerae); assertion is correct (strain-specific immunity), but reason is false (bacteria DO vary genetically via mutations and horizontal gene transfer).

CBSE examiners craft incorrect options to match common misconceptions. Recognizing these patterns will save you marks: **Trap 1: Confusing Beneficial and Harmful Organisms** *Example:* "Which organism causes food spoilage?" Option (A) Lactobacillus, (B) Yeast, (C) Clostridium, (D) Azotobacter. *Why it traps you:* Students remember Lactobacillus as "friendly" and assume it cannot spoil food. Truth: Lactobacillus is generally beneficial, but Clostridium (anaerobic, causes botulism) spoils food. *Lesson:* Don't rely on "good" vs. "bad" labels—check the specific function in the question. **Trap 2: Mixing Up Nitrogen Cycle Steps** *Example:* "Nitrogen fixation converts atmospheric N₂ to…?" Options: (A) Ammonia, (B) Nitrate, (C) Nitrite, (D) Nitrous oxide. *Why it traps you:* All are part of the cycle, but fixation yields ammonia directly. Students who skipped this detail pick (B) or (C). *Lesson:* Memorise the exact products of each step: fixation→NH₃, nitrification→NO₃⁻, denitrification→N₂. **Trap 3: Confusing Preservation Mechanisms** *Example:* "Refrigeration prevents food spoilage because it…?" Options: (A) Kills microorganisms, (B) Slows microbial metabolic rate, (C) Removes water, (D) Increases pH. *Why it traps you:* Students think "cold = dead." Fact: Refrigeration only slows growth; it doesn't sterilise. *Lesson:* Differentiate: **Heat kills** (boiling, pasteurisation), **cold slows** (refrigeration), **drying removes water** (osmotic stress), **salt increases osmolarity** (dehydration). **Trap 4: Asserting Viruses Are Cells** *Example:* "Why are viruses classified as living microorganisms?" Options: (A) They have cell membranes, (B) They replicate independently, (C) They respond to stimuli, (D) None of the above. *Why it traps you:* Viruses are in microbiology units, so students assume they're "living." Correct answer: (D) None—viruses lack membranes, cannot replicate alone, and blur the line between living and non-living. *Lesson:* Viruses are *obligate intracellular parasites*, not cells. **Trap 5: Anaerobic ≠ Fermentation** *Example:* "In anaerobic conditions, bacteria undergo…?" Options: (A) Photosynthesis, (B) Fermentation, (C) Aerobic respiration, (D) Chemosynthesis. *Why it traps you:* Anaerobic can mean facultative respiration (using nitrate), not always fermentation. *Lesson:* Fermentation is *specific anaerobic pathway producing lactate or ethanol*; anaerobic respiration is broader. **Trap 6: Assumption That All Saprophytes Are Harmful** *Example:* "Saprophytic organisms are…?" Options: (A) Always pathogenic, (B) Decomposers recycling nutrients, (C) Parasites in living hosts, (D) Viruses. *Why it traps you:* "Saphro" sounds diseased; students pick (A). Fact: Saprophytes are essential decomposers. *Lesson:* Saprophytes ≠ parasites ≠ pathogens. They're nutrients recyclers. **Trap 7: Temperature and Sterilisation Confusion** *Example:* "Pasteurisation at 72°C kills…?" Options: (A) All microorganisms, (B) Vegetative cells but not spores, (C) Only viruses, (D) Fungi but not bacteria. *Why it traps you:* Students assume higher temperature = complete kill. Pasteurisation is *not* sterilisation. *Lesson:* Pasteurisation: 72°C, 15s → reduces pathogens. Sterilisation: 121°C, 15–30 min under pressure → kills all, including spores. **Trap 8: Mycorrhizae Confusion** *Example:* "Mycorrhizal fungi provide plants with…?" Options: (A) Water only, (B) Nitrogen and phosphorus, (C) Protection from light, (D) Reduced water uptake. *Why it traps you:* Students confuse role with nitrogen-fixing bacteria (Rhizobium). Mycorrhizae specialise in phosphorus and micronutrient mobilisation. *Lesson:* Rhizobium = nitrogen fixation; Mycorrhizae = phosphorus + mineral uptake. **Quick Checklist Before You Answer:** 1. **Read the stem carefully.** Hidden words like "NOT," "EXCEPT," or "MOST" change the answer. 2. **Eliminate two wrong options first.** Don't jump to your gut answer. 3. **In Assertion-Reason, check both statements AND the link.** A true statement with false reasoning is wrong. 4. **If two options seem correct, pick the more specific/narrow one.** CBSE favours precision. 5. **Double-check your vocabulary.** Saprophyte, parasite, obligate, facultative—these words have exact meanings.

With 30–40 MCQs in a 90-minute Science paper, speed and accuracy are both critical. Here's a battle-tested strategy: **Phase 1: Scan & Sort (First 5 minutes)** Before starting, flip through all questions and mentally mark them: - **Green flag (Easy):** Definitions, direct NCERT facts. Aim: 30 seconds per Q. - **Yellow flag (Medium):** Application, multiple-step thinking. Aim: 45–60 seconds per Q. - **Red flag (Hard/Assertion-Reason):** Complex scenarios, linked statements. Aim: 90+ seconds per Q. *Why?* You'll hit a rhythm and avoid panic if you see a tough Q early. Psychologically, starting with "Green" builds confidence. **Phase 2: Green-Flag Questions (10–12 minutes)** Answer all easy questions first. These are your safety net—200–300 assured marks (if 40 MCQs × 1 mark each). Don't over-think; if you're wavering >30 seconds, move on. **Phase 3: Yellow-Flag Questions (20–25 minutes)** Now tackle medium-difficulty questions. Use the "eliminate 2 wrong" method: 1. Read the stem once carefully. 2. Eliminate the most obviously wrong option. 3. Eliminate the second-most obvious wrong option. 4. Choose between the two remaining. *Why eliminate first?* Research shows this is faster and more accurate than "finding the right answer" from scratch. **Phase 4: Red-Flag Questions (15–20 minutes)** For Assertion-Reason questions: 1. **Read A alone.** Is it true? (Yes/No) 2. **Read R alone.** Is it true? (Yes/No) 3. **Is R the cause of A?** (Yes/No) 4. **Cross-check against the 4 options.** *Example:* A: Yeast is used in brewing. (TRUE) R: Yeast respires anaerobically, producing ethanol. (TRUE) Link: Is anaerobic respiration the reason yeast is used in brewing? (YES—we want ethanol as a product.) → Answer: (A) Both A and R are true, and R is the correct explanation of A. **Phase 5: Review & Guess (Last 5 minutes)** With 5 minutes left: - **Revisit flagged questions** (marked with a light pencil tick). - **If stuck, guess intelligently:** - Never leave blanks (no negative marking in Class 9). - If unsure, pick the option with the most unfamiliar term (examiners often hide the correct answer in specialist vocabulary). - Avoid consecutive identical options (ABAB pattern is rare). **Pro Tips:** 1. **Use the NCERT margin:** Chapter 2 of your NCERT has diagrams of the nitrogen cycle and food preservation methods. Mentally revisit them during the exam—visual recall is faster. 2. **Mark as you go.** Use a light pencil mark (✓ for confident, ? for unsure). This saves re-reading in Phase 5. 3. **Avoid the "changed answer" trap.** Studies show first instincts are right ~70% of the time. Only change if you spot a clear factual error. 4. **Manage anxiety.** If you're stuck for >60 seconds on a red-flag question, mark it, move on, and return if time permits. One skipped question is better than losing 3 minutes and rushing through 4 others. 5. **Practice this strategy with mock tests.** Time yourself strictly. The goal is to finish Phase 4 by minute 65, leaving buffer time for Phase 5. **Sample Timeline for 40 MCQs in 90 minutes:** - 0–5 min: Scan & sort. - 5–17 min: Green-flag (12 Qs). - 17–42 min: Yellow-flag (15 Qs). - 42–75 min: Red-flag (10 Qs). - 75–90 min: Review, revisit flagged Qs, final check. This rhythm keeps you calm, avoids rushing, and maximises marks. Start a 3-day free trial at cbsetutor.ai to practise this strategy with timed quizzes and instant feedback from expert tutors.

Before your exam, ensure you can answer these without hesitation: **Types of Microorganisms:** - [ ] Name 5 types: Bacteria, Viruses, Fungi, Algae, Protozoa. - [ ] Know one example of each and whether it's unicellular or multicellular. - [ ] Recall: Viruses are NOT cells. **Beneficial Microorganisms:** - [ ] Lactobacillus → Yogurt, cheese, fermentation (lactic acid). - [ ] Yeast (Saccharomyces) → Bread, beer, wine (alcoholic fermentation). - [ ] Penicillium → Antibiotic (penicillin). - [ ] Rhizobium → Nitrogen fixation in legume root nodules. - [ ] Azotobacter → Free-living nitrogen fixer in soil. - [ ] Mycorrhizal fungi → Phosphorus uptake in plant roots. - [ ] Cyanobacteria/Algae (Spirulina, Chlorella) → Food supplements, oxygen production. **Harmful Microorganisms:** - [ ] Bacteria causing typhoid (Salmonella), cholera (Vibrio cholerae), diarrhoea (E. coli O157:H7). - [ ] Fungal infections (Athlete's foot, ringworm). - [ ] Viruses causing influenza, COVID-19, measles. - [ ] Plasmodium (protozoan) → Malaria. **Food Preservation Methods:** - [ ] Heat: Boiling (kills all), Pasteurisation (72°C, 15s—no spores), Sterilisation (121°C, pressure—all killed). - [ ] Cold: Refrigeration (slows growth, doesn't kill). - [ ] Drying: Removes water (osmotic stress on microbes). - [ ] Salt: Creates hypertonic environment (dehydration). - [ ] Pickling: Low pH (acidic) inhibits growth. - [ ] Smoking: Antimicrobial compounds + reduced water. - [ ] Canning: Vacuum seal prevents microbial entry after sterilisation. **Nitrogen Cycle (Steps in Order):** 1. **Fixation** (N₂ → NH₃): Azotobacter, Rhizobium, Cyanobacteria. Enzyme: Nitrogenase. 2. **Assimilation** (NH₃ → Amino acids): Plants and bacteria incorporate nitrogen into proteins. 3. **Ammonification** (Protein → NH₃): Saprophytic bacteria decompose dead organisms. 4. **Nitrification** (NH₃ → NO₂⁻ → NO₃⁻): Nitrosomonas (NH₃ → NO₂⁻), Nitrobacter (NO₂⁻ → NO₃⁻). 5. **Denitrification** (NO₃⁻ → N₂): Anaerobic bacteria (Pseudomonas) in waterlogged soil; N₂ escapes. **Assertion-Reason Patterns:** - [ ] Both true + linked logically → **(A)** - [ ] Both true + NOT linked → **(B)** - [ ] A true, R false → **(C)** - [ ] Both false → **(D)** **Common Misconceptions to Unlearn:** - Viruses are not cells and are not "alive" in the classical sense. - Refrigeration slows but does not kill (except psychrophiles). - Saprophytes are decomposers, not parasites. - Fermentation ≠ all anaerobic respiration. - Nitrogen fixation yields ammonia, not nitrate. - Pasteurisation is not sterilisation (spores survive). Master this checklist, practise the 30 MCQs above, and apply the time-management strategy in mock exams. You're ready for Chapter 2!