Microorganisms Friend and Foe — Class 8 Science NCERT Solutions (Free)
Free step-by-step NCERT solutions for Class 8 Science chapter "Microorganisms Friend and Foe" — 8 important questions with detailed answers for CBSE board exam preparation.
TL;DR: Free step-by-step NCERT solutions for Class 8 Science chapter "Microorganisms Friend and Foe" — 8 important questions with detailed answers for CBSE b…
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NCERT Solutions for Class 8 Science — Microorganisms Friend and Foe. Step-by-step answers to all 8 textbook questions from this chapter, written for CBSE board preparation and free to use.
Microorganisms Friend and Foe — All 8 Questions Solved
Q1. What are microorganisms?
Microorganisms are living organisms too small to be seen with naked eye, visible only under a microscope. They include bacteria, viruses, fungi, algae, and protozoa.
Q2. Define bacterium and mention its shape.
Bacteria are single-celled, prokaryotic microorganisms lacking a membrane-bound nucleus. Shapes: cocci (spherical), bacilli (rod-shaped), spirilla (spiral-shaped), and vibrio (comma-shaped).
Q3. Name five beneficial uses of microorganisms.
1) Yogurt and cheese production (lactic acid bacteria), 2) Bread and beer fermentation (yeast), 3) Vaccine production, 4) Antibiotic synthesis (penicillin from mold), 5) Decomposition of dead matter and nutrient cycling in ecosystems.
Q4. What are diseases caused by harmful bacteria? Give two examples.
Bacterial diseases: tuberculosis (Mycobacterium tuberculosis), typhoid (Salmonella), cholera (Vibrio cholerae), pneumonia, food poisoning. Examples: tuberculosis damages lungs; typhoid causes intestinal fever.
Q5. What are viruses? Name three viral diseases in humans.
Viruses are non-cellular obligate parasites containing nucleic acid (DNA/RNA) surrounded by protein coat. Cannot reproduce independently; require host cells. Three viral diseases: measles, mumps, influenza (flu).
Q6. Explain fermentation. Name two products made by fermentation.
Fermentation is anaerobic respiration carried out by microorganisms converting sugars into alcohol or acid without using oxygen. Products: wine (yeast fermentation of grapes), vinegar (acetic acid fermentation), yogurt (lactic acid fermentation).
Q7. What is food preservation? Name three methods using microorganisms.
Food preservation prevents microbial growth and food spoilage. Three methods: refrigeration (slows microbial growth), salting (creates hostile environment for bacteria), and smoking (preserves food while removing moisture).
Q8. What is nitrogen fixation and which microorganism performs it?
Nitrogen fixation is the conversion of atmospheric nitrogen (N2) into usable nitrogen compounds (ammonia). Bacteria like Azotobacterium and Rhizobium in soil/plant roots perform this. Essential for nitrogen cycle and plant nutrition.
Microorganisms Friend and Foe — Practice MCQs with Answers
Attempt these 20 multiple-choice questions after working through the solutions above. Each carries the correct option and the reasoning behind it, so a wrong answer tells you which idea to revisit.
MCQ 1. What is the scientific term for disease-causing microorganisms?
- A. Pathogens
- B. Antigens
- C. Antibodies
- D. Microbes
Answer: A. Pathogens — Pathogens are microorganisms that can cause diseases in humans, animals, or plants. The term 'microbes' is a general word for all microorganisms.
MCQ 2. Which microorganism is responsible for the formation of curd from milk?
- A. Yeast
- B. Lactobacillus
- C. Penicillium
- D. Rhizobium
Answer: B. Lactobacillus — Lactobacillus bacteria convert the lactose sugar in milk into lactic acid. This lactic acid then coagulates the milk proteins to form curd.
MCQ 3. The process of conversion of sugar into alcohol by microorganisms is known as:
- A. Pasteurization
- B. Fermentation
- C. Nitrogen fixation
- D. Decomposition
Answer: B. Fermentation — Fermentation is an anaerobic process carried out by yeast and certain bacteria, converting sugars into alcohol and carbon dioxide, which is essential for making alcoholic beverages.
MCQ 4. Which of the following acts as a carrier of the malaria-causing protozoan?
- A. Housefly
- B. Female Anopheles mosquito
- C. Aedes mosquito
- D. Cockroach
Answer: B. Female Anopheles mosquito — The female Anopheles mosquito transmits the Plasmodium protozoan, which is the causative agent of malaria, from an infected person to a healthy one.
MCQ 5. The vaccine for smallpox was discovered by which scientist?
- A. Louis Pasteur
- B. Robert Koch
- C. Edward Jenner
- D. Alexander Fleming
Answer: C. Edward Jenner — Edward Jenner developed the world's first successful vaccine against smallpox in 1796, using material from cowpox lesions.
MCQ 6. Which of the following is an antibiotic?
- A. Alcohol
- B. Penicillin
- C. Baking soda
- D. Sugar
Answer: B. Penicillin — Penicillin is a widely known antibiotic, produced from the fungus Penicillium, used to treat various bacterial infections by inhibiting their growth or killing them.
MCQ 7. Pasteurization is a process used to preserve milk by:
- A. Boiling it at high temperature for a long time
- B. Heating it to about 70°C for 15 to 30 seconds and then suddenly chilling it
- C. Adding large amounts of salt
- D. Freezing it immediately after milking
Answer: B. Heating it to about 70°C for 15 to 30 seconds and then suddenly chilling it — Pasteurization involves heating milk to approximately 70°C for a short duration and then rapidly cooling it. This process kills harmful microbes without significantly affecting its nutritional value or taste.
MCQ 8. The bacterium responsible for nitrogen fixation in leguminous plants is:
- A. Lactobacillus
- B. Penicillium
- C. Rhizobium
- D. Yeast
Answer: C. Rhizobium — Rhizobium bacteria live symbiotically in the root nodules of leguminous plants, converting atmospheric nitrogen into forms usable by the plant, thereby enriching the soil.
MCQ 9. Which of these is a common fungus often seen on stale bread?
- A. Algae
- B. Bread mould
- C. Amoeba
- D. Bacteria
Answer: B. Bread mould — Bread mould, such as species of Rhizopus or Mucor, is a common fungus that grows on stale bread, causing it to spoil.
MCQ 10. Which of the following diseases is caused by a virus and can spread through air?
- A. Cholera
- B. Tuberculosis
- C. Common cold
- D. Typhoid
Answer: C. Common cold — The common cold is a viral infection that primarily affects the upper respiratory tract. It spreads easily through airborne droplets released when an infected person coughs or sneezes.
MCQ 11. Consumption of food spoiled by microorganisms can lead to:
- A. Nitrogen fixation
- B. Fermentation
- C. Food poisoning
- D. Vaccination
Answer: C. Food poisoning — Food poisoning occurs when harmful microorganisms or their toxins, produced during food spoilage, are ingested, leading to symptoms like nausea, vomiting, and diarrhea.
MCQ 12. Microorganisms that break down dead organic matter are called:
- A. Producers
- B. Consumers
- C. Decomposers
- D. Pathogens
Answer: C. Decomposers — Decomposers, mainly bacteria and fungi, play a vital role in ecosystems by breaking down dead plants and animals, returning essential nutrients to the soil.
MCQ 13. Which of the following is an example of an alga?
- A. Amoeba
- B. Yeast
- C. Spirogyra
- D. Bacterium
Answer: C. Spirogyra — Spirogyra is a common genus of filamentous green algae found in freshwater environments. Amoeba is a protozoan, yeast is a fungus, and bacterium is a prokaryotic microorganism.
MCQ 14. The process by which nitrates are converted back into atmospheric nitrogen by certain bacteria is called:
- A. Ammonification
- B. Nitrification
- C. Denitrification
- D. Assimilation
Answer: C. Denitrification — Denitrification is a microbial process where denitrifying bacteria convert nitrates in the soil into gaseous nitrogen, which is then released back into the atmosphere.
MCQ 15. Adding sugar or salt is a common method for preserving food because:
- A. They kill all bacteria instantly
- B. They create a hypertonic environment, drawing out water from microbes
- C. They add flavour to the food
- D. They convert food into alcohol
Answer: B. They create a hypertonic environment, drawing out water from microbes — High concentrations of sugar and salt create a hypertonic environment, causing microbial cells to lose water through osmosis and dehydrate, thereby inhibiting their growth and activity.
MCQ 16. Citrus canker is a plant disease caused by:
- A. Virus
- B. Fungus
- C. Bacterium
- D. Protozoan
Answer: C. Bacterium — Citrus canker is a serious bacterial disease, caused by Xanthomonas axonopodis pv. citri, which affects citrus plants, causing lesions on leaves, stems, and fruits.
MCQ 17. How does a vaccine protect us from diseases?
- A. By directly killing the pathogens in the body
- B. By strengthening our digestive system
- C. By introducing weakened or dead microbes to stimulate antibody production
- D. By increasing the number of red blood cells
Answer: C. By introducing weakened or dead microbes to stimulate antibody production — Vaccines introduce a weakened or inactive form of a pathogen, or its components, to the body. This safely stimulates the immune system to produce antibodies and memory cells, providing future immunity.
MCQ 18. Who discovered the process of fermentation in 1857?
- A. Robert Koch
- B. Louis Pasteur
- C. Alexander Fleming
- D. Edward Jenner
Answer: B. Louis Pasteur — Louis Pasteur, a French microbiologist, made significant discoveries regarding the role of microorganisms in fermentation in 1857, demonstrating that yeast converts sugar into alcohol.
MCQ 19. Which of the following helps in increasing soil fertility?
- A. Pathogens
- B. Harmful bacteria
- C. Nitrogen-fixing bacteria
- D. Viruses
Answer: C. Nitrogen-fixing bacteria — Nitrogen-fixing bacteria, such as Rhizobium and blue-green algae, convert atmospheric nitrogen into usable nitrogen compounds, which are essential nutrients for plants and thus increase soil fertility.
MCQ 20. Dry fruits and vegetables are stored in airtight packets to:
- A. Make them look attractive
- B. Prevent the growth of microorganisms by removing moisture and air
- C. Increase their weight
- D. Maintain their temperature
Answer: B. Prevent the growth of microorganisms by removing moisture and air — Airtight packing prevents the reabsorption of moisture and limits exposure to oxygen, both of which are critical factors for the growth and survival of most food-spoiling microorganisms.
How to Use These Solutions
Attempt each question yourself first and only then compare with the worked answer. Marks in the CBSE board exam are awarded for the METHOD as much as the final result, so reproduce the steps rather than memorising the last line. Where a solution states a law, a formula or a definition, learn that wording — examiners look for it.
Related practice for this chapter: chapter MCQs, previous-year questions, revision notes and sample papers.