Class 8 Science Chapter 2 Microorganisms : Friend and Foe

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I. Chapter Summary:

This chapter introduces microorganisms, which are tiny living organisms that are mostly invisible to the naked eye. It explains the types of microorganisms (bacteria, fungi, protozoa, algae, and viruses) and classifies them as useful (friends) and harmful (foes). It covers the beneficial roles of microbes in the preparation of food (e.g., curd, bread), production of alcohol, medicine (antibiotics, vaccines), and compost. The chapter also highlights harmful effects, such as causing diseases in humans, animals, and plants, and spoilage of food. Methods of food preservation and the importance of hygiene are also emphasized.

II. Key Concepts Covered:

III. Important Questions:

(A) Multiple Choice Questions (1 Mark):

1. The microorganism used in the production of alcohol is:
   a) Rhizobium
   b) Lactobacillus
   c) Yeast ✔️
   d) Virus

2. The first antibiotic discovered was:
   a) Streptomycin
   b) Tetracycline
   c) Penicillin ✔️
   d) Ampicillin

3. Which microorganism helps in nitrogen fixation?
   a) Fungi
   b) Virus
   c) Algae
   d) Rhizobium ✔️

4. Which method is used to preserve milk for several days?
   a) Salting
   b) Drying
   c) Pasteurization ✔️
   d) Freezing

(B) Short Answer Questions (2/3 Marks):

1. Name any two diseases caused by bacteria and two caused by viruses. (PYQ 2020)

2. Define antibiotics. Give one example.

3. Write any three uses of microorganisms in the food industry.

4. What is nitrogen fixation? Which microorganism helps in this process?

(C) Long Answer Questions (5 Marks):

1. Describe five major uses of microorganisms. (PYQ 2021)

2. How do microorganisms cause harm? Mention at least three harmful effects.

3. What are vaccines? How do they work? Give two examples.

4. Describe any five methods of food preservation. (PYQ 2019)

(D) HOTS (Higher Order Thinking Skills):

1. Why are viruses considered both living and non-living?

2. A person fell sick after eating uncovered food. Explain how microorganisms might be responsible.

IV. Key Formulas/Concepts:

V. Deleted Portions (CBSE 2025–2026):

No portions have been deleted from this chapter as per the rationalized NCERT textbooks.

VI. Chapter-Wise Marks Bifurcation (Estimated – CBSE 2025–2026):

VII. Previous Year Questions (PYQs):

VIII. Real-World Application Examples to Connect with Topics:

• Yeast used in baking and brewing shows commercial use of microorganisms.

• Lactobacillus in curd is an example of helpful bacteria in home kitchens.

• COVID-19 vaccine development shows real-world importance of viral research.

• Pasteurization of milk is followed in all milk processing plants.

• Spoilage of bread and fruits due to fungi shows the need for food preservation.

IX. Student Tips & Strategies for Success (Class-Specific):

Time Management:

• Dedicate short sessions to memorizing microbe names and classifications.

• Use flashcards to differentiate between helpful and harmful microbes.

Exam Preparation:

• Practice diagram-based questions (e.g., yeast, bacterial cell).

• Prepare a chart of “Microorganism – Use – Harm – Example” for revision.

Stress Management:

• Watch short science videos to visualize the unseen world of microbes.

• Engage in DIY experiments like bread mould observation (with safety).

X. Career Guidance & Exploration (Class-Specific):

For Class 9–10 Students:

Explore:

• NTSE, CBSE Science Olympiads, Young Scientist Programs, DIY Microbiology Kits

XI. Important Notes:

• Use ncert.nic.in and cbseacademic.nic.in for updates.

• Always draw and label diagrams neatly in long answers.

• Connect microbial roles to everyday life for better retention.