CBSE · Class 8 · Science · Chapter 2

Microorganisms:
Friend and Foe

Complete chapter resources for CBSE Class 8 Science — topic breakdown, key concepts, sample questions, previous year board questions, and instant AI question paper generation.

4Topics
4–5Board marks
8Sample questions
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Key Concepts — Chapter 2
  • Microorganism groups: Bacteria, Fungi, Protozoa, Algae, Viruses
  • Useful bacteria: Lactobacillus (curd), Rhizobium (nitrogen fixation)
  • Pathogens: Bacteria, Fungi, Protozoa, Viruses cause diseases
  • Food preservation: Salt, Sugar, Oil, Vinegar, Pasteurisation, Refrigeration
  • Antibiotic: Kills/inhibits bacteria (e.g., Penicillin by Fleming)
  • Vaccine: Weakened pathogen → stimulates immunity (Jenner, smallpox)
Chapter Overview

What this chapter covers

Microorganisms are tiny living organisms invisible to the naked eye, studied using a microscope. NCERT Class 8 Chapter 2 introduces four major groups — bacteria, fungi, protozoa, and algae — along with viruses, which are considered on the boundary of living and non-living. Students learn where microorganisms live (soil, water, air, body of other organisms) and how they are classified based on structure and nutrition.

The chapter explores the friendly roles of microorganisms: fermentation (curd, bread, wine, cheese), nitrogen fixation by Rhizobium in legume root nodules, decomposition of organic matter, and use in medicines (antibiotics such as penicillin discovered by Alexander Fleming). It then covers harmful effects — pathogens causing diseases in humans (cholera, tuberculosis, malaria, COVID-19), plants (citrus canker, rust of wheat), and food spoilage through microbial action.

A major applied section covers food preservation — the principles behind using salt, sugar, oil, vinegar, pasteurisation, cold storage, and dehydration to slow or stop microbial growth. The chapter closes with the body's immune response, vaccination (Edward Jenner's smallpox vaccine as the historic first), and the critical distinction between antibiotics (for bacterial infections) and antivirals/vaccines (for viral infections).

Topics

What's inside Chapter 2

As per NCERT Class 8 Science (CBSE syllabus)

Topic 1
Types of Microorganisms
Classification into bacteria, fungi, protozoa, algae, and viruses. Habitats (soil, water, air, host bodies). Use of microscope and culture media for observation.
Topic 2
Microorganisms as Friends
Fermentation (yeast, Lactobacillus), nitrogen fixation by Rhizobium, decomposition of organic waste, production of antibiotics, vaccines, and use in industrial processes.
Topic 3
Microorganisms as Foes
Pathogens causing diseases in humans (cholera, TB, malaria, dengue), plants (citrus canker, rust of wheat), and animals. Food spoilage by bacteria and fungi.
Topic 4
Food Preservation & Immunity
Principles of food preservation — salt, sugar, oil/vinegar, pasteurisation, cold storage, dehydration. Immune system, vaccines (Jenner's smallpox), antibiotics vs. antiviral treatments.
Connections

How this chapter fits in

Useful for setting question difficulty and cross-chapter papers.

Builds on
Class 7 · Nutrition in Plants
Soil fertility, decomposers, and nutrient cycles introduced
Class 7 · Transportation in Animals
Blood and immune system basics — white blood cells as defenders
Chapter 2 Microorganisms:
Friend & Foe
Leads to
Class 8 · Crop Production
Biofertilisers (Rhizobium) and biocontrol agents revisited
Class 9 · Why Do We Fall Ill
Infectious diseases, pathogens, immunity and vaccination in depth
Board Blueprint

Marks & question-type breakdown

Typical pattern based on CBSE Class 8 Science school examination papers.

Question type Marks Typical count What's usually tested
MCQ / Fill in the Blank 1 1–2 Name a microorganism group, disease–pathogen matching, or discoverer identification
Very Short Answer 1–2 1 Define fermentation, name an antibiotic, or state one use of microorganisms
Short Answer 3 1 Explain food preservation methods, nitrogen cycle role, or vaccine vs. antibiotic
Long Answer 4–5 0–1 Describe harmful effects of microorganisms with examples, or explain the nitrogen cycle
Total (approximate) 4–5 3–5 Weightage varies across school question paper patterns
Sample Questions

8 sample questions — generated by MarksZen AI

Aligned to CBSE Class 8 Science Chapter 2. Covers all question types across Easy, Medium, and Hard difficulty.

Q1 Easy 1 mark MCQ
Which of the following microorganisms is responsible for the formation of curd from milk? (a) Aspergillus (b) Lactobacillus (c) Rhizobium (d) Plasmodium
Q2 Easy 2 marks Short Answer
Name the scientist who discovered the first antibiotic and state the name of that antibiotic.
Q3 Medium 2 marks Short Answer
Explain why salt and sugar are used as preservatives to prevent food spoilage. State the principle involved.
Q4 Medium 3 marks Short Answer
Distinguish between a vaccine and an antibiotic. Give one example of each and state why antibiotics should not be taken for viral infections.
Q5 Medium 3 marks Short Answer
Name the four major groups of microorganisms studied in this chapter. For each group, give one example and state whether it is unicellular or multicellular.
Q6 Hard 4 marks Long Answer
Describe the role of microorganisms in the nitrogen cycle. How does Rhizobium help maintain soil fertility? Why are leguminous plants important for farmers who do not use chemical fertilisers?
Q7 Hard 4 marks Long Answer
List any four diseases caused by microorganisms in humans. For each disease, name the causative microorganism, its type (bacteria/virus/protozoa/fungi), and one method of prevention or control.
Q8 Hard 5 marks Case-Based
A food processing factory wants to preserve a batch of fruit jams and milk products for long-term distribution across India without refrigeration. (i) Suggest two suitable preservation methods for fruit jams. Explain the principle behind each. (ii) Suggest one method suitable for milk. Who developed this process and what does it involve? (iii) Why is cold storage alone insufficient for long-distance transport of perishable food?
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Previous Year Questions

From CBSE board examinations

Board-style questions from past Class 8 Science papers — Microorganisms chapter.

Board 20223 marks
Explain the process of pasteurisation. How does it differ from sterilisation? Name the scientist who developed the pasteurisation process. (CBSE School Exam 2022)
Board 20232 marks
What is nitrogen fixation? Name two microorganisms that fix atmospheric nitrogen and state where they are found. (CBSE School Exam 2023)
Board 20204 marks
Write a short note on the harmful effects of microorganisms. Give two examples each of human diseases and plant diseases caused by microorganisms, naming the causative organism in each case. (CBSE School Exam 2020)
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FAQ

Questions teachers ask

How many marks does Microorganisms: Friend and Foe carry in the CBSE Class 8 Science exam? +
This chapter typically carries 4–5 marks in CBSE Class 8 Science school exams — usually one 1-mark MCQ or fill-in-the-blank, one 2-mark short answer on uses or harmful effects of microorganisms, and occasionally a 3-mark question on food preservation or diseases. The exact weightage depends on the school's question paper pattern.
What are the four major groups of microorganisms students must know for exams? +
NCERT Class 8 Chapter 2 covers four major groups: Bacteria (e.g., Lactobacillus for curd, nitrogen-fixing Rhizobium), Fungi (e.g., Aspergillus, yeast for bread), Protozoa (e.g., Amoeba, Plasmodium causing malaria), and Algae (e.g., Spirogyra, Chlamydomonas). Viruses are also discussed separately as they are not classified as living organisms by all scientists but are studied alongside microorganisms.
Which food preservation methods are asked most frequently in board-style questions? +
The most frequently tested food preservation methods are: (1) use of salt — osmosis removes water from microbes; (2) use of sugar — jams and jellies; (3) use of oil and vinegar (acidic medium); (4) pasteurisation — heating milk to 70°C for 15–30 seconds then rapid cooling; (5) cold storage / refrigeration; (6) dehydration / sun drying. Questions often ask students to explain the principle behind why a particular method works.
What is the difference between a vaccine and an antibiotic, and how is it examined? +
A vaccine contains weakened or killed pathogens (or their proteins) that stimulate the immune system to produce antibodies without causing disease — it prevents infection. An antibiotic (e.g., penicillin, streptomycin) is a chemical produced by microorganisms that kills or inhibits the growth of bacteria — it treats an existing bacterial infection. Board questions commonly ask students to distinguish between the two, name examples, or explain why antibiotics do not work on viral infections.
How do I generate a custom question paper for this chapter using MarksZen? +
Sign up for a free MarksZen account, choose CBSE Class 8 Science, select Chapter 2 (Microorganisms: Friend and Foe), set your preferred question-type mix (MCQ, short answer, long answer) and total marks — the AI generates a complete board-aligned paper with answer key in under 2 minutes, ready for PDF export.