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CBSE Class 8 Science Question 8 of 14

Microorganisms : Friend and Foe — Question 8

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Question

Question 3

Match the organisms in Column A with their action in Column B.

AB
1. Bacteria(a) Fixing nitrogen
2. Rhizobium(b) Setting of curd
3. Lactobacillus(c) Baking of bread
4. Yeast(d) Causing malaria
5. A protozoan(e) Causing cholera
6. A virus(f) Causing AIDS
 (g) Producing antibodies
Answer
AB
1. Bacteria(e) Causing cholera
2. Rhizobium(a) Fixing nitrogen
3. Lactobacillus(b) Setting of curd
4. Yeast(c) Baking of bread
5. A protozoan(d) Causing malaria
6. A virus(f) Causing AIDS

Chapter Overview: Microorganisms: Friend and Foe

Microorganisms are tiny living organisms visible only under a microscope. This chapter classifies them into five groups (bacteria, viruses, fungi, protozoa, algae) and explores their dual role as both beneficial (food production, medicine, agriculture) and harmful (causing diseases). Food preservation methods and the nitrogen cycle are key examination topics.

Board Exam Weightage: 5-7 marks | Difficulty: Moderate

Key Formulas & Concepts

ConceptDetails
FermentationSugar converted to alcohol + CO2 by yeast (anaerobic process)
PasteurisationHeating milk to 70°C for 15-30 seconds, then rapid cooling
Nitrogen FixationN2 (atmospheric) converted to NH3/NO3- by Rhizobium or lightning
AntibioticsMedicines that kill or stop growth of disease-causing bacteria; e.g., Penicillin from Penicillium

Must-Know Concepts

  • Viruses are not truly living; they reproduce only inside host cells
  • Lactobacillus converts milk to curd by producing lactic acid
  • Yeast is used in bread making (CO2 makes dough rise) and alcohol production
  • Rhizobium in legume root nodules fixes atmospheric nitrogen
  • Female Anopheles mosquito transmits malaria (Plasmodium)
  • Antibiotics work ONLY against bacteria, NOT against viruses

Types of Microorganisms

TypeCell StructureUnicellular/MulticellularExamples
BacteriaProkaryoticUnicellularLactobacillus, Rhizobium, Salmonella
FungiEukaryoticBothYeast, Penicillium, Mushroom
ProtozoaEukaryoticUnicellularAmoeba, Plasmodium, Paramecium
AlgaeEukaryoticBothChlamydomonas, Spirogyra
VirusNon-cellularNeitherInfluenza virus, HIV, TMV

Common Mistakes to Avoid

  • Calling viruses living organisms (they are borderline)
  • Thinking all bacteria are harmful (many are very useful)
  • Confusing antibiotics (treat bacteria) with vaccines (prevent diseases)
  • Believing antibiotics can cure viral infections

Scoring Tips

  • Create a disease-pathogen-transmission table for quick reference
  • Learn the nitrogen cycle as a flow diagram with all 5 steps
  • Remember preservation methods using everyday kitchen examples
  • Focus on NCERT exercise questions on food preservation

Frequently Asked Questions

Q: How many marks does this chapter carry in the exam?
A: Approximately 5-7 marks in the annual exam.

Q: What question types are commonly asked?
A: MCQs (1 mark), Very Short Answer (2 marks), Short Answer (3 marks), and Long Answer / Diagram (5 marks).

Q: Is this chapter important for competitive exams?
A: Yes, concepts from this chapter appear in NTSE, Olympiad, and other science competitions.