Science · Class 8
Microorganisms
Useful and harmful microbes; fermentation, disease, and food preservation.
4 sections15–25 min read6 MCQs · 2 examples
Chapter Overview
Microorganisms are too small to see without microscope — bacteria, fungi, protozoa, algae, viruses. This chapter covers friendly microbes (curd, bread, antibiotics) and harmful ones (diseases, food spoilage).
Types and Uses
- Bacteria: Lactobacillus in curd; Rhizobium fixes nitrogen
- Yeast: bread, alcohol fermentation
- Algae: nitrogen fixation in soil
- Antibiotics from fungi (Penicillium) and bacteria
Harmful Microbes
- Pathogens cause cholera, tuberculosis, malaria (protozoa)
- Food poisoning from contaminated food
- Preservation: refrigeration, pasteurisation, chemicals, oil, sugar
Nitrogen Cycle
- Nitrogen-fixing bacteria convert N₂ to nitrates
- Denitrifying bacteria return nitrogen to air
- Lightning also fixes nitrogen
Solved Examples
Step-by-step solutions — read each step before checking the final answer.
Curd formation
How does milk turn to curd?
- 1Lactobacillus bacteria added/inoculated
- 2Multiplies in warm milk
- 3Converts lactose to lactic acid — curdles
Answer
Bacterial fermentation
Food preservation
Why does sugar preserve jams?
- 1High sugar concentration
- 2Removes moisture from microbes by osmosis
- 3Microbes cannot grow
Answer
Low water activity inhibits microbes
Key Points to Remember
- ✓ Viruses need host to reproduce
- ✓ Pasteurisation kills harmful bacteria in milk
- ✓ Vaccines prevent viral/bacterial diseases
Exam Tips
- • Name useful and harmful microorganisms
- • Explain nitrogen fixation
- • List food preservation methods
Practice MCQs — Microorganisms
Test your understanding with topic-wise multiple choice questions. Explanations appear after each answer.
Question 1 of 6Score: 0/0
Yeast is used in: