Exercise Solutions Available
Step-by-step answers for all textbook questions in this chapter
Quick Summary
Microorganisms is part of Term 3 in Class 8 Science. Microorganisms are living things too small to be seen with the naked eye. This chapter covers the major types — bacteria, viruses, fungi, algae, and protozoa — their characteristics, and their useful and harmful roles.
Key Definitions
- Microorganism: An organism too small to be seen with the naked eye; visible only under a microscope.
- Bacteria: Single-celled prokaryotes; no true nucleus; can be useful or harmful.
- Virus: Non-cellular; contains DNA or RNA wrapped in a protein coat; reproduces only inside living cells.
- Fungi: Eukaryotic organisms; include moulds, yeasts, and mushrooms; decompose organic matter.
- Algae: Photosynthetic microorganisms found in water and moist places.
- Protozoa: Unicellular eukaryotes found in water and soil; some cause diseases.
- Antibiotic: A substance produced by microorganisms (usually fungi) that kills or inhibits bacteria.
Types of Microorganisms
| Type | Example | Special Feature |
|---|---|---|
| Bacteria | E. coli, Lactobacillus | Prokaryote, most numerous |
| Virus | HIV, influenza, SARS-CoV-2 | Non-cellular, needs host to reproduce |
| Fungi | Yeast, Penicillium, Aspergillus | Decomposers; yeast used in fermentation |
| Algae | Chlorella, Spirogyra | Photosynthetic, found in water |
| Protozoa | Amoeba, Plasmodium | Unicellular; Plasmodium causes malaria |
Useful Microorganisms
- Food industry: Yeast (bread, wine, beer); Lactobacillus (curd/yogurt); bacteria in cheese-making.
- Medicine: Penicillin from Penicillium fungus; vaccines (dead/weakened pathogens).
- Agriculture: Rhizobium (nitrogen fixation in legume roots); decomposers enrich soil.
- Biotechnology: Used in making insulin, growth hormones, and other medicines.
Harmful Microorganisms
| Organism | Disease |
|---|---|
| Vibrio cholerae (bacteria) | Cholera |
| Mycobacterium tuberculosis (bacteria) | Tuberculosis (TB) |
| Salmonella typhi (bacteria) | Typhoid |
| Plasmodium (protozoa) | Malaria (via Anopheles mosquito) |
| HIV (virus) | AIDS |
| Influenza virus | Flu |
| Aspergillus (fungi) | Spoils food |
Prevention and Control
- Vaccines stimulate the immune system to produce antibodies.
- Antibiotics treat bacterial infections (not viral).
- Proper sanitation, clean water, and personal hygiene prevent many diseases.
- Pasteurisation kills harmful bacteria in milk.
- Refrigeration slows microbial growth in food.
Important Questions
One Mark
- Name the microorganism that causes malaria.
- What is an antibiotic?
- Which microorganism is used to make curd?
- Name the first antibiotic discovered.
- Are antibiotics effective against viruses?
Short Answer (3 marks)
- Distinguish between bacteria and viruses with two differences.
- Name four useful microorganisms and state one use of each.
- Explain how vaccines protect us from diseases.
Long Answer (5 marks)
- Describe the five types of microorganisms with their characteristics, one example, and one use or harm associated with each.
- Explain the harmful effects of microorganisms. Describe four diseases caused by microorganisms, their causative organisms, and methods of prevention.
Common Mistakes
- Saying antibiotics kill viruses — antibiotics only work on bacteria.
- Confusing algae with plants (algae are microorganisms, plants are macroscopic).
- Mixing up the causative organisms of malaria (Plasmodium, a protozoan, not a bacterium).
Revision Checklist
- [ ] Textbook reading completed
- [ ] Five types of microorganisms with examples noted
- [ ] Useful microorganisms and their roles listed
- [ ] Disease-causing microorganisms with diseases memorised
- [ ] Important questions answered in notebook
Frequently Asked Questions
Bacteria are single-celled living organisms that can reproduce on their own. Viruses are non-cellular entities (not true living organisms) that can only reproduce inside a host cell.