Microorganisms: Friend and Foe — Class 8 MCQs with Answers
Class 8 CBSE Science · Chapter 102
Pre-2026 syllabusThis chapter is from the older NCERT book. See the 2026-27 Class 8 Science chapters →
72 practice questions · 24 Easy · 24 Medium · 24 Hard · Updated
Practise the most important Class 8 CBSE Science questions from Chapter 102, "Microorganisms: Friend and Foe". You get 6 timed quizzes made from 72 NCERT-based MCQs, with answers and explanations. The questions are split into 24 Easy, 24 Medium and 24 Hard. Warm up on the basics, then move on to the exam-level questions that set top scorers in CBSE Board exams apart.
To score well in "Microorganisms: Friend and Foe", focus on clear concepts in physics, chemistry and biology basics. Each MCQ here is timed and uses exam-style marking (+4 correct, −1 wrong, 0 skipped). This trains you to stay accurate under time pressure, as real papers need. Every question has a short explanation, so a wrong answer becomes a quick lesson. It is the fastest way to fix gaps before a test.
Use this chapter for focused revision. Start with the Easy set to check your basics on Microorganisms: Friend and Foe, then move to Medium and Hard to practise applying them. Your accuracy, streaks and XP save automatically. This chapter also adds to your overall Class 8 Science mastery score. 14 sample questions are solved in full below, with the answer and a worked explanation. Sign in free to start practising.
Key concepts: Microorganisms: Friend and Foe (Class 8 Science)
This chapter covers microorganisms — their types, their helpful and harmful roles, and food preservation.
- Types of microorganisms
- Bacteria, fungi, protozoa, algae and viruses; viruses reproduce only inside a host.
- Friendly microbes
- Used in making curd, bread and alcohol, in medicines (antibiotics), and in nitrogen fixation by Rhizobium.
- Harmful microbes
- Cause diseases in humans (e.g. TB), animals and plants, and spoil food.
- Disease transmission
- Communicable diseases spread through air, water, food and carriers like the mosquito and housefly.
Microorganisms: Friend and Foe — important questions & MCQs with answers (Class 8 Science)
14 solved questions from this chapter's difficulty levels, each with its answer and explanation. The other 58 are timed and scored when you sign in.
- Q1Easy
Which group does yeast belong to?
A.BacteriaB.ProtozoaC.Fungi✓ CorrectD.AlgaeAnswer: C. Fungi
Explanation: Yeast is a single-celled fungus, widely used in baking and fermentation.
- Q2Easy
Which bacterium converts milk into curd?
A.YeastB.Lactobacillus✓ CorrectC.PenicilliumD.RhizobiumAnswer: B. Lactobacillus
Explanation: Lactobacillus multiplies in milk and converts it into curd.
- Q3Easy
Malaria is caused by a protozoan carried by the:
A.Female Anopheles mosquito✓ CorrectB.HouseflyC.CockroachD.AntAnswer: A. Female Anopheles mosquito
Explanation: The malarial parasite Plasmodium is carried by the female Anopheles mosquito.
- Q4Easy
The nitrogen-fixing bacterium found in root nodules of leguminous plants is:
A.Rhizobium✓ CorrectB.YeastC.LactobacillusD.AmoebaAnswer: A. Rhizobium
Explanation: Rhizobium lives in root nodules of legumes and fixes atmospheric nitrogen.
- Q5Easy
Heating milk to about 70°C and then cooling it quickly is called:
A.Pasteurisation✓ CorrectB.SterilisationC.FermentationD.CanningAnswer: A. Pasteurisation
Explanation: Pasteurisation (devised by Louis Pasteur) heats milk to kill microbes, then cools it quickly.
- Q6Easy
Which microorganism can reproduce only inside a host cell?
A.SpirogyraB.MushroomC.EarthwormD.Virus✓ CorrectAnswer: D. Virus
Explanation: Viruses reproduce only inside the cells of a host organism; outside they are inactive.
- Q7Medium
Which statement about viruses is correct?
A.They are the largest microbesB.They reproduce freely in soilC.They reproduce only inside a host cell✓ CorrectD.They are always harmlessAnswer: C. They reproduce only inside a host cell
Explanation: Viruses are inactive outside a host and reproduce only inside living host cells.
- Q8Medium
Bread dough rises mainly because yeast:
A.Absorbs waterB.Adds saltC.Heats the doughD.Ferments sugar, releasing CO2 that forms bubbles✓ CorrectAnswer: D. Ferments sugar, releasing CO2 that forms bubbles
Explanation: Yeast ferments sugar in the dough, producing CO2 whose bubbles make it spongy and raised.
- Q9Medium
Why are antibiotics ineffective against the common cold?
A.The cold is caused by a virus, while antibiotics act on bacteria✓ CorrectB.Antibiotics are too weakC.The cold has no causeD.Antibiotics cause the coldAnswer: A. The cold is caused by a virus, while antibiotics act on bacteria
Explanation: Antibiotics target bacterial processes; the common cold is viral, so antibiotics do not work on it.
- Q10Medium
Blue-green algae (cyanobacteria) benefit farmers by:
A.Killing weedsB.Adding saltC.Producing oxygen onlyD.Fixing atmospheric nitrogen in the soil✓ CorrectAnswer: D. Fixing atmospheric nitrogen in the soil
Explanation: Cyanobacteria fix atmospheric nitrogen, naturally enriching the soil, especially in paddy fields.
- Q11Medium
Refrigeration preserves food because low temperature:
A.Adds preservativesB.Slows the growth and multiplication of microbes✓ CorrectC.Removes waterD.Kills all microbes instantlyAnswer: B. Slows the growth and multiplication of microbes
Explanation: Cold does not kill microbes but slows their multiplication, so food stays fresh longer.
- Q12Hard
A vaccinated child does not fall ill on later exposure to polio because the vaccine:
A.Made the body produce antibodies and memory cells against the polio virus✓ CorrectB.Killed all viruses foreverC.Added antibiotics permanentlyD.Supplied vitamin CAnswer: A. Made the body produce antibodies and memory cells against the polio virus
Explanation: Vaccination primes the immune system to make antibodies and memory cells, giving lasting protection.
- Q13Hard
Assertion (A): Antibiotics should be taken only on a doctor's advice and the full course completed. Reason (R): Incomplete or unnecessary use can make microbes resistant. Choose the correct option.
A.A is true, R is falseB.A is false, R is trueC.Both A and R are falseD.Both A and R are true and R explains A✓ CorrectAnswer: D. Both A and R are true and R explains A
Explanation: Misuse breeds resistance, which is exactly why full, doctor-advised courses are needed — R explains A.
- Q14Hard
Leguminous farmers can reduce nitrogen fertiliser because:
A.Legumes need no nitrogenB.Legumes make their own fertiliser pelletsC.Legumes fix nitrogen using yeastD.Rhizobium in their root nodules fixes atmospheric nitrogen✓ CorrectAnswer: D. Rhizobium in their root nodules fixes atmospheric nitrogen
Explanation: Rhizobium in legume root nodules supplies fixed nitrogen, cutting the need for fertiliser.
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Start this chapter free →Microorganisms: Friend and Foe — FAQs
What are the key concepts in Class 8 Science Microorganisms: Friend and Foe?+
This chapter covers microorganisms — their types, their helpful and harmful roles, and food preservation. Key ideas include Types of microorganisms, Friendly microbes, Harmful microbes, Disease transmission.
What does Class 8 Science Chapter 102 (Microorganisms: Friend and Foe) cover on XamBaaz?+
It has 72 NCERT-based MCQs on "Microorganisms: Friend and Foe": 24 Easy, 24 Medium and 24 Hard. Together they make 6 timed quizzes, and you never get the same set twice. Every question has an instant explanation. They help you prepare for CBSE Board exams.
Are these "Microorganisms: Friend and Foe" questions free to practise?+
Yes. Sign in with Google to practise "Microorganisms: Friend and Foe" free. Full unlimited access is ₹999/year on a launch offer until 1 December 2026. No chapter is charged separately.
How should I revise "Microorganisms: Friend and Foe" for the exam?+
Start with the Easy quiz to check your basics, then try Medium and Hard to practise applying them. There are 6 timed quizzes on this chapter, so you can come back for a fresh set instead of one you have seen. Read each explanation, retry the questions you miss, and track your accuracy until it stays high.
Are these "Microorganisms: Friend and Foe" MCQs available with answers?+
Yes. 14 sample questions are shown here in full, each with the correct option and a step-by-step "Why" explanation. Sign in free with Google to start practising, with instant scoring.
Is there negative marking in the "Microorganisms: Friend and Foe" quizzes?+
Yes. The timed quizzes use exam-style marking: +4 for a right answer, −1 for a wrong one and 0 for a skip. MHT-CET and CBSE board papers have no negative marking. Our mocks for those are scored their way.
What are the important questions from Microorganisms: Friend and Foe (Class 8 Science)?+
The questions that matter most test Types of microorganisms, Friendly microbes, Harmful microbes, Disease transmission. This page shows 14 solved important MCQs with answers and explanations; all 72 questions on the chapter are available as timed quizzes once you sign in.
Is there an online quiz for Microorganisms: Friend and Foe?+
Yes — Class 8 Science Microorganisms: Friend and Foe has timed online quizzes at Easy, Medium and Hard levels, with instant scoring and a worked explanation on every question. The first quiz on the chapter is free.