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Practice Class 8 Science Chapter 2 Microorganisms: Friend and Foe MCQ Questions with Answers and Useful Details based on the NCERT syllabus. Revise food preservation, beneficial microorganisms, nitrogen fixation, fermentation, and important scientific concepts from the chapter.
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Chapter Information
Subject: Science
Class: 8
Chapter: Microorganisms: Friend and Foe
Question Type: Multiple Choice Questions (MCQs)
Practice Set: 7
Difficulty Level: Moderate to Difficult
Based On: NCERT Syllabus
Introduction
Microorganisms perform many important functions in nature and in human life. Some are used to prepare food, medicines, and agricultural products, while others cause diseases and food spoilage.
The usefulness of microorganisms depends on their type and activity. Yeast helps in fermentation, Lactobacillus helps in curd formation, and Rhizobium contributes to nitrogen fixation. At the same time, harmful microorganisms can cause diseases and spoil food.
This Practice Set 7 combines important concepts from different parts of the chapter and focuses on understanding how microorganisms are used in everyday life, agriculture, and food preservation.
What You Will Learn
✔ Fermentation
✔ Lactobacillus
✔ Yeast
✔ Food preservation
✔ Rhizobium
✔ Nitrogen fixation
✔ Useful microorganisms
✔ Harmful microorganisms
✔ Microbial growth conditions
Why This Topic Is Important
Microorganisms are connected with many everyday activities. The preparation of curd, bread, pickles, medicines, and several agricultural processes involves microorganisms or their products.
Understanding their different roles helps students avoid the common misconception that all microorganisms are harmful.
This topic also explains how science is applied in food production, agriculture, medicine, and environmental processes.
5 MCQs with Answers and Useful Details
Q1. Which microorganism converts milk into curd?
A) Lactobacillus
B) Rhizobium
C) Plasmodium
D) Yeast
Answer:
A) Lactobacillus
Useful Detail
Lactobacillus is a bacterium that helps convert milk into curd.
It produces lactic acid, which causes changes in milk proteins and helps in the formation of curd. This is one of the common examples of a useful microorganism in our daily life.
The activity of Lactobacillus demonstrates that some bacteria are beneficial rather than harmful.
Correct Answer: A) Lactobacillus
Q2. Which condition generally supports faster growth of many microorganisms?
A) Suitable warmth and moisture
B) Complete absence of water
C) Extremely dry conditions
D) Constant freezing
Answer:
A) Suitable warmth and moisture
Useful Detail
Many microorganisms grow well when conditions such as suitable temperature, moisture, nutrients, and other environmental factors are favourable.
This is one reason why food can spoil faster in warm and moist conditions.
Food preservation methods such as drying, refrigeration, salting, and sugaring work partly by creating conditions that are less favourable for microbial growth.
Correct Answer: A) Suitable warmth and moisture
Q3. Which pair is correctly matched?
A) Yeast — Bread making
B) Rhizobium — Curd formation
C) Lactobacillus — Nitrogen fixation
D) Plasmodium — Bread making
Answer:
A) Yeast — Bread making
Useful Detail
Yeast is a fungus used in bread making. During fermentation, it produces carbon dioxide, which helps dough rise.
Other useful microorganism examples include:
- Lactobacillus — Curd formation
- Rhizobium — Nitrogen fixation
- Plasmodium — Causes malaria
Learning these associations is useful for both MCQs and school examinations.
Correct Answer: A) Yeast — Bread making
Q4. Which method is most suitable for preserving food by keeping it at a low temperature?
A) Refrigeration
B) Sugaring
C) Salting
D) Pickling
Answer:
A) Refrigeration
Useful Detail
Refrigeration keeps food at a low temperature. Low temperatures generally slow down the growth and multiplication of many microorganisms.
This does not necessarily kill all microorganisms, but it can delay food spoilage and increase the usable storage period.
That is why many perishable foods are kept in refrigerators.
Correct Answer: A) Refrigeration
Q5. Which statement about microorganisms is correct?
A) All microorganisms are harmful.
B) All microorganisms are useful.
C) Some microorganisms are useful while others can be harmful.
D) Microorganisms exist only inside human bodies.
Answer:
C) Some microorganisms are useful while others can be harmful.
Useful Detail
Microorganisms have both beneficial and harmful effects.
Useful microorganisms are involved in curd formation, bread making, fermentation, medicine, nitrogen fixation, and decomposition.
Harmful microorganisms can cause diseases and spoil food.
Therefore, microorganisms are appropriately described as both friends and foes.
Correct Answer: C) Some microorganisms are useful while others can be harmful.
Important Formulas & Concepts
Science Chapter 2 does not contain mathematical formulas. Important scientific concepts are given below.
1. Lactobacillus
A useful bacterium involved in the formation of curd.
2. Yeast
A fungus used in fermentation and bread making.
3. Fermentation
A process in which yeast converts sugars into alcohol and carbon dioxide under suitable conditions.
4. Microbial Growth
Microorganisms grow and multiply when suitable environmental conditions and nutrients are available.
5. Refrigeration
Low temperature slows down the growth of many microorganisms.
6. Food Preservation
Food preservation helps prevent or slow spoilage by creating conditions unfavourable for microorganisms.
7. Rhizobium
A nitrogen-fixing bacterium associated with the root nodules of leguminous plants.
8. Nitrogen Fixation
The conversion of atmospheric nitrogen into usable nitrogen compounds.
9. Beneficial Microorganisms
Microorganisms that provide useful products or perform useful functions for humans, plants, or ecosystems.
10. Harmful Microorganisms
Microorganisms that cause diseases or spoil food and other materials.
Related Practice Questions
Q1. How does Lactobacillus help in curd formation?
Q2. Why is yeast used in bread making?
Q3. Why does refrigeration slow food spoilage?
Q4. What is the role of Rhizobium?
Q5. Why are microorganisms called both friends and foes?
Mini Quiz Challenge
Try answering these questions within 60 seconds.
Q1. Which bacterium helps in curd formation?
A) Lactobacillus
B) Rhizobium
C) Plasmodium
D) Salmonella
Q2. Which microorganism is used in bread making?
A) Yeast
B) Amoeba
C) Rhizobium
D) Plasmodium
Q3. Which condition generally favours microbial growth?
A) Suitable moisture and warmth
B) Complete dryness
C) Freezing conditions
D) Absence of nutrients
Q4. Refrigeration mainly:
A) Slows microbial growth
B) Increases microbial growth
C) Removes all microorganisms
D) Produces bacteria
Q5. Microorganisms can be:
A) Only harmful
B) Only useful
C) Both useful and harmful
D) Only found in food
Exam Tips
✔ Remember the difference between Lactobacillus, yeast, and Rhizobium.
✔ Learn the role of yeast in bread making.
✔ Remember that Lactobacillus helps in curd formation.
✔ Understand why refrigeration slows food spoilage.
✔ Learn that microorganisms require favourable conditions for growth.
✔ Revise examples of beneficial microorganisms.
✔ Revise examples of harmful microorganisms.
✔ Remember that Rhizobium is associated with leguminous plants.
✔ Do not assume that all bacteria are harmful.
✔ Use examples to understand the concept rather than memorising isolated facts.
Quick Revision Notes
✔ Lactobacillus: Helps in curd formation.
✔ Yeast: Used in bread making and fermentation.
✔ Fermentation: Produces alcohol and carbon dioxide from sugar through yeast under suitable conditions.
✔ Rhizobium: Helps in nitrogen fixation.
✔ Refrigeration: Slows microbial growth.
✔ Food Preservation: Helps prevent or delay food spoilage.
✔ Moisture: Many microorganisms require suitable moisture for growth.
✔ Beneficial Microorganisms: Used in food, medicine, agriculture, and other processes.
✔ Harmful Microorganisms: Can cause diseases and food spoilage.
✔ Friends and Foes: Microorganisms can have both beneficial and harmful effects.
Common Mistakes Students Make
❌ Thinking Lactobacillus is used to make bread.
❌ Confusing yeast with Rhizobium.
❌ Thinking refrigeration kills every microorganism.
❌ Assuming dry conditions always support faster microbial growth.
❌ Forgetting that microorganisms can be useful.
❌ Thinking all bacteria cause diseases.
❌ Confusing fermentation with nitrogen fixation.
❌ Forgetting that microorganisms are found in many different environments.
Key Takeaways
✔ Microorganisms have many beneficial uses.
✔ Lactobacillus helps make curd.
✔ Yeast is useful in bread making and fermentation.
✔ Rhizobium helps in nitrogen fixation.
✔ Suitable warmth and moisture can support microbial growth.
✔ Refrigeration slows the growth of many microorganisms.
✔ Food preservation reduces or delays microbial spoilage.
✔ Some microorganisms cause diseases and food spoilage.
✔ Microorganisms can therefore be both friends and foes.
FAQs
Q. Why is Lactobacillus useful?
Answer:
Lactobacillus helps convert milk into curd by producing lactic acid.
Q. Why is yeast used in bread making?
Answer:
Yeast carries out fermentation and produces carbon dioxide, which makes the dough rise.
Q. Why does refrigeration preserve food?
Answer:
Low temperature slows the growth and multiplication of many microorganisms, thereby delaying food spoilage.
Q. Are all microorganisms harmful?
Answer:
No. Many microorganisms are useful in food production, medicine, agriculture, and natural processes, while some can cause diseases and spoil food.
Q. Why are microorganisms called friends and foes?
Answer:
They are called friends because many are useful to humans and the environment, while they are called foes because some cause diseases and food spoilage.
Conclusion
Class 8 Science Chapter 2 – Microorganisms: Friend and Foe provides a mixed revision of important concepts related to useful microorganisms and microbial growth.
Students revised Lactobacillus, yeast, fermentation, Rhizobium, nitrogen fixation, refrigeration, food preservation, and the beneficial and harmful effects of microorganisms.
These concepts are important for understanding the practical importance of microorganisms in food, agriculture, medicine, and everyday life.
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