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CBSE Class 8 Science Chapter 2 Microorganisms: Friend and Foe Worksheet with Answers

Microorganisms are tiny living beings invisible to the naked eye, yet they shape our world in profound ways. NCERT Class 8 Science Chapter 2 explores how these microscopic organisms act as both friends and foes. This printable worksheet provides extensive practice for CBSE students, covering all essential concepts from the chapter with questions designed to test understanding, application, and analytical skills across multiple formats.

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Key takeaways

✓Four main types of microorganisms are bacteria, fungi, protozoa, and algae; viruses reproduce only inside host cells
✓Friendly microorganisms help in making curd, bread, antibiotics, and fixing atmospheric nitrogen in the soil
✓Harmful microorganisms cause diseases like tuberculosis, cholera, typhoid, and malaria in humans, animals, and plants
✓Food preservation methods include pasteurisation, refrigeration, salting, adding sugar, drying, and using chemical preservatives
✓The nitrogen cycle involves nitrogen fixation by bacteria like Rhizobium, conversion to nitrates, and return to atmosphere
✓Antibiotics like penicillin kill or stop the growth of disease-causing microorganisms but do not work against viruses
✓Vaccines contain dead or weakened microbes that help our body develop immunity against specific diseases

Quick Chapter Recap: Microorganisms Friend and Foe

Before attempting this worksheet, revisit the core concepts from Class 8 Science Chapter 2. Microorganisms include bacteria, fungi, protozoa, algae, and viruses. Bacteria exist in various shapes – rod-shaped (bacilli), spherical (cocci), comma-shaped (vibrio), and spiral (spirilla). Friendly microbes help produce curd, cheese, bread, and antibiotics. They also assist in making vaccines and cleaning the environment. Harmful microbes cause communicable diseases such as tuberculosis (bacteria), measles (virus), malaria (protozoa), and ringworm (fungi). Food preservation techniques exploit conditions unfavourable for microbial growth. The nitrogen cycle demonstrates how bacteria convert atmospheric nitrogen into usable forms for plants, completing an essential biogeochemical cycle. Understanding these concepts is crucial for scoring well in CBSE Class 8 Science examinations.

Microorganisms are classified into bacteria, fungi, protozoa, algae, and viruses
Lactobacillus converts milk to curd; yeast ferments sugar to produce alcohol and carbon dioxide
Diseases spread through air, water, direct contact, or vectors like mosquitoes and houseflies
Pasteurisation kills microbes in milk by heating to 70°C for 15-30 seconds
Rhizobium bacteria in legume root nodules fix atmospheric nitrogen into nitrates

Worksheet Information and Usage Guidelines

This CBSE Class 8 Science Chapter 2 worksheet has been designed according to the latest NCERT syllabus and CBSE exam pattern. The difficulty level is moderate, suitable for both regular practice and pre-exam revision. Students should allocate 60-75 minutes to complete all sections without interruption. The worksheet contains five sections (A to E) plus one case study, totalling over 40 questions that progressively increase in complexity. Section A tests basic recall through MCQs, while Sections D and E assess application and higher-order thinking skills. Teachers can use this as an in-class assessment tool, and parents can print it for home practice. All questions are mapped to specific learning outcomes from the NCERT Class 8 Science textbook. A complete answer key with explanations follows the question sections, enabling self-assessment and identifying areas needing revision.

Difficulty Level: Moderate (suitable for average to above-average students)
Suggested Time: 60-75 minutes for complete worksheet
Total Questions: 40+ across multiple question types
Marks Distribution: Section A (12 marks), B (5 marks), C (5 marks), D (15 marks), E (15 marks), Case Study (5 marks)
Best used as: Weekly practice, chapter-end revision, or periodic test preparation

Section A: Multiple Choice Questions (1 mark each)

This section contains 12 multiple-choice questions testing fundamental concepts from Microorganisms: Friend and Foe. Each question has four options with only one correct answer. These MCQs cover microorganism types, their beneficial and harmful roles, disease causation, food preservation methods, and the nitrogen cycle. Students should read each question carefully and select the most appropriate answer. In CBSE Class 8 Science exams, MCQs typically carry 1 mark each and form the foundation for objective assessment. Practice these questions to strengthen conceptual clarity and improve speed for competitive exams. Remember that NCERT Class 8 Science emphasises understanding over rote memorisation, so focus on why an answer is correct rather than just memorising it.

Q1. Which of the following is NOT a microorganism? (a) Bacteria (b) Virus (c) Mosquito (d) Fungi
Q2. The process of conversion of sugar into alcohol is called: (a) Nitrogen fixation (b) Fermentation (c) Pasteurisation (d) Decomposition
Q3. Which microorganism is used in making bread? (a) Bacterium (b) Yeast (c) Protozoa (d) Virus
Q4. Common cold is caused by: (a) Bacteria (b) Fungi (c) Virus (d) Protozoa
Q5. Lactobacillus is used to make: (a) Bread (b) Cake (c) Curd (d) Alcohol
Q6. The antibiotic penicillin is obtained from: (a) Bacteria (b) Virus (c) Fungus (d) Algae
Q7. Malaria is caused by a: (a) Virus (b) Protozoan (c) Bacterium (d) Fungus
Q8. Food poisoning is caused by: (a) Bacteria (b) Virus (c) Protozoa (d) Algae

Section B: Fill in the Blanks (1 mark each)

Section B contains 5 fill-in-the-blank questions designed to test recall of specific terms, processes, and facts from Class 8 Science Chapter 2. Students must write the precise scientific term or concept that completes each statement accurately. This question type appears frequently in CBSE Class 8 Science examinations and requires exact terminology from the NCERT textbook. While attempting these questions, pay attention to singular and plural forms, correct spelling, and scientific nomenclature. These questions assess whether students have grasped key vocabulary and definitions related to microorganisms. Unlike MCQs, fill-in-the-blanks require active recall without the aid of options, making them slightly more challenging. Review your NCERT Class 8 Science notes thoroughly before attempting this section.

Q1. Microorganisms that reproduce only inside the cells of host organisms are called __________.
Q2. The process of heating milk to about 70°C for 15 to 30 seconds to kill harmful microbes is called __________.
Q3. __________ is an antibiotic produced from the fungus Penicillium.
Q4. The carrier of disease-causing microorganisms from one person to another is called a __________.
Q5. Blue-green algae fix __________ directly from the atmosphere to enrich the soil.

Section C: Match the Following / True or False (5 marks)

Section C tests your ability to connect related concepts through a matching exercise. Column A lists microorganisms, diseases, or processes, while Column B contains their corresponding characteristics, causative agents, or outcomes. Students must draw connections based on understanding rather than guessing. Each correct match carries 1 mark. This question format appears regularly in CBSE Class 8 Science worksheets and term exams. Approach this systematically: first match the ones you are absolutely certain about, then use elimination for the remaining items. The matching format assesses relational understanding – whether students can link a disease to its causative microorganism, a preservation method to its principle, or a beneficial microbe to its application. This section requires thorough revision of tables and classifications from the NCERT Class 8 Science textbook.

Column A: (i) Tuberculosis, (ii) Foot and mouth disease, (iii) Citrus canker, (iv) Cholera, (v) Typhoid
Column B: (a) Virus, (b) Bacteria causing plant disease, (c) Bacteria causing human disease transmitted through water, (d) Bacteria affecting lungs, (e) Bacteria causing intestinal infection

Section D: Short Answer Questions (3 marks each)

Section D comprises 5 short-answer questions, each carrying 3 marks. Answers should be precise, typically 3-4 sentences or 50-70 words, addressing all parts of the question. CBSE Class 8 Science marking schemes award marks for key points, so structure your answer in clear points wherever possible. These questions test understanding of processes, reasons, differences, and applications. Common question patterns include 'Explain how...', 'Why is...', 'Differentiate between...', and 'What happens when...'. Use scientific terminology from NCERT Class 8 Science and support your answers with examples when asked. Avoid writing lengthy paragraphs; instead, present information in a logical sequence. This section bridges the gap between objective questions and long answers, requiring both factual knowledge and the ability to explain concepts concisely.

Q1. Explain how microorganisms are useful in cleaning up the environment. (3 marks)
Q2. Write three ways through which communicable diseases spread from an infected person to a healthy person. (3 marks)
Q3. Why should we not take antibiotics without consulting a doctor? Give two reasons. (3 marks)
Q4. How does food preservation by salting and adding sugar prevent microbial growth? (3 marks)
Q5. Differentiate between friendly and harmful microorganisms with two examples each. (3 marks)

Section E: Long Answer and HOTS Questions (5 marks each)

Section E contains 3 long-answer questions worth 5 marks each, demanding detailed explanations, multiple examples, and application of concepts. These questions assess higher-order thinking skills (HOTS) such as analysis, evaluation, and synthesis. Answers should be approximately 100-120 words, well-structured with an introduction, main body with labeled points or sub-headings, and a brief conclusion where relevant. CBSE Class 8 Science examiners look for depth of understanding, logical flow, and the ability to connect different concepts from the chapter. Include diagrams wherever asked or when they add value (for example, the nitrogen cycle). Questions may ask you to explain processes in detail, justify statements, analyze situations, or suggest solutions to problems. Manage your time wisely – allocate about 7-8 minutes per question in this section. Referring to NCERT Class 8 Science examples and using scientific vocabulary will earn you full marks.

Q1. Explain the nitrogen cycle in detail with the help of a diagram. Why is it important for plants? (5 marks)
Q2. Describe the role of microorganisms in increasing soil fertility. How do farmers use this knowledge? (5 marks)
Q3. What are antibiotics? How do they work? Why is it important to complete the full course of antibiotics as prescribed by doctors? (5 marks)

Case Study Question: Real-World Application (5 marks)

Case study questions present a real-life scenario or experimental observation requiring students to apply chapter concepts to analyze and answer related sub-questions. This format has gained prominence in recent CBSE Class 8 Science assessments as it tests comprehension, analytical ability, and application skills simultaneously. Read the case study passage carefully, identify the key concepts from Microorganisms: Friend and Foe being tested, and answer each sub-question based on the information provided and your chapter knowledge. The passage typically runs 80-100 words and is followed by 3-4 questions carrying 1-2 marks each. This integrated approach mirrors real scientific thinking where observations lead to questions and conclusions. NCERT Class 8 Science emphasises learning through inquiry, and case studies perfectly align with this pedagogical approach. Take your time to understand the scenario before jumping to answers.

Complete Answer Key with Explanations

The answer key provides correct answers for all sections along with brief explanations to help students understand why a particular answer is correct. This self-assessment tool is crucial for independent learning. After completing the worksheet, students should check their answers, analyze mistakes, and revisit relevant sections in their NCERT Class 8 Science textbook. For MCQs, the explanation clarifies why the correct option is right and why others are incorrect. Fill-in-the-blank answers include the exact term expected. Short and long answers present model responses with all key points that would earn full marks in CBSE examinations. Parents and teachers can use this answer key for quick evaluation. CBSETUTOR.ai offers an even more powerful learning experience – students can photograph any question from this worksheet and receive instant step-by-step solutions with concept explanations, available 24x7 at just ₹999 per month with a 3-day free trial for all classes from 6 to 12.

Section A Answers: 1(c), 2(b), 3(b), 4(c), 5(c), 6(c), 7(b), 8(a), 9(c), 10(b), 11(d), 12(a)
Section B Answers: 1-Viruses, 2-Pasteurisation, 3-Penicillin, 4-Vector, 5-Nitrogen
Section C Answers: (i)-(d), (ii)-(a), (iii)-(b), (iv)-(c), (v)-(e)
Detailed explanations follow for all short-answer, long-answer, and case-study questions below

Detailed Answers for Section D (Short Questions)

Here are model answers for all Section D questions, structured to earn full 3 marks each. Students should note the key points highlighted and ensure they include similar content depth in examinations. D1: Microorganisms help clean the environment by decomposing organic waste such as dead plants, animals, and kitchen refuse. This decomposition returns nutrients to the soil, making it fertile. Certain bacteria break down harmful chemicals and pollutants in water bodies, a process called bioremediation, which is used in sewage treatment plants. D2: Communicable diseases spread through: (i) Air – when infected persons cough or sneeze, droplets containing microbes travel through air (example: tuberculosis, common cold); (ii) Water – drinking contaminated water spreads cholera and typhoid; (iii) Direct contact – touching infected persons or using their belongings spreads ringworm and scabies; (iv) Vectors – mosquitoes and houseflies carry disease-causing microbes from one person to another (malaria, dengue). D3: We should not take antibiotics without consulting a doctor because: (i) Wrong antibiotics may not work against the specific disease-causing microbe, delaying proper treatment; (ii) Overuse and misuse of antibiotics makes bacteria resistant to them, creating 'superbugs' that become difficult to treat in future. Antibiotics do not work against viral infections.

D4 Answer: Salt and sugar draw out moisture from food through osmosis, creating conditions where microorganisms cannot survive and multiply. Microbes need water for growth and reproduction. By reducing available moisture, these preservatives prevent bacterial and fungal growth, thus preserving pickles, jams, and salted fish for long periods without refrigeration.
D5 Answer: Friendly microorganisms: (i) Lactobacillus – converts milk to curd; (ii) Yeast – used in baking bread and brewing. Harmful microorganisms: (i) Vibrio cholerae bacterium – causes cholera; (ii) Plasmodium protozoan – causes malaria. Both types significantly impact human life and economy.

Detailed Answers for Section E (Long Questions)

E1: The Nitrogen Cycle: Atmospheric nitrogen (78% of air) cannot be used directly by plants. The nitrogen cycle converts it into usable forms through several steps: (i) Nitrogen Fixation – Rhizobium bacteria in root nodules of legumes and blue-green algae convert atmospheric nitrogen into ammonia and nitrates that plants absorb; (ii) Assimilation – Plants use nitrates to make proteins, which pass to animals through food chains; (iii) Ammonification – When plants and animals die, decomposer bacteria convert proteins and waste into ammonia; (iv) Nitrification – Soil bacteria convert ammonia into nitrites and then nitrates; (v) Denitrification – Certain bacteria convert nitrates back to atmospheric nitrogen, completing the cycle. Importance: Plants need nitrogen for making proteins and chlorophyll; the cycle ensures continuous availability of usable nitrogen for plant growth. E2: Microorganisms increase soil fertility through: (i) Nitrogen fixation by Rhizobium bacteria in legume roots and blue-green algae, adding usable nitrogen compounds to soil; (ii) Decomposition of organic matter by bacteria and fungi, releasing nutrients like phosphorus, potassium, and nitrogen back into soil; (iii) Formation of humus, which improves soil texture, water retention, and nutrient content. Farmers use this knowledge by: (i) Practicing crop rotation with legumes to naturally enrich soil; (ii) Adding compost and farmyard manure containing beneficial microbes; (iii) Using biofertilizers containing nitrogen-fixing bacteria instead of chemical fertilizers, which is cost-effective and environmentally friendly.

E3 Answer: Antibiotics are medicines that kill or stop the growth of disease-causing microorganisms, particularly bacteria. They are obtained from bacteria and fungi – for example, penicillin comes from Penicillium fungus, streptomycin from Streptomyces bacteria.
How they work: Antibiotics interfere with bacterial cell wall formation, protein synthesis, or DNA replication, thus killing bacteria or preventing their multiplication. They are specific to bacteria and do not work against viral diseases like common cold or flu.
Importance of completing the course: (i) Stopping antibiotics midway kills only weak bacteria, allowing resistant ones to survive and multiply, leading to antibiotic resistance; (ii) Incomplete courses cause disease relapse; (iii) Following prescription ensures complete elimination of infection and prevents development of drug-resistant bacteria that pose serious public health threats.

Case Study Answers with Explanations

Case Study Answers: A1. The small swellings on the roots of gram and pea plants are called root nodules. These are specialized structures formed through symbiotic association between legume plants and nitrogen-fixing bacteria. A2. The microorganism present in root nodules is Rhizobium bacteria. This bacterium lives in a mutually beneficial relationship with leguminous plants like gram, pea, beans, and pulses. A3. Rhizobium bacteria present in root nodules have the ability to convert atmospheric nitrogen (which plants cannot use directly) into ammonia and nitrates through a process called nitrogen fixation. These nitrogen compounds are absorbed by plants to make proteins. When legume plants die and decompose, the nitrogen-rich compounds remain in soil, enriching it naturally. This increases soil nitrogen content without chemical fertilizers. A4. Wheat cultivation is more successful after legume crops because the soil is enriched with usable nitrogen compounds left behind by Rhizobium bacteria in legume root nodules. Wheat requires nitrogen for growth, and this naturally enriched soil provides it, reducing the need for chemical fertilizers and lowering cultivation costs while increasing yield. This practice is called crop rotation and is widely used by farmers.

Additional explanation: Leguminous plants belong to family Fabaceae and include pea, gram, beans, groundnut, and pulses
Rhizobium bacteria and legume plants have symbiotic relationship – both benefit: bacteria get food and shelter, plants get nitrogen
This biological nitrogen fixation is eco-friendly and sustainable compared to chemical fertilizers
Farmers practicing crop rotation maintain soil health and reduce dependence on costly chemical inputs

Frequently asked questions

What topics from Class 8 Science Chapter 2 are most important for CBSE exams?+

The nitrogen cycle, food preservation methods, differences between friendly and harmful microorganisms, and disease prevention are high-weightage topics. Questions on antibiotics, vaccines, and the role of microbes in daily life appear frequently in CBSE Class 8 Science exams. Practice diagrams of the nitrogen cycle and be ready to explain processes like pasteurisation and fermentation with examples.

How much time should I spend on this Microorganisms worksheet?+

Allocate 60-75 minutes for completing the entire worksheet without breaks. Spend approximately 20 minutes on Sections A, B, and C (objective questions), 25 minutes on Section D (short answers), 20 minutes on Section E (long answers and case study), and keep 10 minutes for revision. Regular timed practice builds exam temperament and improves speed.

Are answers to this Class 8 Science Chapter 2 worksheet based on NCERT?+

Yes, all questions and answers strictly follow the NCERT Class 8 Science textbook syllabus and terminology. The worksheet covers every important concept from Microorganisms: Friend and Foe including types of microorganisms, beneficial and harmful roles, disease causation and prevention, food preservation, nitrogen cycle, antibiotics, and vaccines as outlined in NCERT curriculum.

What is the difficulty level of this microorganisms worksheet?+

This worksheet is moderate difficulty, suitable for average to above-average Class 8 students. It includes a balanced mix of easy recall questions (MCQs, fill-ups), application-based short answers, and HOTS long-answer questions. Students who have read NCERT Class 8 Science Chapter 2 thoroughly and practiced NCERT exercises should be able to score 70-80% comfortably.

How can I score full marks in the nitrogen cycle question?+

To score full marks: (1) Draw a clear, labeled diagram showing atmosphere, plants, animals, soil bacteria, and arrows indicating nitrogen movement; (2) Explain all five processes – fixation, assimilation, ammonification, nitrification, denitrification; (3) Mention Rhizobium bacteria and legumes specifically; (4) State importance for plants. Practice drawing the cycle multiple times for speed and accuracy.

What are common mistakes students make in Chapter 2 Microorganisms questions?+

Common errors include: confusing bacteria and virus (viruses need host cells to reproduce), writing wrong microorganism names for diseases (example: saying bacteria causes malaria instead of protozoa), incomplete explanation of food preservation methods, missing steps in nitrogen cycle, and not completing antibiotic courses. Use correct scientific terms from NCERT Class 8 Science textbook to avoid such mistakes.

Should I memorize all disease names and their causative microorganisms?+

Yes, CBSE Class 8 Science exams frequently ask you to match diseases with causative agents or identify the type of microorganism causing a particular disease. Memorize at least 10-12 examples: tuberculosis, cholera, typhoid (bacteria); common cold, polio, measles (virus); malaria, amoebic dysentery (protozoa); ringworm, athlete's foot (fungi). Use mnemonics or flashcards for easier retention.

How do I differentiate between antibiotics and vaccines in answers?+

Antibiotics are medicines that kill or stop growth of bacteria after infection occurs; they treat disease. Vaccines contain dead or weakened microbes given before infection to build immunity; they prevent disease. Antibiotics come from microorganisms like Penicillium fungus. Vaccines were discovered by Edward Jenner (smallpox vaccine). Antibiotics do not work on viruses; vaccines are specific to each disease. State these differences clearly.

Can CBSETUTOR.ai help me with doubts from this worksheet?+

Absolutely. CBSETUTOR.ai provides 24x7 AI-powered tutoring where students can simply photograph any question from this worksheet and receive instant step-by-step solutions with detailed concept explanations. At a flat rate of ₹999 per month for all classes 6-12, with a 3-day free trial, it works like having a personal Science tutor available anytime on your phone.

Is this worksheet enough for CBSE Class 8 Science Chapter 2 preparation?+

This worksheet provides comprehensive practice covering all question types and difficulty levels. However, supplement it with NCERT textbook reading, NCERT exercise solutions, and previous years' question papers. Practice drawing and labeling diagrams. For complete mastery, solve 2-3 worksheets from different sources and revise your Class 8 Science notes regularly. Quality practice matters more than quantity.

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