CBSE Important Questions for Class 8 Science Microorganisms: Friend and Foe - 2025-26
Very Short Answer Questions
1. Which one of the following is caused by bacteria?
Cold
Polio
Chicken pox
Tuberculosis
Ans: (A) Cold
2. Which one of the following is an antibiotic?
Streptomycin
Tetracycline
Erythromycin
All of these
Ans: (D) All of these
3. Bread mould is an example of ……...
Bacteria
Fungi
Virus
Algae
Ans: (B) Fungi
4. Disease causing microorganisms are called ……….
Antibodies
Vaccines
Pathogens
Carriers
Ans: (C) Pathogens
5. ……….. is a bacterial disease in human and cattle
Anthrax
Foot and mouth
Rust
Citrus canker
Ans: (A) Anthrax
6. The temperature at which milk is heated in order to preserve is ……..
$70{}^\circ $
$110{}^\circ $
$45{}^\circ $
$250{}^\circ $
Ans: (A) $70{}^\circ $
7. Air contains …….. of nitrogen.
$65\%$
$78\%$
$95\%$
$80\%$
Ans: (B) $78\%$
Very Short Answer Questions
1. Name the five groups of microorganisms.
Ans: Five groups of microorganisms are bacteria, fungi, protozoa, algae and virus.
2. Name any two serious diseases caused by virus.
Ans: Polio and chickenpox are caused by viruses.
3. Name any two diseases caused by protozoa.
Ans: Dysentery and malaria are caused by protozoa.
4. Name any two domestic uses of microorganisms.
Ans: Two domestic uses of microorganisms are as follows:
Setting of curd,
Fermentation of idlis.
5. Which bacteria promotes the formation of curd?
Ans: Lactobacillus promotes the formation of curd.
6. Name the bacteria used in making bread, pastries and cakes.
Ans: Yeast is used in making bread, pastries and cakes.
7. Name the commercial use of yeast.
Ans: Yeast is used in the production of alcohol and wine.
8. Name the scientist who discovered penicillin.
Ans: Alexander Fleming discovered penicillin.
9. Name any two communicable diseases.
Ans: Cholera and common cold.
10. Name any two carriers of diseases.
Ans: Two common carriers of diseases are mosquitoes and houseflies.
11. Name any two food preservatives.
Ans: Two food preservatives are sugar and vinegar.
12. Name the nitrogen fixation bacteria.
Ans: Rhizobium is the nitrogen fixation bacteria.
13. Who discovered pasteurisation?
Ans: Louis Pasteur.
14. Why milk is boiled before storage and consuming?
Ans: Milk is boiled to prevent spoilage due to development of microorganisms.
Short Answer Questions
1. What are microorganisms?
Ans: Extremely small living organisms that cannot be seen by naked eye are called microorganisms. They may be unicellular or single-celled or multicellular.
2. Define virus.
Ans: Viruses are tiny transferable agents that act as non-living outside host cells and act living inside host cells and show reproduction. Viruses can affect all kinds of organisms including animals, plants and bacteria.
3. Why are viruses different from other microorganisms?
Ans: Viruses are also tiny but are different from other microorganisms as they show reproduction. They reproduce inside the host cell which may be a bacterium, plant or animal.
4. Where are microorganisms found?
Ans: Microorganisms are found in all kinds of environments, ranging from ice cold climate to hot springs; and deserts to marshy lands. They are also found inside the bodies of animals and humans. Some microorganisms grow on other organisms while others exist freely.
5. How do microorganisms help in agriculture?
Ans: Microorganisms increase the soil fertility by combining the air, nitrogenous compounds and minerals. Bacteria like Rhizobium and blue green algae are microorganisms which can fix atmospheric nitrogen in the soil and increase soil fertility, which helps in agriculture.
6. What is fermentation?
Ans: Fermentation is the process in which conversion of sugar into alcohol takes place through the action of enzymes.
7. Why is yeast used in the baking industry for making bread, cakes and pastries?
Ans: Yeast reproduces quickly and produces carbon dioxide during respiration. Yeast makes breads, cakes and pastries soft and spongy. This is the basis of using yeast in the baking industry.
8. What are antibodies?
Ans: An antibody is a protein released by the body's immune system in response to the microbes carrying diseases in our body. Antibody provides the strength to fight against the disease-causing microbe. Antibodies protect our body against infectious diseases.
9. What are communicable diseases?
Ans: Infectious diseases that can spread from an infected person to a healthy person through mediums like air, water, food or physical contact are known as communicable diseases. Examples of communicable diseases are cholera, common cold, chicken pox and tuberculosis.
10. Name the modes of transmission of communicable disease.
Ans: The modes of transmission of communicable diseases can be direct and indirect. In direct transmission diseases can be spread by direct contact, by droplet infection, sharing infected needles, syringes and razors, infected blood transfusion. In indirect transmission diseases can be spread by infected food, water or air, through carriers like the housefly, mosquitoes, and rats or through dirty hands.
11. Write any four ways in which spread of communicable diseases can be prevented.
Ans: Communicable diseases can be prevented by using following ways:
Always keep food covered and do not eat exposed food.
Avoid breeding of mosquitoes by keeping the surroundings dry.
Always cover your mouth while sneezing and coughing.
Wash your hands before and after meals.
12. Why are some microorganisms considered as harmful?
Ans: Some species of bacteria, viruses cause diseases in human beings, plants and animals. Some bacteria cause tuberculosis and typhoid, some kinds of virus cause common cold and influenza. Some microorganisms also cause decomposition of food and clothing. Thus this kind of microorganisms are considered as harmful.
13. How is food poisoning caused?
Ans: Food poisoning is caused due to the eating of contaminated food. Some microorganisms like bacteria, viruses grow on our food and produce toxic substances. These make the food toxic and consuming that food causes serious illness and even death.
14. How do we preserve cooked food at home?
Ans: We preserve cooked food at home by adding some preservatives. We can add salt, sugar and edible oil in cooked food. To preserve meat and fish, raw mango, tamarind and amla, common salt has been used. Sugar is added to jams, jellies and squashes to preserve them. Vegetables, fruits, and pickles are preserved by adding oil and vinegar.
15. Explain preservation by sugar.
Ans: Sugar maintains the moisture content in food which prevents the growth of bacteria. Sugar is used in jams and jellies to preserve the color, flavor and texture.
16. What is pasteurization?
Ans: In the pasteurization process milk is heated at about $70{}^\circ $ for 15 to 30 seconds and cooled suddenly. By pasteurization the growth of microbes is prevented.
17. What are preservatives? State their importance?
Ans: Preservatives are natural or synthetic substances that are used to prevent the growth of microorganisms. Common salt, vinegar, oil and sugar used as preservatives to prevent the growth of microorganisms. Preservatives are important to prevent food from spoilage. Sodium benzoate and sodium metabisulphite are common preservatives.
Long Answer Questions
1. What are antibiotics? What precautions must be taken while taking antibiotics?
Ans: Antibiotics are medicines which kill or stop the growth of microorganisms in our body. These are very effective to prevent the effects of microorganisms. Antibiotics are made from bacteria and fungi. Antibiotics are used to treat various human and animal diseases as they target bacteria and viruses. Examples of common antibiotics are streptomycin, tetracycline, erythromycin etc.
Precautions to be taken while taking antibiotics:
Antibiotics should be taken only by the advice of a qualified doctor.
Antibiotics should only be taken when needed and in an adequate amount. Otherwise, they can be harmful and become less effective in future.
2. What is nitrogen cycle? Draw a neat labelled diagram.
Ans: The organic process by which nitrogen is converted between its various chemical forms is the process of nitrogen cycle. Our atmosphere contains 78% nitrogen gas. Nitrogen is one among the essential constituents of all living organisms as a part of proteins, chlorophyll, nucleic acids and vitamins. The atmospheric nitrogen can't be taken directly from plants and animals. Some bacteria and blue chlorophytes present within the soil fix nitrogen from the atmosphere and convert it into compounds of nitrogen. Once nitrogen is converted into these usable compounds, it can be used by plants from the soil through their roots. Nitrogen is then used for the synthesis of plant proteins and other compounds. Animals feeding on plants get these proteins and other nitrogen compounds. When plants and animals die, bacteria and fungi present within the soil convert the nitrogenous wastes into nitrogenous compounds to be employed by plants again. Certain other bacteria convert some parts of them to nitrogen gas which matches back to the atmosphere. As a result, the share of nitrogen within the atmosphere remains more or less constant.
A labelled diagram of nitrogen cycle is as follows:
Nitrogen Cycle
3. Write any ten uses of microorganisms.
Ans: Ten uses of microorganisms are as follows:
Microorganisms help in the preparation of curd, bread, cake etc.
They are used in the production of alcohol on a huge scale.
Yeast is used to make vinegar.
Microbes are used to reduce pollution.
They act as cleaning agents and decay the waste products into compost.
They decompose the plant and animal dead bodies and reduce the waste.
They increase soil fertility by fixing nitrogen in the atmosphere.
Some microbes are used in production of medicines like antibiotics and vaccines.
They are used as preservatives for food items.
They help in various metabolic activities in our body.
4. Write short notes on increasing soil fertility by the action of microorganisms.
Ans: Microorganisms increase the soil fertility by combining the air, nitrogenous compounds and minerals. Bacteria like Rhizobium and blue green algae are microorganisms which can convert atmospheric nitrogen into nitrates and fix it in the soil and increase soil fertility. These microorganisms are generally called biological nitrogen fixers. Rhizobium is nitrogen fixing bacteria that live in the root nodules of leguminous plants, such as beans and peas, with which it has a symbiotic relationship. Sometimes nitrogen gets stable through the action of lightning.
5 Important Topics of Class 8 Science Chapter 2 You Shouldn’t Miss!
Benefits of Class 8 Chapter 2 Science Important Questions
Clarification of Key Concepts: Important questions help students understand key ideas about microorganisms and what roles they play.
Enhanced Critical Thinking: Engaging with these questions encourages careful thinking about how microorganisms affect health and the environment.
Improved Retention of Information: Answering questions helps students remember important facts about microorganisms better.
Better Exam Preparation: Practising important questions gives students a clear idea of what to expect in their exams, helping them do better.
Real-World Application: The questions connect classroom learning to real-life issues, showing how microorganisms are relevant in different areas.
Development of Research Skills: Important questions encourage students to explore beyond their textbooks, helping them become more curious and knowledgeable.
Increased Confidence: Regular practice makes students feel more confident in their knowledge, preparing them well for tests and discussions.
Conclusion
Bacteria have been on Earth long before humans. These tiny organisms can survive in extreme environments, such as hot mud pots, icy waterways, and places with high levels of chemicals like sulfuric acid and caustic soda.
You can download and add questions and answers about microorganisms to your study materials in a PDF format. Focus on the answers to learn how to use them effectively. Fill in the spaces with these questions and answers to check how ready you are. Make sure to practise and prepare well before your exam.
Important Study Materials for Class 8 Science Chapter 2
CBSE Class 8 Science Important Questions for All Chapters
Additional Study Materials for Class 8 Science
FAQs on CBSE Important Questions for Class 8 Science Microorganisms: Friend and Foe - 2025-26
1. What are the five major groups of microorganisms as per the Class 8 syllabus? Name one important example for each group.
For the CBSE 2025-26 exams, it's important to know the five major groups of microorganisms. These are:
- Bacteria: Single-celled organisms, e.g., Lactobacillus (used in making curd).
- Fungi: Mostly multicellular, non-green organisms, e.g., Yeast (used in baking).
- Protozoa: Unicellular organisms, e.g., Amoeba.
- Algae: Simple, plant-like organisms, e.g., Chlamydomonas.
- Viruses: Microscopic infectious agents that replicate only inside living cells, e.g., Influenza virus.
2. Explain how microorganisms are considered "friends" in the food industry. Provide at least three examples that are frequently asked in exams.
Microorganisms play a crucial role in the production of various food items, making them our 'friends'. This is an important topic for 3-mark questions. Key examples include:
- Curd Making: The bacterium Lactobacillus promotes the formation of curd from milk.
- Baking Industry: Yeast is a fungus used in baking bread, cakes, and pastries. It reproduces rapidly and produces carbon dioxide during respiration, which makes the dough rise.
- Fermentation: Yeast is also used for the large-scale commercial production of alcohol, wine, and vinegar (acetic acid) through the process of fermentation.
3. What are antibiotics, and what is one crucial precaution that must be taken while using them?
Antibiotics are medicines produced by certain microorganisms (like fungi and bacteria) to kill or stop the growth of other disease-causing microorganisms. An example is Penicillin. A crucial precaution, often highlighted in HOTS questions, is that antibiotics should be taken only on the advice of a qualified doctor and the prescribed course must be completed. Taking them unnecessarily can kill beneficial bacteria in the body and may make the drug ineffective in the future due to the development of antibiotic resistance.
4. How do vaccines work to protect us from diseases? Explain the principle of vaccination.
Vaccination is a highly important concept for preventing diseases. A vaccine contains dead or weakened microbes of a particular disease. When introduced into a healthy body, the body's immune system is stimulated to produce antibodies to fight and destroy these invaders. These antibodies remain in the body, providing immunity. If the actual disease-causing microbes enter the body later, the existing antibodies can fight them off quickly, preventing the disease. This is the principle of immunisation.
5. List any five common methods of food preservation. Explain the scientific principle behind using 'sugar' and 'pasteurisation' as preservatives.
This is a potential 5-mark question. Five common methods of food preservation are:
- Chemical Method (using preservatives like sodium benzoate)
- Preservation by Common Salt
- Preservation by Sugar
- Preservation by Oil and Vinegar
- Heat and Cold Treatments (Pasteurisation and Refrigeration)
The principles for two of these methods are:
- Preservation by Sugar: Sugar reduces the moisture content in food items like jams and jellies. This lack of moisture inhibits the growth of bacteria and other microbes that spoil food.
- Pasteurisation: This method involves heating milk to about 70°C for 15 to 30 seconds and then suddenly chilling and storing it. This process kills most of the harmful bacteria without significantly affecting the milk's nutritional value.
6. Why are viruses considered to be on the borderline between living and non-living things? How does this unique characteristic make them a foe?
Viruses are considered unique because they exhibit characteristics of both living and non-living entities. Outside a host organism, they behave like non-living particles and can be crystallised. However, once they enter the cells of a living host (like a plant, animal, or bacterium), they use the host's cellular machinery to reproduce and multiply, which is a key characteristic of living organisms. This dual nature makes them formidable foes because they cannot be treated with antibiotics (which target bacterial cell processes), making viral infections like the common cold or flu difficult to cure.
7. A farmer notices the leaves of his wheat crop are turning yellow with rust-coloured spots. Identify the type of microorganism likely causing this and explain how such plant diseases are transmitted.
The described condition is likely 'rust of wheat,' which is a plant disease caused by a type of fungus. Understanding the transmission of plant diseases is important for application-based questions. Such diseases are primarily transmitted through:
- Air: Fungal spores are lightweight and can be easily carried by wind over long distances.
- Water: Rain or irrigation water can splash spores from infected plants to healthy ones.
- Seeds: Infected seeds can carry the pathogen and spread the disease to a new crop.
- Insects: Some insects can act as carriers, transferring pathogens from one plant to another.
8. Explain the role of microorganisms in the Nitrogen Cycle. Why is this cycle essential for an ecosystem?
Microorganisms are central to the Nitrogen Cycle. Certain bacteria like Rhizobium (in the root nodules of leguminous plants) and blue-green algae present in the soil can fix atmospheric nitrogen, converting it into usable compounds like nitrates. When plants and animals die, other bacteria and fungi in the soil act as decomposers, converting the nitrogenous wastes back into nitrogen compounds to be used again by plants. Some bacteria (e.g., Pseudomonas) perform denitrification, releasing nitrogen gas back into the atmosphere. This cycle is essential because nitrogen is a vital component of proteins, nucleic acids, and vitamins, and this cycle ensures a continuous supply of it for all living organisms.
9. While making dough, your mother adds yeast and keeps it in a warm place. Why does the dough rise? What would happen if she forgot to add sugar?
This is an excellent HOTS question based on a real-world application. The dough rises because the yeast, a type of fungus, undergoes a process called anaerobic respiration (or fermentation). During this process, the yeast breaks down the sugar present in the flour to produce energy, releasing carbon dioxide (CO₂) gas and alcohol. The bubbles of CO₂ gas get trapped in the dough, causing it to increase in volume, or 'rise'. If sugar was not added, the fermentation process would be very slow or might not happen effectively, as yeast needs sugar as its food source to respire rapidly and produce enough CO₂ to make the dough rise.
