Exam Master-NCERT-Class-8-Science-Curiosity-Chapter-2-The Invisible Living World: Beyond Our Naked Eye-CBSE

NCERT Probe & Ponder Questions (Page 8) :

Question 1. Have you ever wondered what you might see if the invisible world around you became visible?

Answer :

Yes,

If we could see microorganisms like bacteria, fungi, protozoa and viruses, it would change our understanding of life.
We would realize that these tiny organisms are present everywhere—in air, water, soil, on objects, and even inside our bodies.
This would make us more careful about cleanliness and health.
At the same time, we would also understand that many microbes are useful and play an important role in digestion, production of vitamins, and recycling of nutrients in nature.
Thus, microorganisms are essential for both health and ecosystems.

Question 2. How do you think your observation of this hidden world might change the way you think about size, complexity, or even what counts as 'living'?

Answer :

If we could observe microorganisms, we would see that they also show living characteristics like movement, reproduction, response to stimuli, and forming colonies.
This would help us understand that even very small organisms are truly living beings.
It also shows that life can be complex, no matter how small the organism is.

Question 3. Have you thought how these tiny living beings interact with each other?

Answer :

Yes,

Microorganisms are always interacting with each other and with other living beings.
Some live in a symbiotic way, while others compete or even prey on one another.
For example, certain bacteria help in digestion, while others cause diseases.
Fungi decompose dead matter and enrich the soil.
These interactions are very important as they form the base of many ecological processes.

Keep The Curiosity Alive (Pages 25-26) :

Question 1. Various parts of a cell are given below. Write them in the appropriate places in the following diagram.

Nucleus, Cytoplasm, Chloroplast, Cell wall, Cell membrane, Nucleoid

Answer :

(1) Nucleus: Present in plant and animal cells (not in bacterial cells), controls the cell's activities.

(2) Cytoplasm: Jelly-like substance that fills the cell and contains all organelles.

(3) Chloroplast: Found only in plant cells, responsible for photosynthesis.

(4) Cell wall: Found in plant and bacterial cells, forms the outermost layer.

(5) Cell membrane: Found in all cells, located inside the cell wall in plant/bacterial cells or as the outer layer in animal cells.

(6) Nucleoid: Found only in bacterial cells; region where DNA is located since bacteria don't have a true nucleus.

Question 2. Aanandi took two test tubes and marked them A and B. She put two spoonfuls of sugar solution in each of the test tubes. In test tube B, she added a spoonful of yeast. Then she attached two incompletely inflated balloons to the mouth of each test tube. She kept the set-up in a warm place, away from sunlight.

(i) What do you predict will happen after 3–4 days? She observed that the balloon attached to test tube B was inflated. What can be a possible explanation for this?

(a) Water evaporated in test tube B and filled the balloon with the water vapour.

(b) The warm atmosphere expanded the air inside the test tube B, which inflated the balloon.

(c) Yeast produced a gas inside the test tube B which inflated the balloon.

(d) Sugar reacted with warm air, which produced gas, eventually inflating the balloon.

(ii) She took another test tube, 1/4 filled with lime water. She removed the balloon from test tube B in such a manner that the gas inside the balloon did not escape.

She attached the balloon to the test tube with lime water and shook it well. What do you think she wants to find out?

Answer :

(i) Option (c) is correct : Yeast produced a gas inside the test tube B which inflated the balloon.

Explanation: The yeast fermented the sugar, releasing carbon dioxide gas (CO₂), which inflated the balloon.

(ii) She wants to test whether the gas produced is carbon dioxide (CO₂). If the lime water turns milky, it confirms the presence of CO₂, because CO₂ reacts with lime water to form calcium carbonate.

Question 3. A farmer was planting wheat crops in his field. He added nitrogen-rich fertiliser to the soil to get a good yield of crops. In the neighbouring field, another farmer was growing bean crops, but she preferred not to add nitrogen fertiliser to get healthy crops. Can you think of the reasons?

Answer :

Beans are leguminous plants that have a symbiotic relationship with Rhizobium bacteria in their root nodules.
These bacteria fix atmospheric nitrogen into the soil, making it available for the plant.
Because of this, farmers growing beans do not need to add nitrogen fertilizers.

Question 4. Snehal dug two pits, A and B, in her garden. In pit A, she put fruit and vegetable peels and mixed it with dried leaves. In pit B, she dumped the same kind of waste without mixing it with dried leaves. She covered both the pits with soil and observed after 3 weeks. What is she trying to test?

Answer :

Snehal is trying to test the role of dried leaves (which provide air spaces and carbon-rich material) in the process of composting.
In Pit A, the mixture of green waste (fruit/vegetable peels) and dry leaves provides the correct carbon-nitrogen balance needed for decomposition.
Pit B lacks this balance and will decompose slower and may smell.
By comparing pit A and pit B, she can observe whether mixing fruit and vegetable peels with dried leaves helps the waste decompose faster and form better compost.

Question 5. Identify the following microorganisms:

(i) I live in every kind of environment, and inside your gut.

(ii) I make bread and cakes soft and fluffy.

(iii) I live in the roots of pulse crops and provide nutrients for their growth.

Answer :

(i) Bacteria (live everywhere, even in our gut)

(ii) Yeast (makes bread fluffy)

(iii) Rhizobium (lives in roots of pulses)

Question 6. Devise an experiment to test that microorganisms need optimal temperature, air, and moisture for their growth.

Answer :

Experiment:
Take bread slices and keep them under different conditions:

Temperature: one in refrigerator, one at room temp, one in a warm place.
Moisture: one dry, one slightly moistened.
Air: one in a sealed bag, one in a bag with holes.

Observation:

Bread in warm, moist, and airy conditions shows faster mold growth.
Bread in cold, dry, or air-tight conditions shows little or delayed growth.

Conclusion:

Microorganisms need optimal temperature, moisture, and air for proper growth

Question 7. Take 2 slices of bread. Place one slice in a plate near the sink. Place the other slice in the refrigerator. Compare after three days. Note your observations. Give reasons for your observations.

Answer :

Observation:

The bread near the sink shows fungal (mold) growth.
The bread in the refrigerator shows little or no growth.

Reason:

Microorganisms grow faster in warm and moist conditions (near the sink).
In the refrigerator, the low temperature slows down their growth.

Question 8. A student observes that when curd is left out for a day, it becomes more sour. What can be two possible explanations for this observation?

Answer :

The lactic acid bacteria in curd keep multiplying when left outside.
These bacteria produce more lactic acid, which makes the curd taste sour.

Question 9. Observe the set-up given in Fig. and answer the following questions.

(i) What happens to the sugar solution in flask A?

(ii) What do you observe in test tube B after four hours? Why do you think this happened?

(iii) What would happen if yeast was not added in flask A?

Answer :

(i) The yeast ferments the sugar in the warm solution, producing carbon dioxide gas and a small amount of alcohol.

(ii) Lime water in test tube B turns milky. This happens because CO₂ produced

in flask A travels through the delivery tube into flask B and reacts with lime water, confirming the presence of carbon dioxide.

(iii) Fermentation would not occur, so no carbon dioxide would be produced. As a result, lime water would remain clear in test tube B.

Objective Type Questions

Multiple Choice Questions (MCQs) :

Choose the correct option in the following questions:

Question 1. Which scientist first used the term "cells" in science after observing cork slices?

(a) Antonie van Leeuwenhoek (b) Louis Pasteur

Question 2. What is the function of Rhizobium?

(a) Produces alcohol (b) Kills viruses

Question 3. Who is known as the "Father of Microbiology" for being the first to see and describe bacteria and blood cells?

(a) Robert Hooke (b) Antonie van Leeuwenhoek

Question 4. Which of the following is not one of the three main parts of a typical cell?

(a) Cell membrane (b) Cytoplasm

Question 5. Which condition is most favourable for maximum microbial activity in the decomposition of fruit waste?

(a) Dry and cold (b) Moist and warm

Question 6. What is the jelly-like substance that fills the cell and contains all organelles, where most life processes occur?

(a) Nucleus (b) Cell membrane

Question 7. Which cell part is porous and allows the entry of essential materials and the exit of waste material?

(a) Nucleus (b) Cell membrane

Question 8. Which structure is found only in plant cells and is responsible for photosynthesis?

(a) Nucleus (b) Cell membrane

Question 9. The nucleoid is found only in which type of cell?

(a) Plant cells (b) Animal cells

Question 10. Which microorganism helps in curd formation?

(a) Lactobacillus (b) Rhizobium

Question 11. Which of the following is a difference between plant and animal cells regarding their outermost layer?

(a) Animal cells have a cell wall, plant cells do not.

(b) Plant cells have a cell wall, animal cells do not.

(d) Neither plant nor animal cells have a cell wall.

Question 12. Which type of cell is described as long with branches to carry messages quickly?

(a) Muscle cell (b) Cheek cell

Question 13. What is the correct hierarchy of organization in complex living organisms, starting from the basic unit?

(a) Organism → Organ System → Organ → Tissue → Cell

(b) Cell → Organ → Tissue → Organ System → Organism

(d) Tissue → Cell → Organ → Organ System → Organism

Question 14. What is the largest known single cell in the living world?

(a) Human nerve cell (b) Amoeba

Question 15. Microorganisms require magnification of about how many times to be seen clearly?

(a) 10 to 20 times (b) 50 to 100 times

Question 16. Which of the following is not listed as a major group of microorganisms?

(a) Protozoa (b) Viruses

Question 17. What characteristic makes Algae look green?

(a) Presence of chlorophyll

(b) Presence of specialized structures for movement

(d) Its single-celled nature

Question 18. What kind of organism is Bread mould?

(a) Protozoa (b) Algae

Question 19. What distinguishes viruses from other microorganisms?

(a) They are unicellular and have a nucleus.

(b) They are multicellular and perform photosynthesis.

(d) They are prokaryotic and can fix nitrogen.

Question 20. Which of the following lacks a well-defined nucleus?

(a) Algae (b) Fungi

Question 21. What is the role of Fungi and bacteria in cleaning the environment?

(a) They produce oxygen for animals.

(b) They break down waste into nutrient-rich manure.

(d) They prevent nutrient recycling.

Question 22. Which microorganism is used in baking bread to make dough soft and fluffy?

(a) Lactobacillus bacteria (b) Rhizobium bacteria

Question 23. What type of microorganism is yeast?

(a) Algae (b) Protozoa

Question 24. Why is a cell considered the basic unit of life?

(a) It is the largest structure in an organism.

(b) It is the only part visible with the naked eye.

(d) It only performs specialized functions in multicellular organisms.

Question 25. Which gas is released during yeast fermentation that makes the dough rise?

(a) Oxygen (b) Nitrogen

Question 26. Rhizobium bacteria live in the root nodules of which plants?

(a) Wheat and rice (b) Beans and peas

Question 27. Which of the following is unicellular?

(a) Human (b) Yeast

Question 28. Which experiment can prove that yeast is alive and actively growing?

(a) Keeping it in sunlight

(b) Mixing it with cold sugar solution

(d) Boiling it in water

Question 29. How does Spirulina support environmental sustainability?

(a) Fixes nitrogen in roots

(b) Produces alcohol

(d) Breaks down waste in landfills

Question 30. What would happen if all decomposer microorganisms disappeared from Earth?

(a) Food production would increase

(b) No curd or bread could be made

(d) Plants would grow faster

Answer :

Answer 1 : (c) Robert Hooke

Answer 2 : (c) Traps nitrogen from the air

Answer 3 : (b) Antonie van Leeuwenhoek

Answer 4 : (c) Cell wall (Note: Cell wall is an additional part found in plant/bacterial cells, but not a universal "main part" of all typical cells as defined in the source)

Answer 5 : (b) Moist and warm

Answer 6 : (c) Cytoplasm

Answer 7 : (b) Cell membrane

Answer 8 : (d) Chloroplast

Answer 9 : (c) Bacterial cells

Answer 10 : (a) Lactobacillus

Answer 11 : (b) Plant cells have a cell wall, animal cells do not.

Answer 12 : (c) Nerve cell

Answer 13 : (c) Cell → Tissue → Organ → Organ System → Organism

Answer 14 : (c) Ostrich egg yolk

Answer 15 : (c) 100 to 400 times

Answer 16 : (b) Viruses (Viruses are mentioned separately as microscopic and acellular, but not under the "Major Groups of Microorganisms" heading)

Answer 17 : (a) Presence of chlorophyll

Answer 18 : (c) Fungi

Answer 19 : (c) They are microscopic and acellular, multiplying only inside living cells.

Answer 20 : (d) Bacterial cell

Answer 21 : (b) They break down waste into nutrient-rich manure.

Answer 22 : (c) Yeast

Answer 23 : (d) Fungi

Answer 24 : (c) All living beings are made up of one or more cells, and it's the smallest structure performing life processes.

Answer 25 : (c) Carbon dioxide

Answer 26 : (b) Beans and peas

Answer 27 : (b) Yeast

Answer 28 : (c) Mixing it with warm sugar solution and observing gas formation

Answer 29 : (c) Produces oxygen and grows in less space

Answer 30 : (c) Waste and dead matter would accumulate

Fill in the blanks

Q.1. Yeast converts sugar into alcohol and ……….. gas during the process of fermentation.

Q.2. ……….. is the part of the cell that controls all its activities.

Q.3. The ………… in plant cells helps in the process of photosynthesis.

Q.4. When carbon dioxide is passed through lime water, it turns ……….

Q.5. The process of converting sugar into alcohol using microorganisms is called …………..

Q.6. In bread making, yeast helps make the dough rise by producing ……

Q.7. Microorganisms such as Rhizobium help in ……….. fixation in leguminous plants.

Q.8. The ………. provides structural support and shape to plant cells.

Q.9. ………. is a jelly-like substance found inside cells that contains all the cell organelles.

Q.10. In the balloon experiment, the balloon inflates due to the gas released by ………..

Q.11. The ……….. is a selectively permeable membrane that surrounds all cells.

Q.12. Microorganisms grow best when there is enough warmth, moisture, air, and ………..

Q.13. The presence of ……… in test tube B proves that gas was produced during the fermentation process.

Q.14. Dried leaves in compost pits provide ………… to balance the nitrogen-

rich food waste.

Q.15. In an experiment with bread slices, the one near the sink is more likely to grow mould because of higher ………. and temperature.

Answer :

Q. 1. carbon dioxide	Q. 2. Nucleus	Q. 3. chloroplast	Q. 4. milky
Q. 5. fermentation	Q. 6. carbon dioxide	Q. 7. nitrogen	Q. 8. cell wall
Q. 9. Cytoplasm	Q. 10. yeast	Q. 11. cell membrane	Q. 12. nutrients
Q. 13. an inflated balloon	Q. 14. carbon	Q. 15. humidity

State whether True or False :

Q.1. The cytoplasm is a jelly-like substance where most life processes occur within a cell.

Q.2. A true nucleus, which controls cell activities, is present in all types of cells, including bacterial cells.

Q.3. Plant cells have a cell wall, but animal cells do not.

Q.4. Chloroplasts, which contain chlorophyll for photosynthesis, are found in both plant and animal cells.

Q.5. Nerve cells are typically thin and flat, forming a protective lining in the body.

Q.6. Yeast produces oxygen during fermentation, which causes balloons to inflate in test tube experiments.

Q.7. The lime water test is used to detect the presence of carbon dioxide gas.

Q.8. Only bacterial cells have chloroplasts to carry out photosynthesis.

Q.9. Rhizobium bacteria in root nodules of legumes can fix atmospheric nitrogen into a form usable by plants.

Q.10. Fungal growth is faster in cold, dry conditions.

Q.11. In an experiment, if lime water remains clear after gas is passed through it, the gas is definitely carbon dioxide.

Q.12. All microorganisms are harmful and cause diseases.

Q.13. Bread becomes softer and fluffier because yeast releases carbon dioxide gas during dough fermentation.

Q.14. The cytoplasm is only found in animal cells.

Q.15. When sugar is mixed with warm water and yeast, the process of fermentation begins.

Q.16. The nucleoid is a membrane-bound structure found only in plant cells.

Q.17. Dried leaves added to compost help balance the carbon and nitrogen ratio needed for effective decomposition.

Q.18. In the absence of air, microorganisms can still grow efficiently on bread.

Q.19. If yeast is not added to the sugar solution in an experiment, the lime

water will still turn milky.

Q.20. Microorganisms can grow in extreme environments like hot springs, acidic lakes, and deep oceans.

Answer :

1. True	2. False	3. True	4. False	5. False
6. False	7. True	8. False	9. True	10. False
11. False	12. False	13. True	14. False	15. True
16. False	17. True	18. False	19. False	20. True

Short, Very Short Answer Type Questions

Question 1. List the three main parts that a typical cell possesses.

Answer :

A typical cell has three main parts: the cell membrane, cytoplasm, and the nucleus.

Question 2. What is the primary function of the cell membrane concerning materials entering and exiting the cell?

Answer :

The cell membrane is porous and allows the entry of materials essential for life processes and the exit of waste material.

Question 3. Name one commercial use of yeast.

Answer :

Baking bread/manufacture of alcoholic drinks.

Question 4. Name the process in yeast that converts sugars into alcohol.

Answer :

Fermentation.

Question 5. What is the jelly-like substance that fills the cell and is where most life processes occur?

Answer :

The cytoplasm is the jelly-like substance that fills the cell, contains all organelles, and is where most life processes occur.

Question 6. Which part of the cell is responsible for controlling all cell activities and regulating growth?

Answer :

The nucleus controls all cell activities and regulates growth.

Question 7. What specialized region is found only in bacterial cells where DNA is located, as they lack a true nucleus?

Answer :

The nucleoid is found only in bacterial cells and is the region where DNA is located since they don't have a true nucleus.

Question 8. Name two distinct structures that are found in plant cells but are absent in animal cells.

Answer :

Cell walls and chloroplasts are found in plant cells but are absent in animal cells.

Question 9. Describe the typical shape and a key function of nerve cells.

Answer :

Nerve cells are typically long with branches and are specialized to carry messages quickly.

Question 10. What is considered the largest known single cell in the living world?

Answer :

The yolk of an ostrich egg is the largest known single cell in the living world.

Question 11. What range of magnification is generally required to clearly observe microorganisms?

Answer :

Microorganisms require magnification of about 100 to 400 times to be seen clearly.

Question 12. Give two examples of unicellular microorganisms mentioned in the sources.

Answer :

Two examples of unicellular microorganisms are Bacteria and Amoeba.

Question 13. What unique characteristic distinguishes viruses from other microorganisms in terms of their reproduction?

Answer :

Viruses are microscopic and acellular and can only multiply inside living cells (host organisms).

Question 14. How do fungi and bacteria contribute to cleaning the environment?

Answer :

Fungi and bacteria break down plant waste and dead animals into nutrient-rich manure, thereby recycling nutrients and cleaning the environment.

Question 15. Which microorganism is responsible for making bread dough soft and fluffy, and how does it achieve this?

Answer :

Yeast, a type of fungus, makes bread dough soft and fluffy by releasing carbon dioxide during respiration.

Question 16. What specific type of bacteria is used to convert milk into curd, and what substance does it produce during this process?

Answer :

Lactobacillus bacteria convert milk into curd by producing lactic acid from milk sugar.

Question 17. Explain the beneficial role of Rhizobium bacteria in agriculture.

Answer :

Rhizobium bacteria live in the root nodules of legumes and trap nitrogen from the air (nitrogen fixation), making it usable for plants and improving soil fertility naturally.

Question 18. Name two significant contributions of microalgae to the environment and human society.

Answer :

Microalgae produce over half of Earth's oxygen through photosynthesis and serve as a food source for aquatic animals. They are also used by humans as supplements and medicines.

Question 19. Why is the cell considered the basic unit of life?

Answer :

The cell is considered the basic unit of life because all living beings are made up of one or more cells, and it is the smallest structure that can carry out all the processes needed for life.

Question 20. Why do farmers grow legumes in rotation with other crops?

Answer :

Because legumes contain Rhizobium bacteria in their roots that fix nitrogen and naturally enrich the soil.

Question 21. What do you observe when a bread slice is kept in a moist, warm place for 3 days? Why?

Answer :

Fungi grow on the bread due to the favorable warm and moist environment that supports microbial growth.

Question 22. What can be inferred if curd forms in warm milk but not in cold milk?

Answer :

Warm temperatures help bacteria multiply and ferment milk, while cold temperatures slow their activity.

Question 23. Why do microorganisms grow faster in warm, moist conditions?

Answer :

Because these conditions support their metabolism and reproduction, speeding up their growth.

Question 24. What is the difference between unicellular and multicellular microorganisms?

Answer :

Unicellular microbes like yeast have one cell; multicellular microbes like mould have many cells.

Question 25. Why is Spirulina called a "superfood"?

Answer :

It is rich in protein, vitamins, and minerals and is easy to digest with little fat or sugar.

Question 26. How are microorganisms used in the food industry?

Answer :

They are used for fermentation in products like curd, bread, idli, dosa, vinegar, and alcohol.

Question 27. What is the role of microorganisms in the production of biogas?

Answer :

Microbes decompose organic waste without oxygen, producing biogas such as methane that can be used as fuel.

Long Answer Type Questions

Question 1. What are the major groups of microorganisms, and how do viruses differ from them?

Answer :

Microorganisms are very tiny living things. The main groups are Protozoa, Algae, Fungi, Bacteria, and Viruses.

Protozoa – Single-celled organisms that can move around (example: Amoeba, Paramecium).
Algae – Plant-like organisms. Some are very small, others are bigger. They make their own food by photosynthesis.
Fungi – Includes yeast and mold. Yeast is single-celled, mold is many-celled. They cannot make their own food because they don’t have chlorophyll.
Bacteria – Single-celled organisms without a true nucleus. Their DNA is in a region called the nucleoid.
Viruses – Not made of cells. They can only reproduce inside the body of living things (plants, animals, or bacteria). Many viruses cause diseases.

Question 2. Explain the role of microorganisms in the nitrogen cycle, with reference to Rhizobium and soil fertility.

Answer :

Microorganisms help in the nitrogen cycle, which is very important for plant growth.

Rhizobium bacteria live in the root nodules of legume plants (like beans, peas, pulses).
They take nitrogen gas from the air and change it into ammonia, which plants can use to make proteins and grow.
This process is called biological nitrogen fixation.
It makes the soil rich in nitrogen, reduces the need for chemical fertilizers, and keeps the soil healthy.
When legume plants die, the nitrogen stays in the soil and helps the next crop.
That’s why crop rotation with legumes increases soil fertility and improves farming.

Question 3. Describe the fermentation process using yeast and explain its importance in daily life, including experimental evidence

Answer :

Fermentation is a process done by yeast where it breaks sugars into alcohol and carbon dioxide without oxygen (anaerobic).

In baking – Yeast in dough uses sugar and releases CO₂ gas. The gas makes bubbles, so the dough rises and becomes soft.
In brewing – Yeast makes alcohol in drinks like beer and wine.

Experiment to show fermentation:

Mix sugar, yeast, and warm water in a flask.
Close the mouth with a balloon.
After some time, the balloon inflates due to CO₂.
Passing this gas through lime water makes it turn milky, proving CO₂ is formed.

Important points:

Fermentation works best in warm and moist
It improves taste, texture, preservation, and nutrition of food.

Question 4. Compare and contrast unicellular and multicellular microorganisms with examples and structural differences.

Answer :

Microorganisms can be unicellular or multicellular.

Unicellular microbes – Made of a single cell that does all life functions.

Examples: Bacteria, yeast, protozoa
Reproduce quickly and are microscopic.
Yeast is a unicellular fungus with nucleus and cytoplasm; it also does fermentation.

Multicellular microbes – Made of many cells with division of labor (different cells do different tasks).

Examples: Moulds, Spirogyra (algae)
Form visible structures like colonies or filaments.
Moulds have thread-like hyphae for reproduction and absorbing nutrients.

Structural differences:

Unicellular microbes are simpler.
Some (like bacteria) are prokaryotic (no true nucleus).
Others (like fungi, protozoa) are eukaryotic (with nucleus).

Multicellular microbes are always eukaryotic, larger, and adapted to complex environments.

These differences affect how they grow, reproduce, and interact with their surroundings.

Question 5. How do environmental factors affect microbial growth? Discuss with the help of bread mould and curd formation examples.

Answer :

Microbial growth depends on temperature, moisture, air, and food.

Warm & moist conditions → microbes grow fast.
Cold & dry conditions → microbial growth slows or stops.

Examples:

Bread mould – Bread in a warm, moist place grows fuzzy mould in a few days. In dry or refrigerated conditions, mould does not grow.
Curd formation – Warm milk curdles quickly because Lactobacillus bacteria multiply faster. In cold milk, they grow slowly, so curdling is delayed.

Conclusion:
Microorganisms are sensitive to their environment. This fact is used in food preservation (refrigeration, drying, adding salt/sugar) to stop their growth.

Question 6. Discuss the role of micro-organisms in sustainable development, including food, fuel, and waste management.

Answer :

Microorganisms help in food, fuel, waste recycling, and soil fertility. They support an eco-friendly, circular system and are important for sustainable development.

Examples :

Food production – Microbes like Lactobacillus and yeast help make curd, bread, idli, and alcoholic drinks. They improve taste, nutrition, and shelf life of food.
Fuel production – In biogas plants, bacteria break down waste (cow dung, food scraps) without oxygen and produce methane gas. This is a renewable, clean fuel for cooking and lighting.
Waste management & soil health – Fungi and bacteria act as decomposers, breaking down dead plants and animals into nutrients, keeping the environment clean and soil fertile.
Microalgae (e.g., Spirulina) – Rich in nutrients, grow with very few resources, and can be used as sustainable superfood.

Case/Source Based Questions :

Question 1. In a school laboratory experiment based on the textbook, three test tubes were prepared:

Test Tube A contained only sugar solution and water.
Test Tube B contained sugar solution, water, and a pinch of yeast.
Test Tube C contained yeast and water only.

All three test tubes were sealed with balloons on top and left undisturbed for 2 days. After the observation period, the balloon on Test Tube B was inflated. When the gas collected from this tube was passed into lime water, it turned milky.

Questions:

(a) What gas caused the balloon on Test Tube B to inflate?

(b) Why was there no balloon inflation in Test Tube A and Test Tube C?

(c) What chemical reaction caused lime water to turn milky?

(d) What is the process called, in which yeast converts sugar into alcohol and carbon dioxide?

(e) What conclusion can you draw about the role of sugar in this experiment?

Answer :

(a) Carbon dioxide (CO₂)

(b) Test Tube A had no yeast to perform fermentation, and Test Tube C had no sugar as food for yeast, so no gas was produced.

(d) Fermentation

(e) Sugar is essential as an energy source for yeast to carry out fermentation and produce CO₂.

Question 2. A farmer decides to stop using chemical fertilizers and instead uses a compost pit made from kitchen waste, vegetable peels, and dry leaves. He also starts planting leguminous crops such as gram and beans between cereal crop seasons. Over time, he notices improved soil fertility and better plant growth.

Questions:

(a) Which types of microorganisms are responsible for breaking down kitchen waste in the compost pit?

(b) Why is it important to add both dry and wet waste to the compost pit?

(c) Which specific microorganism helps leguminous plants fix nitrogen in the soil?

(d) How does growing leguminous crops reduce the need for chemical fertilizers?

(e) What environmental benefit does composting offer over using synthetic fertilizers?

Answer :

(a) Decomposer microorganisms like fungi and bacteria.

(b) Dry waste adds carbon and balances moisture, while wet waste provides nitrogen-both are needed for efficient decomposition.

(d) Rhizobium fixes atmospheric nitrogen into forms plants can use, naturally enriching the soil with nutrients.

(e) Composting recycles organic waste, reduces landfill use, and avoids harmful chemical runoff into water bodies.

Question 3. The Sharma family is having a busy weekend. Mrs. Sharma is diligently preparing bread dough and idli batter for breakfast, knowing that tiny living organisms are crucial for making these foods light and fluffy. Meanwhile, Mr. Sharma is setting up a compost pit in their garden, intending to convert kitchen scraps and garden waste into rich soil. He recalls learning about how certain unseen organisms help break down organic matter. Their daughter, Priya, is also studying biology and asks about the microscopic life that helps their leguminous plants grow so well without much chemical fertilizer.

Questions:

(a) What specific processes are occurring in Mrs. Sharma's bread dough and idli batter.

(b) What is the key microorganism responsible for the changes in these foods?

(c) What types of microorganisms are involved in Mr. Sharma’s composting processes.

(d) What types of microorganisms are involved in Priya’s leguminous plants.

Answer :

(a) In both Mrs. Sharma's bread dough and idli batter, the process of fermentation is occurring.

(b) The key microorganism responsible for this change, especially in bread dough, is yeast, which is a type of fungus.

(d) For Priya's leguminous plants, a specific type of bacterium called Rhizobium is involved in nitrogen fixation.

Key Features of Kitabcd Exam Master :

NCERT Probe and Ponder/Intext Questions with Solutions,
Keep the Curiosity Alive/Exercise Questions with Solutions.
Objective Type Questions with Answers (MCQ, Fill in blanks, True or False)
Activities with answers.
Exam Oriented Questions and Answers
Case/Source Based Questions

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PDF : Curiosity-Chapter-2-The Invisible Living World: Beyond Our Naked Eye– Notes

PDF : Curiosity-Chapter-2-The Invisible Living World: Beyond Our Naked Eye– Exam Master

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Exam Master-NCERT-Class-8-Science-Curiosity-Chapter-2-The Invisible Living World: Beyond Our Naked Eye-CBSE

The Invisible Living World: Beyond Our Naked Eye

NCERT-Class-8-Science (Curiosity)-Chapter-2

Notes

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