NCERT Solutions for Class 8 Science Curiosity Chapter 2 The Invisible Living World: Beyond Our Naked Eye

Keep the curiosity alive

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
See AnswerFor a Plant Cell (typically rectangular):
Cell wall – The thick outermost boundary
Cell membrane – Just inside the cell wall
Cytoplasm – The fluid-filled space inside the cell
Nucleus – The central, round structure inside the cytoplasm
Chloroplast – Green oval-shaped structures in the cytoplasm
For a Bacterial Cell (simpler structure):
Nucleoid – Irregular region without a membrane, contains genetic material.

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 testtube 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.
See AnswerThe balloon attached to test tube B will be inflated, while the balloon on test tube A will show no change.
The Correct Option is (c): Yeast produced a gas inside the test tube B which inflated the balloon.
Explanation:
Yeast is a microorganism that feeds on sugar and in warm conditions, carries out fermentation. During this process, carbon dioxide (CO₂) gas is released, which fills up the balloon. No gas is produced in test tube A as it doesn’t contain yeast.

(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?
See AnswerAanandi wants to identify the gas that was produced by the yeast in test tube B. By passing the gas from the balloon into lime water, she can check for the presence of carbon dioxide (CO₂). If the lime water turns milky, it confirms that the gas produced is carbon dioxide. This is a common test to detect CO₂ and helps prove that yeast ferments sugar to release this gas.

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?
See AnswerYes, there is a clear reason for the difference in approach by the two farmers.
Wheat is a cereal crop that does not fix nitrogen naturally, so the farmer added nitrogen-rich fertiliser to the soil to support plant growth and increase crop yield. These fertilisers supply the essential nitrogen needed for healthy leaves, proteins and grain formation.
On the other hand, bean crops are legumes, and leguminous plants like beans, peas and lentils have a special ability to fix atmospheric nitrogen naturally. They have root nodules that contain Rhizobium bacteria, which convert nitrogen from the air into a form the plant can absorb. This natural process removes the need for adding external nitrogen fertilisers. Hence, the second farmer could grow healthy bean crops without using chemical nitrogen fertilisers.

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?
See AnswerSnehal is trying to test the role of dried leaves (carbon-rich material) in the decomposition process and formation of manure.
By setting up two pits with the same kind of kitchen waste but only pit A having dried leaves mixed in, she is observing how the presence or absence of such dry matter affects the breakdown of organic waste by microorganisms. After 3 weeks, she can compare both pits to see which one has decomposed better and formed dark, nutrient-rich compost.
Dried leaves help maintain a balanced carbon-to-nitrogen ratio, improve aeration and support the growth of decomposer microbes like fungi and bacteria. So, Snehal is investigating how proper mixing of waste helps in faster and better manure formation.

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.
See Answer(i) Bacteria
(ii) Yeast
(iii) Rhizobium.

6. Devise an experiment to test that microorganisms need optimal temperature, air and moisture for their growth.
See AnswerExperiment: To Test That Microorganisms Need Optimal Temperature, Air, and Moisture for Growth
Objective:
To show that microorganisms grow best when the right temperature, air (oxygen), and moisture are present.
Materials Needed:
4 slices of bread
4 clean zip-lock plastic bags or airtight containers
Water
A spoon
Labels or markers
Steps:
Label the bread slices as A, B, C and D.
Bread A: Keep it dry, uncovered and at room temperature.
Bread B: Sprinkle a few drops of water on it to make it moist. Keep it in an airtight bag at room temperature.
Bread C: Moisten it like B, but store it in a refrigerator (cold condition).
Bread D: Moisten it like B, but keep the bag completely sealed with no air.
Observation Time:
Wait for 4–5 days and observe the bread slices daily.
Expected Results:
Bread B (moist, warm, with air) will show most microbial growth like green or black mold.
Bread A (dry) will show little or no growth.
Bread C (cold) will show slow or no growth, due to low temperature.
Bread D (no air) will show less or no growth, due to lack of oxygen.
Conclusion:
This experiment shows that microorganisms grow best when all three conditions—optimal temperature, moisture and air—are available. Lack of any one of these slows or stops their growth.

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.
See AnswerActivity Setup:
Slice A: Placed near the sink (warm, moist area)
Slice B: Placed inside the refrigerator (cold, dry environment)
Observations after 3 days:
Slice A (Near Sink):
Visible growth of green or black spots (mold)
Slight foul smell
Bread appears soft and damp
Slice B (In Refrigerator):
No mold or very minimal growth
Bread remains dry and unchanged in appearance
Explanation:
Microorganisms like fungi and bacteria grow faster in warm and moist environments, which is why mold developed on Slice A near the sink. The sink area has moisture and a suitable temperature, encouraging microbial activity.
On the other hand, Slice B was kept in a cold and dry environment inside the refrigerator. Cold temperature slows down or stops the growth of microorganisms, so the bread stayed mostly unaffected.
Conclusion:
This experiment proves that temperature and moisture play a key role in the growth of microorganisms. Mold grows quickly in warm and moist places, while cold slows down microbial growth.

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?
See AnswerIncreased growth of bacteria:
Curd contains Lactobacillus bacteria, which feed on the sugar (lactose) present in milk. When curd is left outside at room temperature, these bacteria multiply faster. As their number increases, they produce more lactic acid, which makes the curd taste more sour.
Warm temperature speeds up fermentation:
Leaving curd out in a warm environment provides the ideal conditions (temperature and moisture) for bacterial activity. The warm temperature accelerates the fermentation process, increasing the acid content and thus the sourness of the curd over time.
So, both the increase in bacterial activity and the favorable temperature contribute to the curd becoming more sour when left out.

9. Observe the set-up given in Fig. 2.15 and answer the following questions.
(i) What happens to the sugar solution in flask A?
See AnswerThe sugar solution in flask A undergoes fermentation. The yeast added to it feeds on the sugar and breaks it down into carbon dioxide (CO₂) and a small amount of alcohol.

(ii) What do you observe in test tube B after four hours? Why do you think this happened?
See AnswerThe lime water in test tube B turns milky after four hours.
This happens because the carbon dioxide gas released by the yeast during fermentation in flask A is passed into test tube B. Lime water reacts with CO₂ to form calcium carbonate, which causes the milky appearance.

(iii) What would happen if yeast was not added in flask A?
See AnswerIf yeast was not added, no fermentation would take place.
As a result, no carbon dioxide would be produced, and the lime water in test tube B would remain clear, showing that CO₂ is essential for the milky change.

For centuries, the human eye remained blind to a vast invisible world teeming with life. The invention of lenses and later microscopes, revolutionised our understanding by magnifying tiny objects. Robert Hooke’s Micrographia introduced the world to cells through his observation of cork under a microscope. He coined the term “cell”, while Antonie van Leeuwenhoek became the first to see living microorganisms, including bacteria. These discoveries established that all living beings, whether plants, animals or tiny microbes, are made up of cells. Cells have three main parts: the cell membrane, cytoplasm and nucleus. In plant cells, additional structures like the cell wall, vacuoles and plastids such as chloroplasts are present, enabling photosynthesis. Animal cells lack these features but have their own specialised structures. The cell is rightfully called the basic unit of life.

Who is known as the father of microbiology?
See AnswerAntonie van Leeuwenhoek.

What is the basic unit of life?
See AnswerCell.

Which scientist coined the term “cell”?
See AnswerRobert Hooke.

Name one unicellular organism.
See AnswerAmoeba.

What pigment makes algae green?
See AnswerChlorophyll.

Which microorganism helps in curd formation?
See AnswerLactobacillus.

What is the name of the green plastid in plant cells?
See AnswerChloroplast.

Which part of the cell controls all its functions?
See AnswerNucleus.

What is the outer covering of plant cells called?
See AnswerCell wall.

Which gas is released by yeast during fermentation?
See AnswerCarbon dioxide.

What are the microorganisms that live in root nodules of legumes?
See AnswerRhizobium.

Name one fungus used in making bread.
See AnswerYeast.

Which cell structure helps in giving strength to plant cells?
See AnswerVacuole.

What is the shape of a muscle cell?
See AnswerSpindle-shaped.

Which type of cell lacks a well-defined nucleus?
See AnswerBacterial cell.

Microorganisms or microbes are tiny organisms that cannot be seen with the naked eye. They exist in diverse forms such as bacteria, fungi, protozoa, algae and even viruses. Microorganisms may be unicellular (like Amoeba or Lactobacillus) or multicellular (like moulds or some algae). They thrive in water, soil, air and even extreme environments like hot springs and snowfields. Their shapes and structures vary – rod-shaped, spherical, spiral or irregular. Using foldscopes or microscopes, students can observe microorganisms in pond water or soil suspensions, identifying forms like algae, paramecium, fungi and bacteria. These microbes are not just abundant but also functional. For instance, Rhizobium bacteria in legume root nodules help in nitrogen fixation, while fungi and bacteria decompose waste, returning nutrients to the soil and helping clean the environment naturally.

What did Robert Hooke observe in cork under a microscope?
See AnswerRobert Hooke observed tiny compartments in cork that reminded him of a honeycomb. He called each compartment a “cell”, marking the first scientific use of the term.

What is the function of the cytoplasm in a cell?
See AnswerThe cytoplasm contains various components and is the site where most life processes occur, such as storage of nutrients and chemical reactions.

Why do onion peel cells and cheek cells appear different under the microscope?
See AnswerOnion peel cells are rectangular with a cell wall, while cheek cells are round and have only a cell membrane. This is due to structural differences between plant and animal cells.

How do microorganisms help in cleaning the environment?
See AnswerMicroorganisms like fungi and bacteria decompose dead plants and animals into simpler substances, thus recycling nutrients and cleaning up waste.

What is the role of yeast in making bread?
See AnswerYeast ferments the sugars in the dough, releasing carbon dioxide which makes the dough rise and become soft and fluffy.

What is the difference between unicellular and multicellular organisms?
See AnswerUnicellular organisms are made up of only one cell (e.g., bacteria), while multicellular organisms have many cells performing specialised functions (e.g., humans, plants).

How do Rhizobium bacteria help plants?
See AnswerRhizobium bacteria fix atmospheric nitrogen into a form usable by plants, enriching soil fertility especially in leguminous plants like peas and beans.

Why is Spirulina called a superfood?
See AnswerSpirulina is rich in proteins, vitamins (like B12), and nutrients. It produces oxygen and is used in health supplements, making it highly beneficial.

What are the three main parts of a typical cell?
See AnswerA typical cell has a cell membrane, cytoplasm and nucleus.

What is the function of the cell membrane?
See AnswerThe cell membrane surrounds the cell, controls the entry and exit of substances and separates it from other cells.

Microorganisms play essential roles in both household and environmental settings. In the kitchen, yeast, a fungus, is used to ferment dough for bread, idli or bhatura by releasing carbon dioxide, making it fluffy. Similarly, bacteria like Lactobacillus convert milk into curd by producing lactic acid. Microorganisms also help in natural waste disposal. Fungi and bacteria decompose organic waste like fruit peels and dead leaves into manure, enriching the soil. Anaerobic bacteria in biogas plants decompose waste to release methane, which is used as a clean fuel. Microalgae like Spirulina not only produce oxygen but are also superfoods rich in protein and nutrients. However, the same diversity also includes harmful microbes that cause disease, a topic introduced at the chapter’s end. Thus, microorganisms, though invisible, are deeply woven into life processes, ecosystems and even our food.

Describe the structure and function of a plant cell.
See Answer• Rectangular in shape.
• Has a cell wall for strength and protection.
• Contains chloroplasts for photosynthesis.
• Has a large vacuole for storage and shape maintenance.
• Nucleus controls cell activities; cytoplasm is the site for life processes.

Write a few uses of microorganisms in daily life.
See Answer• Lactobacillus is used to make curd.
• Yeast helps in baking bread and cakes.
• Microorganisms decompose waste to form manure.
• Rhizobium enriches soil by nitrogen fixation.
• Algae like Spirulina are used as food supplements.

Explain how microorganisms are observed in water and soil.
See Answer• Use a dropper to collect pond water or soil suspension.
• Place it on a slide and cover with a coverslip.
• Observe under a microscope or foldscope.
• Identify various microbes like Amoeba, Paramecium, algae, bacteria.

What are the differences between plant and animal cells?
See Answer• Plant cells have a cell wall; animal cells do not.
• Plant cells contain chloroplasts; animal cells do not.
• Plant cells have large vacuoles; animal cells have small or no vacuoles.
• Shape of plant cells is fixed; animal cells vary in shape.

How do microbes help in making manure and biogas?
See Answer• Microbes break down organic waste into simple nutrients (manure).
• In the absence of oxygen, some bacteria release methane gas.
• This gas is collected and used as biogas for fuel.
• Helps in waste disposal and energy production.