Class 8 Science Curiosity Chapter 2 Solutions (NCERT 2026–27) – The Invisible Living World: Beyond Our Naked Eye

These Class 8 Science Curiosity Chapter 2 solutions cover The Invisible Living World: Beyond Our Naked Eye from the new NCF-2023 textbook (2026–27). You get every “Keep the curiosity alive” exercise question solved step by step, the “Probe and ponder” prompts answered, plus extra practice, MCQs, Assertion–Reason and FAQs.

Class: 8 Subject: Science Book: Curiosity Chapter: 2 Exercise: Keep the curiosity alive (9 Qs) Session: 2026–27
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Class 8 Science Curiosity Chapter 2 Solutions – Overview

Chapter 2 of Curiosity, The Invisible Living World: Beyond Our Naked Eye, opens the hidden world of tiny living things that the unaided eye cannot see. It traces how the lens and the microscope (Robert Hooke’s Micrographia and Antonie van Leeuwenhoek, the Father of Microbiology) revealed the cell — the basic unit of life. You study the parts of a cell (cell membrane, cytoplasm, nucleus and the plant cell wall), how cells vary in shape and function, the levels of organisation (cell → tissue → organ → organ system → organism) and the four main groups of microorganisms — bacteria, fungi, protozoa and algae — along with viruses. The chapter also shows how microbes decompose waste, fix nitrogen, ferment food and make curd, bread and biogas.

Key Concepts & Definitions

Lens: a curved piece of glass (thick in the middle, thin at the edge) that makes small objects look bigger; improved lenses led to the microscope.

Cell: the basic structural and functional unit of life; the word was first used by Robert Hooke (1665) for the box-like spaces he saw in cork.

Parts of a cell: cell membrane (porous outer boundary), cytoplasm (where most life processes occur) and nucleus (controls activities and growth). Plant cells have an extra rigid cell wall, plus plastids (e.g. chloroplasts with chlorophyll) and a large vacuole.

Levels of organisation: Cell → Tissue → Organ → Organ system → Organism.

Microorganisms (microbes): tiny living beings not visible to the naked eye — bacteria, fungi, protozoa and some algae. They may be unicellular (bacteria, protozoa) or multicellular (most fungi, many algae).

Viruses: microscopic and acellular; they multiply only inside a living host cell and may cause disease.

Useful microbes: bacteria and fungi decompose waste into manure; Rhizobium fixes nitrogen in legume root nodules; yeast (a fungus) makes dough rise; Lactobacillus turns milk into curd; microalgae release oxygen and make biofuel.

“Probe and ponder” — Answers

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

ANSWERWe would see vast numbers of tiny living things everywhere — in water, soil, air, on food and even on our skin and inside our gut. A single drop of pond water would be full of moving protozoa like Amoeba and Paramecium, green algae, threadlike fungi and countless bacteria. The world would look far busier and more crowded than it appears to our naked eye.

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’?

ANSWERIt shows that something can be alive even when far too small to see, and that even a single cell is a complex, working structure with a membrane, cytoplasm and nucleus. We realise that “living” is not about size — a microscopic bacterium and a giant tree are both built from cells. It also makes us question borderline cases like viruses, which are acellular and reproduce only inside a host.

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

ANSWERMicroorganisms interact closely with one another and with plants and animals. Some, like bacteria and fungi, break down dead plants and animals and recycle nutrients; Rhizobium bacteria live inside legume roots and supply nitrogen; helpful bacteria live in our gut and aid digestion; and yeast and Lactobacillus change food during fermentation. Such interactions can be helpful or, in the case of some microbes, harmful.

(The book also invites you to “Share your questions” — write your own curiosities, e.g. “How do bacteria move without legs?”)

Class 8 Science Curiosity Chapter 2 Solutions — 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 (The diagram is a Venn diagram with regions: “Only in Animal Cell”, “Common to all three cells”, “Only in Bacterial Cell” and “Only in Plant Cell”.)

ANSWER Place each part according to where it is found in animal, bacterial and plant cells:
Region of the diagramCell parts to write
Common to all three cellsCell membrane, Cytoplasm
Only in Animal CellNucleus (present, with no cell wall or chloroplast)
Only in Plant CellCell wall, Chloroplast (plant also has a nucleus, shared with the animal cell)
Only in Bacterial CellNucleoid (instead of a true nucleus)
Note: The nucleus is common to animal and plant cells but is replaced by a nucleoid in bacteria. The cell wall is found in plant and bacterial cells but not in animal cells; the chloroplast is only in plant cells.

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 hours? 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) Prediction: the balloon on test tube B (with yeast) will inflate, while the balloon on test tube A (no yeast) will stay almost unchanged. The correct explanation is (c) Yeast produced a gas inside the test tube B which inflated the balloon. Yeast feeds on the sugar, respires and releases carbon dioxide, which collects in the balloon and inflates it. (ii) She wants to find out (test) whether the gas produced by the yeast is carbon dioxide. When carbon dioxide is shaken with lime water, the lime water turns milky — this is the standard test for carbon dioxide.

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 a legume. The roots of legumes (beans, peas, lentils) form swollen root nodules that house Rhizobium bacteria. These bacteria trap nitrogen from the air and convert it into a form the plant can use. Because the bean plants get their nitrogen naturally from these bacteria, the second farmer does not need to add nitrogen fertiliser. Wheat, however, is not a legume and has no such bacteria, so the first farmer must add nitrogen-rich fertiliser. This is also why farmers grow legumes in rotation with other crops — to keep the soil fertile.

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 She is testing how the kind/mixture of organic waste affects decomposition (manure formation) by microorganisms — in particular, whether mixing dried leaves with the peels helps the waste break down faster or better into nutrient-rich manure. Pit B acts as a comparison (control). Microbes such as bacteria and fungi decompose the waste; the different mixtures let Snehal compare how well manure forms in each pit.

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 — they are found in water, soil, air, extreme habitats and inside our gut, where they help in digestion. (ii) Yeast (a fungus) — it releases carbon dioxide that makes bread and cakes rise and become soft and fluffy. (iii) Rhizobium bacteria — they live in the root nodules of pulses (legumes) and fix nitrogen for the plant’s growth.

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

ANSWER Aim: to show that microbes (mould) grow best when temperature, air and moisture are suitable. Method: Take four similar slices of bread. (A) keep a moist slice in a warm, open place; (B) keep a moist slice in the refrigerator (low temperature); (C) keep a dry slice in a warm, open place; (D) seal a moist slice in an air-tight polythene bag with no air. Observe all four after 3–4 days. Expected result: the most mould grows on slice A (warm + moist + air). Less or no growth on B (too cold), C (no moisture) and D (no air). This proves microorganisms need a suitable temperature, moisture and air to grow. (Changing only one factor at a time makes the test fair.)

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 slice kept near the sink develops a cottony or powdery growth (mould, often greenish or black) after three days, while the slice in the refrigerator stays almost fresh with little or no growth. Reason: The area near the sink is warm and moist, which are ideal conditions for the growth of fungi (mould). The refrigerator is cold and dry, which slows down microbial growth — that is why food kept in a fridge stays good for longer.

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 Explanation 1: The curd contains bacteria such as Lactobacillus. When the curd is left out (especially in a warm place), these bacteria keep multiplying and ferment more of the milk sugar (lactose) into lactic acid, which makes the curd more sour. Explanation 2: Warmth speeds up this bacterial activity, so leaving the curd at room temperature (instead of in a cool fridge) allows even faster acid production — making it more sour over the day.

9. Observe the set-up given in Fig. 2.15 and answer the following questions. (Flask A holds warm sugar solution + yeast, connected by a tube to test tube B containing lime water.) (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) In flask A the yeast ferments the sugar solution. The yeast respires using the sugar, producing carbon dioxide gas (and a little alcohol). Bubbles form and the solution becomes frothy, while the gas travels through the tube into test tube B. (ii) After four hours, the lime water in test tube B turns milky (cloudy). This happens because the carbon dioxide produced by the yeast passes into the lime water — carbon dioxide turns lime water milky, confirming that the gas given off is CO2. (iii) If no yeast was added, there would be no fermentation, so no carbon dioxide would be produced. The lime water in test tube B would stay clear (not turn milky), showing that the gas is released only because of the living yeast.

Common Misconceptions to Avoid

Watch out for these

  • Thinking microorganisms are always harmful — many are beneficial (decomposers, nitrogen fixers, yeast, Lactobacillus, microalgae).
  • Treating viruses as ordinary microorganisms — viruses are acellular and multiply only inside a living host cell.
  • Saying bacteria have a nucleus — bacteria have no true nucleus; they have a nucleoid instead.
  • Confusing the cell wall with the cell membrane — the wall is the extra rigid outer layer in plant, fungal and bacterial cells; the membrane is present in all cells.
  • Believing all fungi are multicellular — yeast is a unicellular fungus, while mould is multicellular.
  • Thinking yeast makes dough rise by producing oxygen — it actually produces carbon dioxide (which turns lime water milky), not oxygen.

Extra Practice Questions

Very Short Answer Type Questions

Q1. Who is known as the “Father of Microbiology”?

ANSWERAntonie van Leeuwenhoek.

Q2. Name the bacterium that helps in the formation of curd.

ANSWERLactobacillus.

Q3. What is the genetic region of a bacterial cell called?

ANSWERThe nucleoid (bacteria have no true nucleus).

Short Answer Type Questions

Q1. List the levels of organisation in the body of a multicellular organism.

ANSWERCell → Tissue → Organ → Organ system → Organism. A group of similar cells forms a tissue, different tissues form an organ, several organs form an organ system, and all the systems together make a complete organism.

Q2. How do microorganisms help in cleaning the environment?

ANSWERBacteria and fungi decompose dead plants, animal bodies and other organic waste into simpler, nutrient-rich substances (manure). This recycles nutrients back into the soil, helps plants grow and keeps the surroundings clean.

Long Answer Type Question

Q1. Describe the basic parts of a cell and state the function of each.

ANSWER Cell membrane: the porous outer boundary that separates one cell from another and controls the entry of useful materials and the exit of wastes. Cytoplasm: the jelly-like region between the membrane and nucleus that holds other components and where most life processes take place. Nucleus: the round structure (covered by a thin membrane) that controls all activities of the cell and regulates growth. Cell wall (in plant cells): an extra rigid outer layer that gives strength and shape to the cell. Plant cells also have plastids (e.g. chloroplasts with chlorophyll for photosynthesis) and a large vacuole for storage and support.

MCQs & Assertion–Reason

1. The basic structural and functional unit of life is the:

(a) tissue    (b) organ    (c) cell    (d) nucleus

2. Who first used the word “cell” after observing a thin slice of cork?

(a) Antonie van Leeuwenhoek    (b) Robert Hooke    (c) A. M. Chakrabarty    (d) Louis Pasteur

3. Which of these is NOT a part of a typical cell?

(a) cell membrane    (b) cytoplasm    (c) nucleus    (d) lens

4. Amoeba and Paramecium are examples of:

(a) fungi    (b) protozoa    (c) algae    (d) viruses

5. Which microorganism helps in making the dough of bread rise?

(a) Lactobacillus    (b) Rhizobium    (c) yeast    (d) Amoeba

6. Bacteria lack a true nucleus; instead they have a:

(a) vacuole    (b) nucleoid    (c) chloroplast    (d) cell wall only

7. Rhizobium bacteria living in the root nodules of legumes help by:

(a) fixing nitrogen    (b) producing oxygen    (c) causing disease    (d) making curd

8. Which of the following is acellular (not made of cells)?

(a) bacterium    (b) virus    (c) yeast    (d) alga

9. The gas produced by yeast during fermentation, which turns lime water milky, is:

(a) oxygen    (b) hydrogen    (c) carbon dioxide    (d) nitrogen

10. Microalgae such as Spirulina are important because they:

(a) cause food to rot    (b) release oxygen and serve as food/supplements    (c) have no nucleus    (d) cannot live in water

Answer key: 1-(c), 2-(b), 3-(d), 4-(b), 5-(c), 6-(b), 7-(a), 8-(b), 9-(c), 10-(b).

For each Assertion–Reason question, choose: (A) Both true and the Reason correctly explains the Assertion; (B) Both true but the Reason is not the correct explanation; (C) Assertion true, Reason false; (D) Assertion false, Reason true.

A-R 1. Assertion: Yeast makes dough rise and become fluffy.

Reason: Yeast respires on sugar and releases carbon dioxide, which forms bubbles in the dough.

A-R 2. Assertion: Bacteria are eukaryotic cells with a well-defined nucleus.

Reason: Bacteria have a nucleoid and lack a nuclear membrane.

A-R 3. Assertion: Legumes can grow well without nitrogen fertiliser.

Reason: Rhizobium bacteria in their root nodules fix nitrogen from the air for the plant.

A-R 4. Assertion: Viruses are considered different from other microorganisms.

Reason: Viruses are acellular and multiply only inside a living host cell.

A-R 5. Assertion: Pickles and murabbas do not get infected by microorganisms easily.

Reason: High concentrations of salt or sugar do not allow microbes to grow on them.

Answer key: 1-(A), 2-(D), 3-(A), 4-(A), 5-(A).

Quick Revision Summary

  • Microorganisms are tiny organisms not visible to the unaided eye; they live in water, soil, air and inside plants and animals.
  • They are unicellular or multicellular: bacteria and protozoa are unicellular; fungi may be unicellular (yeast) or multicellular (mould); plants and animals are multicellular.
  • The cell is the basic unit of life. A typical cell has a cell membrane, cytoplasm and a nucleus; plant, fungal and bacterial cells have an extra cell wall, and bacteria lack a true nucleus.
  • Cells differ in shape and size, and their shape suits their function.
  • Bacteria, fungi and protozoa are the main kinds of microorganisms; viruses are small but different because they reproduce only inside a host.
  • Microbes can be beneficial or harmful: they decompose waste, fix nitrogen in root nodules, and make curd (Lactobacillus), bread and cakes (yeast) and biogas.

Real-life Applications

Microbes touch daily life everywhere: yeast makes bread, cakes, idli, dosa and bhatura soft and fluffy; Lactobacillus turns milk into curd; Rhizobium lets farmers grow legumes without nitrogen fertiliser, which is why crops are rotated; bacteria and fungi turn kitchen and garden waste into manure and produce biogas for cooking and electricity; microalgae like Spirulina are used as health supplements and to make biofuel; and high salt or sugar in pickles and murabbas keeps spoilage microbes away so food lasts longer.

How to score full marks in this chapter

Remember the four groups of microbes (bacteria, fungi, protozoa, algae) with one example each, and keep viruses separate as “acellular”. Know the three basic cell parts plus the plant cell wall, and that bacteria have a nucleoid (not a nucleus). For experiment questions on yeast, always state that the gas is carbon dioxide and that it turns lime water milky. Link each useful microbe to its job: Rhizobium → nitrogen fixing, yeast → fermentation/rising dough, Lactobacillus → curd.

Frequently Asked Questions

What is Class 8 Science Curiosity Chapter 2 about?

It is about the invisible living world — how microscopes revealed the cell, the parts of a cell, the levels of organisation (cell to organism), and microorganisms such as bacteria, fungi, protozoa and algae, plus viruses, and how microbes help us.

What is the exercise of Chapter 2 called and how many questions does it have?

The end-of-chapter exercise is titled “Keep the curiosity alive” and has 9 questions (some with sub-parts), all solved step by step on this page.

Why does the balloon on the test tube with yeast inflate?

Yeast feeds on the sugar and respires, releasing carbon dioxide gas. This gas collects and inflates the balloon. Shaking the gas with lime water turns it milky, proving it is carbon dioxide.

Are these Class 8 Science Curiosity Chapter 2 solutions free?

Yes. All ClearStudy NCERT Solutions for Class 8 Science Curiosity are free and follow the official NCERT textbook for 2026–27.

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