NCERT Solutions for Class 8 Science Chapter 2: The Invisible Living World – Free PDF Download

Chapter 2 — The Invisible Living World — introduces cells as the basic unit of life and microorganisms (bacteria, fungi, protozoa, algae) — how microscopes revealed them, their beneficial roles (fermentation, decomposition, nitrogen fixation), and food preservation. Unlike Chapter 1, this chapter has a genuine, numbered end-of-chapter exercise (‘Keep the Curiosity Alive’, 9 questions) plus many in-text discovery questions. Below are complete, verified answers, sourced directly from the official NCERT PDF. These Class 8 Science Chapter 2 solutions are also useful as quick revision notes before exams.

NCERT Solutions for Class 8 Science Chapter 2: The Invisible Living World

Exercise: Keep the Curiosity Alive

1. Various parts of a cell are given: Nucleus, Cytoplasm, Chloroplast, Cell wall, Cell membrane, Nucleoid. Place them correctly — common to all cells, or unique to animal/plant/bacterial cells.
Common to all three: Cytoplasm, Cell membrane. Animal + Plant cells: Nucleus. Plant only: Chloroplast. Plant + Bacterial: Cell wall. Bacterial only: Nucleoid (bacteria lack a true nucleus).

2. Aanandi set up two test tubes with sugar solution; she added yeast only to tube B, and attached a deflated balloon to each. (i) What happens after 3–4 hours, and why does B’s balloon inflate? (ii) She then tests the gas from B’s balloon with limewater — what is she trying to find out?
(i) Yeast in tube B ferments the sugar and releases carbon dioxide gas, which inflates its balloon; tube A (no yeast) shows no change. (ii) She wants to confirm the gas is CO₂ — CO₂ turns limewater milky (forms calcium carbonate), so shaking the gas with limewater is a test for carbon dioxide.

3. One farmer adds nitrogen fertiliser for wheat; his neighbour growing beans doesn’t. Why?
Beans are legumes — their root nodules host Rhizobium bacteria, which fix atmospheric nitrogen into a usable form, naturally enriching the soil. Wheat isn’t a legume and lacks this partnership, so it needs added fertiliser.

4. Snehal buries fruit/vegetable peels mixed with dried leaves in pit A, and the same peels alone (unmixed) in pit B. What is she testing?
She’s testing how mixing dry, carbon-rich matter with moist, nitrogen-rich peels affects decomposition/manure formation — pit A (mixed) is expected to decompose into manure faster than pit B.

5. Identify the microorganism: (i) lives in every environment, including your gut (ii) makes bread and cakes soft and fluffy (iii) lives in the roots of pulse crops and provides nutrients.
(i) Bacteria (ii) Yeast (iii) Rhizobium.

6. Design an experiment to test that microorganisms need optimal temperature, air and moisture to grow.
Keep three identical bread slices: one warm, moist and exposed to air; one sealed dry at room temperature; one refrigerated (cold, moist). Observe for 3–5 days. The warm, moist, aerated slice should show visible mould, while the dry and the cold slices show little or none — showing microbes need warmth, moisture and (for most fungi) air.

7. Place one bread slice near the sink and another in the refrigerator; compare after three days.
The slice near the sink develops visible mould because warmth and moisture favour microbial growth; the refrigerated slice shows little or no growth because cold slows microbial activity.

8. Curd left out for a day becomes more sour — give two possible explanations.
(1) Lactobacillus bacteria keep multiplying and fermenting lactose into lactic acid over time. (2) Warmer room temperature (versus refrigeration) speeds up bacterial activity, increasing acid production.

9. In a set-up where yeast+sugar solution (flask A) is connected to limewater (test tube B): (i) what happens to the sugar solution? (ii) what happens in tube B after four hours, and why? (iii) what if yeast wasn’t added?
(i) Yeast ferments the sugar, producing CO₂ and a little alcohol. (ii) The limewater turns milky, because CO₂ from flask A reacts with it to form calcium carbonate. (iii) Without yeast, no fermentation and no CO₂ would form, so the limewater would stay clear.

Key In-Text Discovery Questions (by section)

How did we discover the invisible world of microbes?
Through the invention of the microscope — a convex lens bends light to form an enlarged image, making tiny objects visible. Robert Hooke’s compound microscope (1665) first showed plant cells in cork; Antonie van Leeuwenhoek’s stronger single-lens microscopes (1670s) first revealed bacteria.

What did onion peel cells and human cheek cells show under a microscope, and how do they compare?
Both are made of small, compact units called cells. Onion peel cells are rectangular and compact with a rigid cell wall; cheek cells are irregular/polygon-shaped, more flexible, and lack a cell wall. Both share a cell membrane, cytoplasm and nucleus.

Why do different cells (like muscle cells and nerve cells) look so different?
Because a cell’s shape is adapted to its function — muscle cells are spindle-shaped for contraction, while nerve cells are long and branched to carry signals over distances.

Why doesn’t a lemon or tomato left out for a while get infected by microbes evenly, and why don’t pickles or murabbas spoil the same way?
Airborne fungi/bacteria settle on exposed food and grow, causing spoilage. Pickles and murabbas use high concentrations of salt or sugar as preservatives — these draw water out of microbial cells by osmosis, preventing them from growing.

What would happen if microorganisms did not exist on Earth?
Dead matter and waste would not decompose, nutrients would not recycle back into soil, and the environment would fill up with undecomposed waste — soil fertility and ecological balance would suffer badly.

What is the role of yeast in making bread dough rise, and why do we add sugar and warm water?
Yeast ferments the sugar, releasing carbon dioxide (which makes the dough rise and turn fluffy) and a little alcohol (which changes its smell). Sugar is the yeast’s food, and warm water creates favourable conditions for yeast activity.

Why does milk turn into curd?
Lactobacillus bacteria in the starter feed on lactose (milk sugar), multiply in warm conditions, and produce lactic acid, which curdles and sours the milk. Cold conditions slow this process, which is why curd doesn’t form well in the refrigerator.

Why This Chapter Matters

This chapter builds the cell concept and introduces microorganisms’ beneficial roles — fermentation, decomposition, and nitrogen fixation — which are the foundation for the next chapter on microbial diseases, and for later biology topics on nutrition, reproduction and ecosystems.

More on This Chapter

Extra Questions (HOTS) | Revision Notes | Formulas Handbook | Class 8 Science Book

Frequently Asked Questions

Does this chapter cover diseases caused by microbes?
No — this chapter focuses only on beneficial microorganisms. The NCERT book explicitly says disease-causing microbes are covered in the next chapter.

Are all the ‘extra questions’ circulating online for this chapter genuine NCERT content?
Not always — some third-party sites publish self-authored practice questions not found in the actual textbook. Everything above (the exercise and the in-text questions) has been verified directly against the official NCERT PDF.

Written by Satish

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