Keep the Curiosity Alive — Chapter Exercises — "The Invisible Living World: Beyond Our Naked Eye"

1. 14 marksCuriosity Grade 8, Chapter 2, Keep the curiosity alive, page 25, Q1

   Various parts of a cell are given below. Write them in the appropriate places in the Venn diagram, whose three circles are labelled 'Only in Animal Cell', 'Only in Bacterial Cell' and 'Only in Plant Cell', with the central overlap labelled 'Common to all three cells'. The parts are: Nucleus, Cytoplasm, Chloroplast, Cell wall, Cell membrane, Nucleoid.

   Hint. Check each part against all three cell types before deciding — two of the six do not belong in any 'only' region.

   Test each part against all three cell types, using the chapter's own statements:

   Part	Animal	Plant	Bacterial	Where it goes
   Cytoplasm	✓	✓	✓	Common to all three
   Cell membrane	✓	✓	✓	Common to all three
   Nucleus	✓	✓	✗	Animal ∩ Plant overlap
   Cell wall	✗	✓	✓	Plant ∩ Bacterial overlap
   Chloroplast	✗	✓	✗	Only in Plant Cell
   Nucleoid	✗	✗	✓	Only in Bacterial Cell

   Filling the labelled regions:

   • Common to all three cells: Cytoplasm, Cell membrane — exactly two, matching the two ruled lines printed there
   • Only in Plant Cell: Chloroplast
   • Only in Bacterial Cell: Nucleoid
   • Only in Animal Cell: nothing from this list

   The two traps in this question.

   Cell wall is not plant-only. The chapter's summary says plant, fungal and bacterial cells have a cell wall, so it cannot go in the plant-only region — it belongs where the plant and bacterial circles overlap.

   Nucleus is not animal-only. Plant cells have nuclei too; it is bacteria that lack a well-defined nucleus, having a nucleoid instead. So the nucleus goes in the animal-plant overlap.

   That leaves no listed part unique to the animal cell — the animal cell is the one defined by what it lacks (no cell wall, no chloroplast) rather than by anything it alone has.

   ✦ Answer: Common to all three — Cytoplasm and Cell membrane; Only in Plant Cell — Chloroplast; Only in Bacterial Cell — Nucleoid; Only in Animal Cell — nothing; with Nucleus in the animal-plant overlap and Cell wall in the plant-bacterial overlap.

2. 24 marksCuriosity Grade 8, Chapter 2, Keep the curiosity alive, page 25, Q2

   Aanandi put sugar solution in test tubes A and B, added yeast to B only, attached partly inflated balloons to both, and kept them warm away from sunlight. (i) What do you predict after 3-4 hours? The balloon on B inflated — which explanation is correct: (a) water vapour, (b) warm air expanding, (c) yeast produced a gas, (d) sugar reacting with warm air? (ii) She then attached B's balloon to a test tube of lime water and shook it. What does she want to find out?

   Hint. Test tube A is the control — use it to eliminate the wrong options.

   (i) Prediction. The balloon on B (with yeast) inflates; the balloon on A does not.

   The correct explanation is (c) — yeast produced a gas inside test tube B which inflated the balloon.

   Why the others are ruled out, and the reasoning is the same each time — test tube A had everything B had except yeast, and A's balloon stayed flat:

   Option	Why it fails
   (a) Water evaporated	A also contains water at the same temperature, so its balloon would inflate too
   (b) Warm air expanded	Both tubes are equally warm, so this would affect A identically
   (d) Sugar reacted with warm air	Both tubes contain sugar solution and warm air; only the yeast differs

   The single difference between the two tubes is the yeast, so the yeast must be the cause. This is exactly the fair-test reasoning from Chapter 1.

   What the yeast did. It respired, breaking down the sugar for energy and releasing carbon dioxide, which collected and inflated the balloon.

   (ii) The lime water test. She wants to identify the gas — specifically, to find out whether the gas produced by the yeast is carbon dioxide. Lime water is the standard test: carbon dioxide turns lime water milky. Shaking brings the gas into contact with the lime water so the change can be seen.

   ✦ Answer: (i) the balloon on B inflates, and the explanation is (c) — yeast respired and produced a gas — since A differed only in having no yeast and stayed flat; (ii) she wants to test whether that gas is carbon dioxide, which she can tell because carbon dioxide turns lime water milky.

3. 33 marksCuriosity Grade 8, Chapter 2, Keep the curiosity alive, page 26, Q3

   A farmer growing wheat added nitrogen-rich fertiliser to his field. A neighbouring farmer growing bean crops preferred not to add nitrogen fertiliser and still got healthy crops. Can you think of the reasons?

   Hint. Beans are legumes. What lives on their roots?

   The key difference is the crop. Beans are a legume, like peas and lentils; wheat is not.

   What that means for the soil. The roots of legumes carry swollen structures called root nodules, and inside them live Rhizobium bacteria. These bacteria trap nitrogen from the air and make it useful for the plant.

   So the bean farmer already has a nitrogen supply. Her crop obtains nitrogen through the bacteria in its own root nodules, which is why it grows healthily without chemical fertiliser. The wheat farmer has no such partnership, so he must supply nitrogen from outside.

   Why nitrogen has to be 'trapped' at all. The air is full of nitrogen, but plants cannot use it directly in that form. What the bacteria contribute is converting it into a form the plant can take up.

   The practical consequence. Since the legume also leaves the soil richer in nitrogen, farmers grow legumes in rotation with other crops, keeping the soil healthy for the next planting and reducing fertiliser costs.

   ✦ Answer: because beans are legumes, whose root nodules contain Rhizobium bacteria that trap nitrogen from the air and supply it to the plant — so no nitrogen fertiliser is needed, whereas wheat has no such bacterial partner.

4. 43 marksCuriosity Grade 8, Chapter 2, Keep the curiosity alive, page 26, Q4

   Snehal dug two pits. In pit A she put fruit and vegetable peels mixed with dried leaves; in pit B she put the same waste without dried leaves. She covered both with soil and observed after 3 weeks. What is she trying to test?

   Hint. Ask what is the same in both pits and what is different.

   Reading the design. Both pits get the same kind of waste, both are covered with soil, both are left for the same 3 weeks, both are in the same garden. Exactly one thing differs: pit A has dried leaves mixed in, pit B does not.

   So the question she is asking is: does mixing dried leaves with the fruit and vegetable peels affect how they decompose into manure? She is testing whether adding dried leaves speeds up the process, or improves the manure formed.

   Why it is a good experiment. It is a fair test — one variable changed, everything else held the same — so whatever difference she finds after 3 weeks can be attributed to the dried leaves.

   What she would compare after 3 weeks. How much of the waste has turned into dark, crumbly manure in each pit, and how it looks and smells.

   Why the answer is not obvious in advance. Decomposition by fungi and bacteria needs suitable temperature and moisture. Dried leaves change how loosely the material sits and how much moisture it holds, so they might help or hinder — which is precisely why the comparison has to be made rather than guessed.

   ✦ Answer: she is testing whether mixing dried leaves with the peels changes how they decompose into manure — a fair test, since the dried leaves are the only difference between the two pits.

5. 53 marksCuriosity Grade 8, Chapter 2, Keep the curiosity alive, page 26, Q5

   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.

   Hint. Each clue names the organism's habitat or its job.

   (i) Bacteria. The chapter says microorganisms are found in water, soil, air and even inside our body, and that our intestine has many bacteria that help in digestion — a point carried over from the Grade 7 chapter on life processes in animals. Living in every kind of environment, including extremes like hot springs and snow-cold zones, also points to bacteria.

   (ii) Yeast. A fungus. It respires by breaking down sugar and releases carbon dioxide, whose bubbles make dough soft and fluffy, along with a little alcohol that gives the characteristic smell. This property is used in making breads, cakes and pastries.

   (iii) Rhizobium. Pulse crops are legumes. Rhizobium lives in the root nodules of legumes such as beans, peas and lentils, and traps nitrogen from the air, supplying nutrients that help the plant grow.

   A check on (ii). Lactobacillus also ferments food, but it produces lactic acid and sets curd; it does not make baked goods fluffy. The clue 'soft and fluffy' points to a gas, and therefore to yeast.

   ✦ Answer: (i) Bacteria, (ii) Yeast, (iii) Rhizobium.

6. 65 marksCuriosity Grade 8, Chapter 2, Keep the curiosity alive, page 26, Q6

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

   Hint. Three factors means three separate comparisons, each changing one thing.

   The principle first. Three factors are being tested, so you need three comparisons, and in each one you change only that factor while keeping everything else the same. Trying to vary all three at once would make the result uninterpretable.

   A workable design. Use bread slices, each moistened equally and kept in identical containers, and look for mould growth after 3-4 days.

   Test	Set-up A	Set-up B	Everything else
   Temperature	Moist bread in a warm place	Moist bread in a refrigerator	Same bread, same moisture, both covered
   Moisture	Moist bread, warm place	Dry bread, warm place	Same bread, same temperature, both covered
   Air	Moist bread in an open/loosely covered container	Moist bread in an airtight sealed container	Same bread, same moisture, same warm place

   Expected results. Mould grows well on the moist, warm, air-exposed slice, and poorly or not at all on the cold one, the dry one, and the sealed one.

   Conclusion drawn. Each comparison isolates one requirement, so together they show that microorganisms need warmth, moisture and air to grow well.

   Precautions that make it honest. Use several slices per condition rather than one, since bread is not uniform. Record the day mould first appears rather than only whether it appeared. Do not open the sealed container to check, since that defeats the test. And handle mouldy bread carefully, disposing of it without inhaling the spores.

   Why this connects to the chapter. It is the same logic as Activity 2.9 — warm bowl A set into curd, cold bowl B did not — extended to three factors instead of one.

   ✦ Answer: run three separate fair tests, each changing one factor only — moist bread warm versus refrigerated (temperature), moist versus dry bread both kept warm (moisture), and moist warm bread open versus airtight (air) — and compare mould growth after 3-4 days.

7. 73 marksCuriosity Grade 8, Chapter 2, Keep the curiosity alive, page 26, Q7

   Take 2 slices of bread. Place one near the sink and the other in the refrigerator. Compare after three days. Note your observations and give reasons.

   Hint. Two conditions differ between those two places, not one.

   Observation. The slice kept near the sink shows growth after three days — patches of mould, often white or greenish and cottony, and the bread becomes soft and smells stale. The slice in the refrigerator shows little or no growth and stays much as it was, though it may go dry and hard.

   Reasons. Mould is a fungus, and like all microorganisms it needs suitable conditions to grow. Near the sink it gets both of the things it needs:

   • Warmth — room temperature, at which fungi grow well
   • Moisture — the air near a sink is damp

   In the refrigerator the low temperature slows the growth of the microorganisms greatly, so mould does not develop. This is the same effect as Activity 2.9, where warm bowl A set into curd but cold bowl B did not.

   An honest caution about this experiment. The two places differ in more than one way — the refrigerator is both colder and drier than the area near a sink. So strictly this comparison shows that the combination of conditions matters, not that temperature alone is responsible. To separate them you would need the three-way design of question 6.

   Why refrigeration preserves food. Not by killing microbes, but by making conditions unfavourable for their growth — the cold counterpart of the salt and sugar in pickles and murabbas.

   ✦ Answer: the slice near the sink grows mould because it has the warmth and moisture fungi need, while the refrigerated slice stays largely unchanged because the low temperature slows microbial growth — though note that the fridge is also drier, so this test does not isolate temperature by itself.

8. 83 marksCuriosity Grade 8, Chapter 2, Keep the curiosity alive, page 26, Q8

   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?

   Hint. What made it sour in the first place, and what happens if that process keeps going?

   Explanation 1 — the bacteria are still working. Curd contains Lactobacillus, which feeds on the milk sugar lactose and produces lactic acid; that acid is what makes curd sour in the first place. Setting the curd does not stop the process. Left out, the bacteria continue to multiply and ferment the remaining lactose, so more lactic acid accumulates and the curd tastes more sour.

   Explanation 2 — being left out is warm. These bacteria grow well in warm conditions, which is why bowl A set and refrigerated bowl B did not in Activity 2.9. Curd left out sits at room temperature rather than in a refrigerator, so the bacteria multiply faster and acid builds up more quickly than it would in the cold.

   How the two fit together. They are not rivals — the first says what is happening (continued fermentation producing lactic acid), and the second says why it is fast (warmth suits the bacteria). Together they explain both the souring and its speed.

   A testable prediction. Keep one bowl of curd out and an identical bowl in the refrigerator for a day, then taste both. If warmth matters, the one left out should be distinctly more sour — a fair test with one variable changed.

   ✦ Answer: (1) Lactobacillus continues fermenting the remaining lactose, producing more lactic acid, and (2) room temperature is warm enough for those bacteria to multiply rapidly, so the acid accumulates faster than it would in a refrigerator.

9. 94 marksCuriosity Grade 8, Chapter 2, Keep the curiosity alive, page 26, Q9

   In Fig. 2.15, flask A holds warm sugar solution with yeast, connected 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, and why? (iii) What would happen if yeast was not added in flask A?

   Hint. Part (iii) is the control — and it is what makes the whole set-up count as evidence.

   (i) In flask A. The yeast respires, breaking down the sugar to release energy for its growth and life processes. The solution becomes frothy and bubbly and turns cloudy as the yeast multiplies, and it develops a slightly different smell because a small amount of alcohol is also produced. The warmth is deliberate — yeast grows well in warm conditions.

   (ii) In test tube B. The lime water turns milky. This happens because the carbon dioxide released by the respiring yeast in flask A travels along the connecting tube into the lime water, and carbon dioxide turns lime water milky. That colour change is the standard test for the gas, so it identifies the gas rather than merely showing that some gas was made.

   (iii) Without yeast in flask A. Nothing would happen. With no yeast there is no respiration, so no carbon dioxide is produced, no gas passes into test tube B, and the lime water stays clear. The sugar solution would simply sit there.

   Why part (iii) matters most. It is the control. Without it, someone could argue the milkiness came from warm air being pushed over, or from something in the sugar solution. Showing that the effect disappears when only the yeast is removed is what pins the carbon dioxide on the yeast — the same reasoning as test tube A in question 2.

   ✦ Answer: (i) the yeast respires, breaking down the sugar and making the solution frothy, with a faint alcoholic smell; (ii) the lime water turns milky, because the carbon dioxide produced passes into it; (iii) with no yeast there is no respiration, no carbon dioxide, and the lime water stays clear — which is what proves the gas came from the yeast.

Solutions written by the tuition.in editorial team and checked against NCERT Curiosity - Textbook of Science for Grade 8 (hecu102.pdf), Chapter 2 'The Invisible Living World: Beyond Our Naked Eye', pages 8-27, Reprint 2026-27. HAND-WRITTEN throughout. Page count verified against the printed contents page (hecu1ps.pdf p.18): 20 pages, 8-27, matching exactly. Covers the 3 'Probe and ponder' prompts (p.8), all 9 activities (2.1 to 2.9), every in-text section (2.1 to 2.5), the 9 numbered 'Keep the curiosity alive' exercises (pp.25-26) and the 4 'Discover, design, and debate' projects (p.27). EXERCISE Q1 VENN DIAGRAM READ OFF A 500-DPI RENDER of p.25: three circles labelled 'Only in Animal Cell' (top), 'Only in Bacterial Cell' (lower left) and 'Only in Plant Cell' (lower right), with an arrow labelling the central three-way overlap 'Common to all three cells'. Each of those four labelled regions carries two ruled lines; the three PAIRWISE overlap regions carry none. Working from the book's own statements (p.13 'the cell wall in the plant cell provides rigidity'; p.24 'bacteria do not have a well-defined nucleus... instead they have a nucleoid'; and the Snapshots line 'Plant, fungal, and bacterial cells have an extra covering, called a cell wall'), the correct placement is: cytoplasm and cell membrane in the three-way overlap (exactly two items, matching the two ruled lines); chloroplast in plant-only; nucleoid in bacterial-only; NUCLEUS in the animal-plant pairwise overlap (plant cells have nuclei, bacteria do not); CELL WALL in the plant-bacterial pairwise overlap (not plant-only, since the book says bacterial and fungal cells have walls too); and NO listed part is unique to the animal cell. The solution states both traps explicitly. FACTUAL FIGURES TAKEN VERBATIM FROM THE BOOK: Hooke's Micrographia 1665, microscope 200-300x, cork, honeycomb, first use of 'cell'; Leeuwenhoek 1660s, Father of Microbiology; ordinary microscope 100-400x; electron microscope about 10,00,000x; ostrich egg yolk 130-170 mm as the largest known cell; Ananda Mohan Chakrabarty 1938-2020, oil-degrading bacterium 1971, patent 1980; biogas mainly carbon dioxide with a high proportion of methane; one of India's oldest biogas plants late 1850s; microalgae produce more than half of Earth's oxygen; Spirulina over 60 per cent protein by body weight and a source of vitamin B12, harvested after 3-6 weeks. The MNRE biogas-programme project answer deliberately does NOT assert current programme details - it names the programmes to search for and tells the student to verify against the current official MNRE page, since names and subsidy rules change.. Questions are referenced from the NCERT textbook for identification.