NCERT Solutions Curiosity Chapter 2 End-of-chapter question set — Keep the curiosity alive
Book page 25 Updated on2026-09-05
Q1.
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
Fill the diagram like this:
Region of the diagram
What to write there
Common to all three cells
Cell membrane, Cytoplasm
Only in Plant Cell
Chloroplast, Cell wall
Only in Bacterial Cell
Nucleoid
Only in Animal Cell
Nucleus
Where each part belongs. The overlap in the middle is what all three cells share.
Why each one goes where it does:
Cell membrane and cytoplasm — every cell has them. Like animal and plant cells, the cells of microorganisms are also surrounded by a cell membrane.
Chloroplast — only plant cells have chlorophyll-containing plastids, so only they can make food by photosynthesis. Fungal and bacterial cells have no chloroplasts.
Nucleoid — bacteria do not have a well-defined nucleus or a nuclear membrane; instead they have a nucleoid. This is the feature that distinguishes bacteria from the cells of yeast, protozoa, algae, fungi, plants and animals.
Nucleus — a well-defined nucleus is exactly what the bacterial cell lacks, so it goes on the animal side of the diagram.
Cell wall — the animal cell has none, so it goes on the plant side.
Be accurate about two of them. Strictly, the plant cell also has a nucleus, and the book itself says that plant, fungal and bacterial cells all have a cell wall. So of the six, only the chloroplast is found in the plant cell alone and only the nucleoid in the bacterial cell alone. Nucleus and cell wall are each shared by two of the three, and the diagram has no space marked for a shared pair — so they are written in the region that shows which cell does not have them.
Q2.
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: after 3–4 hours the balloon on test tube B (the one with yeast) will swell up, while the balloon on test tube A will stay as it was.
The correct explanation is (c) — Yeast produced a gas inside the test tube B which inflated the balloon.
Why the others are wrong:
(a) is wrong. Test tube A holds the same sugar solution at the same warmth. If evaporation were the cause, balloon A would have inflated too.
(b) is wrong for the same reason — both test tubes are in the same warm place, so warm air would expand equally in both.
(d) is wrong. Sugar does not react with warm air to give a gas; and again, tube A had sugar and warm air and nothing happened.
(c) is right because the only difference between A and B is the yeast. Yeast respires, breaking down the sugar to release energy, and releases carbon dioxide — the same gas that makes dough rise in Activity 2.8.
The reasoning behind it: A and B are identical in every way except one — the yeast. When two set-ups differ in only one thing and only one of them shows an effect, that one difference must be the cause. Test tube A is the comparison, and it is what rules out (a), (b) and (d) in a single stroke.
(ii) She wants to find out which gas the yeast produced — specifically, whether it is carbon dioxide.
Lime water is the test for carbon dioxide: when carbon dioxide is shaken with clear lime water, the lime water turns milky. So if the gas collected in the balloon turns the lime water milky, the gas released by the yeast is carbon dioxide. She takes care not to let the gas escape while transferring the balloon, because a test on the wrong gas — ordinary air — would prove nothing.
Q3.
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
Because the bean plant carries its own supply of nitrogen in its roots, and the wheat plant does not.
Beans are a legume. The roots of certain legumes — beans, peas and lentils — have swollen regions called root nodules.
These nodules contain Rhizobium bacteria, which live inside them.
These bacteria trap nitrogen from the air and make it useful for the plant.
So the bean crop gets its nitrogen naturally, and grows well without chemical fertilisers. Adding nitrogen fertiliser would be an unnecessary expense.
Wheat has no root nodules and no Rhizobium, so it must take its nitrogen from the soil. The farmer therefore adds nitrogen-rich fertiliser to get a good yield.
Why farmers rotate crops: the Rhizobium in the bean field goes on adding nitrogen to that soil. So farmers grow legumes in rotation with other crops — this naturally increases the nitrogen in the soil and keeps it healthy for the next crop, such as the wheat. It is a way of letting a microorganism do the fertiliser's job.
Q4.
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 whether mixing dried leaves with the wet peels makes decomposition work better — that is, whether the dried leaves change how quickly and how well the waste turns into manure.
Notice how carefully the experiment is built: the two pits have the same kind of waste, the same covering of soil, the same garden and the same three weeks. Only one thing is different — the dried leaves. So whatever difference she finds after 3 weeks must be due to the dried leaves alone.
What she is likely to find:
Pit A — peels + dried leaves
Pit B — peels only
After 3 weeks
Dark, crumbly, earthy-smelling manure; the peels are no longer recognisable
A wet, packed, slimy mass; decomposition slower and less complete, with a bad smell
Why the dried leaves help: microorganisms need air and an appropriate moisture level to do their work, and manure formation occurs at optimal temperature and moisture. Wet peels alone settle into a solid, soggy lump — water fills every gap and air cannot reach the microbes inside. Dry leaves mixed in keep the heap loose, so air spaces remain, and they soak up the extra moisture. The fungi and bacteria then get both the air and the moisture they need, and the waste is broken down faster and more completely.
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.
Answer
Clue
Microorganism
How the clue gives it away
(i) I live in every kind of environment, and inside your gut
Bacteria
Bacteria are found everywhere — in water, soil and air, in hot water springs and snow cold zones — and our intestine has many bacteria that help in digestion
(ii) I make bread and cakes soft and fluffy
Yeast (a fungus)
Yeast respires in the dough and releases carbon dioxide, whose bubbles make the dough rise; this property is used in making breads and cakes
(iii) I live in the roots of pulse crops and provide nutrients for their growth
Rhizobium (a bacterium)
It lives in the root nodules of legumes such as beans, peas and lentils, and traps nitrogen from the air for the plant
Tip: answer (ii) as yeast, not just ‘fungus’ — yeast is the particular unicellular fungus used for this, while mould is a multicellular fungus that spoils bread rather than raising it.
Q6.
Design an experiment to test that microorganisms need optimal temperature, air, and moisture for their growth.
Answer
Use bread as the food for the microbes, and set up four pieces that differ from one another in only one condition at a time.
What you need: four slices of the same bread, four clean transparent containers or polythene bags, a little water, a spoon or gloves.
Set-up
How it is prepared
What is being tested
Expected result after 4–6 days
A (comparison)
Sprinkle a few drops of water on the slice, leave it loosely covered in a warm, dark corner of the room
All three conditions present
Cottony or powdery mould grows well
B (temperature)
Same moist slice, loosely covered, but kept in the refrigerator
Only the temperature is changed
Very little or no growth
C (moisture)
Dry the slice thoroughly in the sun first, then keep it loosely covered in the same warm corner
Only the moisture is changed
Very little or no growth
D (air)
Moist slice sealed in a bag with the air pressed out, kept in the same warm corner
Only the air is changed
Much less growth than A
How to run it: keep all four at the same time; look at them once a day at the same hour; do not open the containers; record what you see by sketching the patch of growth or by tracing its outline on the container.
How to read the result: A grows well. If B, C and D each grow much less than A, then each of the three conditions you removed — warmth, moisture, air — is needed. That is the conclusion the experiment is designed to reach.
Why only one thing is changed at a time: if you had put a dry slice in the refrigerator, and it did not grow mould, you could not say whether the cold or the dryness stopped it. Changing exactly one condition and keeping everything else the same is what lets the result point to a single cause.
Handling: follow the book's own instruction from Activity 2.5 — do not touch the mouldy material with bare hands; use a spoon or gloves. Seal the containers before throwing them away, and wash your hands afterwards.
Q7.
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: after three days the slice near the sink carries patches of growth — first white and cottony, later turning greenish-black or grey — and the bread under the patch has become soft and discoloured. The slice in the refrigerator shows little or no such growth; it has mainly become dry and hard.
Slice near the sink
Slice in the refrigerator
Appearance after 3 days
Cottony or powdery patches, greenish-black or white
Almost unchanged, dry and hard
Smell
Musty, unpleasant
Normal
Reasons:
Microorganisms are present everywhere in the air, so tiny fungal forms settle on both slices. The two slices do not differ in what lands on them.
The area near the sink is warm and moist. These are exactly the conditions in which microorganisms grow, so the fungus multiplies and forms branched filaments — the same structure recorded for bread mould in Table 2.2, ‘branched filament without chlorophyll having sac-like structure’. Enough of them together make a patch big enough for the naked eye.
Fungi have no chlorophyll and cannot make their own food by photosynthesis, so they feed on the bread itself. That is why the bread beneath the patch is spoilt.
In the refrigerator the temperature is low, so the growth of these microorganisms is slowed down almost to a stop. They are still there — the cold has not killed them — which is why the bread will go mouldy quickly once it is brought out and left in the warm.
Check it yourself: this single comparison explains almost everything we do to keep food — the refrigerator slows microbes down with cold, drying removes their moisture, and pickling removes their free water with salt or sugar.
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?
Answer
Explanation 1 — the Lactobacillus already in the curd goes on working. Curd contains several types of bacteria, one of them Lactobacillus. It feeds on the sugar in the milk (lactose), multiplies, and produces lactic acid as it ferments. Left out at room temperature there is still some lactose remaining and the bacteria keep converting it, so more lactic acid collects in the curd. More acid means more sourness.
Explanation 2 — room temperature is exactly what these bacteria like. These bacteria grow well in warm conditions. Out of the refrigerator they multiply much faster than they did inside it, so the number of bacteria — and therefore the rate at which acid is produced — rises sharply over a day. Other acid-producing microorganisms from the air, the spoon or the vessel can also settle in the open curd and add to the souring.
The common idea: in both explanations the sourness comes from lactic acid. The first says the same bacteria produce more of it; the second says warmth lets more bacteria produce it faster. This is also why curd is kept in the refrigerator once it has set — the cold does not stop the bacteria, it only slows them enough that the curd stays pleasant for a day or two.
Q9.
Observe the set-up given in Fig. 2.15 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
The set-up: flask A holds a warm sugar solution with yeast; a delivery tube carries whatever gas is formed from A into test tube B, which holds lime water.
Fig. 2.15 set-up: the gas made by the yeast in flask A is bubbled through the lime water in test tube B.
(i) In flask A the yeast begins to respire. It breaks down the sugar of the solution to release the energy it needs to grow, so the sugar is gradually used up. Carbon dioxide is released and a small amount of alcohol is formed. You see bubbles rising through the liquid and a froth collecting on top, the solution turns cloudy as the yeast multiplies, and it begins to smell slightly of alcohol.
(ii) In test tube B the clear lime water turns milky after four hours.
This happened because the carbon dioxide released in flask A cannot escape anywhere else — it travels along the delivery tube and bubbles through the lime water. Lime water turns milky when carbon dioxide is passed through it, and that is the test that identifies the gas. So test tube B proves that the gas coming out of A is carbon dioxide.
(iii) If yeast was not added to flask A, nothing would happen. There would be no organism to break down the sugar, so no carbon dioxide would be produced, no bubbles would appear in the sugar solution, no gas would pass down the tube, and the lime water in B would stay clear. The sugar solution itself would be unchanged.
Why this set-up is convincing: it links a cause to an effect at two removes. The yeast is in A, the milkiness is in B, and the only path between them is the tube. Part (iii) is the control that closes the argument — remove the yeast and the whole chain stops.