Chapter 2: The Invisible Living World: Beyond Our Naked Eye (Biology)
Answers to all "Keep the curiosity alive" questions of Chapter 2, The Invisible Living World: Beyond Our Naked Eye (NCERT Class 8 Science, Curiosity, 2026-27): parts of animal, plant and bacterial cells, yeast and carbon dioxide, nitrogen-fixing bacteria, composting, microorganisms and food spoilage, curd and fermentation. All 9 questions are answered, with the key answer highlighted.
Microorganisms are living things too small to see without a microscope: bacteria, fungi (such as yeast), protozoa, algae and viruses. They grow fastest when they have food, moisture, air (for most of them) and a suitable warm temperature.
Write these cell parts in the right places in the diagram (only animal cell, only plant cell, only bacterial cell, common to all three): nucleus, cytoplasm, chloroplast, cell wall, cell membrane, nucleoid.
Solution
Common to all three: cytoplasm and cell membrane.
Animal and plant cells only: nucleus. A bacterial cell has no proper nucleus; its genetic material lies in a region called the nucleoid.
Plant and bacterial cells only: cell wall.
Only in plant cell: chloroplast.
Only in bacterial cell: nucleoid.
Only in animal cell: none of the listed parts. Every part given is found in a plant cell or a bacterial cell as well.
Parts of animal, plant and bacterial cells
Common: cytoplasm and cell membrane. Only plant: chloroplast. Only bacterial: nucleoid. Nucleus: animal and plant. Cell wall: plant and bacterial. Nothing in the list is only in the animal cell.
Aanandi put sugar solution in test tubes A and B, added yeast to B, fixed a balloon on each and kept them warm. (i) What happens after 3–4 hours, and why did balloon B inflate? (ii) Why did she shake the gas from balloon B with lime water?
Solution
(i) The balloon on test tube B swells up, while balloon A stays the same. The correct explanation is (c): the yeast fed on the sugar and gave out a gas (carbon dioxide). This is fermentation.
The other options are wrong. Both tubes were equally warm, so warmth or evaporation would have affected A as well. Sugar does not react with warm air.
(ii) She wants to find out which gas the yeast produced. If the lime water turns milky, the gas is carbon dioxide.
(i) (c) Yeast produced a gas (carbon dioxide) that inflated balloon B. (ii) She is testing whether the gas is carbon dioxide: it turns lime water milky.
A wheat farmer added nitrogen fertiliser to his field, but the bean farmer next to him did not. Why?
Solution
Beans are legumes. Their roots have small swellings called root nodules, which contain Rhizobium bacteria. These bacteria take nitrogen from the air and change it into compounds the plant can use. So bean plants get their own nitrogen and also make the soil more fertile.
Wheat has no such bacteria, so the farmer must add nitrogen to the soil as fertiliser.
Bean plants have nitrogen-fixing bacteria (Rhizobium) in their root nodules, so they need no nitrogen fertiliser; wheat does not.
Snehal put fruit and vegetable peels in pit A mixed with dried leaves, and the same waste without dried leaves in pit B. She covered both with soil. What is she testing?
Solution
She is testing whether mixing in dried leaves helps the waste decompose (turn into compost) faster or better.
Dried leaves add carbon-rich material and leave air spaces in the heap. Both help the decomposers (bacteria and fungi) break down the waste. Pit B is the control for comparison.
Whether dried leaves help microorganisms decompose kitchen waste into compost faster (pit B is the control).
Identify: (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.
Solution
Bacteria, for example Lactobacillus in the gut.
Yeast, a fungus. Its carbon dioxide makes the dough rise.
Rhizobium, the nitrogen-fixing bacteria in root nodules.
Design an experiment to test that microorganisms need a suitable temperature, air and moisture to grow.
Solution
Use slices of the same bread (or boiled potato), all from the same packet, and change one condition at a time:
Set-up
Temperature
Air
Moisture
A (control)
room, warm
open dish
sprinkle a little water
B
refrigerator
open dish
a little water
C
room, warm
sealed airtight jar (air removed as far as possible)
a little water
D
room, warm
open dish
dry (toasted until crisp)
Observe every day for a week and note when mould (fuzzy patches) first appears and how much grows.
Expected result: A grows mould first and most. B, C and D show little or no growth. This shows that warmth, air and moisture are all needed.
Do not open or smell the mouldy samples. Throw them away safely at the end.
Compare identical bread pieces kept warm, moist and open (control) with pieces kept cold, airtight or dry; mould grows well only when all three conditions are present.
Put one bread slice on a plate near the sink and another in the refrigerator. Compare them after three days.
Solution
Near the sink: the slice develops mould, seen as green, black or white fuzzy patches. The place is warm and moist, so the fungal spores in the air land on it and grow quickly.
In the refrigerator: the slice shows little or no mould. The low temperature slows the growth of microorganisms almost to a stop.
This is why we keep food in a refrigerator.
The slice near the sink grows mould (warm and moist); the refrigerated slice hardly changes (cold slows microbial growth).
Flask A has warm sugar solution with yeast, and its gas bubbles into lime water in test tube B. (i) What happens to the sugar solution? (ii) What do you see in B after four hours, and why? (iii) What if no yeast were added?
Solution
(i) The yeast ferments the sugar into alcohol and carbon dioxide. The solution froths, gets warm, and smells of alcohol.
(ii) The lime water turns milky. The carbon dioxide from flask A passes into it and reacts with the lime water.
(iii) There would be no fermentation: no gas would be produced and the lime water would stay clear. This set-up works as the control.
(i) Yeast ferments the sugar into alcohol and carbon dioxide. (ii) Lime water turns milky because of the carbon dioxide. (iii) No gas, and the lime water stays clear.
