🌾 The Invisible Living World: Beyond Our Naked Eye

📘 Class 8 Science – Chapter 2 | Complete Questions & Answers

Q1. Various parts of a cell are given below. Write them in the appropriate places in the following diagram.

(Diagram reference: animal cell, plant cell, bacterial cell)

✅ Answer

Only in Animal Cell: Nucleus
Common to all three cells (Animal, Plant, Bacterial): Cytoplasm, Cell membrane
Only in Bacterial Cell: Nucleoid
Only in Plant Cell: Chloroplast

💡 Note: Bacterial cells lack a true nucleus (nucleoid region), while plant cells have chloroplasts for photosynthesis.

Q2. Aanandi's yeast & balloon experiment

(i) What do you predict will happen after 3–4 days? She observed that the balloon attached to test tube B was inflated. What can be a possible explanation?

• (a) Water evaporated in test tube B and filled the balloon with water vapour.
• (b) The warm atmosphere expanded the air inside test tube B, which inflated the balloon.
• (c) Yeast produced a gas inside test tube B which inflated the balloon.
• (d) Sugar reacted with warm air, which produced gas, eventually inflating the balloon.

📌 Answer (i)

✔ Correct option: (c) — Yeast ferments sugar, producing carbon dioxide gas which inflates the balloon.

(ii) She took another test tube with lime water, transferred the gas from balloon into lime water and shook it. What did she want to find out?

🧪 Answer (ii)

She wants to test whether the gas produced by yeast is carbon dioxide. Lime water turns milky in the presence of CO₂, confirming that the gas is CO₂.

Q3. Wheat farmer vs bean farmer – why no nitrogen fertiliser for beans?

🌱 Reason

Bean crops (legumes) have a symbiotic relationship with Rhizobium bacteria that live in their root nodules. These bacteria fix atmospheric nitrogen into a form plants can use. Therefore, bean plants enrich the soil naturally and don't require additional nitrogen fertilisers. Wheat, being a non-legume, cannot fix nitrogen and needs external fertilisers for healthy growth.

Q4. Snehal's composting pits – what is she testing?

🧑‍🌾 Inference

Snehal is testing the role of carbon-rich material (dried leaves) in composting. In pit A, dried leaves + vegetable peels provide a balanced carbon-nitrogen ratio for effective microbial decomposition. Pit B (only peels) lacks carbon source, which slows down composting. After 3 weeks, she compares how dried leaves improve compost quality.

Q5. Identify the following microorganisms

🔍 "I live in every kind of environment, and inside your gut." → Bacteria

🍞 "I make bread and cakes soft and fluffy." → Yeast

🌿 "I live in the roots of pulse crops and provide nutrients for their growth." → Rhizobium

Answer

Bacteria, Yeast, Rhizobium (in order)

Q6. Devise an experiment to test that microorganisms need optimal temperature, air, and moisture for growth.

🔬 Experiment Design

Materials: 4 slices of bread, 4 zip-lock/plastic bags, water, labels A–D.

• Slice A (dry + warm): Dry bread, kept at warm place (room temperature).
• Slice B (moist + warm): Sprinkle water, kept in warm place.
• Slice C (moist + cold): Sprinkle water, kept in refrigerator.
• Slice D (moist + warm + no air): Spray few drops water, seal in air-tight bag, keep warm.

Observation after 3–5 days: Slice B shows maximum mould growth (warmth + moisture + air). Slice A shows little growth (lack of moisture). Slice C shows very slow/no growth (cold inhibits microbes). Slice D shows reduced growth (lack of oxygen/air).

Conclusion: Microorganisms require optimal warmth, moisture, and air for active growth. Absence of any factor slows down or prevents growth.

Q7. Bread slice near sink vs bread slice in refrigerator – compare after 3 days.

🍞 Observations & Reasons

Observation:
- Slice near sink → Green/black mould patches, bad smell (active microbial growth).
- Slice in refrigerator → Almost no mould, looks fresh.

Reason: The area near sink is warm and humid — ideal conditions for mould spores to germinate. The refrigerator’s cold temperature slows down microbial metabolism, preventing spoilage. Hence, microbes thrive in warmth + moisture.

Q8. Why does curd become more sour when left out for a day?

🥛 Two possible explanations

1. Increased growth of Lactobacillus bacteria: At room temperature, bacteria multiply rapidly and convert more lactose into lactic acid, increasing sourness.
2. Longer fermentation time: Leaving curd outside extends fermentation, leading to higher lactic acid concentration and a sharper sour taste.

Q9. Observe the set-up (Flask A with sugar + yeast, test tube B with lime water).

(Setup description: Flask A contains sugar solution with yeast, a tube delivers gas into test tube B containing lime water.)

(i) What happens to the sugar solution in flask A?

🧫 Answer (i)

Yeast ferments the sugar, breaking it down to produce carbon dioxide gas and alcohol. The sugar solution develops bubbles and froth due to CO₂ release.

(ii) What do you observe in test tube B after four hours? Why?

🥛 Answer (ii)

The lime water turns milky/cloudy. The carbon dioxide from flask A reacts with lime water (calcium hydroxide) to form insoluble calcium carbonate:
CO₂ + Ca(OH)₂ → CaCO₃ (milky) + H₂O. This confirms CO₂ production.

(iii) What would happen if yeast was not added in flask A?

❓ Answer (iii)

No fermentation would occur → no carbon dioxide gas produced. The lime water in test tube B would remain clear, and no change would be observed. The sugar solution would stay unchanged.

🌟 Learning beyond the naked eye — microorganisms play vital roles in our lives, from curd formation to composting, nitrogen fixation, and fermentation.
📚 Class 8 Science | Chapter 2: The Invisible Living World (complete Q&A)