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SCIENCE CLASS- 8

CHAPTER-2 (The Invisible Living World: Beyond Our Naked Eye)

CBSEChapter 2 Keep the curiosity alive

Question 1. Various parts of a cell are given below. Write them in the appropriate places in the following diagram.

Given Parts: Nucleus, Cytoplasm, Chloroplast, Cell membrane, Cell wall, Nucleoid

Answer:

Category Cell Parts
Common to all three cells (Plant Cell, Animal Cell and Bacterial Cell) Cell membrane, Cytoplasm
Only in Animal Cell Nucleus
Only in Plant Cell Cell wall, Chloroplast
Only in Bacterial Cell Nucleoid

Question 2(i). What do you predict will happen after 3–4 hours? The balloon attached to test tube B was inflated. What can be a possible explanation for this?

Answer:

The correct answer is:

(c) Yeast produced a gas inside the test tube B which inflated the balloon.

Explanation:

Test tube A contained only sugar solution, whereas test tube B contained sugar solution and yeast. Yeast is a microorganism that feeds on sugar and carries out the process of fermentation. During fermentation, yeast converts sugar into alcohol and carbon dioxide gas.

The carbon dioxide gas released by yeast accumulated inside the balloon, causing it to inflate.

Reaction:

Sugar + Yeast → Alcohol + Carbon dioxide + Energy

Question 2(ii). She attached the balloon to a test tube containing lime water. What does she want to find out?

Answer:

Anandi wants to find out whether the gas produced by yeast is carbon dioxide.

When carbon dioxide is passed through lime water, the lime water turns milky due to the formation of calcium carbonate.

Reaction:

Calcium hydroxide + Carbon dioxide → Calcium carbonate + Water

Therefore, if the lime water turns milky, it confirms that the gas produced by yeast is carbon dioxide.

Question 3. 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 he preferred not to add nitrogen fertiliser. Can you think of the reasons?

Answer:

Bean plants belong to the legume family. Their roots contain special structures called root nodules which house Rhizobium bacteria.

These bacteria fix atmospheric nitrogen into nitrogen compounds that can be used by plants.

Therefore, bean plants can obtain nitrogen naturally and usually do not require additional nitrogen-rich fertilisers.

Wheat plants do not possess such nitrogen-fixing bacteria and therefore require nitrogen fertilisers for healthy growth.

Question 4. Snehal dug two pits, A and B, in her garden. In pit A, she put fruit and vegetable peels and mixed them with dried leaves. In pit B, she dumped the same kind of waste without mixing it with dried leaves. She covered both pits with soil and observed after 3 weeks. What is she trying to test?

Answer:

Snehal is trying to test whether adding dried leaves helps in faster and better compost formation.

Dried leaves provide air spaces and maintain a proper balance between green waste and dry waste.

This creates favourable conditions for decomposer microorganisms, resulting in faster decomposition and better compost production.

She is comparing the rate of decomposition in the two pits.

Question 5(i). Identify the microorganism: "I live in every kind of environment, and inside your gut."

Answer:

Bacteria

Bacteria are found in soil, water, air and inside the digestive system of humans and animals.

Question 5(ii). Identify the microorganism: "I make bread and cakes soft and fluffy."

Answer:

Yeast

Yeast produces carbon dioxide during fermentation, causing dough to rise and making bread and cakes soft and fluffy.

Question 5(iii). Identify the microorganism: "I live in the roots of pulse crops and provide nutrients for their growth."

Answer:

Rhizobium

Rhizobium bacteria live in root nodules of leguminous plants and fix atmospheric nitrogen.

Question 6. Design an experiment to test that microorganisms need optimal temperature, air and moisture for their growth.

Answer:

Aim: To show that microorganisms require suitable temperature, air and moisture for growth.

Materials Required:

  • Three slices of bread
  • Water
  • Plastic bags
  • Refrigerator

Procedure:

  1. Sprinkle water on all three bread slices.
  2. Keep the first slice in a warm place.
  3. Keep the second slice in a refrigerator.
  4. Keep the third slice in a dry place without moisture.
  5. Observe all slices after 3–4 days.

Observation:

  • The warm and moist bread develops mould rapidly.
  • The refrigerated bread develops little or no mould.
  • The dry bread shows very little mould growth.

Hence

Microorganisms require suitable temperature, air and moisture for growth.

Question 7. Take two slices of bread. Place one slice on 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:

  • The bread kept near the sink develops mould.
  • The bread kept in the refrigerator develops little or no mould.

Reason:

The area near the sink is warm and moist, providing ideal conditions for microbial growth. The refrigerator has a low temperature which slows down the growth and reproduction of microorganisms.

Question 8. 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:

  1. Lactic acid bacteria continue to multiply in the curd.
  2. More lactic acid is produced, increasing the sour taste of the curd.

Warm conditions further enhance bacterial activity and acid production.

Question 9(i). What happens to the sugar solution in flask A?

Answer:

The yeast present in flask A ferments the sugar solution and converts sugar into alcohol and carbon dioxide gas.

Question 9(ii). What do you observe in test tube B after four hours? Why do you think this happened?

Answer:

After four hours, the lime water in test tube B turns milky.

This happens because carbon dioxide gas produced during fermentation passes into the lime water and reacts with it to form calcium carbonate.

Formation of calcium carbonate makes the lime water appear milky.

Question 9(iii). What would happen if yeast was not added in flask A?

Answer:

If yeast was not added:

  • Fermentation would not occur.
  • Carbon dioxide gas would not be produced.
  • The lime water would not turn milky.
  • No noticeable change would occur in the set-up.

Hence Yeast is essential for fermentation and the production of carbon dioxide gas.