The Invisible Living World: Beyond Our Naked Eye Class 8 QUESTION-ANSWERS Science Chapter 2 II CBSE Notes II NCERT Notes II Curiosity Class 8 II Complete notes

Back Exercise Question Answers

1.Various parts of a cell are given below. Write them in the appropriate places in the following diagram. Nucleus Chloroplast Cell membrane Cytoplasm Cell wall Nucleoid

Ans: Nucleus: Present in plant and animal cells (not in bacterial cells), controls the cell’s activities.

Cytoplasm: Jelly-like substance that fills the cell and contains all organelles.

Chloroplast: Found only in plant cells, responsible for photosynthesis.

Cell wall: Found in plant and bacterial cells, forms the outermost layer.

Cell membrane: Found in all cells, located inside the cell wall in plant/bacterial cells or as the outer layer in animal cells.

Nucleoid: Found only in bacterial cells; the region where DNA is located since bacteria don’t have a true nucleus.

2. 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?

Ans: Option (c) Yeast produced a gas inside the test tube B which inflated 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?

Ans. She wants to find out whether the gas collected in the balloon is carbon dioxide (CO₂).

When the gas is passed into lime water and shaken, if the lime water turns milky, it confirms the presence of carbon dioxide. This shows that the process in test tube B produced CO₂.

3. A farmer was planting wheat crops in his field. He added nitrogen-rich fertilizer to the soil to get a good yield of crops. In the neighboring field, another farmer was growing bean crops, but she preferred not to add nitrogen fertilizer to get healthy crops. Can you think of the reasons?

Ans. The farmer in the neighbouring field did not add nitrogen fertiliser because bean is a leguminous plant which can make use of nitrogen on their own. The bean plants have Rhizobium bacteria in their roots that fix nitrogen from the air and provide it to the plant, so extra fertiliser is not needed. So, Wheat crops need nitrogen fertiliser from outside. But bean plants get nitrogen naturally with the help of Rhizobium.

4. 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?

Ans: Snehal is trying to test the effect of adding dried leaves on the process of composting by Decomposition.

By mixing fruit and vegetable peels with dried leaves in pit A, she is creating better conditions for decomposition. In pit B, without dried leaves, decomposition may be slower or less effective.

Microorganisms help in breaking down the waste into compost. Their activity is faster in pit A due to better conditions, leading to quicker and better compost formation.

5. 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.

Answers:

i.         Bacteria

ii.         Yeast

iii.         Rhizobium

 

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

Ans:

Materials:
Bread slices, water, 3–4 airtight containers or zip-lock bags

Procedure:

1. Take 4 bread slices and place them in separate containers (A, B, C, D).
2. Keep conditions different for each:

• Slice A : Slightly moist bread, kept at room temperature (normal air)
• Slice B (No moisture): Dry bread, kept at room temperature
• Slice C (Low temperature): Moist bread, kept in refrigerator
• Slice D (No air): Moist bread, kept in an airtight container

3. Leave them undisturbed for 3–5 days and observe daily.

Observation: Slice A will have maximum microbial (fungal) growth.
Conclusion: Microorganisms grow best when temperature, moisture, and air are optimal.

7. 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.

Ans: Bread near the sink develops green/black patches of fungus and gives a bad smell while bread in refrigerator shows little or no fungal growth and remains almost unchanged. This is because microorganisms grow faster in warm and moist conditions, while low temperature slows their growth.

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?

Ans: Curd becomes more sour because microorganisms continue to grow and produce more lactic acid over time. So, the two possible explanations are:

Bacteria like Lactobacillus keep multiplying in the curd and convert lactose (milk sugar) into lactic acid, making it more sour.

When curd is left out for a long time at room temperature, the warm conditions allow microorganisms to grow faster and produce more acid.

9. 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?

Ans:

(i) The yeast ferments the sugar in the warm solution, producing carbon dioxide gas and a small amount of alcohol.
(ii) Lime water in test tube B turns milky. This happens because CO₂ produced in flask A travels through the delivery tube into flask B and reacts with lime water, confirming the presence of carbon dioxide.
(iii) Fermentation would not occur, so no carbon dioxide would be produced. As a result, lime water would remain clear in test tube B.