Exercises - Chapter 2: The Invisible Living World

Questions and Answers

1. 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: (i) Bacteria (e.g., Lactobacillus in the gut) (ii) Yeast (a type of fungus) (iii) Rhizobium (nitrogen-fixing bacteria)

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2. 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:

• Only in Animal Cell: No specific part from the list is only in animal cells among these, but they lack Cell wall, Chloroplast, and Nucleoid.
• Common to all three (Plant, Animal, Bacterial): Cell membrane, Cytoplasm.
• Common to Plant and Animal: Nucleus.
• Only in Bacterial Cell: Nucleoid.
• Only in Plant Cell: Chloroplast, Cell wall (Cell wall is also in bacterial and fungal cells, but not in animal cells).

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3. Aanandi took two test tubes and marked them A and B. She put two spoonfuls of sugar solution in each. In test tube B, she added yeast. She attached balloons to both.

(i) What do you predict will happen after 3–4 hours? The balloon in B was inflated. Why? (ii) She attached the balloon from B to lime water. What do you think she wants to find out?

Answer: (i) (c) Yeast produced a gas inside the test tube B which inflated the balloon. Yeast breaks down sugar through fermentation, releasing Carbon Dioxide ($CO_2$) gas. (ii) She wants to test for the presence of Carbon Dioxide. When $CO_2$ is passed through lime water, it turns milky. This confirms that the gas produced by yeast is indeed carbon dioxide.

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4. A farmer adds nitrogen-rich fertiliser to wheat soil. In the neighbouring field, another farmer grows bean crops but does not add nitrogen fertiliser. Why?

Answer: Bean crops are legumes. Their roots have root nodules containing Rhizobium bacteria. These bacteria have the ability to trap nitrogen from the air and convert it into a form that plants can use. This naturally increases the nitrogen content of the soil, so nitrogen-rich fertilisers are not needed for bean crops.

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5. Snehal dug two pits, A and B. In pit A, she put fruit and vegetable peels mixed with dried leaves. In pit B, she dumped the same waste without mixing. She covered both with soil. What is she trying to test?

Answer: She is trying to test the process of decomposition and manure formation. Specifically, she may be testing how the addition of dry leaves (carbon source) affects the rate or quality of decomposition of kitchen waste (nitrogen source) by microorganisms in the soil.

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

Answer:

• Temperature: Place one slice of moist bread in a refrigerator and another in a warm cupboard. Observe which one grows mould first.
• Moisture: Place one dry slice of bread and one moist slice of bread in a warm place. Observe the difference in mould growth.
• Air: Place one moist slice of bread in a vacuum-sealed bag and another in an open container. Observe the results.
• Conclusion: Growth will be most rapid on the moist bread kept in a warm, airy place, proving these factors are essential.

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7. Take 2 slices of bread. Place one near the sink and the other in the refrigerator. Compare after three days. Give reasons.

Answer:

• Observation: The slice near the sink will likely have greenish-black fuzzy growth (bread mould), while the one in the refrigerator will remain relatively clean.
• Reason: The area near the sink provides moisture and a warm temperature, which are ideal for the growth of fungi like bread mould. The cold temperature in the refrigerator inhibits the growth of microorganisms.

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8. A student observes that when curd is left out for a day, it becomes more sour. What can be two possible explanations?

Answer:

1. Multiplication of Bacteria: The warm room temperature allows Lactobacillus bacteria to multiply rapidly.
2. Increased Lactic Acid Production: More bacteria ferment more lactose (milk sugar) into lactic acid, which increases the acidity and makes the curd sourer.

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9. Observe the set-up (Flask A: Warm sugar + Yeast → Test tube B: Lime water).

(i) What happens to the sugar solution in flask A? (ii) What do you observe in test tube B after four hours? Why? (iii) What would happen if yeast was not added in flask A?

Answer: (i) The sugar solution undergoes fermentation. It will start frothing due to $CO_2$ release and smell like alcohol. (ii) The lime water in test tube B turns milky. This happens because the $CO_2$ gas produced in Flask A travels to Test tube B and reacts with the lime water. (iii) If yeast was not added, no fermentation would occur, no gas would be produced, and the lime water would remain clear.