Chapter 2: The Invisible Living World: Beyond Our Naked Eye

Key placements:

• Cell wall → Outermost rigid layer (plant cell only)
• Cell membrane → Just inside the cell wall (both plant and bacterial cells)
• Cytoplasm → Space between cell membrane and nucleus
• Nucleus → Round structure in the centre (plant/animal cell)
• Chloroplast → Small oval structures inside cytoplasm (plant cell only)
• Nucleoid → Central region in bacterial cell where genetic material is found (no proper nucleus)

✔ Correct Answer: (c)

Why (c) is correct: Yeast is a microorganism that breaks down sugar (fermentation) and produces carbon dioxide gas as a by-product. This carbon dioxide gas collected inside the test tube and rose up to inflate the balloon attached to test tube B.

Why other options are wrong:

• (a) Water evaporation cannot inflate a balloon significantly, and both test tubes have the same amount of water, yet only B's balloon inflated.
• (b) Both test tubes were kept in the same warm place, so warm air expansion would affect both equally — but only B's balloon inflated.
• (d) Sugar does not react with warm air to produce gas; this is not a chemical reaction that generates gas.

Aanandi is trying to find out whether the gas produced by yeast is carbon dioxide. When carbon dioxide is passed through lime water, it turns the lime water milky (cloudy). By shaking the balloon filled with the gas over lime water, she can confirm whether CO₂ was the gas produced during yeast fermentation.

Bean plants belong to the legume family (pulse crops). Their roots contain special bacteria called Rhizobium, which live in small swellings called root nodules. These bacteria trap nitrogen gas from the air and convert it into a form that plants can use, so the bean crop does not need extra nitrogen fertiliser. Wheat plants do not have this relationship with Rhizobium, so the wheat farmer must add nitrogen fertiliser from outside to meet the crop's needs.

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?

"Snehal is trying to test whether mixing dried leaves with fruit and vegetable peels speeds up the process of decomposition (composting). Dried leaves add carbon-rich material and improve aeration in pit A. She is comparing the rate and quality of compost formation in the two pits to see which method produces better manure in 3 weeks."

(i) Bacteria — Bacteria are found in almost every environment (soil, water, air, extreme conditions) and also live inside the human gut where they help in digestion.

(ii) Yeast — Yeast is a fungus that ferments sugar in dough and releases carbon dioxide gas, which makes bread and cakes rise and become soft and fluffy.

(iii) Rhizobium — Rhizobium is a bacterium that lives in the root nodules of pulse (legume) crops like beans and peas. It fixes atmospheric nitrogen and makes it available to the plant, acting as a natural fertiliser.

Aim: To test that microorganisms need optimal temperature, air, and moisture for their growth.

Materials needed: 6 slices of bread, 6 zip-lock bags or containers, water spray bottle, labels.

Setup:

Steps:

1. Label all slices/containers according to the condition being tested.
2. Spray water on bread slices meant to be moist.
3. Place them in the described conditions and leave for 3–5 days.
4. Observe daily and note growth of mould or other microorganisms.

Expected Observation: Maximum microbial growth (mould) will be seen in the slice kept moist, open to air, and at room temperature. Little or no growth in dry, sealed, or refrigerated conditions.

Conclusion: Microorganisms grow best when they have optimal moisture, air, and temperature. Absence of any one condition slows or stops their growth.

Observation: The bread slice placed near the sink will show visible mould growth (greenish/blackish fuzzy patches) after three days, while the slice in the refrigerator will remain mostly unchanged with little or no mould.

Reason: Microorganisms (like bread mould fungi) need warmth, moisture, and air to grow. The slice near the sink is exposed to warm temperature, moisture from the surroundings, and open air — all ideal conditions for microbial growth. The refrigerator keeps the bread cold and dry, which slows down or stops the growth of microorganisms.

Explanation 1: Curd contains Lactobacillus bacteria, which continue to ferment the lactose (milk sugar) present in the curd even after it is formed. This fermentation produces more lactic acid, which increases the sourness of the curd over time.

Explanation 2: When curd is left out at room temperature (which is warmer than a refrigerator), the bacteria multiply faster. A higher number of bacteria means more lactic acid is produced at a quicker rate, making the curd increasingly sour within a day.

(i) In flask A, the yeast ferments the sugar present in the warm sugar solution. Yeast breaks down the sugar in the absence of sufficient oxygen, producing carbon dioxide gas and a small amount of alcohol. As a result, the sugar solution in flask A slowly decreases in sugar content, and bubbles of CO₂ gas are released, which travel through the tube into test tube B.

(ii) After four hours, the lime water in test tube B turns milky (cloudy). This happens because the carbon dioxide gas produced by yeast fermentation in flask A travels through the tube and bubbles into the lime water. Carbon dioxide reacts with lime water (calcium hydroxide solution) to form calcium carbonate, which is a white precipitate that makes the solution appear milky.

(iii) If yeast was not added to flask A, no fermentation would take place, and therefore no carbon dioxide gas would be produced. Without CO₂ travelling to test tube B, the lime water would remain clear and unchanged (not turn milky). This shows that yeast is essential for the fermentation process that produces carbon dioxide gas.