The Invisible Living World: Beyond Our Naked Eye

Given: Labels to be placed on a cell diagram.

Concept: A typical plant cell has a cell wall (outermost), cell membrane (just inside the cell wall), cytoplasm (fluid filling the cell), nucleus (control centre, surrounded by nuclear membrane), and chloroplasts (green plastids for photosynthesis). A prokaryotic cell (like a bacterium) has a nucleoid instead of a true nucleus and lacks a membrane-bound nucleus.

Placement of labels:

| Label | Location in the diagram |
|---|---|
| Cell wall | Outermost rigid layer (plant cell) |
| Cell membrane | Layer just inside the cell wall |
| Cytoplasm | Fluid-filled space between cell membrane and nucleus |
| Nucleus | Round/oval structure in the centre of the cell (eukaryotic cell) |
| Chloroplast | Green oval structures in the cytoplasm (plant cell only) |
| Nucleoid | Region in the cytoplasm of a prokaryotic cell (e.g., bacterium) where DNA is located, without a membrane |

Key distinction: The nucleus (with a membrane) is found in plant and animal cells (eukaryotes), while the nucleoid (without a membrane) is found in bacteria (prokaryotes). Chloroplasts and cell wall are features of plant cells only.

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

Justification: Yeast is a microorganism (fungus) that breaks down sugar in the presence of warmth through a process called fermentation. During fermentation, yeast converts sugar into alcohol and carbon dioxide (CO₂) gas. This CO₂ gas accumulates inside the test tube and inflates the balloon attached to test tube B. Test tube A has no yeast, so no gas is produced and its balloon remains uninflated. Options (a), (b), and (d) are incorrect because water evaporation, atmospheric warmth, and sugar alone cannot produce enough gas to inflate the balloon under these conditions.

Given: The balloon from test tube B (containing gas produced by yeast) is attached to a test tube of lime water and shaken.

Concept: Lime water (calcium hydroxide solution) turns milky/white when carbon dioxide (CO₂) is passed through it. This is a standard test for CO₂:
$$\text{Ca(OH)}_2 + \text{CO}_2 \rightarrow \text{CaCO}_3 \downarrow + \text{H}_2\text{O}$$
The white precipitate of calcium carbonate (CaCO₃) makes the lime water appear milky.

What Aanandi wants to find out: She wants to identify the gas produced by yeast during fermentation. Specifically, she wants to confirm whether the gas produced is carbon dioxide (CO₂). If the lime water turns milky after shaking, it confirms that the gas collected in the balloon is CO₂, which was released by yeast during the fermentation of sugar.

Expected observation: The lime water will turn milky, confirming that yeast produces carbon dioxide gas during fermentation.

Given: Wheat farmer adds nitrogen fertiliser; bean (legume) farmer does not.

Concept: Leguminous plants (beans, peas, lentils, etc.) have a special relationship with nitrogen-fixing bacteria called *Rhizobium*. These bacteria live in root nodules of leguminous plants.

Explanation:

- Wheat is a non-leguminous crop. Its roots do not have nitrogen-fixing bacteria. Therefore, the soil around wheat crops does not get naturally replenished with nitrogen. The farmer must add nitrogen-rich fertilisers externally to provide the nitrogen needed for healthy crop growth.

- Bean (legume) plants have *Rhizobium* bacteria living in nodules on their roots. These bacteria can trap (fix) atmospheric nitrogen (N₂) from the air and convert it into nitrogen compounds that the plant can use. This process is called biological nitrogen fixation. As a result, the soil around legume crops actually becomes richer in nitrogen over time.

Conclusion: The bean farmer does not need to add nitrogen fertiliser because the *Rhizobium* bacteria in the root nodules of her bean plants naturally fix atmospheric nitrogen and supply it to the plant, making the soil fertile. This is why leguminous crops are also grown in rotation with other crops to improve soil fertility.

Given: Pit A — fruit/vegetable peels mixed with dried leaves; Pit B — fruit/vegetable peels only (no dried leaves). Both covered with soil and observed after 3 weeks.

What Snehal is trying to test: Snehal is trying to test whether the presence of dried leaves (which contain microorganisms and provide additional organic matter and aeration) affects the rate of decomposition of fruit and vegetable peels.

Detailed explanation:
- Dried leaves contain microorganisms (bacteria and fungi) that help break down organic waste.
- Mixing dried leaves with the peels may also improve aeration and moisture balance in the pit, creating better conditions for microbial activity.
- By comparing pit A (with dried leaves) and pit B (without dried leaves), she is investigating whether adding dried leaves speeds up the decomposition process and leads to better/faster manure formation.

Expected observation: Pit A is likely to show faster and more complete decomposition (better manure formation) compared to pit B, because the dried leaves provide additional microorganisms and improve the conditions needed for decomposition.

Conclusion: Snehal is testing the role of dried leaves (and the microorganisms they carry) in enhancing the decomposition of organic waste into manure.