2. The Invisible Living World: Beyond Our Naked Eye

• Common to all three cells: Cell membrane and Cytoplasm
• Only in bacterial cell: Nucleoid
• Only in plant cell: Chloroplast
• Only in animal cell: none of the given parts.

Two parts are shared by only two cells:

• Nucleus — in animal and plant cells. Bacteria lack a well-defined nucleus.
• Cell wall — in plant and bacterial cells. Animal cells have no cell wall.

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

Yeast breaks down sugar. In this process, carbon dioxide gas is released. This gas filled the balloon. In test tube A there was no yeast, so no gas was formed.

She wants to find out which gas was produced by the yeast.

If the lime water turns milky, the gas is carbon dioxide.

Beans are leguminous plants. Their roots have swollen root nodules. These nodules contain Rhizobium bacteria.

Rhizobium traps nitrogen from the air and makes it available to the plant. It also increases soil fertility. So bean crops do not need nitrogen fertiliser.

Wheat is not a legume. It has no such bacteria in its roots. So the farmer has to add nitrogen-rich fertiliser for a good yield.

She is testing whether adding dried leaves helps the waste decompose faster into compost.

Microorganisms (bacteria and fungi) in the soil decompose the waste. Dried leaves keep the waste loose and airy. They also absorb extra moisture. So the waste in pit A is expected to turn into compost faster than in pit B.

Materials: four slices of bread, four clean plates or containers, an airtight box, water, a refrigerator.

1. Set-up A (control): slightly moist bread in an open plate, kept in a warm place.
2. Set-up B (temperature): slightly moist bread kept in the refrigerator.
3. Set-up C (air): slightly moist bread sealed in an airtight box, with the air pressed out.
4. Set-up D (moisture): completely dry bread kept in a warm place.

Change only one condition in each set-up. Keep all other conditions the same. Observe all four daily for a week.

Expected result: mould grows fastest on A. Growth is very slow or absent on B, C and D.

Conclusion: microorganisms need suitable temperature, air and moisture to grow.

Observation: the slice near the sink shows white, green or black cottony patches. This is bread mould. The slice in the refrigerator shows little or no mould.

Reason: the place near the sink is warm and moist. Mould (a fungus) grows quickly in such conditions. The low temperature of the refrigerator slows down the growth of microorganisms.

1. The bacterium Lactobacillus in curd keeps multiplying. It keeps producing lactic acid. More acid makes the curd more sour.
2. Warm room temperature speeds up the growth of these bacteria. In a refrigerator, curd stays less sour because bacteria grow slowly.