If the invisible world suddenly became visible, we would see that water, soil, air, and even our own skin and gut are full of countless tiny living beings — bacteria, protozoa, algae, and fungi — constantly moving, feeding, and multiplying. Surfaces we think of as “empty” or “clean” (a table top, a drop of pond water, the lining of our cheek) would actually appear crowded with life. This would completely change our idea of what “empty space” means.

It would make us realise that “living” does not require being large or visible — a single cell, far too small to see, can carry out all the activities of life: feeding, growing, responding, and reproducing. We would also understand that complexity isn’t only about size; a single microscopic cell (like an Amoeba) is itself a highly organised, complex structure with a membrane, cytoplasm, and genetic material, capable of independent life. This expands our definition of “living” beyond what is visible to the naked eye.

Microorganisms interact with each other and with larger organisms in many ways: some compete for the same food/space, some live together for mutual benefit (e.g., Rhizobium bacteria living in the root nodules of legume plants and exchanging nutrients), some microbes decompose the same dead matter together (fungi and bacteria both act on plant waste to form manure), and some microbes (like gut bacteria) live inside larger organisms, helping them digest food while getting shelter and nutrients in return.

The smallest living things most of us can spot with the unaided eye are very tiny insects like ants, lice, or mites, and fine specks like a grain of pollen or a tiny seed. On average, the human eye can resolve (distinguish) objects that are roughly 0.1 mm (100 micrometres) or larger. Anything smaller than this — such as bacteria (about 1–5 micrometres) — is far too small for our eyes to see directly, which is why a microscope is needed.

The round-bottom flask filled with water has a curved (convex) shape, just like a simple lens. When light rays from the letters pass through the curved water surface, they bend (refract) and converge in a way that makes the letters appear magnified to our eye. This is exactly the principle used in a basic magnifying glass — a thick-middle, thin-edged curved piece of glass (a lens) bends light to enlarge the image of small objects.

Both images show small, similarly shaped units arranged compactly and regularly, one right next to another, without gaps in between — just as bricks are tightly packed together to build a strong wall, cells are tightly packed together to build the “wall” (structure) of a living tissue. This comparison helps us understand that, just like a brick is the basic structural unit of a wall, a cell is the basic structural and functional unit of a living organism.

Similarity: Both are made of the same three basic parts — cell membrane, cytoplasm, and nucleus. Difference: The onion peel cell has a rigid, regular rectangular shape due to the extra cell wall, while the cheek cell is irregular/flat and lacks a cell wall (typical of plant vs. animal cells).

• Cell membrane: Forms the outer boundary of the cell, separates it from its surroundings/other cells, and is porous — it controls the entry of useful materials (like food, oxygen) and the exit of waste materials.
• Cytoplasm: The jelly-like substance between the membrane and nucleus that contains carbohydrates, proteins, fats, and mineral salts; most life processes (chemical reactions that keep the cell alive) take place here.
• Nucleus: Regulates and controls all the activities occurring within the cell, and also regulates the cell’s growth.

Yes — these functions are essential for the maintenance of life, because together they allow the cell to take in nutrients, carry out chemical reactions for energy and growth, remove waste, and coordinate/control all of its internal activities. Without any one of these parts functioning properly, the cell could not survive.

Similarity: Both are animal cells with a cell membrane, cytoplasm, and nucleus, and both are elongated rather than round.

Difference: A muscle cell is spindle-shaped (pointed at both ends) to allow it to contract and relax efficiently for movement, whereas a nerve cell (neuron) is very long with many branches, allowing it to carry electrical messages quickly across long distances in the body.

Yes — the shape and structure of a cell is closely related to the function it performs. This is a key concept called “structure–function relationship“: e.g., flat cheek cells form a protective lining; branched, elongated nerve cells transmit signals fast; spindle-shaped muscle cells contract for movement; long tube-like plant cells transport water.

Just like plants, the body of an animal is also made up of cells — specifically, many different types of animal cells (such as muscle cells, nerve cells, skin/cheek lining cells, blood cells, etc.), each specialised for a different function, working together to keep the animal alive.

Pickles and murabbas are preserved using a high concentration of salt or sugar. This high concentration draws water out of microbial cells (by osmosis) and creates an environment in which microorganisms cannot survive or grow, so the food does not spoil even when kept for a long time without refrigeration.

• Dead plants, animals, and waste material would never decompose, so nutrients would never return to the soil, and soil fertility would gradually decline.
• Plants like legumes would not get nitrogen fixed naturally by bacteria such as Rhizobium, making farming much harder without artificial fertilisers.
• Foods like bread, dosa, idli, curd, and cheese (which depend on fermentation by yeast and bacteria) could not be made.
• Many animals, including humans, would lose the helpful gut bacteria that assist in digestion.
• Overall, the natural recycling of matter in ecosystems would break down, severely disturbing the balance of life on Earth.

Note: Both animal and plant cells have a true, membrane-bound Nucleus — bacterial cells do not (they have a nucleoid instead). So strictly: Nucleus should be placed in the overlapping region shared by “Animal Cell” and “Plant Cell” only (not bacterial). Cell wall is present in plant cells (and also in fungal/bacterial cell walls in general biology, but for this chapter’s comparison, treat Cell wall as a plant-cell feature and Chloroplast as exclusively a plant-cell feature). Nucleoid is found only in bacterial cells, since bacteria lack a true nucleus.

(i) The balloon on test tube B inflates because yeast (a living microorganism) respires and breaks down the sugar to release energy, producing carbon dioxide gas as a by-product. This gas collects in the test tube and inflates the balloon.

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

(Options a, b, and d are incorrect — there is no significant water evaporation to inflate a balloon at room/warm temperature in this short time, warm air expansion alone would be far too small an effect, and sugar does not react with warm air to produce gas.)

(ii) By transferring the gas from the balloon into a test tube containing lime water and shaking it, Aanandi wants to test/confirm whether the gas produced is carbon dioxide (CO₂). Lime water (calcium hydroxide solution) turns milky/cloudy in the presence of carbon dioxide — so if the lime water turns milky after shaking, it confirms that the gas released by the yeast was indeed CO₂.

Beans belong to the legume family. The roots of legume plants (like beans, peas, and lentils) form swollen structures called root nodules, which house nitrogen-fixing bacteria such as Rhizobium. These bacteria trap (fix) nitrogen gas from the air and convert it into a form that plant roots can absorb and use directly. Because of this natural nitrogen supply from Rhizobium, the bean farmer’s soil already gets enriched with nitrogen without needing chemical fertiliser. Wheat, however, is not a legume and does not have this nitrogen-fixing bacterial partnership, so the wheat farmer needs to add nitrogen-rich fertiliser separately to get a good yield.

Snehal is testing whether mixing dry leaves with wet kitchen waste (fruit/vegetable peels) affects the rate or quality of decomposition (manure formation) by microorganisms. Dry leaves add carbon-rich material and improve aeration/moisture balance in the compost mixture, which usually helps microorganisms decompose the waste more effectively. By comparing pit A (with dried leaves) and pit B (without dried leaves) after 3 weeks, she can observe and compare how completely/quickly each pit’s waste turns into manure, and conclude whether dried leaves help speed up or improve decomposition.

1. Bacteria — they are found in almost every environment (water, soil, air, hot springs, cold zones) and also live inside the human gut, helping in digestion.
2. Yeast — a unicellular fungus that releases carbon dioxide during respiration, which makes bread/cake dough rise and become soft and fluffy.
3. Rhizobium (bacteria) — lives in root nodules of legumes/pulse crops (peas, beans, lentils) and fixes nitrogen from air, supplying nutrients to the plant.

Aim: To show that temperature, air, and moisture all affect the growth of microorganisms (using bread/dough as a simple test material).

Materials: 4 identical, fresh slices of bread (or small equal portions of dough); 4 zip-lock bags or covered containers; water; refrigerator.

Method: Set up all four samples at the same time, label them, and observe daily for 3–5 days, noting any visible mould/fungal growth (cottony or powdery patches).

Expected result: Sample 1 (warm + moist + air) shows the fastest, most visible microbial (mould) growth. Samples 2, 3, and 4 — where air, moisture, or warmth is missing respectively — show slower or little to no growth, confirming that microorganisms need optimal temperature, air, and moisture together to grow well.

Precaution: Keep all other conditions identical between samples (same type/amount of bread, same starting time) so that only the one tested factor differs — this keeps it a fair test.

Observation: The bread slice left near the sink (at room temperature, in a relatively warm and possibly humid spot) will show visible mould growth (fuzzy white, green, or black patches) within 2–3 days. The bread slice kept in the refrigerator will show little to no visible mould growth in the same period, and will look mostly unchanged (though it may dry out or harden slightly).

Reason: Microorganisms such as moulds (fungi) grow best in warm, moist conditions. The refrigerator’s low temperature slows down the metabolism and reproduction of these microorganisms dramatically, so spoilage happens much more slowly. The slice near the sink remains warm and may also pick up more moisture/airborne spores, providing favourable conditions for the mould to grow rapidly.

1. Continued bacterial fermentation: Curd already contains live Lactobacillus bacteria. When left out at room temperature (rather than refrigerated), these bacteria remain active and continue to multiply, feeding on the remaining lactose (milk sugar) in the curd and converting more of it into lactic acid, which increases the sourness over time.
2. Warmer temperature speeds up the process: Bacteria grow and ferment faster at warmer room temperatures than in a refrigerator. Since the curd is left out (not refrigerated), the warmer surrounding temperature provides favourable conditions for the bacteria to act more quickly, producing extra lactic acid and making the curd more sour within a day.

(i) In flask A, the yeast (a living microorganism) respires using the sugar in the warm solution as food, breaking it down to release energy for its own growth. During this process, the yeast produces carbon dioxide gas (and a small amount of alcohol) as a by-product. So, bubbles of gas form in the sugar solution, and the gas escapes through the connecting tube towards flask/test tube B.

(ii) After about four hours, the lime water in test tube B turns milky/cloudy white. This happens because the carbon dioxide gas produced by the respiring yeast in flask A travels through the tube and bubbles into the lime water; carbon dioxide reacts with lime water (calcium hydroxide) to form a white, insoluble precipitate of calcium carbonate, turning the clear lime water milky. This confirms that the gas released by the yeast is carbon dioxide.

(iii) If yeast was not added to flask A, there would be no living microorganism present to respire and break down the sugar. As a result, no carbon dioxide gas would be produced, no gas would travel to test tube B, and the lime water in test tube B would remain clear (would not turn milky) even after several hours.