1. Convert field diameter: 5 mm × 1000 = 5000 µm (since 1 mm = 1000 µm)
2. Size of one cell = field diameter ÷ number of cells: 5000 µm ÷ 25 = 200 µm
3. Total magnification = eyepiece × objective: 10X × 10X = 100X

The alveolar cell membrane is selectively permeable. Gases move across it by diffusion — each gas travels from where it is more concentrated to where it is less concentrated. Oxygen is high in the alveolar air and low in the blood, so O₂ diffuses into the blood; CO₂ is high in the blood and low in the air, so it diffuses out. The thin lipid–protein membrane lets these small gas molecules pass while still blocking many other substances.

This happens by osmosis — the movement of water through a selectively permeable membrane from a region of more water (dilute) to less water (concentrated). The membrane lets water pass but not the salt or sugar molecules.

After 12 hours of soaking, the seeds have absorbed water and become swollen and turgid. When moved into a concentrated (hypertonic) solution, the surrounding solution has less water than the seed cells. By osmosis, water now moves out of the seed cells. The seeds will shrink, become wrinkled and lose their firmness, and their sprouting/germination will be slowed because the cells lose the water they need.

• Gives a rigid shape and mechanical strength, so the cell can withstand stresses like wind and rain.
• Protects the delicate cell membrane and contents.
• Stops the cell from bursting when too much water enters (prevents over-swelling in hypotonic surroundings).
• Helps the whole plant/organism stay upright and firm since these organisms cannot move away from harsh conditions.

Shape difference: Plant (onion/Rhoeo) cells have a rigid cell wall that gives a fixed, box-like shape and keeps them regularly arranged. Cheek (animal) cells lack a cell wall; bounded only by a flexible membrane, they take irregular, rounded shapes.

In sugar solution: Both lose water by osmosis. In the plant cell the rigid wall holds the outer boundary in place while the inner contents (membrane + cytoplasm) pull away and shrink — leaving a gap. The cheek cell, having no wall, simply shrinks as a whole.

Prokaryotic: the bacterial cell (a) — it has no well-defined nucleus (only a nucleoid) and no membrane-bound organelles.

Eukaryotic: the plant cell (b) and the animal cell (c) — both have a true membrane-bound nucleus and membrane-bound organelles.

• Mature sieve-tube cells in the phloem of plants lose their nucleus but stay alive to transport food.
• Mature mammalian red blood cells (the example given) — extra space for haemoglobin.
• Platelets in blood are cell fragments without a nucleus.
• Cells of the eye lens fibres lose their nucleus to stay transparent.

The root-tip cells are not all identical. Because the growing tip divides continuously by cell division, different cells are caught at different stages — so we see varied internal appearances (some with a clear nucleus, others with thread-like or rod-shaped chromosomes, some splitting). The stage that comes first is the one where the cell is not yet dividing and shows an intact, well-defined nucleus with loose chromatin — before the chromosomes condense and the cell splits.

Plants are rooted in one place, so they cannot escape wind, rain or other stresses. They need a rigid cell wall for support, to stay upright and to hold their shape against these forces and against water pressure.

Animals move around, so they need flexibility rather than rigidity — flexible, wall-less cells let tissues bend, stretch and change shape during movement. A rigid wall would prevent this, so animal cells have only a membrane.

• The cell would lose its fixed shape and rigidity.
• In plenty of water it could swell too much and even burst, because nothing resists the inflow.
• The plant could no longer stand upright — leaves, stem and flowers would droop.
• It would lose protection and the firm support that the rigid wall normally provides.

This makes it a fair test. Equal-sized pieces start with similar surface area and amount of cells, so the only thing differing between them is the liquid they sit in. Recording the initial weight gives a baseline, so that after the experiment we can measure the change in weight and be sure it was caused by osmosis (water moving in or out) and not by the pieces being unequal to begin with.

White petals usually contain leucoplasts — plastids that have no coloured pigment. They appear white mainly because they reflect/scatter all wavelengths of light and lack the coloured pigments (like the orange/red/yellow pigments of chromoplasts or green chlorophyll). So in terms of colour-giving pigment, white flowers essentially have none; their colourless plastids and reflected light make them look white.

Energy reactions happen on the mitochondrial membranes (the folded cristae). Many small mitochondria together provide a much larger total membrane surface area than one big sphere of the same volume — and more surface means more space for respiration reactions, so more energy (ATP) can be made.

Small units can also be moved to exactly where energy is needed in the cell, and if one is damaged the others keep working. (Geometrically, as an object gets bigger its volume grows faster than its surface area, so a single giant mitochondrion would have too little surface for its volume.)

Healing a cut needs new skin cells identical to the originals — that is exactly what mitosis provides (two identical daughter cells with the full chromosome set).

Meiosis instead produces four cells, each with half the chromosomes and genetic variation. Such cells could not function as normal skin cells, so the wound would not heal properly — the new cells would be defective and unable to rebuild healthy skin tissue.

Osmosis is purely about water crossing the selectively permeable membrane. In pure water (hypotonic) water enters X → it swells; in salt solution (hypertonic) water leaves Y → it shrinks. The salt itself does not cross the membrane, which rules out (i) and (ii); (iv) wrongly blames solute movement.

Leucoplasts are present in plant cells, and the cell wall is absent in animal cells — both halves are true. The others fail: ribosomes, Golgi and ER are all present in animal cells, so they are not “absent in animal cells.”

Roots do contain plastids — just not chloroplasts. Because roots are underground and don’t make food, they contain colourless plastids called leucoplasts (e.g. amyloplasts) that store starch. Rohit confused “plastids” with “chloroplasts”: photosynthesis is absent in roots, but other types of plastids are still present.

• Both are double-membrane-bound organelles.
• Both have their own DNA and ribosomes, so can make some of their own proteins.
• Both are linked to energy and are thought to share an evolutionary (endosymbiotic) origin with bacteria.

DNA is found in the nucleus, mitochondria and chloroplasts. Ribosomes contain RNA (not DNA), and Golgi bodies and lysosomes have none — so (ii) Mitochondria + Nucleus is the only pair where both contain DNA.

(i) Hypothesis: that plant tissue gains or loses water by osmosis depending on the concentration of the surrounding solution (water moves toward the more concentrated side).

(ii) Improvements: use carrots of equal size and mass; weigh each before and after; keep the same volume of liquid, same temperature and same time; repeat with several carrots (replicates) for reliable results.

(iii) In plain water (hypotonic) water enters the carrot cells → they become turgid → firm and crunchy. In salt solution (hypertonic) water leaves the cells → they lose turgor (plasmolysis) → the carrot turns limp and rubbery.

(i) The sugar/salt makes the inside of the cavity hypertonic (high solute). Water from the beaker moves through the living potato cells by osmosis toward this higher concentration, so water collects in the hollow of B and C.

(ii) Cup A is the control. With no solute inside, no water should collect. It proves that the gathering of water in B and C is caused by the added solute (osmosis) and nothing else.

(iii) In A there is no concentration gradient, so there’s no reason for net water movement. In D the potato was boiled — the cells are dead and the membranes destroyed, so they are no longer selectively permeable and osmosis cannot occur, even though sugar is present.

SER does synthesise lipids, but it does not make cellulose — cellulose is made for the cell wall, not by the SER. So (ii) is the wrongly matched pair. Options (i) and (iii) are correct.

Mitochondria are the “powerhouses” that carry out cellular respiration to make ATP (the cell’s energy currency). Without them, the cell loses its main energy supply, so it can no longer power activities like transport, building molecules, or dividing. The cell would become weak and eventually die. (Only a small amount of energy could still come from anaerobic glycolysis in the cytoplasm — far too little to sustain the cell.)

Contact inhibition normally inhibits tumour formation in animals: cells stop dividing when they touch neighbouring cells. Cancer arises when cells lose this control and divide uncontrollably.

Plants: plant cells have rigid cell walls and do not show contact inhibition, so they can form abnormal growths (for example galls/crown gall). However, because of the rigid walls and no circulatory system, these abnormal cells cannot spread (metastasise) through the plant the way animal cancer cells do — so plant “tumours” stay localised.

Proteins are made by ribosomes on the RER; lipids are made by the SER. These are packed into vesicles, sent to the Golgi apparatus for modifying and sorting, then carried in vesicles that fuse with and become part of the plasma membrane.

Mitosis keeps the full chromosome number. So gametes made by mitosis would carry the complete set instead of half. At fertilisation two such gametes join, and the chromosome number would double in every generation — never staying constant.

This would cause serious genetic disorders and likely non-viable offspring. It would also remove the genetic variation that meiosis normally creates. This is exactly why gametes must form by meiosis (halving the chromosomes and shuffling genes).

(i) Concept: osmosis — preserving food by creating a high-solute (hypertonic) environment that draws water out of microbes (plasmolysis).

(ii) High salt or sugar makes the surroundings hypertonic to bacteria and fungi. Water is pulled out of the microbial cells by osmosis, so they lose water, shrink (plasmolyse) and cannot grow or multiply. With so little free water available, spoilage organisms cannot survive — the food keeps longer.

(iii) A healthy recipe: e.g. amla murabba made with a modest amount of jaggery instead of refined sugar, or a lemon-and-ginger pickle preserved with rock salt and a little mustard oil; both store well and add nutrients. (Any salt/sugar-based, low-oil preserve is acceptable.)

(iv) Scientific values: reducing food wastage, sustainability and food security, applying scientific knowledge to everyday life, resourcefulness and self-reliance, and blending traditional knowledge with science to support the local economy.