Chapter 2
Cell: The Building
Block of Life

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.