Sec 3 Combined Science (Biology) Cell Structure & Organisation notes

A cell is the smallest unit that shows the features of life: it takes in materials, releases energy, grows, responds to its surroundings and can reproduce. Some organisms — bacteria, yeast — are single cells going it alone; a human is a cooperating society of trillions. Because cells are so small (most between about 10 and 100 micrometres, µm), we only know their structure by magnifying them.

Two microscopes tell two stories. The light microscope (school laboratory) magnifies up to about ×1500 and resolves structures down to roughly 0.2 µm — enough to see the nucleus, cell wall, chloroplasts and large vacuole. The electron microscope magnifies over ×100 000 and resolves structures thousands of times smaller — it revealed the mitochondria and ribosomes that a light microscope simply cannot show. So the full cell picture is a combined one: light-microscope structures plus electron-microscope detail.

Whether animal or plant, every living cell keeps three working parts. The cell membrane is a thin, flexible boundary built of fats and proteins. It is partially permeable: it decides what enters and leaves, letting oxygen and food in, keeping harmful substances out, and holding the cytoplasm together. The cytoplasm is the jelly-like contents inside the membrane — the cell's factory floor, where most of its chemical reactions happen and where the other structures are suspended. The nucleus is the control centre: it contains the genetic material that carries the instructions for every protein the cell makes, and it directs the cell's activities, growth and division.

Animal cells also hold small, temporary vacuoles — little fluid-filled sacs used for storage and transport. They are scattered and impermanent, which is very different from the single huge vacuole of a plant cell.

Plant cells keep every universal part and add three of their own. Outside the membrane sits the cell wall, a rigid box of cellulose. It gives the cell (and so the plant) its shape and strength, and — crucially for exams — it is freely permeable: water and dissolved substances pass through it without being selected. Just inside, chloroplasts are green discs containing the pigment chlorophyll, which absorbs light energy so the cell can build sugars by photosynthesis. Filling most of the cell's interior is the large central vacuole, a storage tank of water and dissolved substances. When it is full it presses the cytoplasm against the wall, keeping the cell firm — this is why lettuce is crisp and a wilted plant is limp.

A common diagram trap: not every plant cell shows all three. Root cells are underground — they have a wall and a vacuole but no chloroplasts, because no light reaches them. Chloroplasts belong in the green, light-exposed parts.

Electron micrographs show two structures in both animal and plant cells. Mitochondria are oval, sausage-shaped bodies with folded inner walls. They are the power stations: aerobic respiration happens here, releasing energy from food for everything the cell does. A cell that works hard is crowded with mitochondria — a muscle cell or a liver cell has far more than a resting skin cell. Ribosomes are the tiniest bodies of all, appearing as dark grains, often dotted along membrane channels. They are the assembly lines: ribosomes build proteins — enzymes, haemoglobin, structural fibres — by following instructions sent out from the nucleus.

Keep the pairing straight: nucleus issues the instructions, ribosomes build the protein, mitochondria supply the energy to do it. In an electron micrograph, mitochondria are the large ovals with folded linings; ribosomes are the small dark dots — size and shape alone identify them.

The comparison comes down to what the plant adds and what the animal keeps flexible. Both have a membrane, cytoplasm, nucleus, mitochondria, ribosomes and vacuoles. A typical plant cell adds a cellulose wall, chloroplasts and one large permanent vacuole; its shape is therefore regular and box-like. A typical animal cell has none of those three; small temporary vacuoles only, an irregular shape, and it relies on membranes and skeleton proteins (not a wall) for structure.

Exam phrasing matters: say a structure is "present" or "absent", not just "bigger" or "smaller". The wall, chloroplasts and permanent vacuole are the three standard differences — anything else you add should be a real detail you can defend.

A specialised cell trades flexibility for performance at one job, and its shape and organelle count tell you the job. The muscle cell is a long fibre packed with mitochondria and containing protein filaments that can slide past each other: abundant energy release plus a contractile machine lets it shorten and pull. The root hair cell is a normal plant root cell with one enormously elongated extension poking between soil particles — the extension multiplies the surface area for absorbing water and dissolved mineral salts, and the cell has no chloroplasts because it never sees light. The red blood cell is a small, flexible biconcave disc that has lost its nucleus, leaving room to be packed with the red pigment haemoglobin, which binds oxygen; its shape gives a large surface for picking up and releasing oxygen as it squeezes through the narrowest blood vessels.

The pattern to quote in explanations: state the job → match a structural feature → link the feature to the function. "The root hair cell absorbs water; its long extension gives a large surface area; a large surface area increases the rate of uptake." Three linked sentences, three marks.