What you see
down a lens.
microscope at the ready.
A drawing from 1665.
Robert Hooke was a London scientist with a new instrument — a microscope, hand-built and clumsy by our standards. One afternoon, he sliced a piece of cork (the bark of an oak) as thin as he could, lit it from below, and looked. What he saw astonished him. The cork was not a solid wall of stuff. It was built out of tiny, empty boxes — row upon row of them, like the rooms in a monastery.
He reached for the Latin word for those little rooms — cellula — and wrote it down in his 1665 book, Micrographia. The English word that came from it was cell. The name has stuck for three and a half centuries.
What Hooke could not have known is that the empty boxes he was drawing were the dried-out shells of living things — what was left of the cork tree's cells after the tree was dead. Living plants and animals are made of the same little rooms, only filled, busy, working. Today we say it as a rule: every living thing is built from cells. Grass, frogs, the bacteria in a puddle, you. Some living things are one cell on their own, like a bacterium or an amoeba. Others — you, an oak tree — are built from trillions, all coordinating.
cella — a small room, a storeroom, a monk's cell. cellula — the diminutive, a little room. Hooke chose well. Cells really are little rooms — with walls, with contents, with a job.
Hooke's Micrographia was printed as a huge folio — nearly 18 inches tall — and it was the first scientific bestseller in English. Samuel Pepys, the diarist, stayed up reading it until 2 a.m. and wrote that it was "the most ingenious book that ever I read in my life."
A compound microscope, part by part.
A modern light microscope has the same job as Hooke's — to gather light through a sample and bend it into a much bigger image with lenses. Yours is better than his in every way: the optics are sharper, the light is brighter, the focus is steady. But the parts are simple, and you can know them by name before you turn the knob.
The optical line
- Eyepiece — the lens you look through. Usually ×10.
- Body tube — the tube connecting eyepiece to objectives.
- Objective lenses — three or four short lenses on a rotating turret. Common ones are ×4, ×10, ×40.
- Condenser & diaphragm — focuses the light and controls how much hits the slide.
- Light source — a lamp or mirror underneath.
The mechanical line
- Stage — the flat platform the slide sits on.
- Stage clips — the metal arms that hold the slide.
- Coarse focus knob — the bigger knob. Moves the stage a long way.
- Fine focus knob — the smaller knob. Brings the image into perfect sharpness.
- Arm & base — the body and the foot. Carry the microscope by the arm, never the eyepiece.
The eyepiece magnifies, and the objective magnifies again. You multiply the two: total magnification = eyepiece × objective. With a ×10 eyepiece and a ×4 objective, the sample looks ×40. Swap the objective to ×40 and the sample looks ×400.
Light microscopes hit a hard ceiling at around ×1500 — they're limited by the wavelength of visible light itself. To see anything smaller, you need an electron microscope, which fires electrons instead of light. The most powerful ones today can image a single atom — about a million times smaller than a cell.
Plant cells, animal cells — same family, different jobs.
Every cell from every living thing shares a kind of basic furniture — biologists call these built-in parts organelles. There is a soft outer skin holding everything in (the cell membrane). There is a jelly that fills the inside (the cytoplasm). There is a control room (the nucleus) holding the DNA. There are tiny powerhouses releasing energy from food (the mitochondria). And there are ribosomes — the little machines that build proteins. Animal cells have those five things. Plant cells have all five too, plus three more.
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Shared by both
- Cell membrane — the soft skin around the cell.
- Cytoplasm — the jelly inside, where reactions happen.
- Nucleus — the control centre, holds the DNA.
- Mitochondria — release energy from food (respiration).
- Ribosomes — build proteins from instructions.
Plant cells only
- Cell wall — a rigid box of cellulose around the membrane. Holds the cell's shape.
- Chloroplasts — the green ones. Capture light for photosynthesis.
- Large permanent vacuole — a single big bag of cell sap, pushing outwards against the wall, keeping the plant firm.
You will be able to see some of this today. The onion-skin cells you'll look at are a clear case: rectangular, lined up in rows, with a thick wall and a nucleus you can sometimes spot if you stain them. Onion-skin cells are not green — they come from inside the bulb, where there is no light, so they don't bother making chloroplasts. If you found a leaf instead, you'd see the chloroplasts straight away.
Tap each card — what these key organelles do:
The longest cell in the human body is a single nerve cell. One of them can run from the base of your spine all the way down to your big toe — close to a metre long in an adult. The signal that tells your toe to wiggle is one electrical pulse, in one cell, end to end.
Name the parts.
Each part of this microscope is numbered. Pick a label below, then place it in the matching Pin slot. These are the parts you met two pages ago. Two of the labels don't belong on a microscope at all — read carefully before you place them.
Match each label to its pin
Robert Hooke and Isaac Newton had a famous scientific feud. Hooke claimed Newton had taken ideas from him about gravity; Newton denied it. After Hooke died in 1703, Newton became president of the Royal Society — and Hooke's only known portrait quietly disappeared. We still don't know what he looked like.
Magnification — calculate it.
Florence's eyepiece is ×10. She rotates the turret to the ×40 objective. What is the total magnification?
Fresh one. The eyepiece is ×10. Florence rotates to the ×4 objective. What is the total magnification?
Why that page of cork matters.
One cell, or many?
The part only plants have.
The control centre.
Which knob, when?
Your body is built from roughly 37 trillion cells — a 37 with twelve zeros after it. A 2013 study tried to count them properly, organ by organ; before that, every biology textbook was just guessing. The answer turned out smaller than people thought.
Try it — onion skin under your microscope.
The thin papery skin between the curved layers of a fresh onion is the kindest cell sample in the kitchen. The cells are big (for cells), they are arranged in tidy rows, they have a clear cell wall, and if you have a stain, the nucleus shows up as a small dark dot near the edge. You have a microscope at home. Use it.
Onion-skin slide — what you need
Equipment — your microscope, a glass slide, a coverslip, tweezers, a small piece of kitchen paper, a teaspoon of water in a dish, a fresh onion. Iodine solution is ideal as a stain if you have any (it stains nuclei brown). Methylene blue would also work. If you have neither, water alone is fine — the cell walls will still be visible.
Method (short version)
- Cut the onion in half. Pull off one of the curved fleshy layers.
- On the inside of that layer is a clear, thin skin. Peel a small piece off with tweezers — about the size of your little fingernail.
- Lay the skin flat on the slide, with no folds. Add one drop of water on top.
- If you have iodine: add one tiny drop now. Wait thirty seconds.
- Lower the coverslip gently from one edge, like closing a book, so no bubbles get trapped.
- Place the slide on the stage. Clip it. Start on the ×4 objective.
- Use the coarse focus knob to bring the cells into rough focus. Then switch to the fine knob for the sharp image.
- Rotate up to ×10, refocus with the fine knob only. Then to ×40 if you can — small adjustments only.
Every scientific write-up uses four headings, in order. Aim — what you are trying to find out, in one sentence. Method — what you did. Observations — what you saw: numbers, sketches, words. Conclusion — what your observations tell you, and one thing you'd do differently next time.
Aim · what I saw (drawings welcome on paper, words here) · what I think it means · one question I'd want to answer next. 150–200 words is plenty, Florence. No rush.
A few thoughts on your write-up, Florence
strong Your description of the cells as "lined up like bricks" is the right instinct — that's exactly why plant tissue holds its shape. And you used the four headings (Aim, Method, Observations, Conclusion), which is half of how this kind of write-up is read.
try this You said "the cells looked the same" twice, in different words. That's the doom-spiral talking — when you're not sure you've said enough, you say it again. Cross one of them out. Then ask a different question: are some cells bigger than others? can you see anything inside one?
to add In your conclusion, one sentence on why the iodine made a difference (or why water alone wasn't quite enough) would lift this from a description to a real conclusion.
A common onion's cells contain about five times as much DNA as your own cells do. The onion genome is roughly 16 billion base pairs long; the human genome is around 3 billion. Genome size doesn't match how complicated an organism looks — biologists call this the "C-value paradox".
Worth watching.
A short film to watch alongside today's lesson.
The words from today.
- Cell
- The basic unit of life — every living thing is built from one or more of them.
- Organelle
- A specialised structure inside a cell that does a particular job (nucleus, mitochondria, ribosomes are organelles).
- Nucleus
- The control centre of the cell; it holds the DNA.
- Chloroplast
- A green organelle found in plant cells that captures light for photosynthesis.
- Magnification
- How many times larger the image is than the real object: eyepiece × objective.
- Resolution
- The smallest detail a microscope can tell apart; a light microscope resolves to about 200 nm.
You've seen what Hooke saw.
You learned the word "cell" and where it came from. You learned the parts of the microscope, and you know how to work out a total magnification. You met the organelles — the membrane, the nucleus, the mitochondria — and you know that a plant cell carries a few extra: a wall, chloroplasts, a vacuole. Every living thing — yours, the onion's, the bacteria in a puddle — is built from the same little rooms. Now go and look. Florence, this is biology.
There are roughly as many bacterial cells living inside and on your body as there are human cells — about 38 trillion of each. Most of those bacteria are in your gut, and most of them are helping you. By cell count, you are not quite half-human.
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