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# How small is a cell?

Year 8 Biology · Cells

<!-- Speaker note: Students describe the scale of cells relative to organisms and familiar objects, and explain why microscopes were needed to discover and observe them (AC9S8U01, node 1). About 22 minutes (Engage and Explore 6 min · Explain 10 min · Evaluate 3 min · Typical adds 3 min, Wider about 3 min). -->

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## Today

**We are learning to** most cells are about a hundredth to a tenth of a millimetre across, too small to see without a microscope: your body has about 37 trillion of them. Cells were only discovered once microscopes could magnify enough.

- I can explain why cells can't be seen without a microscope.
- I can compare a cell's size with something familiar.
- I can explain why microscopes were needed to discover cells.

<!-- Speaker note: Before class: A concept tutorial for introducing or revising the idea; it is no substitute for looking down a microscope. Set the width with `?width=minimum`, `typical` (default) or `wider`. It opens Year 8 Biology, before Is everything alive made of cells? (cell theory, node 2). Not a hinge node, but it is the entry point: students arrive with organism-level biology, and without a sense of scale "cells" stay a word. The prediction exposes that sense (thousands or millions instead of trillions). Board notes: the zoom board's later levels (nucleus to gene) are Year 10 content; students only need the first step here. The cell diagrams are drawn about 1000 times life size, all at the same size on screen, though real cells vary a lot (bacteria are about ten times smaller than most animal cells). Teacher: In your own words: the 1 mm line with cells, a hair and a bacterium is the evidence. -->

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## Why it matters

> For thousands of years nobody knew that every living thing is made of cells. Then, in the 1660s, someone looked at a thin slice of cork through an early microscope.

<!-- Speaker note: You have studied whole living things. This lesson zooms in. -->

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<span style="background:#dbeafe; color:#1a1a1a; border:1.5px solid #3b82f6; padding:6px 18px; border-radius:8px; font-size:1em; font-weight:500;">💬 Discussion</span>

## Engage: predict

**About how many cells are in your body?**

- About 37 trillion (37 000 000 000 000)
- About 37 thousand
- Just a few, very large ones
- About 37 million

<!-- Speaker note: What happens: About 37 trillion. A typical human cell is about 0.02 mm across, so even a speck of skin you can just see holds dozens of cells. Take two or three predictions before anyone runs it. -->

<!-- Speaker note: Teacher: tally the guesses. Thousands and millions mean cells imagined far too big. -->

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<span style="background:#fef3c7; color:#1a1a1a; border:1.5px solid #f59e0b; padding:6px 18px; border-radius:8px; font-size:1em; font-weight:500;">✍️ Activity</span>

## Explore it

{{embed: https://studio.learningwaypoints.com/tutorials/_playground-board/index.html?id=deck%2Fscience%2Fbiology-8%2Fcell-scale%2Fstep-2&code=widget%3A+cell%0Ashow%3A+zoom%0A&fresh=1}}

[Open this step in the tutorial](https://studio.learningwaypoints.com/tutorials/science/biology-8/cell-scale/?full=step-2#step-2)

<!-- Speaker note:  -->

<!-- Speaker note: Teacher: ask how big the cell would be on the screen if it were drawn life size (about a fiftieth of a millimetre: invisible). -->

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<span style="background:#dcfce7; color:#1a1a1a; border:1.5px solid #22c55e; padding:6px 18px; border-radius:8px; font-size:1em; font-weight:500;">✅ Check</span>

## What happened?

* About 37 trillion. A typical human cell is about 0.02 mm across, so even a speck of skin you can just see holds dozens of cells.

<!-- Speaker note: Reveal after running. Ask who predicted it, and what made them think so. -->

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<span style="background:#dcfce7; color:#1a1a1a; border:1.5px solid #22c55e; padding:6px 18px; border-radius:8px; font-size:1em; font-weight:500;">✅ Check</span>

## Too small to see

**Why can't you see your own skin cells?**

- Each is about 0.02 mm across, smaller than the 0.1 mm or so the eye can see
- They are hidden under a layer of dead skin
- They are see-through

* Answer: Each is about 0.02 mm across, smaller than the 0.1 mm or so the eye can see

<!-- Speaker note: Your eye can just make out things about 0.1 mm across (the width of a hair). Most cells are about 2 to 10 times smaller, and bacteria about 50 times smaller. -->

<!-- Speaker note: Teacher: the 'too small to see' question and the sort are the success criteria; the two number checks are the size comparison. -->

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<span style="background:#dcfce7; color:#1a1a1a; border:1.5px solid #22c55e; padding:6px 18px; border-radius:8px; font-size:1em; font-weight:500;">✅ Check</span>

## Fifty in a millimetre

**A cell is 0.02 mm wide. How many cells, side by side, would stretch across 1 mm?**

* Answer: 50

<!-- Speaker note: 1 ÷ 0.02 = 50 cells in one millimetre. -->

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<span style="background:#dcfce7; color:#1a1a1a; border:1.5px solid #22c55e; padding:6px 18px; border-radius:8px; font-size:1em; font-weight:500;">✅ Check</span>

## Magnification

**A microscope magnifies 400 times. A cell 0.02 mm wide looks how many millimetres wide?**

* Answer: 8

<!-- Speaker note: 0.02 × 400 = 8 mm: big enough to see clearly. -->

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<span style="background:#dcfce7; color:#1a1a1a; border:1.5px solid #22c55e; padding:6px 18px; border-radius:8px; font-size:1em; font-weight:500;">✅ Check</span>

## With or without

**Can you see it without a microscope?**

Groups: Yes · No: you need a microscope

An ant (about 3 mm) · A grain of sand (about 0.5 mm) · A human hair's width (about 0.1 mm) · A human cheek cell (about 0.05 mm) · A red blood cell (about 0.008 mm) · A bacterium (about 0.002 mm)

* Answer: An ant (about 3 mm) → Yes; A grain of sand (about 0.5 mm) → Yes; A human hair's width (about 0.1 mm) → Yes; A human cheek cell (about 0.05 mm) → No: you need a microscope; A red blood cell (about 0.008 mm) → No: you need a microscope; A bacterium (about 0.002 mm) → No: you need a microscope

<!-- Speaker note:  -->

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<span style="background:#dcfce7; color:#1a1a1a; border:1.5px solid #22c55e; padding:6px 18px; border-radius:8px; font-size:1em; font-weight:500;">✅ Check</span>

## Why the 1600s

**Why were cells only discovered in the 1600s?**

- Microscopes good enough to magnify them were only made then
- People weren't interested before then
- Living things didn't have cells before then

* Answer: Microscopes good enough to magnify them were only made then

<!-- Speaker note: Robert Hooke saw tiny boxes in cork in 1665 and called them cells; Antonie van Leeuwenhoek saw living single cells soon after. New technology made new evidence possible. -->

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<span style="background:#fef3c7; color:#1a1a1a; border:1.5px solid #f59e0b; padding:6px 18px; border-radius:8px; font-size:1em; font-weight:500;">✍️ Activity</span>

## Elaborate: a whole living thing in one cell

{{embed: https://studio.learningwaypoints.com/tutorials/_playground-board/index.html?id=deck%2Fscience%2Fbiology-8%2Fcell-scale%2Fstep-4&code=widget%3A+cell%0Ashow%3A+bacterium%2C+yeast%0Afind%3A+membrane%0A&fresh=1}}

[Open this step in the tutorial](https://studio.learningwaypoints.com/tutorials/science/biology-8/cell-scale/?full=step-4#step-4)

<!-- Speaker note: What this step is for: Single-celled organisms are complete living things at microscopic scale; finding their parts prepares the cell theory lesson. -->

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<span style="background:#dcfce7; color:#1a1a1a; border:1.5px solid #22c55e; padding:6px 18px; border-radius:8px; font-size:1em; font-weight:500;">✅ Check</span>

## Smaller still

**A bacterium is about 0.002 mm long and is a whole living thing. How does it compare with one of your cells?**

- The same size, because all cells are the same size
- About ten times smaller, yet it feeds, grows and reproduces on its own
- Bigger, because it is a whole organism

* Answer: About ten times smaller, yet it feeds, grows and reproduces on its own

<!-- Speaker note: Bacteria are among the smallest living things. Next lesson: every living thing, from bacteria to you, is made of cells. -->

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<span style="background:#fef3c7; color:#1a1a1a; border:1.5px solid #f59e0b; padding:6px 18px; border-radius:8px; font-size:1em; font-weight:500;">✍️ Activity</span>

## Elaborate: Hooke and Leeuwenhoek

{{embed: https://studio.learningwaypoints.com/tutorials/_playground-board/index.html?id=deck%2Fscience%2Fbiology-8%2Fcell-scale%2Fstep-5&code=widget%3A+cell%0Ashow%3A+animal%2C+plant%0Afind%3A+nucleus%0A&fresh=1}}

[Open this step in the tutorial](https://studio.learningwaypoints.com/tutorials/science/biology-8/cell-scale/?full=step-5#step-5)

<!-- Speaker note: What this step is for: The history of cell discovery shows evidence waiting on technology, and how a word (cell) can come from what the first observer saw. -->

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<span style="background:#dcfce7; color:#1a1a1a; border:1.5px solid #22c55e; padding:6px 18px; border-radius:8px; font-size:1em; font-weight:500;">✅ Check</span>

## Two discoveries

**Hooke called the boxes in cork 'cells' after small rooms. Leeuwenhoek's better lenses showed living things moving in pond water. What did the two discoveries together suggest?**

- Only plants are made of cells
- Cells are only found in dead things
- Living things are made of tiny units, and some living things are just one of them

* Answer: Living things are made of tiny units, and some living things are just one of them

<!-- Speaker note: Later scientists (Schleiden, Schwann and others, in the 1800s) brought this evidence together as cell theory. -->

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## Evaluate: look back

Write one sentence: "At first I thought… Now I think… because…"

**Revision summary**

- Most cells are about 0.01 to 0.1 mm across; bacteria are smaller still. The eye can't see anything smaller than about 0.1 mm.
- Your body has about 37 trillion cells.
- A microscope **magnifies**: a cell 0.02 mm wide magnified 400 times looks 8 mm wide.
- Cells were discovered only when microscopes were invented (Hooke, 1665; Leeuwenhoek soon after).

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<span style="background:#dcfce7; color:#1a1a1a; border:1.5px solid #22c55e; padding:6px 18px; border-radius:8px; font-size:1em; font-weight:500;">✅ Check</span>

## Exit ticket

**Why couldn't anyone in 1500 have known that plants are made of cells?**

* Cells are smaller than the eye can see (about 0.01 to 0.1 mm), and microscopes strong enough to magnify them hadn't been invented.

<!-- Speaker note: Students answer on paper or a mini whiteboard before the reveal. Anyone who cannot answer needs the main idea again next lesson. Where this leads: Next: Is everything alive made of cells? (cell theory, node 2). In class: an onion skin or cheek cell slide under a light microscope; measuring the field of view with a ruler to estimate cell size; a class scale line from 1 m down to 1 µm. -->