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.
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.
You have studied whole living things. This lesson zooms in.
Engage: Commit to a prediction before you test anything. A wrong guess is fine: it shows you what to look for.
Explore: Try it on the board and compare what happens with your prediction.
Explain: Use the particle model to explain what you saw.
Elaborate: Use the same idea in a new context.
Evaluate: Look back at your first ideas and what changed.
WidthMore contexts, same idea.
Make your prediction first. Help opens once you've locked it in.
EngageCommit to a prediction before you test anything. A wrong guess is fine: it shows you what to look for.
Engage: how many cells?
ExploreTry it on the board and compare what happens with your prediction.
Explore: zoom in
Press Zoom in once. (You can keep zooming: the later levels are for Year 10.)
Click a part of a cell to see its name.
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Zoom in from the person to one cell.
Use the board. This ticks when you get there.
These cells are drawn about 1000 times life size. Find the nucleus in each.
Use the board. This ticks when you get there.
ExplainUse the particle model to explain what you saw.
Explain: too small to see
In your book: draw a 1 mm line and mark how many cells would fit across it. Add a hair's width and a bacterium for comparison, and write one sentence on why a microscope is needed.
A cell is 0.02 mm wide. How many cells, side by side, would stretch across 1 mm?
A microscope magnifies 400 times. A cell 0.02 mm wide looks how many millimetres wide?
Can you see it without a microscope?
An ant (about 3 mm)
A grain of sand (about 0.5 mm)
A human hair's width (about 0.1 mm)Just: about the smallest thing the eye can see.
A human cheek cell (about 0.05 mm)
A red blood cell (about 0.008 mm)
A bacterium (about 0.002 mm)
Elaborate TypicalUse the same idea in a new context.
Elaborate: a whole living thing in one cell
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Find the membrane of the bacterium and the yeast cell.
Use the board. This ticks when you get there.
Elaborate WiderUse the same idea in a new context.
Elaborate: Hooke and Leeuwenhoek
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Find the nucleus in each cell again. Hooke's cork cells were dead: just the walls were left.
Use the board. This ticks when you get there.
EvaluateLook back at your first ideas and what changed.
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).
For teachers
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.
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.
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).
Running the lesson
Engage: tally the guesses. Thousands and millions mean cells imagined far too big.
Explore: ask how big the cell would be on the screen if it were drawn life size (about a fiftieth of a millimetre: invisible).
Explain: the 'too small to see' question and the sort are the success criteria; the two number checks are the size comparison.
In your own words: the 1 mm line with cells, a hair and a bacterium is the evidence.
The hard part (on purpose)
Sense of scale. Ask students to estimate before calculating (how many cells across a millimetre?) and to check their estimate against the answer.
When students get stuck
A student thinks cells are about the size of a grain of sand. Ask: Can you see a grain of sand? How many cells fit across one? Fix: About 10 cheek cells, 25 typical body cells or 250 bacteria would fit across a 0.5 mm grain. Cells are far smaller than anything the eye can pick out.
A student thinks bigger organisms have bigger cells. Ask: Is an elephant's skin cell much bigger than yours? Fix: No: about the same. Bigger organisms have more cells, not bigger ones.
A student mixes up magnification and actual size. Ask: Did the cell grow, or does it only look bigger? Fix: Magnification makes it look bigger: image size = actual size × magnification.
Stretch and support
Stretch: A red blood cell is 0.008 mm across. At what magnification would it look 4 mm across? (500 times.)
Support: Give the reference sizes on a card: hair 0.1 mm, cheek cell 0.05 mm, red blood cell 0.008 mm, bacterium 0.002 mm.
Where it 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.
What each step is for
Engage: how many cells?: Students' sense of scale is the barrier: guesses in the thousands or millions show cells imagined at about the size of a grain of sand.
Explore: zoom in: Stepping from a person to one cell, then looking inside cells drawn a thousand times life size, connects the familiar scale to the microscopic one.
Explain: too small to see: Comparing cell sizes with the smallest thing the eye can see, then using magnification, makes 'microscopic' a measured idea and explains why cells were found so late.
Elaborate: a whole living thing in one cell: Single-celled organisms are complete living things at microscopic scale; finding their parts prepares the cell theory lesson.
Elaborate: Hooke and Leeuwenhoek: The history of cell discovery shows evidence waiting on technology, and how a word (cell) can come from what the first observer saw.
Now you can
Explain why cells can't be seen without a microscope.
Compare a cell's size with something familiar.
Explain why microscopes were needed to discover cells.