Where do cells come from?
You are made of about 37 trillion cells, and nobody had ever seen one until 1665. It took two hundred years, better and better lenses, and one very clever flask to work out three simple facts about them. You'll repeat the important looks yourself.
Grade 8 science · CellsCell theoryAbout 12 min
Part 1 of 4 · The microscope bench
Turn up the lens
Five things are on the bench. Some are made of cells and one is not. Cells are too small for your eye, so the only way to find out is to magnify. Each lens below is a real step in history.
Pick a sample, then step the lens up from 1× to 1000×. Watch for the moment cells appear.
Lens
Your logbook
Fills in by itself as you look. ✓ means you could make out cells at that lens, – means you looked and couldn't.
Which sample never showed a single cell, at any lens?
Part 2 of 4 · 1838 to 1855
Put the evidence together
Once microscopes were good enough, scientists everywhere started looking. Out of all that looking came three claims, now called cell theory. Each claim needed its own evidence.
Tap a piece of evidence, then tap the claim it supports.
Part 3 of 4 · Paris, 1859
Can life start from nothing?
Leave a bowl of meat broth out and in a couple of days it goes cloudy, full of microbes. In the 1850s many people believed those microbes simply appeared from the broth and the air. If that were true, claim 3 would be wrong. Louis Pasteur designed a test.
All three flasks get the same broth. The swan-neck flask is open to the air, so air gets in, but its long bent neck traps falling dust in the bend.
Predict each flask, then run two weeks.
The swan-neck flask let air in and still stayed clear. So air was not the missing ingredient. What was missing was a living microbe to start from. Some of Pasteur's original flasks are kept at the Pasteur Institute in Paris, and the broth in them is still clear more than 150 years later.
Part 4 of 4 · Your turn to judge
Alive, once alive, or never alive?
Growing, moving and "feeding" are not enough. Cell theory gives a sharper test: is it made of cells, and are those cells still alive?
Tap a thing, then tap the shelf it belongs on.
Putting it back together
Every cell has a parent cell
You started as one cell. It split into two, those two into four, and so on. Tap any cell at the bottom to trace it back.
Tap a cell in the bottom row.
Real bodies are messier: cells die and get replaced, and not every cell keeps splitting. But the chain never breaks. Every one of your cells came from a cell, back through your parents' cells, back about 3.5 billion years.
Cell theory, in three lines
- All living things are made of one or more cells.
- The cell is the smallest thing that is alive.
- Every cell comes from another cell.
Hooke's lens showed the first one was possible. Schleiden and Schwann checked it across plants and animals. A single swimming cell in a drop of pond water proved the second. Remak, Virchow and Pasteur's flasks settled the third.