A cell is the smallest unit that is alive. Every living thing is made of one or more.
Cell The smallest unit of a living thing that is itself alive.
Step 1: Something to Notice
Watch first. The explanation comes later.
In 1665 Robert Hooke put a shaving of cork under a new instrument and saw rows of tiny empty boxes. He called them cells, after the small rooms in a monastery.
Why did nobody see this in the previous few thousand years?
Step 2: Find Out
Compare each specimen’s real size with what the eye can resolve.
Step 1 — Predict
Why did nobody discover cells before the 1600s?
Choose what you think will happen. You cannot see the experiment until you do — guessing first is what makes it worth watching.
Step 3: So Here Is Why
Now the explanation, after you have seen it happen.
Smaller than the eye can separate
The eye resolves about 0.1 mm. Most cells are between 0.01 and 0.1 mm, so they fall below it.
The instrument came first
Better lenses in the 1600s made cells visible. The discovery followed within a few years.
Cell theory
All living things are made of cells, cells are the basic unit of life, and cells come from other cells.
Nobody knew until somebody could see
Cells were invisible until the microscope. Hooke saw compartments in cork in 1665 and Leeuwenhoek saw living single cells soon after. The theory followed the instrument, not the other way round.
The three claims of cell theory
All living things are made of cells, the cell is the basic unit of life, and cells come only from existing cells. The third took two centuries to establish and killed spontaneous generation.
Step 4: A Common Mistake
Lots of people think
“People before the 1600s just were not paying attention.”
Step 5: What Hooke Actually Saw
The same idea somewhere new.
Cork is dead, so Hooke saw only the empty boxes left behind by cells that had died — the walls, with nothing inside. He named the thing he could see, which is why we call the living unit of life after an empty room. Living cells were seen a few years later by Leeuwenhoek, in pond water.
Step 6: Think It Through
Practice makes it stick.
Under the Limit
Problem 1 of 2
The eye resolves 0.1 mm. A cell is 0.02 mm. How many times too small is it?
times
Empty Boxes
Problem 2 of 2
Hooke saw empty boxes in cork. Why were they empty?
How We Know
1 of 4
What is a cell?
2 of 4
Roughly what can the unaided eye separate?
mm
3 of 4
Why were cells not discovered earlier?
4 of 4
Which are claims of cell theory?
Step 7: Quick Check
Show what you know.
Question 1 of 1
Why did cell theory arrive in the 1600s?
Step 8: Explain It in Writing
Claim, evidence, then reasoning.
The question
Explain why cell theory arrived in the 1600s rather than in ancient Greece.
Words to use0.06 mm0.1 mmresolvelensinstrumentinvisibleevidence
Fill in all three boxes. The reasoning box is the one that matters most.
What You Found Out
- A cell is the smallest unit of a living thing that is itself alive.
- The eye resolves about 0.1 mm; most cells are smaller than that.
- Cells were invisible until lenses improved, so the discovery waited on technology.
- Cell theory: all living things are cells, cells are the basic unit, cells come from cells.