A hidden world
Microscopes made structures too small for unaided eyes visible. In 1665 Robert Hooke described box-like spaces in cork and called them cells; later, Antonie van Leeuwenhoek observed living microorganisms.
Cell theory
Discover how observations and technology changed our understanding of life.
Class notes
Open PDF →The three principles
All living things are made of one or more cells. Cells are the basic unit of structure and function in living things. New cells come from pre-existing cells.
Science changes with evidence
Cell theory developed as observations accumulated. Better microscopes improved the evidence scientists could collect; the theory was not the work of a single person.
Explore the idea
01All living things
Living things are made of one or more cells.
02The basic unit
The cell is the basic unit of structure and function in living things.
03Existing cells
New cells come from pre-existing cells.
Explore this idea further
Watch: The Wacky History of Cell Theory
Discover how observations and technology changed our understanding of life.
Open on YouTube →How did microscopes change the evidence scientists could collect?
Reveal the reasoning
They made cells visible, allowing scientists to observe structures that cannot be seen unaided.
Try it yourself.
Why did the development of microscopes matter for cell theory?
Check your thinking
Microscopes allowed scientists to observe cells and microorganisms directly and compare evidence across many living things.
Common question: Are viruses made of cells?
No. Viruses are not cells and need a host cell to reproduce, which is one reason their status as living is debated.
Take it further.
A new cell is observed dividing from an existing cell. Which part of cell theory does this support?
See a possible explanation
It supports the principle that new cells arise from pre-existing cells.