Big Ideas: Cell Theory
The Cell Theory states that all organisms are composed of similar units of organization,
called cells. The theory came about over the course of nearly two hundred years of discoveries
by biologists who were giants in their field. Even so, you should keep in mind that, at this
moment, you know more about cells and cell theory than any of these legendary biologists did in
their time!
Probably the single most important invention in relation to the cell theory was that of the
compound microscope. A compound microscope uses a series of concave lenses to increase
the magnification that you can see a tiny object with. The first compound microscope was
invented by the Dutch father and son team of Hans and Zacharias Janssen around 1595.
Additional improvements to the microscope came over the next century, most notably at the
hands of Anton von Leewunhoek, a Dutch businessman and amateur scientist who used his
own compound microscope to observe pond water. In his sample of pond water von
Leewunhoek saw tiny organisms moving on their own. He called them “animalcules”. Today we
know them as protists.
During the 16th and seventeenth centuries people widely believed in a theory called
spontaneous generation. This theory held that living things could come from dead matter or
even from non-living things. This theory was disproven by a famous experiment by an Italian
scientist named Francisco Redi. Redi tested two different slabs of meat, one in the open and
one under a glass cover to show that flies did not come from rotten meat, but rather from flies
laying their eggs in the meat.
The next set of important discoveries came from Robert Hooke. Hooke was an English
scientist who observed cork sections under a microscope. He saw that the cork was made up of
countless tiny chambers. He said they reminded him of the tiny rooms that Catholic monks lived
in called “cells”. That is where we get the term today.
The real foundation of the Cell Theory is drawn from the observations of three German
scientists. Matthias Schleiden was a botanist who learned that every plant he looked at under
the microscope was made of cells. At around the same time a zoologist named Theodor
Schwann discovered that all animals are likewise made of cells. The final observation that
completed the theory was made sometime later by a pathologist (studies the causes of
diseases) named Rudolph Virchow. Virchow did a whole series of observations that led him to
believe that all cells only come from already existing cells. This idea proved to be accurate
through many repeated experiments.
Thus the Cell Theory was finalized as follows:
1. The cell is the basic unit of structure and function in living things.
2. All living things are made of cells.
3. Cells only come from already existing cells.
Cells WalkThrough
Name: _____________ Block: ___
Use your resources to answer the following questions
Cell Theory
1. Francisco Redi proved the theory of _______________________ _______________________
was wrong.
2. Hans and Zachariah Janssen’s main contribution was the invention of the
_______________________
_______________________.
3. The German botanist ___________________ _________________ proposed that all plants are
made from cells
4. Anton von Leewunhoek was the Dutch business man who saw tiny moving creatures he called
_______________________ moving in pond water with a microscope of his own making.
5. _______________________ _______________________ was the German pathologist who
said all cells come from other _______________________.
6. Englishman _______________________ _______________________ gave us the term cells
after observing cork under a microscope.
7. German zoologist _______________________ _______________________ said all animals are
made of cells after studying his samples.
8. Without the invention of the ___________________ ___________________ most of the
discoveries about the Cell Theory could not have happened.
9. In your own words why is the Cell Theory important to biologists?
10. Give the three principles that make up the Cell Theory:
a.
b.
c.