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The Cell Theory
Some Random Cell Facts
The average human being is composed
of around 100 Trillion individual cells!!!
 It would take as many as 50 cells to
cover the area of a dot on the letter “i”

WOW!!!
Visualizing Cells
Light Microscope
Can observe living cells
Less Expensive
Don’t magnify as much as an
E.M.
 Resolution
 Magnification

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Objective x eyepiece
Rules:
1.
2.
3.
4.
Always carry using 2
hands, one on the base,
the other holding the arm
Never use coarse adj. in
med or high power
Never clean the lenses with
anything other than lens
paper
Always turn the
microscopes off and put a
cover over them when not
in use
Electron Microscopes
Can magnify much
higher than light
microscopes
 Very expensive
 Big and non-portable
 2 types

S.E.M. (scanning)
 T.E.M. (transmission)

Scanning Electron Microscope
(S.E.M.)
Shows an image of
the scanned surface
 Useful for looking at
“3-D” structure of
specimens
 Butterfly Eye S.E.M.
22,000X

Transmission Electron Microscope
(T.E.M.)
Shows a cross
section view of
specimens
 Useful for
viewing the
“insides”
 Plant Leaf T.E.M.
42,000x
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Discovery of Cells
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1665- English Scientist, Robert Hooke, discovered
cells while looking at a thin slice of cork.
He described the cells as tiny boxes or a honeycomb
He thought that cells only existed in plants and fungi
Anton van Leuwenhoek
1673- Used a handmade
microscope to observe pond
scum & discovered singlecelled organisms
 He called them “animalcules”
 He also observed blood cells
from fish, birds, frogs, dogs,
and humans
 Therefore, it was known that
cells are found in animals as
well as plants

150-200 Year Gap???
Between the Hooke/Leuwenhoek discoveries
and the mid 19th century, very little cell
advancements were made.
 This is probably due to the widely accepted,
traditional belief in Spontaneous Generation.
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Spontaneous Generation
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For much of history, people believed that animals
could come from non-living sources.
Historical documents contain “recipes” for creating
creatures such as: bees, frogs, mice, etc.
 These “recipes” were based on observations.
 When the Nile river flooded, soon many frogs
appeared in the mud.
 If damp rags were left in a pile of wheat grain,
many mice appeared 21 days later.
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Jar-1
Left open:
Maggots
developed
Flies were
observed
laying eggs on
the meat in
the open jar
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Jar-2
Covered with
netting: Maggots
appeared on the
netting
Flies were
observed laying
eggs on the
netting
Francisco Redi
• Jar-3
• Sealed: No
maggots
developed
Lazzaro Spallanzani (1767)
Set up an experiment to test
whether a “vital force” in the air
could give rise to microorganisms.
 Spallanzani’s hypothesis:
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Beef broth becomes cloudy due to
bacteria present in the air. The
bacteria from the air enter the broth
and contaminate it, rather than the
bacteria arising from the air or broth
itself.
Spallanzani’s Experiment
The Big Prize
The Paris Academy of Science offered a
prize to the person who could best resolve
the spontaneous generation debate once
and for all.
 And the winner was…
 Louis Pasteur (1864):
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A scientist who designed an experiment
using a flask that would allow air
containing the “vital force” to enter the
flask, but would keep microorganisms out.
Louis Pasteur Broth Experiment
Development of Cell Theory
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1838- German Botanist, Matthias Schleiden,
concluded that all plant parts are made of cells
1839- German physiologist, Theodor Schwann,
who was a close friend of Schleiden, stated that
all animal tissues are composed of cells.
Development of Cell Theory
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1858- Rudolf Virchow, German physician,
after extensive study of cellular pathology,
concluded that cells must arise from
preexisting cells.
The Cell Theory Complete
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The 3 Basic Components of the Cell Theory
were now complete:

1. All organisms are composed of one or more
cells. (Schleiden & Schwann)(1838-39)
2. The cell is the basic unit of life in all living
things. (Schleiden & Schwann)(1838-39)
3. All cells are produced by the division of
preexisting cells. (Virchow)(1858)
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