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Cells: How their
discovery led to the cell
theory
History
Cells were unknown until the
discovery of microscopes in the
1650’s
two scientists working
independently built the first
microscopes
Anton von Leeuwenhoek in
Holland 1676
Robert Hooke in England 1665
Hooke
Studied cork - a kind
of tree bark
named the structures
he saw “cells”
because they
reminded him of the
small rooms monks
slept in.
Leeuwenhoek
Studied pondwater, and sour
milk,
named moving organisms
“animalcules”
Leeuwenhoek’s
drawings of
“animalcules” set off a
flurry of amateur and
sometimes ridiculous
claims, such as:
- pondwater
animalcules causing
madness!
Mathias Schleiden stated that all
plants are made of cells 1838
Theodor Schwann added that all
animals are made of cells. 1839
5.
Rudolph Virchow
Rudolph Virchow
1858
Stated “Virchow’s law”
Omnis cellula e cellula
Every cell comes from a cell
The Cell Theory
The cell theory has three parts:
All living things contain at least one
cell
Cells are the smallest living units of
matter
Cells can only come from preexisting cells
The cell theory-a closer look
All living things contain at least one cell
Many scientists working after Hooke
and Leeuwenhoek observed different
plants and animals
Each of them noted that no matter
what they observed, if it was alive it
had cells.
Cells are the smallest living units of
matter
scientists quickly realized that when
cells were dissected or broken open
they died
This meant that whatever “life” is, it is
something that happens inside cells
Cells can only come from pre-existing
cells
does not answer the question of
where the first cell came from or how
it came to be.
has not been disproved yet- no
scientist has ever built a living cell
from nonliving organic molecules
Spontaneous Generation
From pre-historic times to about 1850,
most people believed that under the
right conditions, living things could
spontaneously appear from non-living
material.
People throughout the
Middle Ages believed
that mice could be
“created”
spontaneously by
putting grain in dark,
quiet place and leaving
it for a few weeks.
The discovery of
cells only
confused people
more- If cells are
alive, then where
do they come
from?
Can these
almost invisible
things appear
spontaneously
from the air?
Francesco Redi
Born 1626 in Italy
First to challenge the
idea of spontaneous
generation
Did not accept the
common belief that
flies magically
appeared from rotting
meat
Redi’s Experiment
IV=cover
DV=presence of flies
Hypothesis: If a jar
containing rotting
meat is covered,
then it will produce
no flies
Redi’s Conclusions
Flies lay eggs, which
grow into maggots,
which
metamorphose into
flies
If flies can’t lay
eggs, then no new
flies can grow
Fly eggs
Objections to Redi
Many people rejected Redi’s claim that
flies do not spontaneously generate
Their reasoning:
“sealing the jar closed prevented a
magical essence from entering the
rotting meat and brining it to life”
“Scientists seek only to challenge
belief systems and stir things up”
Lazzaro Spallanzani
1729. Italian
Believed microbes that spoil food
come from the air and can be killed
by boiling
IV= air, DV=food spoilage
Hypothesis: If air is allowed to reach
food, then microbes will get in and
cause it to spoil
Spallanzani’s Experiment
Flask 1: boiled broth, open
Flask 2: boiled broth, sealed shut
Results
Flask 1 spoiled
Flask 2 did not spoil
Objections to Spallanzani
Sealing the flask shut
blocked the entrance of a
magical life force in the air
from getting to the broth.
Since few people had seen
these microbes, few people
believed him.
Louis Pasteur
1822. France
Supported that
spontaneous
generation is a myth
Invented
pasteurization
(sterilization by heat)
Pasteur’s Experiment
An improvement on Spallanzani’s work
Used special “swan-neck flasks” that allowed
air in but kept bacteria out
IV= bacteria, DV=spoiling broth
Hypothesis; If boiled broth is kept free of
bacteria, then it will not spoil even if air can
reach it.
The curved neck allows air in but traps
bacteria-carrying dust and dirt particles
Spontaneous generation is
dead!
Redi didn’t believe in it, and did an
experiment using flies
Spallanzani didn’t believe in it, and
experimented with broth
Pasteur disproved it conclusively with
his open-air yet spoilage free flasks.
Life from dead stuff= 0
Biogenesis = 3
Experimental method wins out over
superstition
the cell theory takes hold among
ordinary people
Pasteur is the father of
modern microbiology
supported the last part of the cell theory:
cells only come from pre-existing cells
identified yeasts as the microbes that
change grape juice into wine
showed that heat can be used to
sterilize foods and preserve them in
sealed glass containers, and later cans.
Finally...
in the late 1870’s,
doctors start to wash
their hands and
instruments before
operating...some even
start wearing gloves!
Joseph Lister’s wacky
idea that antiseptics kill
germs catches on...
The cell theory is one of the
most important theories in
biology.
All living things contain at least one cell
Cells are the smallest living units of matter
Cells can only come from pre-existing cells
Organization of life
Atom
molecule
cell
Organ
Organ system
tissue
organism