Cells: How their discovery led to the cell theory

Cell Theory –
and the history
behind it.
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
People throughout the
Middle Ages believed
that mice could be
spontaneously by
putting grain in dark,
quiet place and leaving
it for a few weeks.
Francesco Redi
Born 1626 in Italy
First to challenge the
idea of spontaneous
Did not accept the
common belief that
flies magically
appeared from rotting
Redi’s Experiment
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,
metamorphose into
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 bringing it to life”
“Scientists seek only to challenge
belief systems and stir things up”
History of Microscopes
Cells were unknown until the
discovery of microscopes in the
two scientists working
independently built the first
Anton von Leeuwenhoek in
Robert Hooke in England
Studied pondwater, sour milk,
and semen
named moving organisms
scared people and caused a
drawings of
“animalcules” set off a
flurry of amateur and
sometimes ridiculous
claims, such as:
- pondwater
animalcules causing
Human sperm cells
contain tiny human
Today we can look
back and think
“crazy,” but at the time
people took these
ideas very seriously.
Studied cork - a kind of
tree bark
named the structures he
saw “cells” because they
reminded him of the small
rooms monks slept in.
What questions might the
scientist now be asking?
The discovery of
cells only
confused people
more- If cells are
alive, then where
do they come
Can these
almost invisible
things appear
from the air?
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
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
Since few people had
seen these microbes, few
people believed him.
Louis Pasteur
1822. France
Supported that
generation is a myth
(sterilization by heat)
Why would this be
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
How does what Pasteur
learned keep us safe
Pasteur is the father of
modern microbiology
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.
The Cell Theory
The cell theory has three parts:
All living things contain at least one
Cells are the smallest structural and
functional units of life
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.
Development of Cell
1838- German Botanist, Matthias Schleiden,
concluded that all plant parts are made of
Development of Cell
1839- German physiologist, Theodor
Schwann, who was a close friend of
Schleiden, stated that all animal tissues are
composed of cells.
Cells are the smallest structural and
functional units of life
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
What is Life?
Living things are composed of Cells
Living things have different Levels of
Living things use Energy
Living things respond to their
Living things Grow
Living things Reproduce
Living things Adapt/Evolve to their
Environment over Time
Cells can only come from pre-existing
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
Development of Cell
1858- Rudolf Virchow, German physician,
after extensive study of cellular
pathology, concluded that cells must
arise from preexisting cells.
Modern Cell Theory
Modern Cell Theory contains 4 statements, in
addition to the original Cell Theory:
The cell contains hereditary information(DNA) which
is passed on from cell to cell during cell division.
All cells are basically the same in chemical
composition and metabolic activities.
All basic chemical & physiological functions are
carried out inside the cells. (movement, digestion, etc)
Cell activity depends on the activities of sub-cellular
structures within the cell (organelles, nucleus, plasma
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