Transcript The Birth of Experimental Biology

The Birth of
Experimental Biology
Spontaneous Generation
vs.
Biogenesis
Spontaneous generation
Spontaneous generation was a widely-held
idea by both scientists and non-scientists
for a very long time.
 Historical documents contain “recipes” for
creating creatures such as: bees, frogs,
mice, etc.
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These “recipes” were based on observations.
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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.
Spontaneous generation vs. Biogenesis
Spontaneous Generation: The belief that
non-living material(s) give rise to
living organisms.
 Biogenesis: The principle that states
that all living things come from other
living things.
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Spontaneous Generation is put to the test
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Francesco Redi (1698):
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The first scientist to design and
carry out a controlled
experiment to test the idea of
spontaneous generation.
Redi’s hypothesis:
 Flies appear near rotting meat,
not because the meat gives rise
to flies, but because flies lay eggs
on rotting meat.
Redi’s Experiment
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Two sets of flasks.
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Both contain rotting meat.
One set is open to the air (and flies) =
control group
The other set has a covering over the
meat, preventing flies from landing on it =
experimental group
Formal Hypothesis:
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If flies only come from eggs laid by other flies,
then no flies will appear on rotting meat kept
in a sealed container.
Redi’s Experiment
http://www.harlem-school.com/5TH/sci_pdf/graphics/Redi_exp.gif
Identify the Variables of Redi’s Experiment
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Independent Variable:
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Dependent Variable:
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Remember, it’s what differs between the control and
experimental groups.
Covering on Jar
Remember, it’s what will be measured or observed to
see if the hypothesis is supported or disproved.
Appearance of flies inside the jar
Controlled Variables:
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Things that are kept the same in both groups.
Type and size of jar, type and amount of meat,
location of jars, time period, etc.
Redi’s Conclusion
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The results of Redi’s experiment supported his
hypothesis. If the eggs of existing flies were
not able to be laid on rotting meat, no flies
appeared on the meat.
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People were able to accept the idea that “large”
organisms, such as flies, mice, and frogs could not
arise spontaneously.
Some still wondered about the spontaneous generation
of microbes (microscopic organisms).
Score:
Spontaneous Generation
Biogenesis
0
1
Microbes
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Microbes are microscopic (very small), typically
unicellular, organisms.
Examples: yeast, bacteria, etc.
The invention of the microscope allowed
scientists their first glimpse at a world of living
creatures that surround us every day, yet go
unnoticed.
Many people, including scientists, thought that
these simple organisms could arise from pond
water, etc.
 In fact, if you place a bit of hay in water and
look at the water under the microscope,
initially it will be free of microorganims. 2-3
days later it will be teeming with them.
The Spontaneous Generation debate
“heats up”!
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Lazzaro Spallanzani (1767):
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a scientist that 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
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Two sets of Flasks:
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Boil beef broth in both sets. (Kill all
microorganisms initially present.)
Leave one set of boiled broth open to the
air = control group
Seal the flasks of the other set of boiled
broth to prevent air from entering =
experimental group
Formal Hypothesis:
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If bacteria do not arise spontaneously
from air, then the broth in the sealed
flasks will remain sterile and clear.
Spallanzani’s Experiment
Identify the Variables of Spallanzani’s
Experiment
Independent Variable:
 Seal of flask
 Dependent Variable:
 Appearance of broth (clear or
cloudy)
 Controlled Variables:
 Broth boiled for same amount of
time, type and size of flask, type
and amount of broth, location of
flasks, time period, etc.
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Spallanzani’s Conclusion
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The result of Spallanzani’s experiment supported his
hypothesis. The beef broth only became contaminated
with bacteria if the broth was exposed to air that
contained living microorganisms.
 For many people, this experiment confirmed that simple
organisms do not spontaneously arise either.
 But the debate wasn’t settled yet! A fellow scientist,
and others, said that the boiling killed the “life
force” of the air in the flask. Supposedly the
“altered” air inside the sealed flask lacked the
“life force” and was not able to give rise to living
bacteria.
Score:
Spontaneous Generation 0
Biogenesis
2
The Big Prize
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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.
Pasteur’s Experiment
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Designed a modified flask.
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The flask had a long neck that was curved like a
side-ways “S”. (Control Group)
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The curved-neck flask allowed outside air to move into the
flask containing boiled broth.
The curve in the neck forced solid particles, such as
microorganisms, to become trapped; they could not enter
the flask.
More than a year later, for comparison, Pasteur
broke the neck off of a flask. (Experimental Group)
Formal Hypothesis:
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If air does not contain a “vital force”, then the
broth in the curved-neck flask will remain clear of
microorganisms.
Pasteur’s Experiment
Identify the Variables of Pasteur’s
Experiment
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Independent Variable:
 Broken neck of flask
Dependent Variable:
 Appearance of broth (clear or cloudy)
Controlled Variables:
 Broth boiled for same amount of time, air
can enter flask, type and size of flask,
type and amount of broth, location of
flasks, time period, etc.
Pasteur’s Conclusion
The results of his experiment supported Pasteur’s
hypothesis. His curved-neck flask remained
sterile for a year. This proved that air had no
force with which to create life from non-living
material. If the flask was opened, it quickly
became contaminated by bacteria already
present in the air.
 Score:
Spontaneous Generation 0
Biogenesis
3
The debate was finally settled! Biogenesis remains
a key principle of Biology.
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