Cells: How their discovery led to the cell theory

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Cells How their discovery led to the cell theory
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History

• Cells were unknown until the discovery of
  microscopes in the 1650s
• two scientists working independently built the
  first microscopes
• Anton von Leeuwenhoek in Holland
• Robert Hooke in England

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Leeuwenhoek

• Studied pondwater, sour milk, and semen
• named moving organisms animalcules
• scared people and caused a sensation

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Leeuwenhoeks drawings of animalcules set off a
flurry of amateur and sometimes ridiculous
claims, such as - pondwater animalcules causing
madness!
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and... Human sperm cells contain tiny human
beings! Today we can look back and think
crazy, but at the time people took these ideas
very seriously.
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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.

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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 pre-existing cells

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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.

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• 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

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• 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 proven wrong yet- no scientist has
  ever built a living cell from nonliving organic
  molecules

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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.

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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.
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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?
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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

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• Redis Experiment
• IVcover
• DVflies
• Hypothesis If a jar containing rotting meat is
  covered, then it will produce no flies

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• Redis Conclusions
• Flies lay eggs, which grow into maggots, which
  metamorphose into flies
• If flies cant lay eggs, then no new flies can
  grow

Fly eggs
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Objections to Redi

• Many people rejected Redis 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

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Lazzaro Spallanzani

• 1729. Italian
• Believed microbes that spoil food come from the
  air and can be killed by boiling
• IV air, DVmicrobes spoil food
• Hypothesis If air is allowed to reach food, then
  microbes will get in and cause it to spoil

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Spallanzanis Experiment

• Flask 1 boiled broth, open
• Flask 2 boiled broth, sealed shut
• Results
• Flask 1 spoiled
• Flask 2 did not spoil

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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.

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Louis Pasteur

• 1822. France
• Proved that spontaneous generation is a myth
• Invented pasteurization (sterilization by heat)

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Pasteurs Experiment

• An improvement on Spallanzanis work
• Used special swan-neck flasks that allowed air
  in but kept bacteria out
• IV bacteria, DVspoiling broth
• Hypothesis If boiled broth is kept free of
  bacteria, then it will not spoil even if air can
  reach it.

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The curved neck allows air in but traps
bacteria-carrying dust and dirt particles
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Spontaneous generation is dead!

• Redi didnt believe in it, and did an experiment
  using flies
• Spallanzani didnt believe in it, and
  experimented with broth
• Pasteur proved it conclusively with his open-air
  yet spoilage free flasks.

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Life from dead stuff 0Biogenesis 3

• Experimental method wins out over superstition
• the cell theory takes hold among ordinary people

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Pasteur is the father of modern microbiology

• proved 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
• proved that heat can be used to sterilize foods
  and preserve them in sealed glass containers, and
  later cans.

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Finally...

• in the late 1870s, doctors start to wash their
  hands and instruments before operating...some
  even start wearing gloves!
• Joseph Listers wacky idea that antiseptics kill
  germs catches on...

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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