Cells and

The Cell Theory

Cells and

The Cell Theory

Anton von Leeuwenhoek

in Holland

Robert Hooke in

England

Cells were unknown until the invention of microscopes in the

1650’s

two scientists working independently built the first microscopes

Anton Van Leeuwenhoek

1600’s

Made lenses that could magnify more.

Observed many living organisms

Looking at Protists called them “Wee Beasties”

Anton Van Leeuwenhoek

Leeuwenhoek’s drawings of “animalcules” set off a flurry of amateur and sometimes ridiculous claims, such as:

- pondwater animalcules causing madness!

Anton Van Leeuwenhoek

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.

Robert Hooke

in 1665 - First to coin term – “Cell”

Looked at Cork

Looked really at a dead plant cell

Only saw the cell wall

Late 1800’s – Charles Spencer developed the modern day compound light microscope

Can magnify an object up to 1000 times!

Scientists start looking at everything under the microscope and they notice…

Every living thing seems to be made up of small units called cells!

1838 Matthias Schleiden – concludes all plants made up of cells

Birth of the cell theory…

1839 – Theodore Schwann concludes all animals are composed of cells

Birth of the cell theory…Schleiden & Schwann, in 1839, formally espoused cell theory –

Cells are of universal occurrence in living things

and

Cells are the basic units of an organism

Birth of the cell theory…In 1858, Rudolph Virchow forcefully states the paradigm of cell theory

``All cells come from previous cells’’Omnis cellula e cellula

Thus:  all individuals derived from a single celled organism are related

and all cells in a multi-celled organism are descended from one fertilized egg.

The Cell TheoryThe 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

The Cell TheoryAll 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.

The Cell TheoryCells 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

The Cell TheoryCells can only come from pre-existing

cellsdoes 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 GenerationFrom 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 or biogenesisDid not accept the common belief that flies magically appeared from rotting meat

Redi’s Experiment

Independent variable = cover

Dependent variable = presence of flies

Hypothesis: If a jar containing rotting meat is covered, then it will produce no flies

Fly eggs

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

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”

Lazzaro Spallanzani1729. Italian

Believed microbes that spoil food come from the air and can be killed by boiling

Independent variable = air

Dependent variable = food spoilageHypothesis: If air is allowed to reach food, then microbes will get in and cause it to spoil

Spallanzani’s ExperimentFlask 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 Pasteur1859. 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

Independent variable = bacteria

Dependent variable = 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= 0Biogenesis = 3

Experimental method wins out over superstition

the cell theory takes hold among ordinary people

Pasteur is the father of modern microbiologysupported 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