Cells : How their discovery led to the cell theory

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 • Robert Hooke in England

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

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

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.

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.

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

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= 0Biogenesis = 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

Cells : How their discovery led to the cell theory