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 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.

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 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 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.

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 from? Can these almost invisible things appear spontaneously 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 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) Why would this be important?

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 today?

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 cell Cells are the smallest structural and functional units of life 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.

Development of Cell Theory

1838- German Botanist, Matthias Schleiden, concluded that all plant parts are made of cells

Development of Cell Theory

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

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What is Life?

Living things are composed of Cells Living things have different Levels of Organization Living things use Energy Living things respond to their environment Living things Grow Living things Reproduce Living things Adapt/Evolve to their Environment over Time

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

Development of Cell Theory

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 membrane)