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A short class with Sarah and her students. The other ways are under More.
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Imagine it is the year 1600. You leave meat out, and a few days later it is covered in maggots. Where did they come from?
Back then I might have thought the meat turned into maggots.
Many people did. The idea that life arises from non-living matter is called spontaneous generation.
But they never actually tested it?
Not carefully, until Francesco Redi in 1668. He put meat in open jars, sealed jars and jars covered with gauze.
And maggots only appeared where flies could reach the meat?
Exactly. The maggots came from flies' eggs, not from the meat.
Even after Redi, people believed microbes could appear in broth by themselves. How could you test that?
Boil the broth to kill everything, then seal it?
That was tried, but critics said sealing kept out something in the air that life needed. Pasteur's flasks had long S-shaped necks, open to the air.
So air could get in, but the dust and microbes would get stuck in the bend.
Right. The broth stayed clear, until he broke the neck off. Then it became cloudy.
So the only difference was whether microbes could reach the broth.
That is what makes it such a good controlled experiment.
A few years before Pasteur's flasks, in 1855, Rudolf Virchow had summed up the idea for the cells of the body: omnis cellula e cellula. What does that mean?
Every cell comes from a cell.
How do new cells form in your body?
By mitosis, when a cell divides into two identical cells.
Yes, and bacteria divide by binary fission. But if every cell comes from a cell, where did the first cell come from?
From chemicals, a very long time ago, when Earth was very different?
That is the scientific view. The principle describes life today, not the origin of life billions of years ago.
New cells are produced only by the division of existing cells.
The old, disproved idea that life arises from non-living matter.
Their controlled experiments showed that maggots and microbes come from existing organisms.
Virchow's phrase, from 1855, meaning every cell comes from a cell.
Binary fission, mitosis, meiosis and fertilisation link every cell to earlier cells.
Redi's open jars of meat developed maggots, while the sealed and gauze-covered jars did not, because flies could not lay eggs on that meat.
Pasteur's boiled broth stayed clear in a swan-neck flask but turned cloudy once the neck was broken, because microbes could then reach it.
When a cut in your skin heals, the new skin cells are produced by mitosis from the existing skin cells around the wound.
A cell is born from a cell before, never from broth or the dusty floor.
A curious cook leaves soup in a pot with a long, bent spout. Days pass and the soup stays fresh. One day the cook snaps off the spout, and within days the soup is cloudy: the germs were floating in the air all along, waiting for a way in.
Really Very Persuasive: Redi (1668), Virchow (1855) and Pasteur (around 1860), in order of time.
C.F.C. - Cells From Cells.
Term
What does the third principle of the cell theory state?
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Spontaneous generation
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Why did Pasteur use flasks with S-shaped necks?
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Omnis cellula e cellula
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Where did all the cells in your body come from?
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The theory that all living things are made of cells, that the cell is the basic unit of life, and that all cells come from pre-existing cells.
The disproved idea that living things can arise directly from non-living matter.
The principle that living things come only from other living things.
The way bacteria reproduce, by copying their DNA and splitting into two cells.
Cell division that produces two genetically identical cells, used for growth and repair.
The single cell formed when a sperm and an egg fuse; the first cell of a new individual.
An experiment in which only one factor is changed, so its effect can be identified.
The third principle of the cell theory states that every cell arises from the division of a cell that already exists; living cells never form from non-living matter today.
This lesson explores the principle that all cells come from pre-existing cells. It traces how scientists overturned the old belief in spontaneous generation, from Francesco Redi's experiment with meat and flies to Louis Pasteur's swan-neck flasks, and how Rudolf Virchow summed up the idea as "every cell from a cell". It then shows the principle at work today in binary fission, mitosis, meiosis and fertilisation, and considers what it means for the very first cells.
The cell theory has three principles: all living things are made of cells; the cell is the basic unit of structure and function; and all cells come from pre-existing cells. This lesson is about the third principle, which says that a new cell can only be produced when an existing cell divides. Life is therefore an unbroken chain of cells, each one the offspring of another.
For more than two thousand years, many people believed in spontaneous generation: the idea that living things could arise directly from non-living matter. Maggots seemed to appear from rotting meat, and mice from stored grain. The idea seemed to match everyday observations, so it was not seriously tested until the seventeenth century.
The Italian physician Francesco Redi placed meat in several jars. Some were left open, some were sealed and some were covered with fine gauze. Flies landed on the meat in the open jars, and maggots appeared there. No maggots developed on the meat in the sealed jars or under the gauze, although flies laid eggs on the gauze itself. Redi concluded that maggots come from the eggs of flies, not from the meat.
Microscopes had revealed micro-organisms, and many people still believed that these tiny organisms, at least, arose spontaneously in broths. Louis Pasteur tested this with flasks whose long necks he bent into an S shape, like a swan's neck. He boiled nutrient broth in the flasks to kill any microbes. Air could still enter, but dust and microbes were trapped in the bend of the neck. The broth stayed clear. When the neck was broken off, or the flask was tilted so the broth touched the trapped dust, the broth soon became cloudy with microbial growth. The microbes came from the air, not from the broth.
Because air reached the broth in both cases, Pasteur answered the old objection that sealing a flask kept out some "vital force" in the air.
In 1855, a few years before Pasteur's flasks, the German doctor Rudolf Virchow popularised the Latin phrase omnis cellula e cellula, "every cell from a cell", building on the observations of Robert Remak, who had seen cells dividing. This corrected an earlier idea of Schleiden and Schwann, who had thought new cells might form out of non-cellular material.
You began as a single cell, a zygote, and every one of the tens of trillions of cells in your body came from it by division. That zygote came from your parents' cells, which came from their parents' cells, and so on back through the history of life.
The principle describes life as it exists now. The very first cells, more than 3.5 billion years ago, must have formed from non-living chemicals, under conditions very different from today's and over a very long time. That is a separate question from spontaneous generation, which claimed that whole organisms appear quickly from non-living matter all the time. Viruses do not break the rule either: they are not cells, and they can only be copied inside living host cells.