Where do Cells Come

From?From Spontaneous Generation to Cell

Theory - and back to Abiogenesis.

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Spontaneous Generation

Up until the 19th century, “Abiogenesis” (spontaneous generation) was an accepted explanation for the origins of many organisms, especially ones considered vermin.Spontaneous generation is described in Aristotle, who claimed that...

Francisco Redi1627-1697

" I put in four flasks with wide mouths one steak, some fish of river, four small eels of Arno river and a piece of calf and I locked very well the mouths of the flasks with paper and string. Afterwards I placed in other four flasks the same things and left the mouths of flasks open.

A short time later the meat and the fishes inside the open flasks became verminous, and after three weeks I saw many flies around these flasks, but in the locked ones I never saw a worm.”

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And the “Animalcules?”

While many naturalists accepted that large organisms did not arise by spontaneous generation, belief that microorganisms (being more “simple”) could arise spontaneously was widely accepted. For example, Lamarck’s theory of evolution depended on spontaneous generation feeding new microbes into the system, which then strove to become more complex.

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John Needham1713-1781

Knowing that boiling would kill microbes, Needham boiled gravy, broth, and hay infusions.The boiled solutions were placed in sealed flasks. A few days later, all were growing microbes.Needham and his followers claimed this as support of Spotaneous Generation.

Lazzaro Spallanzani1729-1799

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Spallanzani’s Doubts

Spallanzani doubted Spontaneous Generation as a plausible explanation. He suspected that Needham had not boiled the solutions long enough. His own experience showed that long boiling was necessary to kill all microbes. Spallanzani set up his own experiment.

Spallanzani’s experiment was similar to Redi’s original experiment, but using

nutrient broth to grow microbes.

Like Redi, Spallanzani was criticized for sealing off the flasks. If air could not get

in, then the experimental and control flasks were different. The “vital fluid”

thought to be in air could not reach the broth.

Louis Pasteur1822-1895

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Pasteur’s Dilemma

• The problem with Spallanzani’s experiment was the exclusion of air. Pasteur suspected that microbes came in with the air. How could he allow air in and yet exclude tiny microbes?

• Finer nets were not enough. The mesh wouldn’t be small enough. Pasteur needed a better design.

The answer was a swan-necked flask. Air was allowed in freely, but air

currents in the neck would be too weak to carry the microbes up the

long neck.

“I place into a glass flask one of the following liquids, all extremely alterable upon contact with ordinary air: water of brewer’s yeast, water of brewer’s yeast with sugar added, urine, sugar beet juice, pepper water; then I draw out the neck of the flask in such as way

as to give it various curvatures. I then bring the liquid to a boil for several minutes until steam issues freely

through the neck, without any other precautions.”

“I then allow the flask to cool. It is a remarkable thing, likely to astonish everybody used to the

delicacy of experiments relating to the so-called ‘spontaneous’ generation, that the liquid in the flask

will remain indefinitely unchanged.”

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Early Cell Theory

• Cell Theory was first proposed in 1839 by two 19th century scientists working independently:

• Theodore Schwann (1810-1832)• Matthias Jakob Schleiden (1804-1881)

Schwann Schleiden

Tenets of the Schleiden and Schwann Cell Theory1. The cell is the fundamental unit of structure in living

things.2. All living things are made of cells.

3. All cells come from pre-existing cells by cell division.

Schwann also proposed “Free Cell Formation” or spontaneous generation of cells — this was before

Pasteur’s definitive experiments.

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Modern Cell Theory Also

States:• Life’s chemical processes, such as metabolism, occur inside of cells.

• Cells contain hereditary material. Single cells are the units of reproduction.

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But...

• If all cells arise from other cells, where did the first cell come from?

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Abiogenesis Revisited

• Could early cells have formed from nonliving matter? Alexander Oparin and J.B.S. Haldane in the 1920’s thought they could, but not in today’s oxygen-rich atmosphere.

• Could this hypothesis be tested?

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Miller-Urey Experiment

• Stanley Miller and Harold Urey set up an apparatus to test Oparin’s hypothesis. They recreated the best understanding of early Earth’s atmosphere, with sparks to simulate lightning, and got organic molecules.

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Surface or deep

water?• Miller’s studies showed that organic molecules could form on a watery surface, given energy.

• Other setups showed that deep ocean thermal vents could also be a source of organic molecules.

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Repeated Experiments

• Stanley Miller repeated the experiments in multiple ways throughout his life, using different energy sources, and refining the gas mixtures as scientists better understood the early earth’s atmosphere.

• The results still showed that organic molecules can form under early Earth’s conditions.

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What about cells?

• Experiments have shown that lipids (fats and oils, which are organic molecules) spontaneously form cell-sized bubbles when agitated in water.

Cell membranes in all organisms today are made

up largely of lipids.

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Metabolism first?

• Lipid bubbles can collect other organic molecules within. If these formed compartments where chemical reactions took place, could this be the origin of some of the chemical reactions that led to metabolism.

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Genetic material first?

• The “RNA World” hypothesis suggests that RNA was the first self-replicating molecule.

• Unlike DNA, RNA does not need enzymes to replicate itself. It can also act as an enzyme to catalyze some chemical reactions.

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Of the two theories...• ... the RNA world hypothesis has the most support at present.

• However, both processes could contribute to the formation of cells.

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The question is open.

We have tantalizing clues that show that certain cell parts can form under the conditions present on early Earth.At present we do not know exactly which events actually occurred, and in what order.

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