Where do Cells Come From? • From Spontaneous Generation to Cell Theory
Spontaneous Generation • Up until the 19th century, “Abiogenesis” (spontaneous generation) was an accepted explanation for the origins of many organisms, especially ones considered vermin. • Abiogenesis is described in Aristotle, who claimed that...
Aphids arose from the dew of plants Fleas came from putrid material Alligators arose from rotting logs Mice came from spoiled hay
Francisco Redi 1627-1697
" 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.”
Criticisms of Redi’s Experiment • If the jars were closed, the meat had no access to the air, which might contain an essential ingredient or vital force. • Redi responded by placing fine netting over the jars instead of sealing them. Once again he saw no maggots on the meat.
And the Animacules? • 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.
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 Spallanzani 1729-1799
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.
“So microbes cannot grow without air,” the critics said. “Nevertheless, they do grow when air is allowed into the flask.” The matter remained an open question until 1859...
Louis Pasteur 1822-1895
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.”
As Spontaneous Generation was dying a slow death, a new theory was just emerging.... Cell Theory
Henri Dutrochet1776-1847 • As a naturalist and embryologist, Dutrochet had performed microscopic investigations of many organisms. • Dutrochet proposed that “the cell is the fundamental element of organization” in living organisms.
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 Theory 1. 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.
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.