Biology 2201, Unit 1
Early Cell Biologists
For centuries, since at least the time of Aristotle (4th Century BC), people (including scientists) believed that simple living organisms could come into being by “arising” from mud, muck, and other such sources. This was the idea that non-living objects can give rise to living organisms. (Spontaneous Generation).
The debate raged between those that held that life could originate only from living sources - biogenesis and those that held that as per the examples cited in the previous paragraph life could spontaneously arise – abiogenesis
History
In 1668, Francesco Redi, an Italian physician, did an experiment with flies jars containing meat. This was a true scientific experiment. He followed the scientific method and found that when jars with meat were sealed there would be no flies or maggots present. If the jars were uncovered you would get flies gathering around the opening and subsequently maggots and flies. Obviously, the rotting meat that had been hanging in the sun all day was the source of the flies.
After this experiment, people were willing to acknowledge that flies and other "larger" organisms didn’t arise via spontaneous generation, but had to have parents.
With the development and refinement of the microscope in the 1600s, people began seeing all sorts of new life forms - many microscopic and had not been seen before. No one knew from where these organisms came, but people figured out they were associated with things like spoiled broth. This seemed to add new evidence to the idea of spontaneous generation -- it seemed perfectly logical that these minute organisms should arise spontaneously. After all if they did not arise spontaneously where would they have come from?
In 1745 - 1748, John Needham, a Scottish clergyman and naturalist showed that microorganisms flourished in various soups that had been exposed to the air. He claimed that there was a "life force" present in all inorganic matter, including air and the oxygen in it, that could cause spontaneous generation to occur, thus accounting for the presence of bacteria in his soups. He even briefly boiled some of his soup and poured it into "clean" flasks with cork lids, and microorganisms still grew there.
A few years later (1765 - 1767), Lazzaro Spallanzani, an Italian abbot and biologist, tried several variations on Needham’s soup experiments. First, he boiled soup for one hour, then sealed the glass flasks that contained it by melting the mouths of the flasks shut. Soup in those flasks stayed sterile. He then boiled another batch of soup briefly before sealing the flasks, and found that microorganisms grew in that soup. In a third batch, soup was boiled for an hour, but the flasks were sealed with real-cork corks (which, thus, were loose-fitting enough to let some air in), and microorganisms grew in that soup.
Spallanzani concluded that while one hour of boiling would sterilize the soup, only a few minutes of boiling was not enough to kill any bacteria initially present, and the microorganisms in the flasks of spoiled soup had entered from the air.
Needham claimed that Spallanzani’s sterilizing the containers had killed the "life force." He felt that bacteria could not develop (by spontaneous generation) in the sealed containers because the life force could not get in, but in the open container, the broth soured because it had access to fresh air, hence the life force inherent in its molecules, which contained and replenished the life force needed to trigger spontaneous generation. In the minimally-boiled flasks, he felt the boiling was not severe enough to destroy the life force, so bacteria were still able to develop.
By 1860, the debate had become so heated that the Paris Academy of Sciences offered a prize for any experiments that would help settle the matter once and for all. The prize was claimed in 1864 by Louis Pasteur, as he published the results of an experiment he did to disproved spontaneous generation in these microscopic organisms. To test his ideas Pasteur boiled broth in various-shaped flasks to sterilize it, and then let it cool. As the broth and air in the containers cooled, fresh room air was drawn into the containers. None of the flasks were sealed -- all were exposed to the outside air in one way or another.
From the work of these early Biologists and others who expanded upon their discoveries the cell theory was developed.
The Cell Theory:
1. All Living Organisms are composed of one or more cells
2. Cells are the basic units of structure and function in all organisms.
3. All Cells are derived from Pre-Existing Cells.
4. In a multicellular organism, the activity of the organism depends upon the total activity of its independent cells.
Other notable discoveries.
Another new world of extraordinary variety, that of microorganisms, was revealed by Anton van Leeuwenhoek (1632-1723).
The first description of the cell though is generally attributed to Robert Hooke (1635-1702), an English physicist who was also a distinguished microscopist.
In 1838, the botanist Matthias Jakob Schleiden (1804-1881) suggested that every structural element of plants is composed of cells or their products. The following year, a similar conclusion was made for animals by the zoologist Theodor Schwann (1810–1882). He stated that "the elementary parts of all tissues are formed of cells" and that "there is one universal principle of development for the elementary parts of organisms... and this principle is in the formation of cells".
The conclusions of Schleiden and Schwann are considered to represent the official formulation of 'cell theory' and their names are almost as closely linked to cell theory as are those of Watson and Crick with the structure of DNA.