If you're seeing this message, it means we're having trouble loading external resources on our website.

If you're behind a web filter, please make sure that the domains *.kastatic.org and *.kasandbox.org are unblocked.

Main content
Current time:0:00Total duration:11:14

History and development of cell theory

Video transcript

all right so in order to describe cell theory I'm going to go ahead and tell you the story of how cell theory developed now over here on the right side of the screen you're going to notice a timeline that goes from the 1600s to the late 1800s and a couple boxes here and these boxes will fill in as the three major tenets of cell theories and they are correspond to the different periods in time in which these tenets were developed so and throughout this story I'm going to refer to examples of bacteria plants and animals to illustrate some of the points so the story starts off in the Netherlands in the 1600s with a scientist by the name Anton von Lewin oeq around this time the microscope was actually invented in the Netherlands and Van Leeuwen Hoek started using a microscope he actually invented his own version of the microscope and started looking at everything he could get his hands on under the microscope and so one day this curiosity brought him to look at the gunk on his teeth that's right he looked at his dental scrapings under the microscope and when he did so he noticed many small animal-like creatures that were moving around and one of them may have looked something like this and because he thought these kind of looked like little animals he named them Anna Michals and now of course we know that what he was looking at now were actually bacteria but at the time bacteria hadn't been discovered and for this discovery of these animals Anton van Lewin Hoek is frequently referred to as the father of modern microbiology now around the same time there was an English scientist by the name of Robert Hooke and hook also looked at all sorts of things under the microscope and this isn't probably how it actually happened but it's a fun story that I like to think to help remember remind myself of this I'd like to think that Robert Hooke was at a party one night and they decided to pop a couple bottles of champagne and perhaps one of the corks hit Robert Hooke in the head and he then grabbed that cork put it in his pocket and you know the next morning woke up and said I wonder what this cork would look like under a microscope so he sliced it thin and looked at it under a microscope and what he saw was something that looked like this and what he was seeing was the remnants of the cells in the plant that made the cork and he thought that these spaces kind of looked like the little dormitory rooms that monks lived in in a monastery and these rooms in Latin are known as cellular and so this is where the term cells came from and then similarly other scientists were also looking at animal tissues and they also noticed that in different animal tissues there were these similar cells as in bacteria and plants and from this they developed the first tenant of cell theory and that is that the cell is the basic unit of structure in life now over the ensuing years scientists looked at all continue to look at all sorts of things under the microscope and in regards to bacteria what they discovered was that there were all sorts of bacteria ones like the one we saw above here the Lorca maybe kind of rod-shaped or there was bacteria that were spiral-shaped and there's bacteria that were more spherical and what they discovered was that regardless of the shape of these bacteria they all had the same cell as their structure this our basic unit of structure so moving on to plants in the 1830s there was a German botanist by the name of matthias schleiden and like I said schleiden was a botanist and he also was interested in microbiology so he looked at all sorts of plants underneath a microscope and what he realized was it didn't matter whether he was looking at a specific type of flower or if he was looking at a maple tree all the different plants he looked at at the same microscopic structure that Robert Hooke had described a couple hundred years earlier of these cells then at the same time another German scientist by the name of Theodor Schwann was looking at the nervous systems of different animals and what he realized was it didn't matter if he was looking at a human or a cat no matter what type of animal they all had a similar structures that were these cells and luckily for microbiology slide-in and Shrew in nineteen or excuse me in 1837 or actually add a dinner party together and they began discussing each other's work and I got imagine a light bulb just went off in both of their heads and they really discovered you know if all plants are made of cells and all animals are made of cells it must be that all living organisms are made of cells and the next year in 1838 Schwann published and in this book he stated this finding that all living things are composed of cells now this was all well and good because at the time no one really argued that all living things were made of cells however scientists didn't really know where the cells actually came from through observations some of this was known for instance it was known for thousands of years that if a female and a male mated you know over some period of time afterwards they would have an offspring and so it was generally accepted that animals came from animals and similarly in plants it was well known that a tree or a plant say this one right here would produce a seed and that seed could then be planted in the ground and it would then produce a tree or a plant that looked similar to the one that produced the seed so it was pretty well accepted that plants came from plants however in bacteria it wasn't really clear where they came from and the predominant theory at the time was something known as abiogenesis so let me just erase some of our work here and in order to describe this theory of abiogenesis now abiogenesis is actually a very old theory it dates as far back as the 4th century BC with Aristotle and up to the 1800s this was really the predominant theory of how life came into existence so let's just kind of describe abiogenesis here briefly imagine you have let's say a rock this is a rock and what they thought what's up but scientists thought at the time was that there was some unknown substance in the air and that it would combine with these non living materials such as a rock whatever it might be and from that it would produce life the mechanism of this area was thought to be that of spontaneous generation the idea that life was spontaneously made from non-life now in the late 1800s the scientists who were studying cells began to refute this theory and one such scientist was the German physician and pathologist Rudolf Virchow now Virchow was using his microscope to look at cells and what he observed was that some bacteria if you watch them at the right point of time they actually divided and formed two bacteria that were identical at least in appearance to the first bacteria now today we call this mechanism binary fission and it's the means by which bacteria reproduce by the time that wasn't known and so Virchow just observed this and he published this he published in a very famous work a phrase in which he stated omnis cellula is cellula which roughly translated from latin means every cell originates from a cell like it so it's this idea that cells are not made by spontaneous generation but cells produce cells and it is important to note that although Virchow is generally given credit for this theory the phrase omnis cellula celluloid earlier by a french physician by the name of francois vincent raphael and this theory of you know the binary fission was actually likely plagiarized from robert Rumaki a Polish physiologist so ferchaux although we didn't come up with the idea or the term he's still for some reason generally given credit but virtually he received a lot of criticism for this theory and other scientists who believed in this a biogenesis you know they stated or this idea that cells produced cells that might be true but it doesn't by any means disprove abiogenesis i mean both of these mechanisms could be going on in the world and so this is where the very famous French scientist Louis Pasteur enters our story in the 1860's now during this time louis pasteur did a very famous experiment known as the swan --neck bottle experiment which kind of finally laid to rest this theory of abiogenesis so before I describe the swan --neck bottle experiment let me you describe an experiment that some of the proponents of abiogenesis used it's evidence for their theory so imagine you have a flask and you fill this flask with some broth and the broth may have some bacteria in it well it was known that if they boiled the broth they heated up and boiled it that would kill the bacteria you could sterilize the broth in this flask and then over time if the flask was left alone what would happen is that bacteria would start to grow in it you'd get this growth and proponents of abiogenesis said you know see nothing was added to the flask and yet bacteria was formed it must have been formed through spontaneous generation as no living organisms were added to it however peshto disagreed with this and so he came up with his own flask known as a swan --neck bottle it looks something like this and this swan neck flask was also filled with broth and sterilized by boiling it and what Louis Pasteur theorized was that there was actually maybe some bacteria or living particles in the air that would fall into the broth here and that is what caused the growth so by creating this swan --neck he was still allowing the broth to be exposed to air and this potential unknown substance in the air but if there were any microorganisms in the air that fell in they would be collected in this curve of the neck and they wouldn't reach the broth and lo and behold over time there was no growth like I mentioned earlier this kind of once and for all put to rest this theory of abiogenesis and really established the third tenet of cell theory and that is that all cells come from pre-existing cells so this is the story of cell theory and how we how scientists over a period of 200 years really discovered the three major tenets of cell theory that are really the foundation of much of modern biology and these are that the cell is the basic unit of structure and life and that all living organisms are composed of cells and that all cells come from pre-existing cells