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Endosymbiotic theory is an emerging theory explaining how eukaryotic cells originated. Lynn Margulis, an unconventional and undiscouraged scientist, is the founder of this theory. Initially, many scientists did not accept this theory into mainstream biology. Many people dismissed her as a radical, and her work never entered mainstream biology until recently. Margulis indicated that million years ago, chloroplasts and mitochondria were once free-living prokaryotes. These prokaryotes became engulfed by larger free-living cells that did not digest them. These two organisms are associated symbiotically, each benefiting from the other. The word ‘endo’ refers to Marguli’s proposition that the smaller cells (bacteria) got into, the larger cells by means of endocytosis. She postulates that both the large and the small cells had a single membrane. However, as the engulfment occurred, the formation of a second membrane took place. The smaller cell (bacteria) formed the inner membrane while the larger cell formed the outer membrane. These two cells lived in a symbiotic relationship, with the large cell providing ‘housing’ for the small cells as they offered chemical nourishment in return. Margulis argued that these small cells later evolved into mitochondria and chloroplast, each with a special function.
Unfortunately, Margulis could not get other scientists into believing her observations. However, recent studies have supported Marguli’s studies by confirming that there is evidence linking mitochondria and chloroplast to bacteria. For instance, it is now clear that both chloroplast and mitochondria have their own DNA that is circular, just like bacteria DNA. Moreover, these organelles have ribosomes almost similar to that of bacteria that enable them to synthesize their own proteins. There is evidence that these two organelles divide by simple binary fission, just like bacteria.
My measurements of the sizes of some present-day eukaryotic cells and bacteria will indicate that a bacterium can easily “fit” inside the larger cell and, therefore, will provide support for the endosymbiotic theory.
Materials needed to carry out this experiment include a computer with access to the internet and a paper edge.
Go to and open CELLS alive! Homepage. Click on the subject heading “cell Models.” Click on the topic entitled “take me to the animation.” Choose an animal cell diagram first, use the listing of cell constituents to quiz yourself if you can name the organelles, and remember their function. Then click on the plant cell and repeat the self-test. To confirm the answers for the self-test, click on the organelles to see the name of the organelle and its function. Next, click on the bacterial cell diagram and repeat the self-test here. Give particular emphasis on the location, structure, and function of the organelles.
Go back to the opening page and click the topic titled How Big? Familiarize yourself with the description of this page before going any further. Click on “start the animation,” realize the differences in cell sizes, and take some measurements of different cells and organelles. On the ‘How Big?’ page, scroll to the furthest below section of the page and brush up on the information regarding nanometer, millimeter, and micron units of measurements. Scroll back to the photograph and observe the use of arrows to decrease and increase the size of the photograph. Increase the photograph to the uttermost size and reduce it to the minimum size as you familiarize yourself with how the animation works. Go to the right side of the photograph and click on the name of each item on that list from top to bottom. Take several items and measure them using a bar of a given length appearing on the photograph. Use a paper edge to find out the real measurements of the item selected. Place the paper edge on the length of the bar, and then place this crude ruler across the picture of the selected item and take measurements.
- Staphylococcus at 1.0 x 104 measured approximately 1.9 micrometers
- Lymphocyte at 1.0 x 103 measured 19 micrometers
- E. coli at 1.0 x 104 measured approximately 1.9 micrometers
Can the sizes of certain present-day cells and cell components be used to support the endosymbiotic theory?
There is enough evidence in contemporary times that present-day cells like bacterial cells and organelles like mitochondria and chloroplast may be used to support the endosymbiotic theory. For instance, this experiment indicates that a prokaryote cell can ‘fit’ well into a eukaryotic cell. These specimens were taken in modern times.
From the results obtained, it is clear that bacterial cells (prokaryotes) can ‘fit’ in eukaryotic cells. In this case, lymphocytes (eukaryotic cells) measured approximately 0.000019 meters, while staphylococcus (prokaryote) measured 0.00000019 meters at the same magnification. This implies that a eukaryotic cell is a hundred times larger than a prokaryote cell. This is in line with the endosymbiotic theory. These results qualify the hypothesis given above.
Lynn Margulis did an outstanding job concerning endosymbiotic theory. Even though there are numerous differences between eukaryotes and prokaryotes, Margulis chose to focus on the similarities between the two. She found out that eukaryotes originated from a symbiotic relationship between large and small prokaryotes. However, Marguli’s success did not come o a silver plate. Her work faced rejection from other mainstream scientists. For instance, in 1967, she wrote a paper titled “Origin of Mitosing Cells.” This paper received immediate rejection by over 20 publishers. Scientists dismissed her as radical; consequently, her work on endosymbiotic theory never entered mainstream biology until recently. Presently, there is enough data to support the endosymbiotic theory. Firstly, it is now clear that both chloroplast and mitochondria have their own DNA that is circular, just like bacteria DNA. Again, these organelles have ribosomes almost similar to that of bacteria that enable them to synthesize their own proteins. There is evidence that these two organelles divide by simple binary fission, just like bacteria. I like Marguli’s persistence. Despite all the criticisms and rejection, she remained undeterred and focused, had faith in what she believed, and as a consequence, we have the endosymbiotic theory widely accepted today. However, I wouldn’t say I like her idea of foregoing family for the sake of studies. I believe we can always strike a balance between family life and career.
The data collected indicated that a bacterial cell could readily get into a larger cell, thus provided enough backup to the endosymbiotic theory. Thus, the hypothesis is qualified.
The findings in this experiment are very significant with regard to endosymbiotic theory. The far-reaching implications here are that it may be possible that eukaryotes originated from a symbiotic relationship between large prokaryotic cells and smaller ones. Given the observation that eukaryotes are a hundred times larger than bacterial cells, the endosymbiotic theory may be true.
There were few sources of error, given the nature of the experiment. The error could have occurred in making the metric measurement. The primitive ruler may not give accurate measurements.