Endosymbiosis Case Study Questions KEY:
1. Amoebae are single celled eukaryotes that are in the Kingdom Protista. A
bacterium is also single celled but is a prokaryote. Prokaryotes do not have any
membrane bound organelles or a nucleus.
2. Students may have a heard time coming up with two ideas. One hypothesis will
come from the story, that the amoeba “fought off the infection.” Talk with
students about what this means. Discuss how cells engulf large particles by
phagocytosis and perform intracellular digestion with lysosomes. Another
hypothesis will be that the amoeba engulfed the bacterium and the bacterium are
alive with the amoebae. Students may come up with other ideas, such as the
bacterium died on their own.
3. Antibiotics are designed to kill bacteria by exploiting the different characteristics
between prokaryotes and eukaryotes. For example, some antibiotics destroy the
peptidogylcan in the bacterial cell wall. Others affect protein production, since
prokaryotic ribosomes are different enough from eukaryotic ribosomes. Others
affect DNA replication of prokaryotes, since the DNA replication proteins are
different enough between the two cell types.
4. Since antibiotics are designed to kill prokaryotic cells and not eukaryotic cells,
this is unexpected. Even if the antibiotic killed the bacteria within the amoebae,
the amoeba should be left unaffected, unless they had become dependent on the
bacteria living within. You may choose to discuss parasitism vs. mutualism here.
5. Brian added antibiotic to his amoebae that were infected with bacteria. It was
surprising that the amoeba died. As a control, he should add antibiotic to
uninfected amoebae to make sure the antibiotic was not acting as an amoeba
poison. He would expect this application of antibiotic to have no effect on normal
amoebae. He should also have a control in which some infected amoebae do not
receive antibiotic, to ensure that some other variable in the environment was not
killing off amoebae in his lab.
6. Lynn Margulis first proposed the endosymbiosis hypothesis. Endosymbiosis
provides explanation for how mitochondria and chloroplasts may have originated
as independent bacteria that were then engulfed by larger eukaryotic cells. Brian’s
results do provide evidence of endosymbiosis. Because his cells died when the
bacteria within were killed, it indicates that the bacteria were providing some
essential role for the amoebae. These results show that it is possible for cells to
live within cells and for them to become dependent on each other.
7. These three facts are more evidence, although indirect, for endosymbiosis. These
three facts show how similar mitochondria are to bacteria, thus suggesting that
mitochondria evolved from bacteria.
8. Sid was right to mention this because Paramecium bursaria is another example of
one cell living within another. In this case, photosynthetic algae cells live inside a
free-swimming Paramecium.
9. Brian should know that Kwang Jeon is a scientist that performed experiments like
Brian’s with amoebae and bacteria in the 1960s. His experiments provided early
evidence to support the endosymbiosis hypothesis.
