Name: _______________________________________ Date: ____________ Class:___________________
The Endosymbiotic Theory
Imagine if you ate a peanut, but the peanut was not digested; instead, the peanut lived and reproduced in
your body. This peanut seed multiplied, making more peanut plants, spreading all over inside of you. Your
skin turns green and you suddenly have the ability to absorb sunlight and convert this light into food.
Scientists believe that
something like this happened 2 billion
years ago to ancient single-celled
organisms. At this time, there were
only bacteria living on earth.
Remember, bacteria are prokaryotes,
this means that they have no
organelles, no nucleus, and have
circular DNA. Also, prokaryotes are
small, single-celled organisms. There
is great diversity in bacteria and always
has been. Some bacteria make energy
from sunlight by carrying out
photosynthesis. Some do other simple
chemical reactions to gain energy and stay alive.
It is believed that this first happened 2 billion years ago, when a large cell engulfed a bacterium, but
rather than digesting it for food, the bacterium lived inside the cell. The bacteria began to provide energy and
other chemical products for the cell. The bacteria also received protection from the outside environment
because it was in a safe, consistent environment inside the cytoplasm of the cell. The cell and the bacteria
formed a symbiotic relationship. A
symbiotic relationship is a relationship
between two organisms where they usually
both benefit. In this symbiotic relationship
the bacteria received protection and the cell
received energy and chemical products.
The relationship was so successful
that when the cell reproduced, the bacteria
inside it reproduced and the offspring had these helpful bacteria inside the new cells. These cells had a huge
advantage because of their helper bacteria living inside them. As a result, they spread all over ancient Earth.
This idea is called the Endosymbiotic Theory. The prefix “endo-”
means “inside” and the word “symbiotic” explains the helpful
relationship that the bacteria have to the larger cell.
Scientists believe that two organelles found in eukaryotes
came from large cells eating bacteria in the way the
Endosymbiotic Theory describes. One of the organelles is
mitochondria, which provides the cell with energy by breaking
down sugar for the cell. Mitochondria are found in all eukaryotes.
The other organelle is the chloroplasts found only in plant cells.
Chloroplasts convert sunlight into food.
The protobacteria Rickettsiales has much in common with
mitochondria and it is believed to be the ancestor of the
mitochondria found in eukaryotes. There are bacteria that carry
out photosynthesis called cyanobacteria, and these are believed to
be the ones that led to the chloroplasts that plant cells now have.
At first, the idea of Endosymbiotic Theory seems too mind-boggling
to be true, but there is a great deal of evidence. These two
organelles mitochondria and chloroplasts are unique, and have much
in common with their suspected prokaryotic, bacterial ancestors.
First of all, mitochondria and chloroplasts are about the same size as
bacteria. Also, mitochondria and chloroplasts have their own DNA
and ribosomes! The DNA of mitochondria is circular, just like it is
in all prokaryotic bacteria.
The DNA has the code for
proteins that are very similar
in structure to those of
Rickettsiales and
cyanobacteria and other
bacterial relatives. The two
organelles also reproduce
when the eukaryotic cell
divides. Mitochondria and chloroplasts reproduce by binary fission, the
very same process that bacteria use when they reproduce. Additionally,
mitochondria and chlorophyll are the only organelles with a double
membrane. If the mitochondria or chloroplasts in a cell are destroyed,
they cannot be remade. They are their own living cells and the
eukaryotic cell can’t just re-make them from its DNA. The eukaryotic
cell has a membrane surrounding the organelle, and then the organelle
has its own membrane. This is called a double membrane. Mitochondria
and chloroplasts live inside eukaryotic cells, but do many of the same
Lynn Margulis came up with
things and have similar appearance to bacteria living on their own. All
the Endosymbiotic Theory.
of this evidence supports the Endosymbiotic Theory because it suggests that
She had trouble getting her
mitochondria and chloroplasts are bacteria and along time ago they did live
idea accepted at first, but her
on their own.
perseverance paid off. Now
Scientists now believe that 2 billion years ago, when a large cell ate
the theory it is widely
some bacteria, eukaryotes were created. The bacteria and the large cell
believed by scientists.
worked so well together than they spread all over the Earth. The scientist to
propose this idea was met with huge disapproval and ridicule at the time.
Lynn Margulis spent years doing research and wrote a book detailing the evidence for her theory. Publishers
mocked her idea and refused to publish her work. So, she got in touch with other scientists and proposed her
idea to them. Within a few years, a university had published her work and Endosymbiotic Theory was widely
approved in the scientific community.
Now eukaryotes have become very complex with many organelles, but it is believed that mitochondria
and chloroplasts originated from the Endosymbiotic Theory. So, within each and every one of our cells is a
huge population of bacteria, helping us to survive, and we’re helping them survive.
Name: _______________________________________ Date: ____________ Class:___________________
The Endosymbiotic Theory
Article Questions
Answer the following questions by writing one or two complete sentences.
1.
What do the parts of the phrase “Endosymbiotic Theory” mean? Why are they called that?
2.
What was life like on Earth 2 billion years ago? What kinds of organisms were around?
3.
Do you think that humans could survive on the Earth like it was 2 billion years ago? Why or why not?
4.
If endosymbiosis had not happened, would be some organisms that wouldn’t be around today? List at least 4.
Extended Answer Questions.
Answer these questions by writing one short paragraph.
5.
What evidence is there that supports the Endosymbiotic Theory? Provide at least 4 specific pieces of evidence.
6.
Think about the kinds of organisms that were created as a result of endosymbiosis. Why do you think that they were able to
evolve into more complex multi-cellular organisms, like plants, animals, and eventually into humans? Why do you think
prokaryotes were not able to do this?
Name: _______________________________________ Date: ____________ Class:___________________
The Endosymbiotic Theory
Imagine if you ate a peanut, but the peanut was not digested; instead, the peanut lived and reproduced in your
body. This peanut seed multiplied, making more peanut plants, spreading all over inside of you. Your skin turns green
and you suddenly have the ability to absorb sunlight and convert this light into food.
Scientists believe that something
like this happened 2 billion years ago to
ancient single-celled organisms. At this
time, there were only bacteria living on
earth. Remember, bacteria are
prokaryotes, this means that they have no
organelles, no nucleus, and have circular
DNA. Also, prokaryotes are small, singlecelled organisms. There is great diversity
in bacteria and always has been. Some
bacteria make energy from sunlight by
carrying out photosynthesis. Some do
other simple chemical reactions to gain
energy and stay alive.
It is believed that this first
happened 2 billion years ago, when a large cell engulfed a bacterium, but rather than digesting it for food, the bacterium
lived inside the cell. The bacteria began to provide energy and other chemical products for the cell. The bacteria also
received protection from the outside environment because it was in a safe, consistent environment inside the cytoplasm of
the cell. The cell and the bacteria formed a symbiotic relationship. A symbiotic relationship is a relationship between two
organisms where they usually both benefit. In this symbiotic relationship the bacteria received protection and the cell
received energy and chemical products.
The relationship was so successful that when the cell reproduced, the bacteria inside it reproduced and the
offspring had these helpful bacteria inside the new cells. These cells had a huge advantage because of their helper
bacteria living inside them. As a result, they
spread all over ancient Earth. This idea is called
the Endosymbiotic Theory. The prefix “endo-”
means “inside” and the word “symbiotic” explains
the helpful relationship that the bacteria have to
the larger cell.
Scientists believe that two organelles
found in eukaryotes came from large cells eating
bacteria in the way the Endosymbiotic Theory describes. One of the
organelles is mitochondria, which provides the cell with energy by
breaking down sugar for the cell. Mitochondria are found in all
eukaryotes. The other organelle is the chloroplasts found only in plant
cells. Chloroplasts convert sunlight into food.
The protobacteria Rickettsiales has much in common with
mitochondria and it is believed to be the ancestor of the mitochondria
found in eukaryotes. There are bacteria that carry out photosynthesis
called cyanobacteria, and these are believed to be the ones that led to the
chloroplasts that plant cells now have. At first, the idea of Endosymbiotic
Theory seems too mind-boggling to be true, but there is a great deal of
evidence. These two organelles mitochondria and chloroplasts are unique,
and have much in common with their suspected prokaryotic, bacterial
ancestors. First of all, mitochondria and chloroplasts are about the same
size as bacteria. Also, mitochondria and chloroplasts have their own DNA
and ribosomes! The DNA of mitochondria is circular, just like it is in all
prokaryotic bacteria. The DNA has the code for proteins that are very
similar in structure to those of
Rickettsiales and cyanobacteria and
other bacterial relatives. The two
organelles also reproduce when the
eukaryotic cell divides. Mitochondria
and chloroplasts reproduce by binary
fission, the very same process that
bacteria use when they reproduce.
Additionally, mitochondria and
chlorophyll are the only organelles with
a double membrane. If the
mitochondria or chloroplasts in a cell
are destroyed, they cannot be remade.
They are their own living cells and the
eukaryotic cell can’t just re-make them
from its DNA. The eukaryotic cell has
a membrane surrounding the organelle, and then the organelle has its own
membrane. This is called a double membrane. Mitochondria and chloroplasts live
inside eukaryotic cells, but do many of the same
things and have similar appearance to bacteria living on their own. All of this
Lynn Margulis came up with the
evidence supports the Endosymbiotic Theory because it suggests that mitochondria
Endosymbiotic Theory. She had
and chloroplasts are bacteria and along time ago they did live on their own.
trouble getting her idea
Scientists now believe that 2 billion years ago, when a large cell ate some
accepted at first, but her
bacteria, eukaryotes were created. The bacteria and the large cell worked so well
perseverance paid off. Now the
together than they spread all over the Earth. The scientist to propose this idea was
theory it is widely believed by
met with huge disapproval and ridicule at the time. Lynn Margulis spent years
scientists.
doing research and wrote a book detailing the evidence for her theory. Publishers
mocked her idea and refused to publish her work. So, she got in touch with other
scientists and proposed her idea to them. Within a few years, a university had published her work and Endosymbiotic
Theory was widely approved in the scientific community.
Now eukaryotes have become very complex with many organelles, but it is believed that mitochondria and
chloroplasts originated from the Endosymbiotic Theory. So, within each and every one of our cells is a huge population
of bacteria, helping us to survive, and we’re helping them survive.
Discussion Questions
1.
2.
3.
4.
Discuss the answers to the following questions with your group.
What do the parts of the phrase “Endosymbiotic Theory” mean? Why are they called that?
What was life like on Earth 2 billion years ago? What kinds of organisms were around?
Do you think that humans could survive on the Earth like it was 2 billion years ago? Why or why not?
If endosymbiosis had not happened, would be some organisms that wouldn’t be around today? List at least 4.
For your Notebook


Title a section of your notebook, “The Endosymbiotic Theory.”
Make the below table in your notebook and complete it. In the first column, write in pieces of evidence that support the
Endosymbiotic Theory. Include at least 4 separate pieces of evidence. In the second column, challenge the pieces of evidence
that you wrote in the first column with an alternative hypothesis that would undermine the Endosymbiotic Theory. You
creatively come up with these alternative hypotheses. Then, in the third column, explain which position you believe and why.
Endosymbiotic Theory Evidence
Challenges and Alternative
Hypotheses
Which do you believe and why?