Wize AP Biology Textbook > The Plasma Membrane

Endosymbiont Theory

Popular Courses

AP Exam Prep

0:00 / 0:00

Endosymbiont Theory
The endosymbiont theory is an evolutionary theory that suggests several key organelles in eukaryotes (such as mitochondria and chloroplasts) were taken inside another cell to function in the host. It is believed that some prokaryotes were able to phagocytose ("eat") others. The prokaryotes that were eaten up were able to continue living inside the phagocytic cell, giving rise to these organelles.

Photo by Kelvinsong / CC BY
Evidence supporting that mitochondria and chloroplast have prokaryotic origins:
Both organelles proliferate through a process similar to binary fission seen in prokaryotes;
Mitochondria and chloroplast have similar morphologies to bacteria and their genome/DNA is in the form of a circular plasmid, much like bacteria, that can act independently of the nuclear DNA found in the cell.
Endosymbionts (organisms that live within the body or cell of another organism) are naturally occurring in  modern time. 

Example: Mixotricha paradoxa is a protist living within the digestive tract of  termites. It contains endosymbiotic bacteria instead of mitochondria.

Example: Mitochondria
Why are mitochondria thought to come from prokaryotic origin? Name 2 reasons.

This is because they are membrane bound, they produce their own energy and they have their own DNA. They go through their own divisions that replicate their DNA (This process is called Binary Fission).

Practice: Support of Endosymbiont Theory
Which of the following supports endosymbiont theory?

A) Mitochondria lack their own DNA

B) Ribosomes are completely different in prokaryotes and eukaryotes

C) Mitochondria are of similar size to bacteria

D) None of the above

I don't know

Practice: Origin of Mitochondria
According to the endosymbiont theory of the origin of eukaryotic cells, how did mitochondria originate?

A) From the endoplasmic reticulum of the first eukaryotic cells

B) From an engulfed prokaryotic cell that was anaerobic

C) From an engulfed prokaryotic cell that lacked most of the DNA it needed to live on its own

D) From an engulfed prokaryotic cell that was an aerobic heterotroph

E) From an engulfed chloroplast that was modified to help oxidize glucose

I don't know

Practice: Endosymbiosis
Which of the following statements are correct regarding the evidence supporting the theory of endosymbiosis?

A. The DNA found in mitochondria and chloroplasts share many sequence homologies with each other.
B. The plasma membranes of chloroplasts and mitochondria has a lipid monolayer just like in Archaea.
C. The chromosomes in most mitochondria and chloroplasts is circular.
D. Mitochondria and chloroplast divide similarly to their proposed prokaryotic ancestors

I don't know