The Endosymbiotic Theory
Worksheet, January 14, 2002

I. A theory on the Origins of Eukaryotic Cells: Mitochondria and Chloroplasts

There are a great many differences between Eukaryotic cells and Prokaryotic cells in size, complexity, internal compartments.
However, there is a curious similarity between prokaryotic cells and the organelles of eukaryotic cells. Some of these similarities were first noted in the 1880s, but were largely ignored for almost a century!
  Prokaryotes Eukaryotes Mitochondria of
Eukaryotic cells
Chloroplasts of Photosynthetic eukaryotes
DNA 1 single, circular chromosome Multiple linear chromosomes
compartmentalized in a nucleus
1 single, circular
1 single, circular chromosome
Replication Binary Fission
(1 cell splits into 2)
Mitosis Binary Fission
(1 cell splits into 2)
Binary Fission
(1 cell splits into 2)
Ribosomes "70 S" "80 S" "70 S" "70 S"
Electron Transport Chain Found in the plasma membrane around cell Not found in the plasma membrane
around cell (found only in the cell's mitochondria and chloroplasts)
Found in the plasma membrane around mitochondrion Found in the plasma membrane around chloroplast
Size (approximate) ~1-10 microns ~50 - 500 microns ~1-10 microns ~1-10 microns
Appearance on Earth Anaerobic bacteria:
~3.8 Billion years ago
Photosynthetic bacteria:
~3.2 Billion years ago
Aerobic bacteria:
~2.5 Billion years ago
~1.5 billion years ago ~1.5 billion years ago ~1.5 billion years ago
Based on these similarities, could you come up with a hypothesis about the possible ancestors of mitochondria and chloroplasts? Talk it over, and list it here:


II. The Endosymbiotic Theory ... first postulated by Lynn Margulis in the 1967.

The Endosymbiotic Theory was first proposed by former Boston University Biologist Lynn Margulis in the 1960's and officially in her 1981 book "Symbiosis in Cell Evolution".  Although now accepted as a well-supported theory, both she and the theory were ridiculed by mainstream biologists for a number of years.  Thanks to her persistance, and the large volumes of data that support this hypothesis gathered by her and many other scientists over the last 30 years, biology can now offer a plausible explanation for the evolution of eukaryotes. [Quote]

Dr. Margulis was doing reserarch on the origin of eukaryotic cells. She looked at all the data about prokaryotes, eukaryotes, and organelles. She proposed that the similarities between prokaryotes and organelles, together with their appearance in the fossil record, could best be explained by "endo-symbiosis".

[Endo = "within"]
= (cyto = cell) a process of 'cell eating' - cells are engulfed, but then usually digested as food....]
= cells are engulfed, but not digested...cells live together is a mutually benefitting relationship, or symbiosis]

Her hypothesis originally proposed that:

Margulis' original hypothesis proposed that aerobic bacteria (that require oxygen) were ingested by anaerobic bacteria (poisoned by oxygen), and may each have had a survival advantage as long as they continued their partnership.

Each would have performed mutually benefiting functions from their symbiotic relationship.  The aerobic bacteria would have handled the toxic oxygen for the anaerobic bacteria, and the anaerobic bacteria would ingested food and protected the aerobic "symbiote"..

The result = a cell with a double-membrane bound organelle. The inner lipid bilayer would have been the bacterial cell's plasma membrane, and the ouler lipid bilayer came from the cell that engulfed it.

Other evidence that supports this hypothesis:

1. . The timeline of life on Earth:

a. Anaerobic bacteria: Scientists have fossil evidence of bacterial life on Earth ~3.8 billion years ago. At this time, the atmosphere of the Earth did not contain oxygen, and all life (bacterial cells) was anaerobic.

b. Photosynthetic bacteria
: About ~3.2 billion years ago, fossil evidence of photosynthetic bacteria, or cyanobacteria, appears. These bacteria use the sun's energy to make sugar. Oxygen, released as a byproduct, began to accumulate in the atmosphere. However, oxygen is actually pretty toxic to cells, even our cells! As a result, anaerobic cells were now a disadvantage in an oxygen-containing atmosphere, and started to die out as oxygen levels increased. [Image]

c. Aerobic cells appear in the fossil record shortly after that (~2.5 Billion years ago). There cells were were able to use that 'toxic' oxygen and convert it into energy (ATP) and water. Organisms that could thrive in an oxygen-containing atmosphere were now 'best suited to the environment'.

2. Organelles have their own DNA, and divide independently of the cell they live in: When Margulis initially proposed the Symbiotic Theory, she predicted that, if the organelles were really bacterial (prokaryotic) symbionts, they would have their own DNA.  If her theory DID best explain the origin of eukaryotic cells, she reasoned, organelles would have DNA that resembled bacterial DNA and be different from the cell's DNA (located in the nucleus membrane).  Amazingly, in the 1980's this was proven to be the case for two classes of organelles, the mitochondria and chloroplasts.  Further, in the late 1980's a team of Rockefeller University investigators announced their similar discovery regarding centrioles, structures that provide the eukaryotic cell with the ability of locomotion and cell division. [Quote]

Thanks to over 30 years of additional evidence, The Endosymbiotic Theory provides the most plausible explanation for the development of organelles within the eukaryotic cell.

A few last words: Does this mean that there are no longer anaerobic organisms on the Earth? NO! THere are lots of places on Earth where anaerobic environments exist and MANY species of anaerobic bacteria! Your digestive system (and that of other mammals) relies on bacteria deep within your intestines..., plus anaerobes thrive under the ground, in swamps, in garbage dumps, under the ocean, etc. Anaerobic organisms are also responsible for human diseases like botulism, gangrene, and tetanus.

Some weird & wild Eukaryotes: Amitochondrial eukaryotes (no biochemical, molecular, or structural evidence for a mitochondrion): These primitive, 1-celled eukaryotes (protists) are anaerobic, have no mitochondria, and often have 2 nuclei per cell! Stay away from these guys!

Cryptosporidium parvum , Giardia intestinalis


1. Explain 5 similarities between mitochondria & chloroplasts and bacterial cells
2. Describe the benefits to both an aerobic cell and an anaerobic cell that may have allowed for the formation of a mutually benefitting relationship to occur.
3. Explain why there are 2 lipid bilayers around both mitochondria and chloroplasts

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