The Endosymbiotic Theory
February 18, 2004

Since we have just finished photosynthesis and respiration...
I. A theory on the Origins of Eukaryotic Cells: Mitochondria and Chloroplasts

There are a great many differences between Prokaryotic cells and Eukaryotic cells in size, complexity, internal compartments, as you well remember from Cells and Organelles, January 21st, 2004

 

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!

 

Based on these similarities, could you come up with a hypothesis about the possible ancestors of mitochondria and chloroplasts? Talk it over!

 

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

Her hypothesis originally proposed that:

• mitochondria are the result of endocytosis of aerobic bacteria
• chloroplasts are the result of endocytosis of photosynthetic bacteria
• in both cases by large anaerobic bacteria who would not otherwise be able to exist in an aerobic environment.
• this arrangement became a mutually beneficial relationship for both cells (symbiotic).

Why would this arrangement have been favorable? 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.

• 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.
• HOWEVER, 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 waste product, 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.
• Aerobic bacteria 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'.
• Aerobic eukaryotes appear in fossil record shortly after that, (~1.5 billion years ago). These cells contain either mitochondria, or both mitochondria and chloroplasts. The mitochondria and chloroplasts have their own genomes, but can not live outside their host cell. [Image]

A few last words:

1) Does the endosymbiotic theory prove that mitochondria and chloroplasts are direct descendants of bacteria? NO! A theory does not prove, but provides an explanation for a group of observations. Can you think of another theory that would better explain the above group of observations? (and is it testable?)

Interesting sidenote: Theories can also be predictive - for instance, when Lynn Margulis proposed the endosymbiotic theory in the early 1970s, she predicted (but did not investigate herself) that IF mitochondria and chloroplasts really were engulfed bacterial endosymbiotes, THEN there should be evidence of bacteria DNA within those organelles...and sure enough, that is what researchers independently found a few years later! Way cool?! I think so! :)

2) Does the presence of an atmosphere full of free oxygen 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.

3) Some weird & wild eukaryotes: A missing link between proks and euks? 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! WEIRD! Stay away from these guys!

Cryptosporidium parvum , Giardia intestinalis

Objectives:

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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