In Class Exercise 2:
The origins of chloroplasts and mitochondria in eukaryotic cells

What hypothesis would you suggest about the origin of organelles after reading the following pieces of information?

1. The outer membrane of the chloroplast and mitochindria is similar to the eukaryotic plasma membrane...but the inner membrane has lipids and proteins more commonly seen in prokaryotic plasma membranes.

 

2. Mitochondria and chloroplasts both have their own DNA, and it consists of a single, circular, chromosome...just like a bacterial chromosome

 

3. Mitochondria and chloroplasts have their own ribosomes...and their ribosomes have the same size and shape as prokaryotic ribosomes.

 

4. Only photosynthetic bacteria, aerobic bacteria, mitochondria, and chloroplasts have electron transport chain enzymes - these enzymes are NEVER found any where else in the eukaryotic cell.

 

5. Whereas the eukaryotic cell undergoes a specialized process of cell division called mitosis, mitochondria and chloroplasts divide independently of the rest of the eukaryotic cell by a simple mechanism of splitting in two called "binary fission"...or "bacterial reproduction".

 

6. Mitochondria and chloroplasts are about 3 microns or less in size...just like bacteria

 

With a partner or group, get together and discuss the evidence above, and think of a hypothesis that might best explain the evidence above. There may be more that one way the evidence could be explained

Hypothesis: mitochondria and chloroplasts came about in eukaryotic cells when.....

The Endosymbiotic Theory ... first postulated by Lynn Margulis in the 1970s.

Simply stated, the Endosymbiotic Theory postulates that mitochondria and chloroplasts are the result of endocytosis ("within cells") of bacteria and blue-green (photosynthetic) bacteria by anaerobic bacteria, which became mutually beneficial for both cells (symbiotic). Endocytosis + symbiosis = endosymbiotic = "living together - within cells"

The appearance of eukaryotic cells with double membrane-bound organelles appear for the first time in the fossil record right about the time the atmosphere on Earth changed from an anaerobic (no oxygen) environment to an aerobic (oxygen-containing) environment.

Her original hypothesis proposed that aerobic bacteria (that require oxygen) were ingested by anaerobic bacteria (poisoned by oxygen) each would have performed mutually benefiting functions from their symbiotic relationship as the waters and atmosphere of the Precambrian changed.  The aerobic bacteria would have "breathed" for the anareobic bacteria, and the anaerobic bacteria would have navigated through new oxygen-rich waters and ingested food.

Although now widely accepted as a valid Scientific Theory ("way up there" on the scale of certainty), both Dr. Margulis and her hypothesis were scoffed at for a number of years. 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.