Slide 1
The endosymbiotic theory states that mitochondria and chloroplasts are derived from endosymbiotic bacteria that were engulfed by a larger cell.
Mitochondria most likely arose from Rickettsiales (bacteria that are mainly known to live as endosymbionts - modern day representatives of these bacteria cause typhus and Rocky Mountain spotted fever)
Chloroplasts from cyanobacteria (photosynthetic bacteria)
Slide 2
The evidence for endosymbiosis includes:
That mitochondria and chloroplasts have a circular DNA "chromosome" that is the same size as a typical bacterial genome.
They also have two membranes - an and outer that resulted from being engulfed by a larger cell.
Mitochondria and chloroplasts divide by binary fission just like bacteria do.
In eukaryotic cells, nuclear DNA contains genes that probably came from proto-mitochondria and proto-plastids
Some proteins that are encoded in the nucleus are transported to mitochondria and chloroplasts
and the Ribosomes of mitochondria and chloroplasts are bacteria-like - unlike the ribosomes found in the rest of the eukaryotic cell.
Slide 3
Biologists have been able to observe endosymbiosis in the lab. Discovered in 2006, Hatena arenicola is an organism believed to be in the process of an endosybiosis. It is a eukaryote that behaves like a heterotrophic predator - until it ingests a certain species of green algae and then it becomes an autotroph.
Slide 4
Endosymbionts occur all throughout the world. The guts of termites are whacky places full of all kinds of strange organisms - both prokaryotes and eukaryotes.
Trichonympha - for example - helps termites to digest cellulose. However, they require bacterial endosymbionts to produce the enzyme that digest celluose. Without the bacterial endosymbiont they cannot digest cellulose. They also have bacterial endosymbionts that function as flagella and help the organism move through its environment.
Mixotricha is equally as strange. It also has bacterial endosymbionts that help it digest cellulose and function as flagella in motility.
It has additional bacterial endosymbionts that function as mitochondria - because mixotricha do not have their own mitochondria.
Our current secretary of energy Steven Chu is a nobel-prize winning physicist who believes the termite gut may provide insight into alternative energy sources. When these organisms digest celluose they produce ethanol - ethanol that can be used as a biofuel.
Slide 5
The evolution of eukaryotic cells via endosymbiosis implies that there was a selective advantage to this relationship. As an example, we will look at the evolution of the relationship that resulted in mitochondria.
To understand the evolution of this relationship we need to remember that conditions on early earth were very different than they are now.
About 3.5 billion years ago cyanobacteria were polluting the atmosphere with oxygen - oxygen is a by-product of the photosynthesis that was being performed by the cyanobacteria.
By 2 BYA the atmosphere was becoming aerobic-how could obligate anaerobes survive this toxic environment? Remember, many organisms are poisoned by oxygen so in an increasingly aerobic environment these organisms needed to find a way to protect themselves.
The answer was to get rid of oxygen by converting the oxygen to water - remember, this is what happens at the end of the electron transport chain in the mitochondria - oxygen is reduced to water.
Primitive eukaryotes formed symbioses with prokaryotes to protect themselves from the effects of oxygen. This increased the fitness of the primitive eukaryote due to the detoxification of oxygen.
So the ancestral function of the mitochondrion was to detoxify oxygen in the environment. Today the
more derived function of the mitochondrion is energy transduction - converting the energy of the electrons found in the NADH and FADH2 produced by glycolysis and the krebs cycle to energy in the form of ATP.