Endosymbiotic theory tries to explicate about the origins of cell organelles of eukaryotes such as mitochondria and chloroplasts. Endosymbiont theory was originally put forward by biologist L. Margulis in the 1960s. Mitochondria is usually well thought-out to have arisen from proteobacteria (order:Rickettsiales) by endosymbiosis. And Chloroplasts are generally thought to have arisen from cyanobacteria through endosymbiosis. Endosymbiosis has gained more favourable receptions nowadays especially with the relatively new advancements such as sequencing technologies.

The endosymbiont theory argues that the eukaryotic mitochodria evolved from a tiny, autotrophic bacterium that was ingested by a bigger primitive, heterotrophic, eukaryotic cell. This eukaryotic cell originated when an anaerobic prokaryote (not able to utilize oxygen for energy) lost its cell wall. The more elastic membrane beneath the cell wall then started to grow and fold up on its own, which in turn, led to formation of a nucleus and other internal membranes. Endosymbiosis occurred according to;

- The primordial eukaryotic cell was also finally able to engulf prokaryotes, an obvious development to absorbing small molecules from its environment.

- The progression of endosymbiosis occurred when the eukaryote ingested but did not digest the autotrophic bacterium. Proofs imply that this ingested bacterium (alphaproteobacteria), was an autotroph that utilizes photosynthesis to gain energy.

- The eukaryote then started symbiotic (a mutually beneficial ) relationship with the bacterium in which the eukaryote provided protection and nutrients to the prokaryote, and in turn, the prokaryotic endosymbiont supplied additional energy to its eukaryotic host through its respiratory cellular apparatus.

- The association became stable over time implementing primary endosymbiosis as the endosymbiont lost some genes it utilized for its independent life and transferred others to the nucleus of the eukaryote. The symbiont thus became needy on the host cell for organic and inorganic compounds and the genes of the repiratory apparatus became a mitochondrion. Endosymbiotic theory hypothesizes the origin of chloroplasts in the same manner, where a eukaryote with mitochondria ingests a photosynthetic cyanobacteruim in a beneficial relationship concluding to a chloroplast organelle.

Endosymbiosis and Mitochondria - The aerobic bacterium sustained within the cell cytoplasm that supplied plenty of molecular food for its heterotrophic existence. The bacterium disintegrated and assimilated these molecules that produced huge energy in the form of adenosine triphosphate (ATP), and so much was liberated that extra ATP was accessible to the host cell's cytoplasm. This extremely benefited the anaerobic cell that then acquired the ability to aerobically digest food. Gradually, the aerobic bacterium could no longer survive independently from the cell, evolving into the mitochondrion organelle. Such aerobically acquired energy greatly superseded that of anaerobic respiration, allowing the stage for hugely accelerated evolution of eukaryotes.

Endosymbiosis and Chloroplasts - Endosymbiotic theory postulates the analogous origin of the chloroplasts. A cell englufed a photosynthetic cyanobacterium and was unsuccessful to digest it. The cyanobacterium sustained in the cell and finally evolved into the first chloroplast.

Some evidences for Endosymbiotic Theory:

• Mitochondria have many likely features as purple-aerobic bacteria. They both utilize O2 in the liberation of ATP, and both of them do this by means of the Kreb's Cycle and oxidative phosphorylation. Likely, chloroplasts are very alike to photosynthetic bacteria that, both have similar chlorophyll that utilize light energy that is converted into chemical energy. Even though there are many similarities among mitochondria and purple aerobic bacteria and chloroplasts and photosynthetic bacteria, they come out to be slight and can be explained by consequent evolution.

• The size of Chloroplasts and Mitochondria are alike compared to that of bacteria, about 1 to 10 µm.

• Mitochondria and chloroplast deoxyribo nucleic acid, ribosomes, ribo nucleic acid and chlorophyll (in case of chloroplasts), and protein synthesis mechanisms were alike to that of bacteria. This gave the initial essential evidence for the endosymbiotic hypothesis. It was also found that mitochondria and chloroplasts multiply independently of the cell where they lived.

• Mitochondria and chloroplasts have two phospholipid bilayers. This seems to have originated when mitochondria and chloroplasts entered eukaryotic cells by means of endocytosis. Both purple aerobic bacteria and photosynthetic bacteria (similar to mitochondria and chloroplast) have a single phospholipid bilayer, but primitively when they entered another cell by means of endocytosis, they are surrounded by a vesicle which forms the second layer of their double phospholipid bilayer.

-The Endosymbiotic Theory provides the most plausible explanation for the development of organelles within the eukaryotic cell, although there are many variants for the theory.

About Author / Additional Info: