Who proposed endosymbiosis hypothesis to explain the origin of mitochondria and chloroplasts?

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:

Ã¢â‚¬Â¢ 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. Ã¢â‚¬Â¢ The size of Chloroplasts and Mitochondria are alike compared to that of bacteria, about 1 to 10 Ã‚Âµm. Ã¢â‚¬Â¢ 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. Ã¢â‚¬Â¢ 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:

Page ID3220

Gary Kaiser
Community College of Baltimore Country (Cantonsville)

It is thought that life arose on earth around four billion years ago. The endosymbiotic theory states that some of the organelles in today's eukaryotic cells were once prokaryotic microbes. In this theory, the first eukaryotic cell was probably an amoeba-like cell that got nutrients by phagocytosis and contained a nucleus that formed when a piece of the cytoplasmic membrane pinched off around the chromosomes. Some of these amoeba-like organisms ingested prokaryotic cells that then survived within the organism and developed a symbiotic relationship. Mitochondria formed when bacteria capable of aerobic respiration were ingested; chloroplasts formed when photosynthetic bacteria were ingested. They eventually lost their cell wall and much of their DNA because they were not of benefit within the host cell. Mitochondria and chloroplasts cannot grow outside their host cell.

Evidence for this is based on the following:

Chloroplasts are the same size as prokaryotic cells, divide by binary fission, and, like bacteria, have Fts proteins at their division plane. The mitochondria are the same size as prokaryotic cells, divide by binary fission, and the mitochondria of some protists have Fts homologs at their division plane.
Mitochondria and chloroplasts have their own DNA that is circular, not linear.
Mitochondria and chloroplasts have their own ribosomes that have 30S and 50S subunits, not 40S and 60S.
Several more primitive eukaryotic microbes, such as Giardia and Trichomonas have a nuclear membrane but no mitochondria.

Although evidence is less convincing, it is also possible that flagella and cilia may have come from spirochetes.

Figure \(\PageIndex{1}\): One model for the origin of mitochondria and plastids. This model has an amitochondriate eukaryote engulfing an aerobe and then a cyanobacterium. from Kelvinsong

The endosymbiotic theory states that mitochondria and chlopoplasts in today's eukaryotic cells were once separate prokaryotic microbes.

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Biologist Lynn Margulis first made the case for endosymbiosis in the 1960s, but for many years other biologists were skeptical. Although Jeon watched his amoebae become infected with the x-bacteria and then evolve to depend upon them, no one was around over a billion years ago to observe the events of endosymbiosis. Why should we think that a mitochondrion used to be a free-living organism in its own right? It turns out that many lines of evidence support this idea. Most important are the many striking similarities between prokaryotes (like bacteria) and mitochondria:

Membranes — Mitochondria have their own cell membranes, just like a prokaryotic cell does.
DNA — Each mitochondrion has its own circular DNA genome, like a bacteria’s genome, but much smaller. This DNA is passed from a mitochondrion to its offspring and is separate from the “host” cell’s genome in the nucleus.
Reproduction — Mitochondria multiply by pinching in half — the same process used by bacteria. Every new mitochondrion must be produced from a parent mitochondrion in this way; if a cell’s mitochondria are removed, it can’t build new ones from scratch.

When you look at it this way, mitochondria really resemble tiny bacteria making their livings inside eukaryotic cells! Based on decades of accumulated evidence, the scientific community supports Margulis’s ideas: endosymbiosis is the best explanation for the evolution of the eukaryotic cell.

What’s more, the evidence for endosymbiosis applies not only to mitochondria, but to other cellular organelles as well. Chloroplasts are like tiny green factories within plant cells that help convert energy from sunlight into sugars, and they have many similarities to mitochondria. The evidence suggests that these chloroplast organelles were also once free-living bacteria.

The endosymbiotic event that generated mitochondria must have happened early in the history of eukaryotes, because all eukaryotes have them. Then, later, a similar event brought chloroplasts into some eukaryotic cells, creating the lineage that led to plants.

Despite their many similarities, mitochondria (and chloroplasts) aren’t free-living bacteria anymore. The first eukaryotic cell evolved more than a billion years ago. Since then, these organelles have become completely dependent on their host cells. For example, many of the key proteins needed by the mitochondrion are imported from the rest of the cell. Sometime during their long-standing relationship, the genes that code for these proteins were transferred from the mitochondrion to its host’s genome. Scientists consider this mixing of genomes to be the irreversible step at which the two independent organisms become a single individual.

Grabbing take-out: Paramecium bursaria packs a lunch

Paramecium bursaria, a single-celled eukaryote that swims around in pond water, may not have its own chloroplasts, but it does manage to “borrow” them in a rather unusual way. P. bursaria swallows photosynthetic green algae, but it stores them instead of digesting them. In fact, the normally clear paramecium can pack so many algae into its body that it even looks green! When P. bursaria swims into the light, the algae photosynthesize sugar, and both cells share lunch on the go. But P. bursaria doesn’t exploit its algae. Not only does the agile paramecium chauffeur its algae into well-lit areas, it also shares the food it finds with its algae if they are forced to live in the dark.

From prokaryotes to eukaryotes