An endosymbiont is a cell which lives inside another cell with mutual benefit
Eukaryotic cells are believed to have evolved from early prokaryotes that were engulfed by phagocytosis
The engulfed prokaryotic cell remained undigested as it contributed new functionality to the engulfing cell (e.g. photosynthesis)
Over generations, the engulfed cell lost some of its independent utility and became a supplemental organelle
Overview of the Process of Endosymbiosis
Evidence for Endosymbiosis
Mitochondria and chloroplasts are both organelles suggested to have arisen via endosymbiosis
Evidence that supports the extracellular origins of these organelles can be seen by looking at certain key features:
- Membranes (double membrane bound)
- Antibiotics (susceptibility)
- Division (mode of replication)
- DNA (presence and structural composition)
- Ribosomes (size)
Mnmemonic: MAD DR (mad doctor)
Chloroplast and Mitochondrial Evidence
• The origin of eukaryotic cells can be explained by the endosymbiotic theory
Biology, though sometimes boring yet has been teaching us about a number of thought-provoking phenomenon occurring within us and our vicinity. To start it off with the most cardinal concept within this huge concept named endosymbiotic theory of the origin of eukaryotic cells; let us talk about symbiosis. The term symbiosis has a Greek origin, which has a literal meaning; living together. Symbiosis refers to a close relationship between two living cells in which either both or just one gets benefitted. Some common examples of symbiosis include mutualism, commensalism, parasitism, etc. With that being said, let us now move on to our topic of interest i.e. endosymbiosis.
The term endosymbiosis has set its root within the Greek vocabulary, it means to live within. According to the normal vanilla endosymbiosis definition, it is a symbiotic phenomenon in which two living cells live together to mutually benefit each other, but under a constraint that one of them will have to live within the other. An example of endosymbiotic relationship includes Rhizobia, wherein the nitrogen-fixing bacterium lives within the root nodules of leguminous plants in order to benefit each other. Endosymbiosis might seem like most of the other conventional phenomenon of the biological sciences, but is actually the pedestal of an extremely critical phenomenon; Endosymbiotic theory of the origin of eukaryotic cells. The endosymbiotic theory is yet another very complex yet interesting theory within the life sciences discipline. More about endosymbiotic theory is mentioned in the below section of the blog.
Explain the Endosymbiotic Theory of the Origin of Eukaryotic Cells
This theory is a very well-known theory which describes about the way organisms arose and evolved. It also differentiates the eukaryotes from the prokaryotes. The endosymbiotic theory of the origin of eukaryotic cells states that all the individuals are evolved from the same individual. In accordance with the endosymbiotic theory of origin of eukaryotic cells, the eukaryotes have evolved from number of cells that happened to join together and form a single eukaryotic cell.
Now, that we are on the verge to discuss the endosymbiotic theory for the origin of eukaryotes, it is important to state what endosymbiosis is? Endosymbiosis is the process in which one organism lives within the other, and the endosymbiont is the organism that lives within the other organism. So, in accordance with the endosymbiotic theory, every eukaryotic cell that we see today is actually composed of a number of other cells which were once whole in themselves.
It is believed that both chloroplast and mitochondrion were once free living complete beings or cells. According to the endosymbiotic theory, both of them were actually prokaryotes that lived endosymbiotically in some other host and they happened to join the host cell through phagocytosis or by some other means. This eventually led to formation of the eukaryotic cell which actually evolved from an endosymbiotic relationship. Now, in lieu of killing the host cell or getting digested by it, these organelles lived within comfortably. The host cell actually provided these organelles with a safe place to thrive and in return the organelles worked and provided proper energy to the host cell.
According to the endosymbiotic theory of the origin of eukaryotic cells this entire event happened long time ago and since, then the host and the endosymbiont have been evolving together and now they are one single eukaryotic cell or organism composed actually of multiple such types of cells. It is believed that it is not possible to sustain freely within the environment and this led to the evolution of the eukaryotes. Although we still find evidences of free living organisms.
Evidence of Endosymbiosis
The endosymbiosis evidence holds immense importance in the endosymbiotic theory of the origin of eukaryotic cells. There have been multiple endosymbiosis evidences to support the endosymbiotic theory, but very few are considered and accepted by the widely known scientists and researchers. In the year 1883 botany researcher Andreas Schimper observed something odd while looking at the plastids of a green plant. He observed that the plastids or the chloroplasts in particular, were dividing on its own, like that of a single bacterial cell.
Later in time, during the 1960s, a troop of scientists came to a conclusion that the mitochondria as well as the plastids present within a plant cell possess their own DNAs. The DNAs found within these organelles were not similar to the one of the plant cell. Closer examination proved that the genes of both, the mitochondria and the plastids resembled that of the prokaryotes. Hence, it is evident that the cell organelles within the eukaryotic cell actually are related to the prokaryotes.
Hence, according to this evidence of endosymbiosis, it is believed that the eukaryotes actually evolved due to symbiosis and hence, endosymbiotic theory holds true. The cell organelles of the eukaryotes resemble the modern day prokaryotic cells and have their own genetic material. Hence, it is proved that the eukaryotes actually evolved from freely living cells that were once in endosymbiotic relationship with the other.
Although, there are numerous evidences to support Endosymbiotic theory of the origin of eukaryotic cells, but a few theories and evidences have been evoked against it. Hence, Endosymbiotic theory of the origin of eukaryotic cells is actually most controverted and talked about. Majority of evidences are actually in support of the endosymbiotic theory of the origin of eukaryotic cells.