Endosymbiotic theory: the origin of cell types

This theory gives an explanation for the appearance of eukaryotic cells, those of our body.

Human curiosity knows no bounds. He has always needed to appease his need for knowledge about everything around him, either through science or faith. One of the great doubts that has haunted mankind is the origin of life. As a human, wondering about existence, about how it has come to be until today, is a fact.

Science is no exception. Many theories are related to this idea. The theory of evolution or the theory of serial endosymbiosis are clear examples. are clear examples. The latter postulates how the current eukaryotic cells that shape the formation of both animals and plants have been generated.

Prokaryotic and eukaryotic cells.

Before we begin, it is necessary to keep in mind What is a prokaryotic cell and a eukaryotic cell?.

They all have a membrane that separates them from the outside. The main difference between these two types is that in prokaryotes there are no membranous organelles and their DNA is free inside. The opposite is true for eukaryotes, which are full of organelles and whose genetic material is restricted to a region inside a barrier known as the nucleus. These facts must be kept in mind, because endosymbiotic theory is based on explaining the appearance of these differences..

Endosymbiotic theory

Also known as serial endosymbiosis theory (SET), was postulated by the American evolutionary biologist Lynn Margulis, in 1967, to explain the in 1967, to explain the origin of eukaryotic cells. It was not easy, and he was repeatedly denied publication, because at that time the idea that eukaryotes were the result of gradual changes in the composition and nature of the membrane dominated, so this new theory did not fit with the prevailing belief.

Margulis sought an alternative idea of the origin of eukaryotic cells, stating that this was based on the progressive union of prokaryotic cells, where one cell phagocytizes others, but instead of digesting them, it makes them part of it. This would have given rise to the different organelles and structures of today's eukaryotes. In other words, he speaks of endosymbiosis, a cell is introduced into the interior of another cell.The theory of endosymbiosis is based on a symbiotic relationship.

The theory of endosymbiosis describes this gradual process in three major successive incorporations.

1. First incorporation

In this step, a cell that uses sulfur and heat as a source of energy (thermoacidophilic archaea) unites with a swimming bacterium (Spirochete). With this symbiosis, the ability of some eukaryotic cells to move thanks to the flagellum (such as spermatozoa) would begin and the appearance of the nuclear membranewhich gave the DNA greater stability.

Archaea, despite being prokaryotes, are a different domain from bacteria, and evolutionarily it has been described that they are closer to eukaryotic cells.

2. Second incorporation

An anaerobic cell, which found the oxygen increasingly present in the atmosphere toxic, needed help to adapt to the new environment. The second postulated incorporation is the union of aerobic prokaryotic cells to the interior of the anaerobic cell, explaining the appearance of the organelles peroxisomes and mitochondria.. The former have the ability to neutralize the toxic effects of oxygen (mainly free radicals), while the latter obtain energy from oxygen (respiratory chain). With this step, the eukaryotic animal cell and fungi (fungi) would appear.

3. Third incorporation

The new aerobic cells, for some reason, carried out endosymbiosis with a prokaryotic cell that had the capacity for photosynthesis (obtaining energy from light), giving rise to the organelle of plant cells, the chloroplast. With this last incorporation, the origin of the plant kingdom the origin of the plant kingdom.

In the last two incorporations, the introduced bacterium would benefit from protection and the obtaining of nutrients, while the host (eukaryotic cell) would gain the capacity to make use of oxygen and light, respectively.

Evidence and contradictions

Today, the endosymbiotic theory is partially accepted.. There are points that have been found in favor, but others that generate many doubts and discussions.

The clearest one is that both the mitochondrion and the chloroplast have their own circular double-stranded DNA in their interior in a free manner, independent of the nuclear one. This is striking, since they are reminiscent of prokaryotic cells in their configuration. In addition, they behave like bacteria, because they synthesize their own proteins, use ribosomes 70s (and not ribosomes 80s like eukaryotes), develop their functions through the membrane and replicate their DNA and perform binary fission to divide (and not mitosis).

Evidence is also found in their structure. Mitochondria and chloroplasts have a double membrane. This could be due to their origin, the inner one being the membrane that enveloped the prokaryotic cell and the outer one the vesicle from when it was phagocytosed.

The major point of criticism is in the first incorporation. There is no evidence that can demonstrate that this union between cells existed, and without samples, it is difficult to support. The appearance of other organelles in eukaryotic cells, such as the endoplasmic reticulum and the Golgi of eukaryotic cells, such as the endoplasmic reticulum and the Golgi apparatus. And the same is true for peroxisomes, which have neither their own DNA nor a double layer of membranes, so there are no samples as reliable as in the mitochondrion or chloroplast.