Endosymbiotic Theory: The Origin of Cell Types

The curiosity of the human being has no limits. He has always needed to appease that need to have knowledge for everything that surrounds him, whether through science or faith. One of the great doubts that has haunted humanity is the origin of life. As a human, wondering about existence, about how it has come to be today, is a fact.

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

• Related article: "Major cell types of the human body"

Prokaryotic and eukaryotic cells

Before starting, it is necessary to bear 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 is no presence of membranous organelles and their DNA is free inside. The opposite is true of eukaryotes, which are full of organelles and whose genetic material is restricted in a region within a barrier known as the nucleus. These data must be kept in mind, because

endosymbiotic theory is based on explaining the appearance of these differences.

• You may be interested: "Differences between DNA and RNA"

Endosymbiotic theory

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

Margulis sought an alternative idea of ​​the origin of eukaryotic cells, establishing that it was based on the union progression of prokaryotic cells, where one cell phagocytes others, but instead of digesting them, makes them part of her. This would have given rise to the different organelles and structures of current eukaryotes. In other words, it speaks of endosymbiosis, one cell is introduced inside another, obtaining mutual benefits through a symbiotic relationship.

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

1. First incorporation

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

Archaea, despite being prokaryotic, 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, to which the increasingly present oxygen in the atmosphere was toxic, needed help to adapt to the new environment. The second incorporation that is postulated is the union of aerobic prokaryotic cells inside the anaerobic cell, explaining the appearance of peroxisome organelles 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, performed endosymbiosis with a prokaryotic cell that had the capacity of photosynthesis (obtaining energy from light), giving rise to the organelle of plant cells, the chloroplast. With this latest addition, there is the origin of the plant kingdom.

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

Evidences and contradictions

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

The clearest is that both mitochondria and chloroplast have their own circular double-stranded DNA inside it freely, independent of the nuclear one. Something striking, since they resemble prokaryotic cells due to their configuration. In addition, they behave like a bacterium, because they synthesize their own proteins, they use 70s ribosomes (and not 80s ribosomes like eukaryotes), perform their functions through the membrane and replicate their DNA and perform binary fission to divide (and not mitosis).

Evidence is also found in its structure. The mitochondria and chloroplast have a double membrane. This could be due to its origin, the interior being the very membrane that enveloped the prokaryotic cell and the external one being the vesicle from when it was phagocytosed.

The biggest point of criticism is on the first incorporation. There is no evidence that can show that this junction between cells existed, and without samples, it is difficult to substantiate. The appearance of other organelles is not explained either. of eukaryotic cells, such as the endoplasmic reticulum and the Golgi apparatus. And the same happens with peroxisomes, which have neither their own DNA nor a double layer of membranes, so there are no samples as reliable as in the mitochondria or in the chloroplast.