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 persecuted humanity is the origin of life. As a human being, asking about existence, about how it has come to this day, is a fact.
Science is not an exception. Many theories are related to this idea. The theory of evolution or the theory of serial endosymbiosis they are clear examples. The latter postulates how the current eukaryotic cells that shape the formation of both animals and plants have been generated.
- Related article: "Types of major cells of the human body"
Prokaryotic and eukaryotic cells
Before starting, it is necessary to keep in mind what is a prokaryotic cell and a eukaryotic cell .
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 happens to the eukaryotes, which are full of organelles and whose genetic material is restricted in a region inside a barrier known as a nucleus. You have to keep this data in mind, because the endosymbiotic theory is based on explaining the appearance of these differences .
- Maybe you're interested: "Differences between DNA and RNA"
Endosymbiotic theory
Also known as the theory of serial endosymbiosis (SET), was nominated by the American evolutionist biologist Lynn Margulis in 1967, to explain the origin of eukaryotic cells. It was not easy, and he was repeatedly denied his publication, because at that time he dominated the idea that eukaryotes were 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 this was based on the progressive union of prokaryotic cells, where one cell fagocita to others, but instead of digesting them, makes them part of it. This would have given rise to the different organelles and structures of the current eukaryotes. In other words, it talks about endosymbiosis, one cell is inserted into another , obtaining mutual benefits through a relationship of symbiosis.
The theory of endosymbiosis describes this gradual process in three large successive additions.
1. First incorporation
In this step, a cell that uses sulfur and heat as an energy source (thermoacidófila archaea) joins with a swimming bacterium (Espiroqueta). With this symbiosis, the ability to move of some eukaryotic cells would start thanks to the flagellum (how the sperm) and the appearance of the nuclear membrane , which gave the DNA greater stability.
Archaea, in spite of being prokaryotes, are a domain different from bacteria, and evolutionarily it has been described that they are closer to eukaryotic cells.
2. Second incorporation
An anaerobic cell, to which the oxygen increasingly present 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 organelles peroxisomes and mitochondria . The former have the capacity to neutralize the toxic effects of oxygen (mainly free radicals), while the latter obtain oxygen energy (respiratory chain). With this step, the animal eukaryotic cell and fungi (fungi) would already appear.
3. Third incorporation
The new aerobic cells, for some reason, performed endosymbiosis with a prokaryotic cell that had the capacity of photosynthesis (obtain 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 additions, the introduced bacteria would benefit from protection and obtaining nutrients, while the host (eukaryotic cell) would gain the ability 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 is that Both the mitochondria and the chloroplast have their own circular double-stranded DNA in its interior in a free way, independent of the nuclear one.Something striking, as they remind some prokaryotic cells by their configuration. In addition, they behave like bacteria, because they synthesize their own proteins, use 70s ribosomes (and not 80s ribosomes 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 its structure. The mitochondria and the chloroplast have a double membrane. This could be due to its origin, the inner being the own membrane that surrounded the prokaryotic cell and the external one the vesicle of when it was phagocytized.
The biggest 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 sustain. The appearance of other organelles is also not explained of eukaryotic cells, such as the endoplasmic reticulum and the Golgi apparatus. And the same happens with peroxisomes, which do not have their own DNA or a double layer of membranes, so there are no samples as reliable as in the mitochondria or in the chloroplast.
