ENDOSYMBIOTIC THEORY

The endosymbiotic theory talks about the origin of cell organelles from prokaryotic organisms.

Chloroplasts originated from the endosymbiotic relationship between an autotrophic and a heterotrophic organism.

The endosymbiotic theory , popularized by Lynn Margulis in 1981, posits that mitochondria and plastids, such as the chloroplast , originated from small prokaryotic organisms that came to live within other larger organisms, in a symbiotic relationship.

In this type of relationship, one organism lives in an intimate association with the other. In endosymbiosis, a smaller organism, called a symbiont , lives inside a larger one, the host , in a mutually beneficial relationship .

→ Origin of mitochondria

Since all cells have mitochondria , but not all have plastids, a model of endosymbiosis suggests that mitochondria arose before plastids . An anaerobic organism is believed to have engulfed an aerobic organism that utilized oxygen to a very good advantage, releasing more energy per glucose molecule than performing an anaerobic process. Thus, with the host gaining more energy and the symbiotic gaining protection , these organisms became one, dependent on each other and inseparable.

Don’t stop now… There’s more after the publicity 😉

→ Origin of chloroplasts

A similar process happened with the chloroplast , but in several steps, due to the existing diversity of these plastids. An autotrophic organism capable of producing its own food, such as a cyanobacterium , would have been engulfed and started to live harmoniously within the host, aiding in food production and gaining protection .

→ Evidence of endosymbiosis:

  • Mitochondria and chloroplasts have their own cellular machinery and genetic material, DNA being circular, like that of bacteria ;
  • They have ribosomes more similar to those of prokaryotic than eukaryotic cells ;
  • The inner membranes have enzymes and transport systems that resemble those found in the plasma membrane of present-day prokaryotic organisms;
  • The process of dividing these organelles is similar to the process of reproduction in bacteria .

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