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First, according to our text book, the endosymbiotic theory was first thought of by Lynn Margulis. Endosymbiotic theory is the theory that ancestors of mitochondria and chloroplasts were bacteria that was living within other cells in a double sided beneficial partnership. The intracellular bacteria in this double sided beneficial partnership is called an endosymbiont. Over a period of time, each partner became absolutely necessary to each other to function. The endosymbiont eventually lost key features like the ability to replicate independently and the loss of its own cell wall. (Anderson, D. G., et al. 2016)
Now I’ll give you some evidence endosymbiosis. According to WordPress.com, since the endosymbiotic theory states how mitochondria and chloroplasts arose from bacteria entering a eukaryotic cell to form a relationship, common features and abilities between the bacteria and these organelles show strong evidence that this theory is correct. (WordPress)
Something else that shows the relationship between bacteria, the mitochondria and chloroplast is the similarities they share in functioning. Purple aerobic bacteria and the mitochondria share extremely similar characteristics. Both purple aerobic bacteria and the mitochondria use oxygen in the production of ATP. They both do so by utilizing the Krebs Cycle and oxidative phosphorylation. The chloroplasts also show similarities to photosynthetic bacteria. Both chloroplasts and photosynthetic bacteria have similar chlorophyll that use light energy to turn into chemical energy. (WordPress)
These organelles and bacteria also are similar in size. Chloroplasts, mitochondria, and prokaryotes range from about one to ten microns in size. If the chloroplasts, mitochondria, and prokaryotes had a large difference in size the endosymbiotic theory would appear to be false. (WordPress)
Another good piece of evidence that helped in supporting endosymbiosis was whether or not the mitochondria and chloroplasts have their own DNA and to see if this DNA is actually similar to the DNA in bacteria. Eventually, this was proven for ribosomes, chlorophyll, protein synthesis, DNA, and RNA. (WordPress) Chloroplasts, and mitochondria both carry genetic information that is necessary for their function. This also includes genes for certain ribosomal proteins and ribosomal RNAs that make their 70S ribosomes. These ribosomes differ from the typical 80S ribosomes that characterize eukaryotic cells which are in fact equivalent to the bacterial 70S ribosomes. (Anderson, D. G., et al., 2016). Mitochondria and chloroplasts also divide independently of the cell they live in. Mitochondria having their own DNA and dividing independently was big result in only mitochondrial DNA being inherited by ones’ mother since only an egg cell has DNA while sperm cell does not. This ground of independence among these organelles shows that they are indeed not very related to the nucleus or other organelles of a eukaryotic cell and because they are not related, it appears to be even more probable that the mitochondria and chloroplasts were originally bacteria the entered a eukaryotic cell by endocytosis to form this symbiotic relationship. (WordPress)
Another simple piece of evidence pertaining to the endosymbiotic theory was the fact that both mitochondria and chloroplasts have double phospholipid bilayers. This seems to have arisen by when the mitochondria and chloroplasts entered the eukaryotic cells by endocytosis. Photosynthetic bacteria, similar to chloroplast, and purple aerobic bacteria, similar to the mitochondria, only have one phospholipid bilayer, but when either of these enter another cell by endocytosis, they are bound by a vesicle which forms the second layer of their double phospholipid bilayer. (WordPress)
As you can see, there is significant amount of evidence that I provided to show that endosymbiosis does indeed exists.

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