Explaining Symbiogenesis (The Endosymbiotic Theory)


Symbiosis is an equally beneficial partnership between two organisms. An example would be the bacteria in our stomach that help break our food down. Eukaryotic cells are cells that have organelles and a nucleus. These include animal and plant cells. Single celled organisms like bacteria are classified as prokaryotic cells, cells without a nucleus, and have been around for much longer evolutionarily speaking than eukaryotes. The Endosymbiosis Theory states that certain organelles were originally prokaryotic bacteria that were swallowed by a primitive eukaryotic cell for symbiotic properties. These prokaryotes then evolved into what is called the mitochondria in animal cells and chloroplast in plant cells.

Cyanobacteria are a type of photosynthetic bacteria that evolved around 3.5 billion years ago. They preformed photosynthesis and got their energy (ATP) from the intake of carbon dioxide and the sun. Once the process of taking carbon dioxide in and releasing oxygen began some microorganisms acquired the process of cellular respiration. The process that takes in oxygen and releases carbon dioxide producing more energy for the cell than photosynthesis, allowing some of the microorganisms to grow larger and more complex. Cyanobacteria and Purple bacteria (bacteria using cellular respiration) became food sources for other larger and more complex microorganisms. This began the symbiosis partnership.

When larger microorganisms ate purple bacteria the endosymbiosis began. Back then organisms did not have mouths and they just swallowed smaller microorganism whole. This allowed the smaller organisms to stay intact as the membrane surrounded them. The difference in plant and animal cells started with the bacteria that the cells swallowed. Both cyanobacteria and purple bacteria are thought to be the division point between the two eukaryotes. In animal cells, the smaller purple bacteria was swallowed and continued to preform cellular respiration from inside of the larger eukaryotic cell. This gave more energy to the cell while in return the smaller bacteria was given resources and protection. As time went on and the cells evolved, the bacteria gave up some of its genes and became apart that is called the mitochondria. In plant cells, cyanobacteria were swallowed and continued their process of energy production. This allowed the same symbiotic property by giving the cell more energy in return for protection. This evolved into what is the chloroplast in plants.

The Endosymbiosis Theory is backed up by a lot of evidence found in the mitochondria and the chloroplast. This first piece of evidence is the way that they replicate. Both mitochondria and chloroplast divide similarly to the way that bacteria reproduce. The other piece of evidence is DNA. When the DNA extracted from both the mitochondria and the other parts of the cell are compared, they show little resemblance to each other. The fact that the organelle has a different genetic sequence, one that is closer to bacteria, further supports the theory. The ribosomes in the mitochondria are also different from the ribosomes in the rest of the cell. Mitochondrial ribosomes are smaller than the ribosomes in the cytoplasm. Size also reflects the similarly between bacteria and the organelle. They are both 11-10 microns in size, which is similar to their respected parent bacterium.


Endosymbiosis: Lynn Margulis. (nd). Understanding Evolution: The History of Evolutionary Thought. Retrieved from http://evolution.berkeley.edu/evolibrary/article/history_24

Endosymbiosis – the appearance of the eukaryote. (nd). The Fossil Museum. Retrieved from http://www.fossilmuseum.net/Evolution/Endosymbiosis.htm

Koonin, V. E. (2010). The origin and early evolution of eukaryotes in the light of phyogenomics. BioMed Central. Genome Biology 11:209. Retrieved from 10.1186/gb-2010-11-5-209

Origin of eukaryotes. (nd). Shmoop: Biology. Retrieved from http://www.shmoop.com/eukaryotes/eukaryote-origin.html

When did eukaryotic cells (cells with nuclei and other internal organelles) first evolve? What do we know about how they evolved from earlier life forms? (nd). Scientific American. Retrieved from http://www.scientificamerican.com/article/when-did-eukaryotic-cells/



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