Evidence for the Endosymbiotic Hypothesis

Similarities Between Bacteria and Semiautonomous Organelles

Since the symbiotic hypothesis states that mitochondria and chloroplasts arose from bacteria entering a eukaryotic cell to form a symbiotic relationship, similarities between bacteria and these semiautonomous organelles show strong evidence that this hypothesis is correct.

  • Function

Mitochondria share very similar characteristics with purple-aerobic bacteria. They both use oxygen in the production of ATP, and they both do this by using the Kreb’s Cycle and oxidative phosphorylation. (mitochondria on the left and purple aerobic bacteria on the right)

Chloroplasts are very similar to photosynthetic bacteria in that they both have very similar chlorophyll that harness light energy to convert into chemical energy. (Chloroplast on the left and photosynthetic bacteria on the right)

Although there are many similarities between mitochondria and purple aerobic bacteria and chloroplasts and photosynthetic bacteria, they appear to be slight and to have arisen via evolution.

  • Size

Size of mitochondria and chloroplasts in comparison to bacteria is another simple observation that supports the endosymbiotic hypothesis. Mitochondria, chloroplasts, and prokaryotes (bacteria) range from about one to ten microns in size. (1 micron=1X10-6 Meters) This seems very basic, but if there was a large difference in sizes between these three components, the hypothesis would appear to be false.

  • DNA, RNA, Ribosomes and Protein Synthesis

The first piece of evidence that needed to be found to support the endosymbiotic hypothesis was whether or not mitochondria and chloroplasts have their own DNA and if this DNA is similar to bacterial DNA. This was later proven to be true for DNA, RNA, ribosomes, chlorophyll (for chloroplasts), and protein synthesis. This provided the first substantial evidence for the endosymbiotic hypothesis. It was also determined that mitochondria and chloroplasts divide independently of the cell they live in.

Mitochondria having their own DNA and dividing independently of the cell is what ultimately results in only mitochondrial DNA being inherited by one’s mother since only an egg cell has DNA while a sperm cell does not. (This relationship also further proves that the discovered characteristics of mitochondria are true.)

This level of independence among semiautonomous organelles shows that they are not very related to the nucleus or other organelles of a eukaryotic cell. Since they are not related, it appears to be even more probable that mitochondria and chloroplasts were originally bacteria that entered the eukaryotic cell via endocytosis to form a symbiotic relationship.

Evolutionary Drive

Scientists (particularly Lynn Margulis) then began to think that if mitochondria and chloroplasts were truly bacteria that were taken into eukaryotic cells via endocytosis, then there must be a historical drive to promote this symbiotic relationship. About 3.8 billion years ago, there were only anaerobic bacteria in existence because Earth’s atmosphere did not contain any oxygen. The first photosynthetic bacteria arose around 3.2 billion years ago and began producing large quantities of oxygen as a byproduct of photosynthesis. Oxygen is very toxic to cells, and as a result, these anaerobic, photosynthetic bacteria became less effective at surviving in their environment. At this point, some of the anaerobic bacteria evolved into aerobic bacteria. Aerobic bacteria are much better suited to this oxygen containing environment and they even use oxygen in the process of making ATP (a molecule that stores a great amount of easily accessible energy). One important factor that both of these bacteria lacked was the ability to ingest large quantities of nutrients from the surrounding environment via phagocytosis. About 1.5 billion years ago, the first nucleated cell (the eukaryote) was arose through evolution, and this cell had the groundbreaking ability to take in large quantities of nutrients via phagocytosis. The fact that bacteria, which are very similar to mitochondria and chloroplasts, existed before the eukaryotic cell shows evidence that it was bacteria that was integrated into a eukaryotic cell rather than eukaryotes being entirely separate in evolutionary history. This timeline also gives evidence as to why a symbiotic relationship would be beneficial.

The photosynthetic and aerobic bacteria were naturally driven to enter into this relationship because the eukaryotic cell supplies both protection and nutrients, and the bacteria supply ways for eukaryotes to harness more energy than they previously could using only glycolysis.

This (above) is the second stage of the glycolysis process (the only stage that actually produces the ATP), and as you can see it only produces a total of 4 ATP (2 net ATP). When this process is combined with the Krebs cycle and oxidative phosphorylation (which requires mitochondria), the net amount of ATP produced is 36-38 molecules.

By eukaryotic cells engulfing photosynthetic bacteria, they could then create  glucose molecules that could then be used to go through the catabolic processes in the mitochondria, and hence, the eukaryotic cell harnesses even more energy than it would on its own. Having so much energy to drive cellular processes makes this new eukaryotic cell more fit for survival.

Double Phospholipid Bilayer

A fairly simple piece of evidence for the endosymbiotic hypothesis is the fact that both mitochondria and chloroplasts have double phospholipid bilayers. This appears to have arisen by mitochondria and chloroplasts entering eukaryotic cells via endocytosis. Both purple, aerobic bacteria (similar to mitochondria) and photosynthetic bacteria (similar to chloroplasts) only have one phospholipid bilayer, but when they enter another cell via endocytosis, they are bound by a vesicle which forms the second layer of their double phospholipid bilayer.

This video shows the process of endocytosis of aerobic bacteria and photosynthetic bacteria very well.

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