• The theory of endosymbiosis is an idea, supported by quite a bit of evidence, stating that some of the structures found in eukaryotes were once prokaryotes that lived independently of the cell. These prokaryotic cells developed a close relationship with eukaryotic cells, eventually becoming part of the larger cell.

• We call organisms that live within the cells of another organism endosymbionts.

  • A modern-day example of an endosymbiont is found in coral cells. Corals are animals that live in shallow ocean waters. Most corals require clear, shallow water to survive because they have very small algal cells that live within their cells.

  • These algal cells provide energy for the corals and the corals provide protection for the algal cells. These algae are known as zooxanthellae. Researchers have been studying this relationship for many years. Interestingly, some corals get their zooxanthellae from their environment as juveniles and some corals seem to have their algal cells with them when they are “born.” Each coral also has a relationship with a specific zooxanthellae “species” and will only partner with their specific species. All corals, however, will expel the zooxanthellae from their cells when they are stressed. These stressors can be changes in temperature, pH, water clarity, etc.

  • Because corals have evolved to live with their zooxanthellae, if they expel their zooxanthellae, they cannot survive and will die. This is known as coral bleaching because the corals turn white without their algal symbionts. Interestingly, if the zooxanthellae are expelled, they too will die because they have evolved to live in a relationship with corals.

Evidence for Endosymbiosis

• There is quite a bit of evidence supporting the theory of endosymbiosis. Both mitochondria and chloroplasts have their own DNA, which is unique among organelles. Additionally, the DNA within both mitochondria and chloroplasts is small and circular, similar to the DNA found in prokaryotes. They both have their own ribosomes for making proteins. This is also unique among organelles.

• Furthermore, mitochondria and chloroplasts can reproduce independently of the rest of the cell. When they reproduce, they split like prokaryotic cells. Lastly, these two organelles are surrounded by a double membrane, which is similar to the cell membrane found in all cells. No other organelles have a cell membrane like this.

• Endosymbiosis explains some of the differences between mitochondria and chloroplasts from other organelles in a eukaryotic cell. Mitochondria are the site of cellular respiration, the process that turns the energy found in food into a form that is usable by the cell. Chloroplasts are the site of photosynthesis, the process that turns light energy from the Sun into chemical energy (food).

How did this happen?

• Scientists have a hypothesis that explains how the endosymbiosis between mitochondria, chloroplasts, and eukaryotic cells could have occurred.

• Scientists think that the endosymbionts might have entered the eukaryotic cell as either prey or as an internal parasite. Once inside, however, the host cell did not destroy it. The cell might have found that it was beneficial to have the mitochondria and/or chloroplasts. Alternatively, the cell might have had a mutation that didn’t allow the larger cell to notice the smaller cell as different. Having the endosymbiont would have been an advantage to the cell, since both of these “organelles” release energy for the cell. This would have made the cell with the endosymbiont(s) better adapted to its environment, which would have given it an evolutionary advantage. The larger cell would have provided protection for the smaller one, making the relationship mutually beneficial for both organisms.

• As the generations passed, the two cells became increasingly interdependent on each other. Eventually, after many generations, the two cells evolved such a close relationship that they became one cell. The example earlier between corals and zooxanthellae represents an intermediary in this evolutionary process – the coral and zooxanthellae cannot survive without one another, but they are not yet a single cell.

• Scientists think that because all eukaryotes have mitochondria, but not all eukaryotes have chloroplasts, the relationship between eukaryotes and mitochondria developed first. Once the relationship with chloroplasts was formed, the cells started producing oxygen as a by-product of photosynthesis, which filled the atmosphere with oxygen, paving the way for organisms that breathe oxygen.

Secondary Endosymbiosis

• Secondary endosymbiosis is another type of endosymbiosis. However, instead of the relationship occurring between a eukaryote and a prokaryote, the relationship occurs between two eukaryotes.

• Dinoflagellates are a large group of single-celled organisms. The algae that live in corals, zooxanthellae, are members of this group. However, some dinoflagellates are photosynthetic and have chloroplasts. The other half are heterotrophic, which means they need to consume other living things to survive. In the case of the photosynthetic dinoflagellates, is thought that heterotrophic single-celled organisms consumed smaller red algae cells, as a type of secondary endosymbiosis. Over time, these two cells learned to live together and eventually became inseparable.

• Euglena, a protist that can perform photosynthesis, is also thought to be a result of endosymbiosis. Euglena’s story is similar to the dinoflagellates’. A large, heterotrophic, single-celled prokaryote consumed a green algae cell, which allowed the larger cell to receive nutrition in the form of glucose from the algae and the algae gained a safe place to live. At first, this relationship was a mutually beneficial relationship between two different organisms. Over time, the two separate organisms became one.

Edited: 05 October 2022