Endosymbiosis is a unique and intimate relationship between two organisms in which one organism lives inside the other. The term was first coined by botanist Anton de Bary in 1879, when he observed that certain fungi live inside plant cells. De Bary proposed that this symbiotic relationship might explain how new species are formed.
Endosymbiosis is thought to have played an important role in the evolution of eukaryotic cells, which are cells that contain a nucleus and other organelles enclosed within membranes. Eukaryotic cells are more complex than prokaryotic cells (cells without a nucleus), and they are able to perform many more functions. It is believed that endosymbiosis was key to the development of these more complex cell types.
There are two main types of endosymbiotic relationships: obligate and facultative. In an obligate endosymbiotic relationship, both organisms rely on each other for survival; they cannot live without each other. In a facultative endosymbiotic relationship, one organism can survive without the other, but both benefit from the association.
The best-known examples of endosymbiosis involve bacteria living inside eukaryotic cells. These bacterial symbionts are known as mitochondria and chloroplasts, and they play important roles in energy production and photosynthesis respectively. Mitochondria are often referred to as the “powerhouses” of eukaryotic cells because they generate most of the cell’s ATP (adenosine triphosphate), which is used for energy storage and transport. Chloroplasts convert sunlight into chemical energy that plants use to grow and produce food (photosynthesis).
It is thought that mitochondria and chloroplasts were once free-living bacteria that entered into symbiotic relationships with ancient eukaryotic cells. Over time, these relationships became permanent, and the bacteria lost their ability to live independently outside of their host cell . This process is known as serial endosymbiosis because it involves one Symbiont becoming dependent on another Symbiont . For example, mitochondria probably evolved from purple bacteria that were taken up by early eukaryotes . These purple bacteria were then passed on from one generation to the next until they became permanently integrated into their host cell . Chloroplasts may have arisen in a similar way , though their origins are still debated . Some scientists believe that chloroplasts evolved from cyanobacteria , while others think they arose from green algae . Regardless of their origin , it is clearthat chloroplasts and mitochondria now play essential roles in cellular function .