When two or more species share a close physical interaction, we call this a symbiosis. We share symbioses with many different species. For example, living in our guts are millions and millions of bacteria cells. They live inside us in a symbiotic relationship, from which we both benefit. We benefit from our gut bacteria because they have enzymes that can digest food particles that we cannot. Our gut bacteria benefit from us as they get a place to live (our gut), and a share of some of the nutrients from the food we eat.
Endosymbiosis is a special kind of symbiosis. It’s when one organism lives within the cells or tissues of another. Endosymbioses are very common among unicellular organisms. In the video above, you can see a unicellular organism called Paramecium bursaria (eukaryotic). Within it are many green spheres. Those green spheres are completely separate organisms; they're green algae cells (also eukaryotic) called Chlorella.
Sally Warring. (2016, February 24). Endosymbiosis: Living Together. ASU - Ask A Biologist. Retrieved March 31, 2020 from https://askabiologist.asu.edu/fr/endosymbiosis-living-together
Sally Warring. "Endosymbiosis: Living Together". ASU - Ask A Biologist. 24 February, 2016. https://askabiologist.asu.edu/fr/endosymbiosis-living-together
Sally Warring. "Endosymbiosis: Living Together". ASU - Ask A Biologist. 24 Feb 2016. ASU - Ask A Biologist, Web. 31 Mar 2020. https://askabiologist.asu.edu/fr/endosymbiosis-living-together
Chloroplasts found in plant and algae cells evolved from cyanobacteria that were able to live in other cells unharmed.