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3.7: Primary Endosymbiosis

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    33219
  • The water fern, Azolla, belongs to a different domain of life, Domain Eukarya. Eukaryotes have a more complex cellular makeup and can be either unicellular or multicellular. Eukaryotic cells get their name from the nucleus (eu- meaning true and karyo- meaning seed). In addition to the nucleus, eukaryotic cells contain other membrane-bound organelles. The DNA stored inside the nucleus is linear, as opposed to circular.

    Around 1.56 billion years ago, a heterotrophic eukaryote engulfed a cyanobacterial ancestor in an attempt to eat it, a process called phagocytosis (phago- meaning to eat, cyto- meaning cell). Instead of being digested, this photosynthetic prokaryote remained inside the cell and continued to photosynthesize. The eukaryotic host must have benefited from the sugars produced by the prokaryotic captive and this relationship developed into a symbiosis (sym- meaning shared, bio- meaning life), much like the Anabaena and the Azolla.

    Overtime, the captive prokaryote evolved into the chloroplasts and other plastids that we see in plant cells today. This process of converting a free-living organism into an organelle is called endosymbiosis (endo- meaning inner) and is also the way eukaryotes evolved mitochondria. All plants, such as the water fern you are looking at, descended from this primary endosymbiosis event that led to the first chloroplasts.

    clipboard_e61792d3139b17886502675a0ceea7fa3.png
    Figure \(\PageIndex{1}\): Primary endosymbiosis

    In the diagram of endosymbiosis above, label the cyanobacterium, the eukaryote, nucleoid, nucleus, and chloroplast.

    What evidence could we look for in a chloroplast that would indicate evolution from a bacterium?

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