Enlarge / A photo of Braarudosphaera bigelowii with the nitroplast indicated by an arrowhead.Tyler Coale
The complex cells that underlie animals and plants have a large collection of what are called organelles—compartments surrounded by membranes that perform specialized functions. Two of these were formed through a process called endosymbiosis, in which a once free-living organism is incorporated into a cell. These are the mitochondrion, where a former bacteria now handles the task of converting chemical energy into useful forms, and the chloroplast, where photosynthesis happens.
The fact that there are only a few cases of organelles that evolved through endosymbiosis suggests
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